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  • Articles  (315)
  • Institute of Physics (IOP)  (315)
  • 2015-2019  (315)
  • 2017  (315)
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  • Articles  (315)
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  • 2015-2019  (315)
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  • 1
    Publication Date: 2017-08-15
    Description: Absolutely separable states ϱ remain separable under arbitrary unitary transformations ##IMG## [http://ej.iop.org/images/1367-2630/19/8/083010/njpaa7e06ieqn1.gif] {$U\varrho {U}^{\dagger }$} . By example of a three qubit system we show that in a multipartite scenario neither full separability implies bipartite absolute separability nor the reverse statement holds. The main goal of the paper is to analyze quantum maps resulting in absolutely separable output states. Such absolutely separating maps affect the states in a way, when no Hamiltonian dynamics can make them entangled afterwards. We study the general properties of absolutely separating maps and channels with respect to bipartitions and multipartitions and show that absolutely separating maps are not necessarily entanglement breaking. We examine the stability of absolutely separating maps under a tensor product and show that ##IMG## [http://ej....] {${{\rm{\Phi }}}^{\otimes N}$}
    Electronic ISSN: 1367-2630
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  • 2
    Publication Date: 2017-08-15
    Description: We demonstrate experimentally a new type of order in optical magnetism resembling the staggered structure of spins in antiferromagnetic ordered materials. We study hybrid electromagnetic metasurfaces created by assembling hybrid meta-atoms formed by metallic split-ring resonators and dielectric particles with a high refractive index, both supporting optically-induced magnetic dipole resonances of different origin. Each pair (or ‘metamolecule’) is characterized by two interacting magnetic dipole moments with the distance-dependent magnetization resembling the spin exchange interaction in magnetic materials. By directly mapping the structure of the electromagnetic fields, we demonstrate experimentally that strong coupling between the optically-induced magnetic moments of different origin can flip the magnetisation orientation in a metamolecule creating an antiferromagnetic lattice of staggered optically-induced magnetic moments in hybrid metasurfaces.
    Electronic ISSN: 1367-2630
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  • 3
    Publication Date: 2017-09-01
    Description: Description unavailable
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  • 4
    Publication Date: 2017-09-01
    Description: By developing a hydrodynamic formalism, we investigate the expansion dynamics of the single-minimum phase of a binary spin–orbit coupled Bose–Einstein condensate, after releasing from an external harmonic trap. We find that the expansion of the condensate along the direction of the spin–orbit coupling is dramatically slowed down near the transition between the single-minimum phase and the plane-wave phase. Such a slow expansion, resembling a form of an effective localization, is due to the quenching of the superfluid motion which results in a strong increase of the effective mass. In the single-minimum phase the anisotropic expansion of the Bose gas, which is spin balanced at equilibrium, is accompanied by the emergence of a local spin polarization. Our analytic scaling solutions emerging from hydrodynamic picture are compared with a full numerical simulation based on the coupled Gross–Pitaevskii equations.
    Electronic ISSN: 1367-2630
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  • 5
    Publication Date: 2017-09-06
    Description: We investigate exciton-polariton condensates with a rotating potential induced by an electric field in a semiconductor microcavity ring. In the absence of transverse-electric-transverse-magnetic (TE-TM) splitting, we find that there is a critical laser pump rate, above which the quantized phase slips appear with hysteresis, and there exist quantized average angular momenta at fixed angular velocities, regardless of the pump rate. When considering the TE-TM splitting, we find that there are a series of hysteresis loops with the same intervals which are modulated by the magnitude of the splitting for a linear polarization laser. Further, multistability occurs at large pump rate, and there arises a phase slip for only one spin with large absolute value of the polarization degree, owing to the competition of the TE-TM splitting and energy induced by rotation. These results can be verified experimentally with current technology, and are useful for future electro-optic devices.
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  • 6
    Publication Date: 2017-09-06
    Description: We study the appearance and properties of cluster crystals (solids in which the unit cell is occupied by a cluster of particles) in a two-dimensional system of self-propelled active Brownian particles with repulsive interactions. Self-propulsion deforms the clusters by depleting particle density inside, and for large speeds it melts the crystal. Continuous field descriptions at several levels of approximation allow us to identify the relevant physical mechanisms.
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  • 7
    Publication Date: 2017-09-12
    Description: When neutrino masses arise from the exchange of neutral heavy leptons, as in most seesaw schemes, the effective lepton mixing matrix N describing neutrino propagation is non-unitary, hence neutrinos are not exactly orthonormal. New CP violation phases appear in N that could be confused with the standard phase ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093005/njpaa79ecieqn1.gif] {${\delta }_{\mathrm{CP}}$} characterizing the three neutrino paradigm. We study the potential of the long-baseline neutrino experiment DUNE in probing CP violation induced by the standard CP phase in the presence of non-unitarity. In order to accomplish this we develop our previous formalism, so as to take into account the neutrino interactions with the medium, important in long baseline experiments such as DUNE. We find that the expected CP sensitivity of DUNE is somewhat degraded with respect to that characterizing the standard unitary case. However the effect ...
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  • 8
    Publication Date: 2017-08-22
    Description: Novel ion traps that provide either a static or a dynamic magnetic gradient field allow for the use of radio-frequency radiation for coupling internal and motional states of ions, which is essential for conditional quantum logic. We show that the Hamiltonian describing this coupling in the presence of a resonant dynamic gradient, is identical, in a dressed state basis, to the Hamiltonian in the case of a static gradient. The coupling strength is in both cases described by the same effective Lamb-Dicke parameter. This insight can be used to overcome demanding experimental requirements when using a dynamic gradient field for state-of-the-art experiments with trapped ions, for example, in quantum information science. At the same time, this insight opens new experimental perspectives by way of using a single resonant or detuned dynamic gradient field, inducing long-range coupling, for conditional multi-qubit dynamics.
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  • 9
    Publication Date: 2017-08-22
    Description: We study the optical properties of an ensemble of two-level atoms coupled to a 1D photonic crystal waveguide (PCW), which mediates long-range coherent dipole–dipole interactions between the atoms. We show that the long-range interactions can dramatically alter the linear and nonlinear optical behavior, as compared to a typical atomic ensemble. In particular, in the linear regime, we find that the transmission spectrum contains multiple transmission dips, whose properties we characterize. Moreover, we show how the linear spectrum may be used to infer the number of atoms present in the system, constituting an important experimental tool in a regime where techniques for conventional ensembles break down. We also show that some of the transmission dips are associated with an effective ‘two-level’ resonance that forms due to the long-range interactions. In particular, under strong global driving and appropriate conditions, we find that the atomic ensemble is only capable of absorbing...
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  • 10
    Publication Date: 2017-08-22
    Description: We study numerically the geometric entanglement in the Laughlin wave function, which is of great importance in condensed matter physics. The Slater determinant having the largest overlap with the Laughlin wave function is constructed by an iterative algorithm. The logarithm of the overlap, which is a geometric quantity, is then taken as a geometric measure of entanglement. It is found that the geometric entanglement is a linear function of the number of electrons to a good extent. This is especially the case for the lowest Laughlin wave function, namely the one with filling factor of 1/3. Surprisingly, the linear behavior extends well down to the smallest possible value of the electron number, namely, N = 2. The constant term does not agree with the expected topological entropy. In view of previous works, our result indicates that the relation between geometric entanglement and topological entropy is very subtle.
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  • 11
    Publication Date: 2017-09-01
    Description: Continuous monitoring of a cloud of antiprotons stored in a Penning trap for 405 days enables us to set an improved limit on the directly measured antiproton lifetime. From our measurements we extract a storage time of ##IMG## [http://ej.iop.org/images/1367-2630/19/8/083023/njpaa7e73ieqn1.gif] {$3.15\times {10}^{8}$} equivalent antiproton-seconds, resulting in a lower lifetime limit of ##IMG## [http://ej.iop.org/images/1367-2630/19/8/083023/njpaa7e73ieqn2.gif] {${\tau }_{\bar{{\rm{p}}}}\gt 10.2\,{\rm{a}}$} with a confidence level of ##IMG## [http://ej.iop.org/images/1367-2630/19/8/083023/njpaa7e73ieqn3.gif] {$68 \% $} . This result improves the limit on charge-parity-time violation in antiproton decays based on direct observation by a factor of 7.
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  • 12
    Publication Date: 2017-09-14
    Description: Description unavailable
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  • 13
    Publication Date: 2017-08-15
    Description: When a periodically modulated many-body quantum system is weakly coupled to an environment, the combined action of these temporal modulations and dissipation steers the system towards a state characterized by a time-periodic density operator. To resolve this asymptotic non-equilibrium state at stroboscopic instants of time, we use the dissipative propagator over one period of modulations, ‘Floquet map’, and evaluate the stroboscopic density operator as its invariant. Particle interactions control properties of the map and thus the features of its invariant. In addition, the spectrum of the map provides insight into the system relaxation towards the asymptotic state and may help to understand whether it is possible (or not) to construct a stroboscopic time-independent Lindblad generator which mimics the action of the original time-dependent one. We illustrate the idea with a scalable many-body model, a periodically modulated Bose–Hubbard dimer. We contrast the relations between t...
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  • 14
    Publication Date: 2017-08-17
    Description: We investigate high-order harmonic generation (HHG) from noble gas clusters in a supersonic gas jet. To identify the contribution of harmonic generation from clusters versus that from gas monomers, we measure the high-order harmonic output over a broad range of the total atomic number density in the jet (from 3×10 16 to 3 × 10 18 ##IMG## [http://ej.iop.org/images/1367-2630/19/8/083017/njpaa8029ieqn1.gif] {${\mathrm{cm}}^{-3}$} ) at two different reservoir temperatures (303 and 363 K). For the first time in the evaluation of the harmonic yield in such measurements, the variation of the liquid mass fraction, g , versus pressure and temperature is taken into consideration, which we determine, reliably and consistently, to be below 20% within our range of experimental parameters. By comparing the measured harmonic yield from a thin jet with the calculated corresponding yield from monomers alone, we find an increased emission of the harm...
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  • 15
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    Institute of Physics (IOP)
    Publication Date: 2017-08-17
    Description: A major challenge facing existing sequential Monte Carlo methods for parameter estimation in physics stems from the inability of existing approaches to robustly deal with experiments that have different mechanisms that yield the results with equivalent probability. We address this problem here by proposing a form of particle filtering that clusters the particles that comprise the sequential Monte Carlo approximation to the posterior before applying a resampler. Through a new graphical approach to thinking about such models, we are able to devise an artificial-intelligence based strategy that automatically learns the shape and number of the clusters in the support of the posterior. We demonstrate the power of our approach by applying it to randomized gap estimation and a form of low circuit-depth phase estimation where existing methods from the physics literature either exhibit much worse performance or even fail completely.
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  • 16
    Publication Date: 2017-08-22
    Description: Topological superfluid is an exotic state of quantum matter that possesses a nodeless superfluid gap in the bulk and Andreev edge modes at the boundary of a finite system. Here, we study a multi-orbital superfluid driven by an attractive s -wave interaction in a rotating optical lattice. Interestingly, we find that the rotation induces the inter-orbital hybridization and drives the system into topological orbital superfluid in accordance with intrinsically chiral d -wave pairing characteristics. Thanks to the conservation of spin, the topological orbital superfluid supports four rather than two chiral Andreev edge modes at the boundary of the lattice. Moreover, we find that the intrinsic harmonic confining potential forms a circular spatial barrier which accumulates atoms and supports a mass current under the injection of small angular momentum as an external driving force. This feature provides an experimentally detectable phenomenon to verify the topological orbita...
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  • 17
    Publication Date: 2017-09-09
    Description: The influence of proton irradiation on the fluctuation-induced magnetoconductivity of high quality FeSe 1− x Te x ( x = 0.4, 0.55) (FST) thin films has been investigated. The measurements were performed with magnetic fields up to 13 T applied in the two main crystal directions. The results were interpreted in terms of the Ginzburg–Landau approach for three-dimensional materials under a total-energy cutoff. The analysis shows that properly-tuned proton irradiation does not appreciably affect fundamental superconducting parameters like the T c value, the upper critical fields or the anisotropy. This has important consequences from the point of view of possible applications due to the enhancement of vortex pinning induced by irradiation.
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  • 18
    Publication Date: 2017-09-09
    Description: Cytoskeletal filaments provide cells with mechanical stability and organization. The main key players are actin filaments and microtubules governing a cell’s response to mechanical stimuli. We investigated the specific influences of these crucial components by deforming MCF-7 epithelial cells at small (≤5% deformation) and large strains (〉5% deformation). To understand specific contributions of actin filaments and microtubules, we systematically studied cellular responses after treatment with cytoskeleton influencing drugs. Quantification with the microfluidic optical stretcher allowed capturing the relative deformation and relaxation of cells under different conditions. We separated distinctive deformational and relaxational contributions to cell mechanics for actin and microtubule networks for two orders of magnitude of drug dosages. Disrupting actin filaments via latrunculin A, for instance, revealed a strain-independent softening. Stabilizing these filaments by treatment ...
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  • 19
    Publication Date: 2017-09-14
    Description: Close to equilibrium, the exchange of particles and heat between macroscopic systems at different temperatures and different chemical potentials is known to be governed by a matrix of transport coefficients which are positive and symmetric. We investigate the amounts of heat and particles that are exchanged between two small quantum systems within a given time, and find them characterized by a transport matrix which neither needs to be symmetric nor positive. At larger times even spontaneous transport can be observed in the total absence of temperature and chemical potential differences provided that the two systems are different in size. All these deviations from standard transport behavior can be attributed to the fact that work is done on the system in the processes contacting and separating those parts of the system that initially possess different temperatures and chemical potentials. The standard transport properties are recovered for vanishing work and also in the limit o...
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  • 20
    Publication Date: 2017-09-14
    Description: We show that the early time dynamics of easy-axis magnetic domain formation in a spinor condensate is described by percolation theory. These dynamics could be initialized using a quench of the spin-dependent interaction parameter. We propose a scheme to observe the same dynamics by quenching the quadratic Zeeman energy and applying a generalized spin rotation to a ferromagnetic spin-1 condensate. Using simulations we investigate the finite-size scaling behavior to extract the correlation length critical exponent and the transition point. We analyze the sensitivity of our results to the early-time dynamics of the system, the quadratic Zeeman energy, and the threshold condition used to define the positive (percolating) domains.
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  • 21
    Publication Date: 2017-09-14
    Description: Resistive switching (RS) is one of the foremost candidates for building novel types of non-volatile random access memories. Any practical implementation of such a memory cell calls for a strong miniaturization, at which point fluctuations start playing a role that cannot be neglected. A detailed understanding of switching mechanisms and reliability is essential. For this reason, we formulate a particle model based on the stochastic motion of oxygen vacancies. It allows us to investigate fluctuations in the resistance states of a switch with two active zones. The vacancies’ dynamics are governed by a master equation. Upon the application of a voltage pulse, the vacancies travel collectively through the switch. By deriving a generalized Burgers equation we can interpret this collective motion as nonlinear traveling waves, and numerically verify this result. Further, we define binary logical states by means of the underlying vacancy distributions, and establish a framework of writi...
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  • 22
    Publication Date: 2017-09-14
    Description: The physical nature of any quantum source guarantees the existence of an effective Hilbert space of finite dimension, the physical sector, in which its state is completely characterized with arbitrarily high accuracy. The extraction of this sector is essential for state tomography. We show that the physical sector of a state, defined in some pre-chosen basis, can be systematically retrieved with a procedure using only data collected from a set of commuting quantum measurement outcomes, with no other assumptions about the source. We demonstrate the versatility and efficiency of the physical-sector extraction by applying it to simulated and experimental data for quantum light sources, as well as quantum systems of finite dimensions.
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  • 23
    Publication Date: 2017-09-15
    Description: We discuss resonant states with diverging Q factor within the radiation continuum based on the anomalous interaction of leaky guided modes and diffracted waves in suitably designed reflection gratings. We show that these trapped optical states can be understood within the framework of leaky-wave theory, which unveils their generation process and dynamics. Our findings reveal an interesting mechanism to realize embedded eigenstates in periodic structures, shedding light on their electromagnetic properties, and offering the possibility to quantitatively predict their occurrence and systematically design optimal structures that support them. The realization of extraordinary optical trapping in open structures may be important for applications that require strongly confined and enhanced fields and high selectivity in angle and frequency.
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  • 24
    Publication Date: 2017-09-15
    Description: The laws of quantum mechanics allow for the distribution of a secret random key between two parties. Here we analyse the security of a protocol for establishing a common secret key between N parties (i.e. a conference key), using resource states with genuine N -partite entanglement. We compare this protocol to conference key distribution via bipartite entanglement, regarding the required resources, achievable secret key rates and threshold qubit error rates. Furthermore we discuss quantum networks with bottlenecks for which our multipartite entanglement-based protocol can benefit from network coding, while the bipartite protocol cannot. It is shown how this advantage leads to a higher secret key rate.
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  • 25
    Publication Date: 2017-09-25
    Description: We present a quantitative characterization of an electrically tunable Josephson junction defined in an InAs nanowire proximitized by an epitaxially-grown superconducting Al shell. The gate-dependence of the number of conduction channels and of the set of transmission coefficients are extracted from the highly nonlinear current–voltage characteristics. Although the transmissions evolve non-monotonically, the number of independent channels can be tuned, and configurations with a single quasi-ballistic channel achieved.
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  • 26
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    Institute of Physics (IOP)
    Publication Date: 2017-06-24
    Description: We study the existence of the maximal quantum Fisher information matrix in the multi-parameter quantum estimation, which bounds the ultimate precision limit. We show that when the maximal quantum Fisher information matrix exists, it can be directly obtained from the underlying dynamics. Examples are then provided to demonstrate the usefulness of the maximal quantum Fisher information matrix by deriving various trade-off relations in multi-parameter quantum estimation and obtaining the bounds for the scalings of the precision limit.
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  • 27
    Publication Date: 2017-06-24
    Description: In this study, we investigate analytically and experimentally the roles of quasi-linearly polarized (LP), hybrid, plasmonic and photonic modes in optical detection and excitation with aperture tips in scanning near-field optical microscopy. Aperture tips are tapered and metal-coated optical fibers where small circular apertures are made at the apex. In aperture tips, there exist plasmonic modes that are bound at the interface of the metal cladding to the inner dielectric fiber and photonic modes that are guided in the area of the increased index in the dielectric fiber core. The fundamental photonic mode, although excited by the free-space Gaussian beam, experiences cutoff and turns into an evanescent mode. The photonic mode also becomes lossier than the plasmonic mode toward the tip aperture, and its power decay due to absorption and reflection is expected to be at least 10 −9 . In contrast, the fundamental plasmonic mode has no cutoff and thus reaches all the way to t...
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  • 28
    Publication Date: 2017-06-24
    Description: We investigate the utility of the convex hull of many Lagrangian tracers to analyze transport properties of turbulent flows with different anisotropy. In direct numerical simulations of statistically homogeneous and stationary Navier–Stokes turbulence, neutral fluid Boussinesq convection, and MHD Boussinesq convection a comparison with Lagrangian pair dispersion shows that convex hull statistics capture the asymptotic dispersive behavior of a large group of passive tracer particles. Moreover, convex hull analysis provides additional information on the sub-ensemble of tracers that on average disperse most efficiently in the form of extreme value statistics and flow anisotropy via the geometric properties of the convex hulls. We use the convex hull surface geometry to examine the anisotropy that occurs in turbulent convection. Applying extreme value theory, we show that the maximal square extensions of convex hull vertices are well described by a classic extreme value distribution...
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  • 29
    Publication Date: 2017-06-27
    Description: We introduce and demonstrate an experimental method, optically-detected spin-echo (ODSE), to measure ground-state relaxation times of a rubidium (Rb) atomic vapor held in a glass cell with buffer-gas. The work is motivated by our studies on high-performance Rb atomic clocks, where both population and coherence relaxation times ( T 1 and T 2 , respectively) of the ‘clock transition’ (5 2 S 1/2 ##IMG## [http://ej.iop.org/images/1367-2630/19/6/063027/njpaa73c2ieqn1.gif] {$| {F}_{g}\,=\,1,{m}_{F}=0\rangle \leftrightarrow | {F}_{g}=2,{m}_{F}=0\rangle $} ) are relevant. Our ODSE method is inspired by classical nuclear magnetic resonance spin-echo method, combined with optical detection. In contrast to other existing methods, like continuous-wave double-resonance (CW-DR) and Ramsey-DR, principles of the ODSE method allow suppression of decoherence arising from the inhomogeneity of the static magnetic field across...
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  • 30
    Publication Date: 2017-06-27
    Description: Tunneling two-level systems (TLSs) are ubiquitous in amorphous solids, and form a major source of noise in systems such as nano-mechanical oscillators, single electron transistors, and superconducting qubits. Occurance of defect tunneling despite their coupling to phonons is viewed as a hallmark of weak defect–phonon coupling. This is since strong coupling to phonons results in significant phonon dressing and suppresses tunneling in two-level tunneling defects effectively. Here we determine the dynamics of a tunneling defect in a crystal strongly coupled to phonons incorporating the full 3D geometry in our description. We find that inversion symmetric tunneling is not dressed by phonons whereas other tunneling pathways are dressed by phonons and, thus, are suppressed by strong defect–phonon coupling. We provide the linear acoustic and dielectric response functions for a tunneling defect in a crystal for strong defect–phonon coupling. This allows direct experimental determination...
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  • 31
    Publication Date: 2017-09-15
    Description: Strong coupling between a system and its environment leads to the emergence of non-Markovian dynamics, which cannot be described by a time-local master equation. One way to capture such dynamics is to use numerical real-time path integrals, where assuming a finite bath memory time enables manageable simulation scaling. However, by comparing to the exactly soluble independent boson model, we show that the presence of transient negative decay rates in the exact dynamics can result in simulations with unphysical exponential growth of density matrix elements when the finite memory approximation is used. We therefore reformulate this approximation in such a way that the exact dynamics are reproduced identically and then apply our new method to the spin-boson model with superohmic environmental coupling, commonly used to model phonon environments, but which cannot be solved exactly. Our new method allows us to easily access parameter regimes where we find revivals in population dynami...
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  • 32
    Publication Date: 2017-09-15
    Description: Topological magnetic textures—like skyrmions—are major players in the design of next-generation magnetic storage technology due to their stability and the control of their motion by ultra-low currents. A major challenge to develop new skyrmion-based technologies is the controlled creation of magnetic skyrmions without the need of complex setups. We show how to create skyrmions and other magnetic textures in ferromagnetic thin films by means of a homogeneous DC current and without requiring Dzyaloshinskii–Moriya interactions. This is possible by exploiting a static loss of stability arising from the interplay of current-induced spin-transfer-torque and a spatially inhomogeneous magnetization, which can be achieved, e.g., by locally engineering the anisotropy, the magnetic field, or other magnetic interactions. The magnetic textures are created controllably and efficiently with a period that can be tuned by the applied current strength. We propose a specific experimental setup rea...
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  • 33
    Publication Date: 2017-09-15
    Description: Modern high performance circular accelerators require sophisticated corrections of nonlinear lattices. The beam betatron tune footprint may cross many resonances, reducing dynamic aperture and causing particle loss. However, if particles cross a resonance reasonably fast, the beam deterioration may be minimized. This paper describes the experiments with the beam passing through a half-integer resonance stopband via tune modulation by exciting synchrotron oscillations. This is the first time that beam dynamics have been kept under precise control while the beam crosses a half-integer resonance. Our results convincingly demonstrate that particles can cross the half-integer resonance without being lost if the passage is reasonably fast and the resonance stopband is sufficiently narrow.
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  • 34
    Publication Date: 2017-09-25
    Description: We considered various types of potential noise in gates controlling non-adiabatic holonomic transformations of spin-qubits in one and two-dimensional systems with the Rashba interaction. It is shown how exact results can be derived for deviations of spin rotation angle and fidelity of the qubit transformation after a completed transformation. Errors in initial values of gate potentials and time-dependent drivings are considered and exact results for white gate noise are derived and analysed in detail. It is demonstrated how the drivings can be tuned to optimise the final fidelity of the transformation and to minimise the variances of qubit transformations.
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  • 35
    Publication Date: 2017-09-25
    Description: Universal scaling behavior in the relaxation dynamics of an isolated two-dimensional Bose gas is studied by means of semi-classical stochastic simulations of the Gross–Pitaevskii model. The system is quenched far out of equilibrium by imprinting vortex defects into an otherwise phase-coherent condensate. A strongly anomalous non-thermal fixed point is identified, associated with a slowed decay of the defects in the case that the dissipative coupling to the thermal background noise is suppressed. At this fixed point, a large anomalous exponent ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093014/njpaa7eebieqn1.gif] {$\eta \simeq -3$} and, related to this, a large dynamical exponent ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093014/njpaa7eebieqn2.gif] {$z\simeq 5$} are identified. The corresponding power-law decay is found to be consistent with three-vortex-collision induced loss. The article discusses these aspects of non-therm...
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  • 36
    Publication Date: 2017-09-25
    Description: The outcomes of quantum mechanical measurements are inherently random. It is therefore necessary to develop stringent methods for quantifying the degree of statistical uncertainty about the results of quantum experiments. For the particularly relevant task of quantum state tomography, it has been shown that a significant reduction in uncertainty can be achieved by taking the positivity of quantum states into account. However—the large number of partial results and heuristics notwithstanding—no efficient general algorithm is known that produces an optimal uncertainty region from experimental data, while making use of the prior constraint of positivity. Here, we provide a precise formulation of this problem and show that the general case is NP-hard. Our result leaves room for the existence of efficient approximate solutions, and therefore does not in itself imply that the practical task of quantum uncertainty quantification is intractable. However, it does show that there exists a...
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  • 37
    Publication Date: 2017-08-15
    Description: We demonstrate that the introduction of an elemental beam of Mn during the molecular beam epitaxial growth of Bi 2 Se 3 results in the formation of layers of Bi 2 MnSe 4 that intersperse between layers of pure Bi 2 Se 3 . This study revises the assumption held by many who study magnetic topological insulators (TIs) that Mn incorporates randomly at Bi-substitutional sites during epitaxial growth of Mn:Bi 2 Se 3 . Here, we report the formation of thin film magnetic TI Bi 2 MnSe 4 with stoichiometric composition that grows in a self-assembled multilayer heterostructure with layers of Bi 2 Se 3 , where the number of Bi 2 Se 3 layers separating the single Bi 2 MnSe 4 layers is approximately defined by the relative arrival rate of Mn ions to Bi and Se ions during growth, and we present its compositional, structural, and electronic propert...
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  • 38
    Publication Date: 2017-08-15
    Description: The dependence of hydrogen coverage on the bulk doping concentration is investigated for the polar O-terminated (0001̅) ZnO surface. We use hybrid density-functional theory in combination with ab initio thermodynamics to determine a doping-dependent phase diagram of this surface. For hydrogen coverages lower than 50% dangling oxygen bonds remain at the surface, where they subsequently become charged by bulk electrons. For such charged surfaces, a computational first-principles approach is presented, with which long-range band bending can now be included in first-principles supercell calculations. In this work, we use a modified and extended version of the recently introduced charge-reservoir electrostatic sheet technique (Sinai et al 2015 Phys. Rev. B 91 075311) to incorporate band bending effects directly into our first-principles calculations. This allows us to investigate the effect of space charge layers and the resulting band bendin...
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  • 39
    Publication Date: 2017-08-17
    Description: In this work, we study strongly interacting spinor atoms in a lattice subject to a two dimensional (2d) anisotropic Rashba type of spin orbital coupling (SOC) and an Zeeman field. We find the interplay between the Zeeman field and the SOC provides a new platform to host rich and novel classes of quantum commensurate and in-commensurate phases, excitations and phase transitions. These commensurate phases include two collinear states at low and high Zeeman field, two co-planar canted states at mirror reflected SOC parameters respectively. Most importantly, there are non-coplanar incommensurate Skyrmion (IC-SkX) crystal phases surrounded by the four commensurate phases. New excitation spectra above all the five phases, especially on the IC-SKX phase are computed. Three different classes of quantum commensurate to in-commensurate transitions from the IC-SKX to its four neighboring commensurate phases are identified. Finite temperature behaviors and transitions are discussed. The cri...
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  • 40
    Publication Date: 2017-08-17
    Description: In Zeeman deceleration, time-varying spatially inhomogeneous magnetic fields are used to create packets of translationally cold, quantum-state-selected paramagnetic particles with a tuneable forward velocity, which are ideal for cold reaction dynamics studies. Here, the covariance matrix adaptation evolutionary strategy is adopted in order to optimise deceleration switching sequences for the operation of a Zeeman decelerator. Using the optimised sequences, a 40% increase in the number of decelerated particles is observed compared to standard sequences for the same final velocity, imposing the same experimental boundary conditions. Furthermore, we demonstrate that it is possible to remove up to 98% of the initial kinetic energy of particles in the incoming beam, compared to the removal of a maximum of 83% of kinetic energy with standard sequences. Three-dimensional particle trajectory simulations are employed to reproduce the experimental results and to investigate differences in...
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  • 41
    Publication Date: 2017-08-17
    Description: We consider a model of the exciton-polariton condensate based on a system of two Gross–Pitaevskii equations coupled by the second-order differential operator, which represents the spin–orbit coupling in the system. Also included are the linear gain, effective diffusion, nonlinear loss, and the standard harmonic-oscillator trapping potential, as well as the Zeeman splitting. By means of combined analytical and numerical methods, we identify stable two-dimensional modes supported by the nonlinear system. In the absence of the Zeeman splitting, these are mixed modes, which combine zero and nonzero vorticities in each of the two spinor components, and vortex–antivortex complexes. We have also found a range of parameters where the mixed-mode and vortex–antivortex states coexist and are stable. Sufficiently strong Zeeman splitting creates stable semi-vortex states, with vorticities 0 in one component and 2 in the other.
    Electronic ISSN: 1367-2630
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  • 42
    Publication Date: 2017-08-26
    Description: Beams of short-lived radioactive nuclei are needed for frontier experimental research in nuclear structure, reactions, and astrophysics. Negatively charged radioactive ion beams have unique advantages and allow for the use of a tandem accelerator for post-acceleration, which can provide the highest beam quality and continuously variable energies. Negative ion beams can be obtained with high intensity and some unique beam purification techniques based on differences in electronegativity and chemical reactivity can be used to provide beams with high purity. This article describes the production of negative radioactive ion beams at the former holifield radioactive ion beam facility at Oak Ridge National Laboratory and at the CERN ISOLDE facility with emphasis on the development of the negative ion sources employed at these two facilities.
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  • 43
    Publication Date: 2017-08-26
    Description: An optomechanical force sensor for a mechanical oscillator in a non-Markovian environment is presented. By performing homodyne detection, we obtain a general expression for the output signal. It is shown that the weak force detection is sensitive to the non-Markovian environment. The additional noise can be reduced and the mechanical sensitivity can be obviously amplified compared to the Markovian condition even in resolved sideband regimes without using assistant systems or squeezing. Our results provide a promising platform for improving the sensitivity of weak-force ultrasensitive detection.
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  • 44
    Publication Date: 2017-08-26
    Description: The ITER Neutral Beam Test Facility (NBTF), called PRIMA (Padova Research on ITER Megavolt Accelerator), is hosted in Padova, Italy and includes two experiments: MITICA, the full-scale prototype of the ITER heating neutral beam injector, and SPIDER, the full-size radio frequency negative-ions source. The NBTF realization and the exploitation of SPIDER and MITICA have been recognized as necessary to make the future operation of the ITER heating neutral beam injectors efficient and reliable, fundamental to the achievement of thermonuclear-relevant plasma parameters in ITER. This paper reports on design and R&D carried out to construct PRIMA, SPIDER and MITICA, and highlights the huge progress made in just a few years, from the signature of the agreement for the NBTF realization in 2011, up to now—when the buildings and relevant infrastructures have been completed, SPIDER is entering the integrated commissioning phase and the procurements of several MITICA components are at a w...
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  • 45
    Publication Date: 2017-09-06
    Description: We study the critical behavior associated with transient dynamics of vortices near the depinning transition by an ac drive. From ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093001/njpaa819dieqn1.gif] {${I}_{{\rm{ac}},{\rm{e}}}$} – ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093001/njpaa819dieqn2.gif] {${V}_{{\rm{ac}},{\rm{e}}}$} curves at different frequencies f , where ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093001/njpaa819dieqn3.gif] {${I}_{{\rm{ac}},{\rm{e}}}$} and ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093001/njpaa819dieqn4.gif] {${V}_{{\rm{ac}},{\rm{e}}}$} are the effective current and voltage of sinusoidal waveform, respectively, we clearly identify the characteristic voltage separating linear and nonlinear regimes, from which we can estimate the mean diameter of the pinning potential. We also measure the time evolution of the voltage V ac
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  • 46
    Publication Date: 2017-09-09
    Description: It is shown that the concept of topological phase transitions can be used to design nonlinear photonic structures exhibiting power thresholds and discontinuities in their transmittance. This provides a novel route to devising nonlinear optical isolators. We study three representative designs: (i) a waveguide array implementing a nonlinear 1D Su–Schrieffer–Heeger model, (ii) a waveguide array implementing a nonlinear 2D Haldane model, and (iii) a 2D lattice of coupled-ring waveguides. In the first two cases, we find a correspondence between the topological transition of the underlying linear lattice and the power threshold of the transmittance, and show that the transmission behavior is attributable to the emergence of a self-induced topological soliton. In the third case, we show that the topological transition produces a discontinuity in the transmittance curve, which can be exploited to achieve sharp jumps in the power-dependent isolation ratio.
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  • 47
    Publication Date: 2017-06-27
    Description: We discuss the conditions for the classicality of quantum states with a very large number of identical particles. By defining the center of mass from a large set of Bohmian particles, we show that it follows a classical trajectory when the distribution of the Bohmian particle positions in a single experiment is always equal to the marginal distribution of the quantum state in physical space. This result can also be interpreted as a single experiment generalization of the well-known Ehrenfest theorem. We also demonstrate that the classical trajectory of the center of mass is fully compatible with a quantum (conditional) wave function solution of a classical non-linear Schrödinger equation. Our work shows clear evidence for a quantum–classical inter-theory unification, and opens new possibilities for practical quantum computations with decoherence.
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  • 48
    Publication Date: 2017-06-30
    Description: Mechanical metamaterials are those structures designed to convey force and motion in novel and desirable ways. Recently, Kane and Lubensky showed that lattices at the point of marginal mechanical stability (Maxwell lattices) possess a topological invariant that describes the distribution of floppy, zero-energy edge modes. Here, we show that applying force at a point in the bulk of these lattices generates a directional mechanical response, in which stress or strain is induced only on one side of the force. This provides both a bulk metric for mechanical polarization and a design principle to convey stresses and strains towards or away from parts of the structure. We also characterize the effects of removing bonds on the material’s structure and floppy modes, establishing a relationship between edge modes and bulk response.
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  • 49
    Publication Date: 2017-06-30
    Description: Astrophysical fluids are turbulent, magnetized, and frequently partially ionized. As an example of astrophysical turbulence, the interstellar turbulence extends over a remarkably large range of spatial scales and participates in key astrophysical processes happening on different ranges of scales. Significant progress has been achieved in the understanding of the magnetohydrodynamic (MHD) turbulence since the turn of the century, and this enables us to better describe turbulence in magnetized and partially ionized plasmas. In fact, the modern revolutionized picture of MHD turbulence physics facilitates the development of various theoretical domains, including the damping process for dissipating MHD turbulence and the dynamo process for generating MHD turbulence with many important astrophysical implications. In this paper, we review some important findings from our recent theoretical works to demonstrate the interconnection between the properties of MHD turbulence and those of tu...
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  • 50
    Publication Date: 2017-07-06
    Description: We study glide protected topological (GSPT) phases of interacting bosons and fermions in three spatial dimensions with certain on-site symmetries. They are crystalline SPT phases, which are distinguished from a trivial product state only in the presence of non-symmorphic glide symmetry. We classify these GSPT phases with various on-site symmetries such as U (1) and time reversal, and show that they can all be understood by stacking and coupling two-dimensional (2D) short-range-entangled phases in a glide-invariant way. Using such a coupled layer construction we study the anomalous surface topological orders of these GSPT phases, which gap out the 2D surface states without breaking any symmetries. While this framework can be applied to any non-symmorphic SPT phase, we demonstrate it in many examples of GSPT phases including the non-symmorphic topological insulator with ‘hourglass fermion’ surface states.
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  • 51
    Publication Date: 2017-07-06
    Description: A boundary undergoing relativistic motion can create particles from quantum vacuum fluctuations in a phenomenon known as the dynamical Casimir effect (DCE). We examine the creation of particles, and more generally the transformation of quantum field states, due to boundary motion in curved spacetime. We provide a novel method enabling the calculation of the effect for a wide range of trajectories and spacetimes. We apply this to the experimental scenario used to detect the DCE, now adopting the Schwarzschild metric, and find novel resonances in particle creation as a result of the spacetime curvature. Finally, we discuss a potential enhancement of the effect for the phonon field of a Bose–Einstein condensate.
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  • 52
    Publication Date: 2017-07-06
    Description: The dynamics of dark–bright (DB) solitons beyond the mean-field approximation is investigated. We first examine the case of a single DB soliton and its oscillations within a parabolic trap. Subsequently, we move to the setting of collisions, comparing the mean-field approximation to that involving multiple orbitals in both the dark and the bright component. Fragmentation is present and significantly affects the dynamics, especially in the case of slower solitons and in that of lower atom numbers. It is shown that the presence of fragmentation allows for bipartite entanglement between the distinguishable species. Most importantly the interplay between fragmentation and entanglement leads to the splitting of each of the parent mean-field DB solitons, placed off-center within the parabolic trap, into a fast and a slow daughter solitary wave. The latter process is in direct contrast to the predictions of the mean-field approximation. A variety of excitations including DB solitons in...
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  • 53
    Publication Date: 2017-07-06
    Description: The problem of characterizing classical and quantum correlations in networks is considered. Contrary to the usual Bell scenario, where distant observers share a physical system emitted by one common source, a network features several independent sources, each distributing a physical system to a subset of observers. In the quantum setting, the observers can perform joint measurements on initially independent systems, which may lead to strong correlations across the whole network. In this work, we introduce a technique to systematically map a Bell inequality to a family of Bell-type inequalities bounding classical correlations on networks in a star-configuration. Also, we show that whenever a given Bell inequality can be violated by some entangled state ρ , then all the corresponding network inequalities can be violated by considering many copies of ρ distributed in the star network. The relevance of these ideas is illustrated by applying our method to a specific mult...
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  • 54
    Publication Date: 2017-07-12
    Description: In the present study, we applied a regularized inversion method to extract the particle size, magnetic moment and relaxation-time distribution of magnetic nanoparticles from small-angle x-ray scattering (SAXS), DC magnetization (DCM) and AC susceptibility (ACS) measurements. For the measurements the particles were colloidally dispersed in water. At first approximation the particles could be assumed to be spherically shaped and homogeneously magnetized single-domain particles. As model functions for the inversion, we used the particle form factor of a sphere (SAXS), the Langevin function (DCM) and the Debye model (ACS). The extracted distributions exhibited features/peaks that could be distinctly attributed to the individually dispersed and non-interacting nanoparticles. Further analysis of these peaks enabled, in combination with a prior characterization of the particle ensemble by electron microscopy and dynamic light scattering, a detailed structural and magnetic characterizat...
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  • 55
    Publication Date: 2017-07-20
    Description: Osteocytes and their cell processes reside in a large, interconnected network of voids pervading the mineralized bone matrix of most vertebrates. This osteocyte lacuno-canalicular network (OLCN) is believed to play important roles in mechanosensing, mineral homeostasis, and for the mechanical properties of bone. While the extracellular matrix structure of bone is extensively studied on ultrastructural and macroscopic scales, there is a lack of quantitative knowledge on how the cellular network is organized. Using a recently introduced imaging and quantification approach, we analyze the OLCN in different bone types from mouse and sheep that exhibit different degrees of structural organization not only of the cell network but also of the fibrous matrix deposited by the cells. We define a number of robust, quantitative measures that are derived from the theory of complex networks. These measures enable us to gain insights into how efficient the network is organized with regard to i...
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  • 56
    facet.materialart.
    Unknown
    Institute of Physics (IOP)
    Publication Date: 2017-07-21
    Description: A beam of ThO molecules has been used to make the most precise measurement of the electron’s electric dipole moment (EDM) to date. In their recent paper, the ACME collaboration set out in detail their experimental and data analysis techniques. In a tour-de-force, they explain the many ways in which their apparatus can produce a signal which mimics the EDM and show how these systematic effects are measured and controlled.
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  • 57
    Publication Date: 2017-07-22
    Description: The low temperature diffusivity of nanoscale crystal defects, where quantum mechanical fluctuations are known to play a crucial role, are essential to interpret observations of irradiated microstructures conducted at cryogenic temperatures. Using density functional theory calculations, quantum heat bath molecular dynamics and open quantum systems theory, we evaluate the low temperature diffusivity of self-interstitial atom clusters in tungsten valid down to temperatures of 1 K. Due to an exceptionally low defect migration barrier, our results show that interstitial defects exhibit very high diffusivity of order ##IMG## [http://ej.iop.org/images/1367-2630/19/7/073024/njpaa78eaieqn1.gif] {${10}^{3}\,\mu {{\rm{m}}}^{2}\,{{\rm{s}}}^{-1}$} over the entire range of temperatures investigated.
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  • 58
    Publication Date: 2017-07-22
    Description: Direct detection of gravitational waves is opening a new window onto our universe. Here, we study the sensitivity to continuous-wave strain fields of a kg-scale optomechanical system formed by the acoustic motion of superfluid helium-4 parametrically coupled to a superconducting microwave cavity. This narrowband detection scheme can operate at very high Q -factors, while the resonant frequency is tunable through pressurization of the helium in the 0.1–1.5 kHz range. The detector can therefore be tuned to a variety of astrophysical sources and can remain sensitive to a particular source over a long period of time. For thermal noise limited sensitivity, we find that strain fields on the order of ##IMG## [http://ej.iop.org/images/1367-2630/19/7/073023/njpaa78cbieqn1.gif] {$h\sim {10}^{-23}/\sqrt{\mathrm{Hz}}$} are detectable. Measuring such strains is possible by implementing state of the art microwave transducer technology. We show that the proposed sys...
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  • 59
    facet.materialart.
    Unknown
    Institute of Physics (IOP)
    Publication Date: 2017-07-22
    Description: Artificial molecular machines are often driven by the periodic variation of an external parameter. This external control exerts work on the system of which a part can be extracted as output if the system runs against an applied load. Usually, the thermodynamic cost of the process that generates the external control is ignored. Here, we derive a refined second law for such small machines that include this cost, which is, for example, generated by free energy consumption of a chemical reaction that modifies the energy landscape for such a machine. In the limit of irreversible control, this refined second law becomes the standard one. Beyond this ideal limiting case, our analysis shows that due to a new entropic term unexpected regimes can occur: the control work can be smaller than the extracted work and the work required to generate the control can be smaller than this control work. Our general inequalities are illustrated by a paradigmatic three-state system.
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  • 60
    Publication Date: 2017-07-22
    Description: Description unavailable
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  • 61
    Publication Date: 2017-07-23
    Description: Topological insulators, a hallmark of condensed matter physics, have recently reached the classical realm of acoustic waves. A remarkable property of time-reversal invariant topological insulators is the presence of unidirectional spin-polarized propagation along their edges, a property that could lead to a wealth of new opportunities in the ability to guide and manipulate sound. Here, we demonstrate and study the possibility to induce topologically non-trivial acoustic states at the deep subwavelength scale, in a structured two-dimensional metamaterial composed of Helmholtz resonators. Radically different from previous designs based on non-resonant sonic crystals, our proposal enables robust sound manipulation on a surface along predefined, subwavelength pathways of arbitrary shapes.
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  • 62
    Publication Date: 2017-07-25
    Description: Recent discoveries have spurred the theoretical prediction and experimental realization of novel materials that have topological properties arising from band inversion. Such topological insulators are insulating in the bulk but have conductive surface or edge states. Topological materials show various unusual physical properties and are surmised to enable the creation of exotic Majorana-fermion quasiparticles. How the signatures of topological behavior evolve when the system size is reduced is interesting from both a fundamental and an application-oriented point of view, as such understanding may form the basis for tailoring systems to be in specific topological phases. This work considers the specific case of quantum-well confinement defining two-dimensional layers. Based on the effective-Hamiltonian description of bulk topological insulators, and using a harmonic-oscillator potential as an example for a softer-than-hard-wall confinement, we have studied the interplay of band i...
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  • 63
    Publication Date: 2017-07-26
    Description: Initializing a set of qubits to a given quantum state is a basic prerequisite for the physical implementation of quantum-information protocols. Here, we discuss the polarization of the electronic and nuclear spin in a single nitrogen vacancy center in diamond. Our initialization scheme uses a sequence of laser, microwave and radio-frequency pulses, and we optimize the pumping parameters of the laser pulse. A rate equation model is formulated that explains the effect of the laser pulse on the spin system. We have experimentally determined the population of the relevant spin states as a function of the duration of the laser pulse by measuring Rabi oscillations and Ramsey-type free-induction decays. The experimental data have been analyzed to determine the pumping rates of the rate equation model.
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  • 64
    Publication Date: 2017-07-26
    Description: We theoretically investigate graphene plasmons in the presence of a low density of adatoms on the graphene surface. The adatoms can significantly modify the conductivity and plasmonic properties of graphene and may produce a level splitting with the plasmon mode, resulting in two plasmon branches. The high energy branch exhibits large losses and the low energy branch exhibits low losses. Our model may also be considered as a simple model for molecules on graphene and we show that graphene plasmons are sensitive to such changes in the environment. Our microscopic treatment of plasmons and adatoms shows the sensitivity of plasmons and highlights the potential of graphene plasmons for sensing purposes.
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  • 65
    Publication Date: 2017-07-26
    Description: Quantum spin Hall (QSH) effect, a fundamentally new quantum state of matter and topological phase transitions are characteristics of a kind of electronic material, popularly referred to as topological insulators (TIs). TIs are similar to ordinary insulator in terms of their bulk bandgap, but have gapless conducting edge-states that are topologically protected. These edge-states are facilitated by the time-reversal symmetry and they are robust against nonmagnetic impurity scattering. Recently, the quest for new materials exhibiting non-trivial topological state of matter has been of great research interest, as TIs find applications in new electronics and spintronics and quantum-computing devices. Here, we propose and demonstrate as a proof-of-concept that QSH effect and topological phase transitions can be realized in ##IMG## [http://ej.iop.org/images/1367-2630/19/7/073031/njpaa795cieqn1.gif] {${\mathrm{InN}}_{x}{\mathrm{Bi}}_{y}{\mathrm{Sb}}_{1-x-y}$} /InSb ...
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  • 66
    Publication Date: 2017-07-26
    Description: We recently set a new limit on the electric dipole moment of the electron (eEDM) (J Baron et al and ACME collaboration 2014 Science 343 269–272), which represented an order-of-magnitude improvement on the previous limit and placed more stringent constraints on many charge-parity-violating extensions to the standard model. In this paper we discuss the measurement in detail. The experimental method and associated apparatus are described, together with the techniques used to isolate the eEDM signal. In particular, we detail the way experimental switches were used to suppress effects that can mimic the signal of interest. The methods used to search for systematic errors, and models explaining observed systematic errors, are also described. We briefly discuss possible improvements to the experiment.
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  • 67
    Publication Date: 2017-07-26
    Description: The saturated absorption technique is an elegant method widely used in atomic and molecular physics for high-resolution spectroscopy, laser frequency standards and metrology purposes. We have recently discovered that a saturated absorption scheme with a dual-frequency laser can lead to a significant sign reversal of the usual Doppler-free dip, yielding a deep enhanced-absorption spike. In this paper, we report detailed experimental investigations of this phenomenon, together with a full in-depth theoretical description. It is shown that several physical effects can support or oppose the formation of the high-contrast central spike in the absorption profile. The physical conditions for which all these effects act constructively and result in very bright Doppler-free resonances are revealed. Apart from their theoretical interest, results obtained in this manuscript are of great interest for laser spectroscopy and laser frequency stabilization purposes, with applications in laser c...
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  • 68
    Publication Date: 2017-07-28
    Description: Spekkens’ toy model is a non-contextual hidden variable model with an epistemic restriction, a constraint on what an observer can know about reality. The aim of the model, developed for continuous and discrete prime degrees of freedom, is to advocate the epistemic view of quantum theory, where quantum states are states of incomplete knowledge about a deeper underlying reality. Many aspects of quantum mechanics and protocols from quantum information can be reproduced in the model. In spite of its significance, a number of aspects of Spekkens’ model remained incomplete. Formal rules for the update of states after measurement had not been written down, and the theory had only been constructed for prime-dimensional and infinite dimensional systems. In this work, we remedy this, by deriving measurement update rules and extending the framework to derive models in all dimensions, both prime and non-prime. Stabiliser quantum mechanics (SQM) is a sub-theory of quantum mechanics with restr...
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  • 69
    facet.materialart.
    Unknown
    Institute of Physics (IOP)
    Publication Date: 2017-07-29
    Description: Calculation of gravitational forces is essential for many fundamental measurements, such as determining the gravitational constant or investigating violations of the inverse square law. These calculations, even with modern computational power, are slow and tedious. Improved calculation efficiency allows an experimentalist to easily check the effect of possible systematic biases and to ease the process of instrument design. Many gravitational measurements are expanded in terms of multipole moments for efficient calculations, however for many experimental geometries these do not converge, leaving awkward sextuple integrals. In this work we introduce a modified approach to the calculation which reduces the force between a point mass and any arbitrary object to a sum of single integrals. The force between any two objects can then be calculated as a quadruple rather than a sextuple integral.
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  • 70
    Publication Date: 2017-07-29
    Description: We point out a contrasting role the entanglement plays in communication and estimation scenarios. In the first case it brings noticeable benefits at the measurement stage (output super-additivity), whereas in the latter it is the entanglement of the input probes that enables significant performance enhancement (input super-additivity). We identify a weak estimation regime where a strong connection between concepts crucial to the two fields is demonstrated; the accessible information and the Holevo quantity on one side and the quantum Fisher information related quantities on the other. This allows us to shed new light on the problem of super-additivity in communication using the concepts of quantum estimation theory.
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  • 71
    Publication Date: 2017-07-29
    Description: Determining the structure and the internal dynamics of tissues is essential to understand their functional organization. Microscopy allows for monitoring positions and trajectories of every single cell. Those data are useful to extract statistical observables, such as intercellular distance, tissue symmetry and anisotropy, and cell motility. However, this procedure requires a large and supervised computational effort. In addition, due to the large cross-section of cells, the light scattering limits the use of microscopy to relatively thin samples. As an alternative approach, we propose to take advantage of light scattering and to analyze the dynamical diffraction pattern produced by a living tissue illuminated with coherent light. In this article, we illustrate with a few examples that supra-cellular structures produce an exploitable diffraction signal. From the diffraction signal, we deduce the mean distance between cells, the anisotropy of the supra-cellular organization and, ...
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  • 72
    Publication Date: 2017-07-29
    Description: The belief propagation (BP) algorithm is a powerful tool in a wide range of disciplines from statistical physics to machine learning to computational biology, and is ubiquitous in decoding classical error-correcting codes. The algorithm works by passing messages between nodes of the factor graph associated with the code and enables efficient decoding of the channel, in some cases even up to the Shannon capacity. Here we construct the first BP algorithm which passes quantum messages on the factor graph and is capable of decoding the classical–quantum channel with pure state outputs. This gives explicit decoding circuits whose number of gates is quadratic in the code length. We also show that this decoder can be modified to work with polar codes for the pure state channel and as part of a decoder for transmitting quantum information over the amplitude damping channel. These represent the first explicit capacity-achieving decoders for non-Pauli channels.
    Electronic ISSN: 1367-2630
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  • 73
    Publication Date: 2017-08-01
    Description: The participation of individuals in multi-layer networks allows for feedback between network layers, opening new possibilities to mitigate epidemic spreading. For instance, the spread of a biological disease such as Ebola in a physical contact network may trigger the propagation of the information related to this disease in a communication network, e.g. an online social network. The information propagated in the communication network may increase the awareness of some individuals, resulting in them avoiding contact with their infected neighbors in the physical contact network, which might protect the population from the infection. In this work, we aim to understand how the time scale γ of the information propagation (speed that information is spread and forgotten) in the communication network relative to that of the epidemic spread (speed that an epidemic is spread and cured) in the physical contact network influences such mitigation using awareness information. We begin ...
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  • 74
    Publication Date: 2017-08-01
    Description: Minimizing the band splitting energy to approach orbital degeneracy has been shown as a route to improved thermoelectric performance. This represents an open opportunity in some promising layered materials where there is a separation of p orbitals at the valence band edge due to the crystal field splitting. In this work, using ab initio calculations and semiclassical Boltzmann transport theory, we try to figure out how orbital degeneracy influences the thermoelectric properties of layered transition-metal dichalcogenide ZrS 2 . We tune the splitting energy by applying compressive biaxial strain, and find out that near-degeneration at the ##IMG## [http://ej.iop.org/images/1367-2630/19/7/073036/njpaa7b58ieqn1.gif] {${\rm{\Gamma }}$} point can be achieved for around 3% strain. As expected, the enhanced density-of-states effective mass results in an increased power factor. Interestingly, we also find a marked decline in the lattice therma...
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  • 75
    Publication Date: 2017-08-01
    Description: The undamped electron acoustic wave is a newly-observed nonlinear electrostatic plasma wave and has potential applications in ion acceleration, laser amplification and diagnostics due to its unique frequency range. We propose to make the first attempt to excite a monochromatic and stable electron acoustic wave (EAW) by two counter-propagating laser beams. The matching conditions relevant to laser frequencies, plasma density, and electron thermal velocity are derived and the harmonic effects of the EAW are excluded. Single-beam instabilities, including stimulated Raman scattering and stimulated Brillouin scattering, on the excitation process are quantified by an interaction quantity, ##IMG## [http://ej.iop.org/images/1367-2630/19/7/073038/njpaa79b9ieqn1.gif] {$\eta =\gamma {\tau }_{B}$} , where γ is the growth rate of each instability and ##IMG## [http://ej.iop.org/images/1367-2630/19/7/073038/njpaa79b9ieqn2.gif] {${\tau }_{B}$} i...
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  • 76
    Publication Date: 2017-08-01
    Description: The electronic state and transport properties of hot dense iron are of the utmost importance for the understanding of Earth’s interior. Combining state-of-the-art density functional and dynamical mean field theories we study the impact of electron correlations on the electrical and thermal resistivity of hexagonal close-packed ϵ -Fe at Earth’s core conditions and show that the electron–electron scattering in ϵ -Fe exhibit a nearly perfect Fermi-liquid (FL) behavior. Accordingly, the quadratic dependence of the scattering rate, typical of FLs, leads to a modification of the Wiedemann–Franz law and suppresses the thermal conductivity with respect to the electrical one. The consequence is a significant increase of the electron–electron thermal resistivity, which is found to be of comparable magnitude to the electron–phonon one.
    Electronic ISSN: 1367-2630
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  • 77
    Publication Date: 2017-08-01
    Description: Many real-world multilayer systems such as critical infrastructure are interdependent and embedded in space with links of a characteristic length. They are also vulnerable to localized attacks or failures, such as terrorist attacks or natural catastrophes, which affect all nodes within a given radius. Here we study the effects of localized attacks on spatial multiplex networks of two layers. We find a metastable region where a localized attack larger than a critical size induces a nucleation transition as a cascade of failures spreads throughout the system, leading to its collapse. We develop a theory to predict the critical attack size and find that it exhibits novel scaling behavior. We further find that localized attacks in these multiplex systems can induce a previously unobserved combination of random and spatial cascades. Our results demonstrate important vulnerabilities in real-world interdependent networks and show new theoretical features of spatial networks.
    Electronic ISSN: 1367-2630
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  • 78
    Publication Date: 2017-08-01
    Description: We investigate the measurement-only topological quantum computation (MOTQC) approach proposed by Bonderson et al (2008 Phys. Rev. Lett. 101 [http://dx.doi.org/10.1103/PhysRevLett.101.010501] 010501 ) where the braiding operation is shown to be equivalent to a series of topological charge ‘forced measurements’ of anyons. In a forced measurement, the charge measurement is forced to yield the desired outcome (e.g. charge 0) via repeatedly measuring charges in different bases. This is a probabilistic process with a certain success probability for each trial. In practice, the number of measurements needed will vary from run to run. We show that such an uncertainty associated with forced measurements can be removed by simulating the braiding operation using a fixed number of three measurements supplemented by a correction operator. Furthermore, we demonstrate that in practice we can avoid applying the correction operator in hardware by implementing it ...
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  • 79
    Publication Date: 2017-08-08
    Description: Edge modes in topological insulators are known to be robust against defects. We investigate if this also holds true when the defect is not static, but varies in time. We study the influence of defects with time-dependent coupling on the robustness of the transport along the edge in a Floquet system of helically curved waveguides. Waveguide arrays are fabricated via direct laser writing in a negative tone photoresist. We find that single dynamic defects do not destroy the chiral edge current, even when the temporal modulation is strong. Quantitative numerical simulation of the intensity in the bulk and edge waveguides confirms our observation.
    Electronic ISSN: 1367-2630
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  • 80
    Publication Date: 2017-08-08
    Description: In earlier work, we have developed a kinetic field theory (KFT) for cosmological structure formation and showed that the nonlinear density-fluctuation power spectrum known from numerical simulations can be reproduced quite well even if particle interactions are taken into account to first order only. Besides approximating gravitational interactions, we had to truncate the initial correlation hierarchy of particle momenta at the second order. Here, we substantially simplify KFT. We show that its central object, the free generating functional, can be factorised, taking the full hierarchy of momentum correlations into account. The factors appearing in the generating functional, which we identify as nonlinearly evolved density-fluctuation power spectra, have a universal form and can thus be tabulated for fast access in perturbation schemes. In this paper, we focus on a complete evaluation of the free generating functional of KFT, not including particle interactions yet. This implies...
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  • 81
    Publication Date: 2017-08-08
    Description: We present and implement a non-destructive detection scheme for the transition probability readout of an optical lattice clock. The scheme relies on a differential heterodyne measurement of the dispersive properties of lattice-trapped atoms enhanced by a high finesse cavity. By design, this scheme offers a 1st order rejection of the technical noise sources, an enhanced signal-to-noise ratio, and an homogeneous atom-cavity coupling. We theoretically show that this scheme is optimal with respect to the photon shot noise limit. We experimentally realise this detection scheme in an operational strontium optical lattice clock. The resolution is on the order of a few atoms with a photon scattering rate low enough to keep the atoms trapped after detection. This scheme opens the door to various different interrogations protocols, which reduce the frequency instability, including atom recycling, zero-dead time clocks with a fast repetition rate, and sub quantum projection noise frequency...
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  • 82
    Publication Date: 2017-08-08
    Description: In dynamical systems, one may ask how long it takes for a trajectory to reach the attractor, i.e. how long it spends in the transient phase. Although for a single trajectory the mathematically precise answer may be infinity, it still makes sense to compare different trajectories and quantify which of them approaches the attractor earlier. In this article, we categorize several problems of quantifying such transient times. To treat them, we propose two metrics, area under distance curve and regularized reaching time, that capture two complementary aspects of transient dynamics. The first, area under distance curve, is the distance of the trajectory to the attractor integrated over time. It measures which trajectories are ‘reluctant’, i.e. stay distant from the attractor for long, or ‘eager’ to approach it right away. Regularized reaching time, on the other hand, quantifies the additional time (positive or negative) that a trajectory starting at a chosen initial condition needs to...
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  • 83
    Publication Date: 2017-08-08
    Description: We present here an adaptive control scheme with a feedback delay to achieve elimination of synchronization in a large population of coupled and synchronized oscillators. We validate the feasibility of this scheme not only in the coupled Kuramoto’s oscillators with a unimodal or bimodal distribution of natural frequency, but also in two representative models of neuronal networks, namely, the FitzHugh–Nagumo spiking oscillators and the Hindmarsh–Rose bursting oscillators. More significantly, we analytically illustrate the feasibility of the proposed scheme with a feedback delay and reveal how the exact topological form of the bimodal natural frequency distribution influences the scheme performance. We anticipate that our developed scheme will deepen the understanding and refinement of those controllers, e.g. techniques of deep brain stimulation, which have been implemented in remedying some synchronization-induced mental disorders including Parkinson disease and epilepsy.
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  • 84
    facet.materialart.
    Unknown
    Institute of Physics (IOP)
    Publication Date: 2017-08-09
    Description: Yang–Lee edge singularities are the branch point of free energy on the complex plane of physical parameters and have been shown to be the simplest universality class of phase transitions. However, Yang–Lee edge singularities have not been regarded as experimentally observable since they occur at complex physical parameters which are unphysical. A recent discovery regarding the relation between partition functions and probe spin coherence makes it experimentally feasible to access the complex plane of physical parameters. However, how to extract the critical point and the critical exponent of Yang–Lee edge singularities in many-body systems, which occurs only at the thermodynamic limit, has still been elusive. Here we show that the quantum coherence of a probe spin coupled to finite-size Ising-type spin systems presents universal scaling behavior near the Yang–Lee edge singularity. The finite-size scaling behavior of the quantum coherence of the probe spin predicts that one can e...
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  • 85
    Publication Date: 2017-08-09
    Description: The effective manipulation of skyrmion motion introduces the technologically relevant possibility of skyrmion-based spintronics. Here we show how a magnetic skyrmion can be captured by a radially spatial gradient of the magnetic field. A theoretical model governed by the Thiele equation is employed to study the skyrmion motion. The analytical predictions are compared to micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation, showing that the dynamic behavior of skyrmions strongly depends on the gradient strength of magnetic fields as well as the radius of the skyrmion and the Gilbert damping factor. Moreover, we find that the skyrmions can also be dragged by a moving magnetic tip.
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  • 86
    facet.materialart.
    Unknown
    Institute of Physics (IOP)
    Publication Date: 2017-08-09
    Description: A two-dimensional electron system in the presence of a magnetic field and microwave irradiation can undergo a phase transition towards a zero-resistance state (ZRS). A widely used model predicts the ZRS to be a domain state, which responds to applied dc voltages or dc currents by slightly changing the domain structure. Here we propose an alternative response scenario, according to which the domain pattern remains unchanged. Surprisingly, a fixed domain pattern does not destroy zero-resistance, provided that the resistance is direction independent. Otherwise, if the symmetry of the domain pattern allows a direction dependence of the resistance, the domain state can be dissipative. We give examples for both situations and simulate the response behavior numerically.
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  • 87
    Publication Date: 2017-08-09
    Description: We investigate the full-counting statistics (FCS) of energy transport carried by electrons in molecular junctions for the Anderson–Holstein model in the polaronic regime. Using the two-time quantum measurement scheme, the generating function (GF) for the energy transport is derived and expressed as a Fredholm determinant in terms of Keldysh nonequilibrium Green’s function in the time domain. Dressed tunneling approximation is used in decoupling the phonon cloud operator in the polaronic regime. This formalism enables us to analyze the time evolution of energy transport dynamics after a sudden switch-on of the coupling between the dot and the leads towards the stationary state. The steady state energy current cumulant GF in the long time limit is obtained in the energy domain as well. Universal relations for steady state energy current FCS are derived under a finite temperature gradient with zero bias and this enabled us to express the equilibrium energy current cumulant by a lin...
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  • 88
    Publication Date: 2017-08-09
    Description: We present analytical studies of a trapped boson-fermion mixture at zero temperature with spin-polarized fermions. Using the Thomas–Fermi approximation for bosons and the local-density approximation for fermions, we find a large variety of different density shapes. In the case of continuous density, we obtain analytic conditions for each configuration for attractive as well as repulsive boson-fermion interaction. Furthermore, we analytically show that all the scenarios we describe are minima of the grand-canonical energy functional. Finally, we provide a full classification of all possible ground states in the interpenetrative regime. Our results also apply to binary mixtures of bosons.
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  • 89
    Publication Date: 2017-10-03
    Description: Using the highly sensitive anomalous Hall effect we have been able to measure the reversal of a single magnetic island, of diameter 220 nm, in an array consisting of more than 80 of those islands. By repeatedly traversing the hysteresis loop, we measured the thermally induced fluctuation of the switching field of the islands at the lower and higher ends of the switching field distribution. Based on a novel easy-to-use model, we determined the switching field in the absence of thermal activation, and the energy barrier in the absence of an external field from these fluctuations. By measuring the reversal of individual dots in the array as a function of temperature, we extrapolated the switching field and energy barrier down to 0 K. The extrapolated values are not identical to those obtained from the fluctation of the switching field at room temperature, because the properties of the magnetic material are temperature dependent. As a result, extrapolating from temperature dependent...
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  • 90
    Publication Date: 2017-10-03
    Description: The topological properties in topological superconductors are usually characterized by bulk Chern numbers, edge-state spectra, and Majorana zero modes. Whether they are equivalent or inequivalent is not well understood. Here, we investigate this issue with a focus on a checkerboard-lattice model combining the Chern insulator and chiral p-wave superconductivity. Multiple topologically superconducting phases with Chern numbers up to ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093018/njpaa8022ieqn1.gif] {${ \mathcal N }=4$} are produced. We explicitly demonstrate the mismatch between the Chern numbers, edge states, and Majorana zero modes in this two-dimensional topological-superconductor model. The intrinsic reason is that some edge states in the superconducting phases inherited from the Chern-insulator phase are not protected by particle-hole symmetry. We further check the mismatches in vortex states. Our results therefore clarify these different but co...
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  • 91
    Publication Date: 2017-10-03
    Description: The magnetic response related to the paramagnetic Meissner effect (PME) is studied in a high quality single crystal ZrB 12 with non-monotonic vortex–vortex interactions. We observe the expulsion and penetration of magnetic flux in the form of vortex clusters with increasing temperature. A vortex phase diagram is constructed, and shows that the PME can be explained by considering the interplay among the flux compression, the different temperature dependencies of the vortex–vortex and the vortex–pin interactions, and thermal fluctuations. Such a scenario is in good agreement with the results of magnetic relaxation measurements.
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  • 92
    Publication Date: 2017-10-03
    Description: We propose to use the sub-system fidelity, defined by comparing a pair of reduced density matrices, to identify and locate zero modes relating two degenerate ground states in one-dimensional interacting many-body systems, where the degeneracy arises from either symmetry protected topology (SPT), or from discrete symmetry breaking (DSB). A theorem is provided to construct locally indistinguishable (LI) states by linear recombination of the degenerate states of minimal bulk entanglement entropy. The theorem enables us to construct local operators that swap exactly the LI states and are therefore the zero energy modes in the many-body system. Interestingly, they can be located anywhere in the DSB case, but can only be accommodated near the edges in the SPT case. This can be used to identify or distinguish SPT states against the so-called cat states in the DSB case. We illustrate the results for the anisotropic Haldane chain and the interacting Kitaev model.
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  • 93
    Publication Date: 2017-10-04
    Description: Attenuating low-frequency sound remains a challenge, despite many advances in this field. Recently-developed acoustic metamaterials are characterized by unusual wave manipulation abilities that make them ideal candidates for efficient subwavelength sound control. In particular, labyrinthine acoustic metamaterials exhibit extremely high wave reflectivity, conical dispersion, and multiple artificial resonant modes originating from the specifically-designed topological architectures. These features enable broadband sound attenuation, negative refraction, acoustic cloaking and other peculiar effects. However, hybrid and/or tunable metamaterial performance implying enhanced wave reflection and simultaneous presence of conical dispersion at desired frequencies has not been reported so far. In this paper, we propose a new type of labyrinthine acoustic metamaterials (LAMMs) with hybrid dispersion characteristics by exploiting spider web-structured configurations. The developed design ap...
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  • 94
    Publication Date: 2017-10-05
    Description: A first-principles approach based on density functional theory and non-equilibrium Green’s functions is used to study the molecular transport system consisting of benzenedithiolate connected with monoatomic gold and platinum electrodes. Using symmetry arguments we explain why the conductance mechanism is different for gold and platinum electrodes. We present the charge stability diagram for the benzenedithiolate connected with monoatomic platinum electrodes including many-body effects in terms of an extended Hubbard Hamiltonian and discuss how the electrodes and the many-body effects influence the transport properties of the system.
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  • 95
    Publication Date: 2017-10-05
    Description: We develop a mesoscopic field theory for the collective nonequilibrium dynamics of multicomponent mixtures of interacting active (i.e., motile) and passive (i.e., nonmotile) colloidal particles with isometric shape in two spatial dimensions. By a stability analysis of the field theory, we obtain equations for the spinodal that describes the onset of a motility-induced instability leading to cluster formation in such mixtures. The prediction for the spinodal is found to be in good agreement with particle-resolved computer simulations. Furthermore, we show that in active-passive mixtures the spinodal instability can be of two different types. One type is associated with a stationary bifurcation and occurs also in one-component active systems, whereas the other type is associated with a Hopf bifurcation and can occur only in active-passive mixtures. Remarkably, the Hopf bifurcation leads to moving clusters. This explains recent results from simulations of active-passive particle mi...
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  • 96
    Publication Date: 2017-10-13
    Description: Chemotaxic receptors in bacteria form clusters at cell poles and also laterally, and this clustering plays an important role in signal transduction. These clusters were found to be periodically arranged on the surface of the bacterium Escherichia coli , independent of any known positioning mechanism. In this work we extend a model based on diffusion and aggregation to more realistic geometries and present a means based on ‘bursty’ protein production to distinguish spontaneous positioning from an independently existing positioning mechanism. We also consider the case of isotropic cellular growth and characterize the degree of order arising spontaneously. Our model could also be relevant for other examples of periodically positioned protein clusters in bacteria.
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  • 97
    Publication Date: 2017-10-14
    Description: We present inelastic neutron scattering data across the Verwey transition in magnetite, obtained for a single crystal via a detwinning method. We provide direct evidence of the influence of the charge order on the transverse-acoustic phonons, associated with discontinuous hardening and narrowing at the transition temperature, and energy splitting for different polarizations. This contrasts with the behavior of the transverse-optical X 3 mode, which does not present any critical anomaly, contrary to theoretical expectations. Our data indicate that the incommensurate fluctuations occurring above the critical temperature become locked to the lattice at the transition point, through a mechanism similar to the crystallization of a two-dimensional liquid on a solid surface. Our results also contribute to clarify the different dynamics and mutual interactions of the electronic and structural modes in the Verwey transition.
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  • 98
    Publication Date: 2017-10-14
    Description: We study numerically the motion of vortices in nonequilibrium Bose–Einstein condensates, that are described by a generalized Gross–Pitaevskii equation. We analyze how the vortex properties are modified when moving away under deviation from equilibrium. We find that far from equilibrium, the radial component dominates over the azimuthal one in the distribution of vortex currents at large distances from the vortex core. The modification of the current pattern has a strong effect on the vortex–antivortex interaction energy, that can become entirely repulsive. The vortex trajectories are also strongly affected by the driving and dissipation. Self acceleration of vortices is observed in the strong nonequilibrium case.
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  • 99
    Publication Date: 2017-10-14
    Description: The spin splitting of conduction band electrons in inversion-asymmetric InGaAs/InP quantum wells (QWs) is studied by Shubnikov-de Haas measurements combining the analysis of beating patterns and coincidence measurements in doubly tilted magnetic fields. The method allows us to determine the absolute values of the Rashba and linear Dresselhaus spin–orbit interaction (SOI) coefficients, their relative sign and the full Landé g-tensor. This is achieved by analyzing the anisotropy of the beat node positions with respect to both polar and azimuthal angles between the magnetic field direction and the QW normal. We show that the SOI is dominated by a large Rashba coefficient together with a linear Dresselhaus coefficient that is 10% of the Rashba coefficient. Their relative sign is found to be positive. The g-tensor is found to have a marked out-of-plane anisotropy and a smaller but distinct in-plane anisotropy due to SOI.
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  • 100
    Publication Date: 2017-10-13
    Description: Simulating the stochastic evolution of real quantities on a digital computer requires a trade-off between the precision to which these quantities are approximated, and the memory required to store them. The statistical accuracy of the simulation is thus generally limited by the internal memory available to the simulator. Here, using tools from computational mechanics, we show that quantum processors with a fixed finite memory can simulate stochastic processes of real variables to arbitrarily high precision. This demonstrates a provable, unbounded memory advantage that a quantum simulator can exhibit over its best possible classical counterpart.
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