ALBERT

Description: Here, we present x-ray resonant magnetic dichroism and x-ray resonant magnetic scattering measurements of the temperature dependence of magnetism in Pr-doped La-Ca-Mn-O films grown on (110) NdGaO 3 substrates. We observed thermal hysteresis of the ferromagnetism in one film that also showed large thermal hysteresis of ∼18 K in transport measurements. While in a second film of a different nominal chemistry, which showed very small thermal hysteresis ∼3 K in transport measurements, no thermal hysteresis of the ferromagnetism was observed. These macroscopic properties are correlated with evolution of surface magnetization across metal insulator transition for these films as observed by soft x-ray resonant magnetic scattering measurements.

Description: In this study, we have investigated the ion concentration dependent collective dynamics in two series of deep eutectic solvent (DES) systems by femtosecond Raman-induced Kerr effect spectroscopy, as well as some physical properties, e.g., shear viscosity ( η ), density ( ρ ), and surface tension ( γ ). The DES systems studied here are [0.75CH 3 CONH 2 + 0.25{ f  KSCN + (1 – f  )NaSCN}] and [0.78CH 3 CONH 2 + 0.22{ f  LiBr + (1 – f  )LiNO 3 }] with f = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. γ of these DES systems shows near insensitivity to f , while ρ shows a moderate dependence on f . Interestingly, η exhibits a strong dependence on f . In the low-frequency Kerr spectra, obtained via the Fourier transform of the collected Kerr transients, a characteristic band at ∼70 cm −1 is clear in [0.78CH 3 CONH 2 + 0.22{ f  LiBr + (1 – f  )LiNO 3 }] DES especially at the larger f . The band is attributed to the intermolecular hydrogen bond of acetamide. Because of less depolarized Raman activities of intermolecular/interionic vibrational motions, which are mostly translational (collision-induced or interaction-induced) motions, of spherical ions, the intermolecular hydrogen-bonding band is clearly observed. In contrast, the intermolecular hydrogen-bonding band is buried in the other intermolecular/interionic vibrational motions, which includes translational and reorientational (librational) motions and their cross-terms, in [0.75CH 3 CONH 2 + 0.25{ f  KSCN + (1 – f  )NaSCN}] system. The first moment ( M 1 ) of the intermolecular/interionic vibrational band in these DES systems is much higher than that in typical neutral molecular liquids and shows a weak but contrasting dependence on the bulk parameter γ / ρ . The time constants for picosecond overdamped Kerr transients in both the DES systems, which are obtained on the basis of the analysis fitted by a triexponential function, are rather insensitive to f for both the DES systems, but all the three time constants (fast: ∼1–3 ps; intermediate: ∼7–20 ps; and slow: ∼100 ps) are different between the [0.78CH 3 CONH 2 + 0.22{ f  LiBr + (1 – f  )LiNO 3 }] and [0.75CH 3 CONH 2 + 0.25{ f  KSCN + (1 – f  )NaSCN}] systems. These results indicate that the intermolecular/interionic interactions in DES systems is strongly influenced by the ionic species present in these DES systems.

Description: Rugged energy landscapes find wide applications in diverse fields ranging from astrophysics to protein folding. We study the dependence of diffusion coefficient ( D ) of a Brownian particle on the distribution width (ɛ) of randomness in a Gaussian random landscape by simulations and theoretical analysis. We first show that the elegant expression of Zwanzig [Proc. Natl. Acad. Sci. U.S.A.85, 2029 (1988)] for D (ɛ) can be reproduced exactly by using the Rosenfeld diffusion-entropy scaling relation. Our simulations show that Zwanzig's expression overestimates D in an uncorrelated Gaussian random lattice – differing by almost an order of magnitude at moderately high ruggedness. The disparity originates from the presence of “three-site traps” (TST) on the landscape – which are formed by the presence of deep minima flanked by high barriers on either side. Using mean first passage time formalism, we derive a general expression for the effective diffusion coefficient in the presence of TST, that quantitatively reproduces the simulation results and which reduces to Zwanzig's form only in the limit of infinite spatial correlation. We construct a continuous Gaussian field with inherent correlation to establish the effect of spatial correlation on random walk. The presence of TSTs at large ruggedness (ɛ ≫ k B T ) gives rise to an apparent breakdown of ergodicity of the type often encountered in glassy liquids.

Description: A time dependent thermal model for a superconducting constriction based weak-link (WL) is discussed for investigating the deterministic dynamics of its temperature and phase. A new dynamic regime is found where a non-zero voltage exists across the WL, and its temperature stabilizes between the bath temperature and superconductor's critical temperature. This regime exists over a limited bias current range and gives rise to a new hysteretic regime in current-voltage characteristics. We also discuss the effect of fluctuations on the current-voltage characteristics and experimental implications of this dynamic regime.

Description: A semi-molecular theory for studying composition dependent Stokes shift dynamics of a dipolar solute in binary mixtures of (non-dipolar ionic liquid + common dipolar solvent) is developed here. The theory provides microscopic expressions for solvation response functions in terms of static and dynamic structure factors of the mixture components and solute-solvent static correlations. In addition, the theory provides a framework for examining the interrelationship between the time dependent solvation response in and frequency dependent dielectric relaxation of a binary mixture containing electrolyte. Subsequently, the theory has been applied to predict ionic liquid (IL) mole fraction dependent dynamic Stokes shift magnitude and solvation energy relaxation for a dipolar solute, C153, in binary mixtures of an ionic liquid, trihexyltetradecylphosphonium chloride ([P 14,666 ][Cl]) with a common dipolar solvent, methanol (MeOH). In the absence of suitable experimental data, necessary input parameters have been obtained from approximate methods. Dynamic shifts calculated for these mixtures exhibit a linear increase with IL mole fraction for the most part of the mixture composition, stressing the importance of solute-IL dipole-ion interaction. Average solvation rates, on the other hand, show a nonlinear IL mole fraction dependence which is qualitatively similar to what has been observed for such binary mixtures with imidazolium (dipolar) ILs. These predictions should be re-examined in suitable experiments.

Description: Understanding of metal insulator transitions in a strongly correlated system, driven by Anderson localization (disorder) and/or Mott localization (correlation), is a long standing problem in condensed matter physics. The prevailing fundamental question would be how these two mechanisms contrive to accomplish emergent anomalous behaviors. Here, we have grown high quality perovskite SrIrO 3 thin films, containing a strong spin orbit coupled 5 d element Ir, on various substrates such as GdScO 3 (110), DyScO 3 (110), SrTiO 3 (001), and NdGaO 3 (110) with increasing lattice mismatch, in order to carry out a systematic study on the transport properties. We found that metal insulator transitions can be induced in this system; by either reducing thickness (on best lattice matched substrate) or changing degree of lattice strain (by lattice mismatch between film and substrates) of films. Surprisingly these two pathways seek two distinct types of metal insulator transitions; the former falls into disorder driven Anderson type whereas the latter turns out to be of unconventional Mott-Anderson type with the interplay of disorder and correlation. More interestingly, in the metallic phases of SrIrO 3 , unusual non-Fermi liquid characteristics emerge in resistivity as Δ ρ ∝ T ε with ε evolving from 4/5 to 1 to 3/2 with increasing lattice strain. We discuss theoretical implications of these phenomena to shed light on the metal insulator transitions.

Description: This paper explores an interconnection between timescales of dynamic heterogeneity (DH) in a neat ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF 6 ]), and slow solvation of a dipolar solute, coumarin 153 (C153) in it at 298 K and 450 K. Molecular dynamics simulations employing realistic interaction potentials for both the IL and the solute have been performed. DH timescales have been obtained from non-Gaussian and new non-Gaussian (NNG) parameters, and four-point dynamic susceptibilities (χ 4 (k, t)) and overlap functions (Q(t)). Simulated ion displacement distributions exhibit pronounced deviations from Gaussian behaviour and develop bimodality in the timescale of structural relaxation, τ α , indicating ion hopping at long-time. DH timescales from χ 4 (k, t) and Q(t) have been found to be longer than τ NNG although τ α ≈ τ NNG . Maximum cation jump length detected here corresponds to ∼50% of the ion diameter and agrees well with experimental estimates. DH length-scale (ξ) extracted from χ 4 (k, t) spans about an ion diameter and shows correct temperature dependence. Our simulated solvation response functions for C153 in [Bmim][PF 6 ] are tri-exponentials with fast time constants in good agreement with the available experimental and/or simulation data. The slow solvation rate at 298 K, however, is ∼4 times slower than that found in experiments, although the same at 450 K corroborates well with simulation data at similar temperature from different sources. Importantly, our simulated slow solvation rates at these temperatures strongly correlate to longer DH timescales, suggesting DH as a source for the slow solvation at long-time in IL. Moreover, ion jumps at long-time suggests viscosity decoupling of long-time solvation rate in ILs.

Description: Neutron supermirrors and supermirror polarizers are thin film multilayer based devices which are used for reflecting and polarizing neutrons in various neutron based experiments. In the present communication, the in-house development of a 9 m long in-line dc sputtering system has been described which is suitable for deposition of neutron supermirrors on large size (1500 mm × 150 mm) substrates and in large numbers. The optimisation process of deposition of Co and Ti thin film, Co/Ti periodic multilayers, and a-periodic supermirrors have also been described. The system has been used to deposit thin film multilayer supermirror polarizers which show high reflectivity up to a reasonably large critical wavevector transfer of ∼0.06 Å −1 (corresponding to m = 2.5, i.e., 2.5 times critical wavevector transfer of natural Ni). The computer code for designing these supermirrors has also been developed in-house.

Description: Aliovalent doping has been recently shown to remarkably improve energy resolution in some halide scintillators. Based on first-principles calculations we report on the formation of DX-like centers in a well-known scintillator material, Tl-doped NaI (NaI:Tl), when codoped with Ca or Ba. Our calculations indicate a net binding energy favoring formation of the defect complex ( Tl Na − + Ca Na + ) involving a new cation-cation bond, instead of the isolated substitutional defects. The pair has properties of a deep DX-like acceptor complex. Doping with the aliovalent anion impurity Te is also found to induce deep centers, which can act as effective electron or hole traps. The hole trapped as Te I 0 involves large lattice relaxation of the Te and an adjacent iodine, consistent with extrinsic self-trapping of the hole. Thus, in contrast to the positive effect achieved by aliovalent co-doping of the rare-earth tri-halides LaBr 3 :Ce and CeBr 3 :Ca as reported recently, co-doping with donor-like cations Ca, Ba, or the acceptor-like anion Te in monovalent NaI:Tl is found to inhibit scintillation response.

Description: It is thought that the proposed new family of multi-functional materials, namely, the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching, we show that the recently discovered thermoelectric semiconductor AgSbSe 2 has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as AgSbSe 2 crystalizes in cubic rock-salt structure with centro-symmetric space group ( Fm –3 m ), and therefore, no ferroelectricity is expected. However, from high resolution transmission electron microscopy measurement, we found the evidence of local superstructure formation which, we believe, leads to local distortion of the centro-symmetric arrangement in AgSbSe 2 and gives rise to the observed ferroelectricity. Stereochemically active 5 S 2 lone-pair of Sb may also give rise to local structural distortion thereby creating ferroelectricity in AgSbSe 2 .