ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

X
feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    Electronic Resource
    Electronic Resource
    Bogoliubov transformations and exact isolated solutions for simple nonadiabatic Hamiltonians (2002)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 43 (2002), S. 3916-3926 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: We present a new method for finding isolated exact solutions of a class of nonadiabatic Hamiltonians of relevance to quantum optics and allied areas. Central to our approach is the use of Bogoliubov transformations of the bosonic fields in the models. We demonstrate the simplicity and efficiency of this method by applying it to the Rabi Hamiltonian. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1490406
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 2
    Electronic Resource
    Electronic Resource
    Simple accurate coupled cluster results for the linear E⊗e pseudo-Jahn–Teller effect (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4008-4015 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the coupled cluster method (CCM), we present a simple accurate calculation for the energies of the ground- and first excited states of the linear E⊗e Jahn–Teller and pseudo-Jahn–Teller Hamiltonians. From the solution of a single transcendental equation, we obtain results with a maximal error of 1.2%. These results are notably better than previous results obtained both via the CCM and other many-body approximations. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1285806
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 3
    Electronic Resource
    Electronic Resource
    Pairing correlations: I. The ground-state coupled-cluster formalism as a unifying approach (1988)
    Springer
    Few body systems 4 (1988), S. 161-177 
    Details
    ISSN: 1432-5411
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract the general problem of pairing correlations within a many-fermion system of identical particles is discussed at some length. We show in this first work in a series how the ground-state version of the [exp(S) or] coupled-cluster formalism (CCF) of quantum many-body theory may be rather generally applied to this problem, and how it thereby provides a very powerful and unifying approach to it. At the so-called SUB2 level of truncation, which is the lowest natural level of approximation for homogeneous systems, the CCF may be cast as a nonlinear integral equation for a four-point correlation function,S 2, which provides a measure of the two-particle/two-hole component in the true “ground-state” wavefunction. This approximation couples the particle-particle, hole-hole and particle-hole correlations simultaneously, and treats them all on an equal footing. In the present work we concentrate particular attention on particle-particle and hole-hole correlations by focussing on the formulation of generalised ladder approximations within the CCF. These are therefore likely to be physically applicable in the low-density regime. In particular, we show that the well-known Galitskii approximation may be formulated within the CCF as that drastic sub-approximation to the full SUB2 equation which keeps only the so-called complete ladder (CLAD) terms, which we describe. A second paper applies the CCF to the particular case of a general (non-local) separable potential, and obtains exact analytic solutions within the CLAD approximation for the corresponding wavefunctions of the simultaneous particle-particle and hole-hole substructures within the many-fermion system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01075347
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 4
    Electronic Resource
    Electronic Resource
    Pairing correlations: II. Exact model ground-state results for generalised ladders (1988)
    Springer
    Few body systems 4 (1988), S. 179-209 
    Details
    ISSN: 1432-5411
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In this series of papers, the so-called ground-state version of the [exp(S) or] coupled-cluster formalism (CCF) of quantum many-body theory is applied to the general problem of pairing correlations within a many-body system of identical fermions. In this second work in the series we restrict ourselves to exact calculations and concentrate on analytic solutions to the generalised ladder approximations formulated in the first paper. We focus attention on the particular model case of a general (non-local) separable potential, and work within the so-called complete ladder (CLAD) approximation which was shown in the earlier paper to be the CCF formulation of the well-known Galitskii approximation. We show how the CLAD approximation reduces in this case to a highly non-trivial pair of coupled nonlinear integral equations for the four-point correlation function,S 2, which provides a measure of the two-particle/two-hole component in the true “ground-state” wave-function. In the further derivation of exact analytic solutions for bothS 2 and the corresponding “ground-state” energy, we also see how various types of composite pairs within the many-body medium manifest themselves as “virtual (de-)excitations”. We thus show how our CCF provides an efficient and unified framework in which to describe all aspects of pairing, such as: (i) a possible free bound pair and its gradual approach to “dissolution” as the density is increased; (ii) the possible appearance of a second bound pair of predominantly hole-like quasi-particles above some lower critical density (which depends on the total momentum of the pair); (iii) the unstable but bound resonant pairs that can exist for densities above a comparable upper critical density at which the two previous types of real bound pairs have “dissolved”; and (iv) Cooper pairs. Even though each of these composite pairs leads to a new “condensed-pair phase” of lower energy, we further show that our so-called ground-state CCF leads only to the fluid-like state of uncondensed particles. In a third paper in this series we use the solutions obtained here as input to the analogous excited-state version of the CCF, and show how these various composite pairs materialise as “negative energy (de-)excitations”.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01076993
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 5
    Electronic Resource
    Electronic Resource
    An Efficient Implementation of High-Order Coupled-Cluster Techniques Applied to Quantum Magnets (1998)
    Springer
    Journal of statistical physics 90 (1998), S. 327-361 
    Details
    ISSN: 1572-9613
    Keywords: Coupled-cluster method ; quantum magnets ; strongly correlated spin lattices ; high-order LSUBm approximations ; generalized Néel model state ; square-lattice XXZ model ; triangular-lattice Heisenberg antiferromagnet ; ket-state parametrization ; bra-state parametrization ; lattice animals and fundamental configurations ; ground-state energy ; sublattice magnetization ; critical points ; quantum order ; quantum phase transitions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We illustrate how the systematic inclusion of multi-spin correlations of the quantum spin–lattice systems can be efficiently implemented within the framework of the coupled-cluster method by examining the ground-state properties of both the square-lattice and the frustrated triangular-lattice quantum antiferromagnets. The ground-state energy and the sublattice magnetization are calculated for the square-lattice and triangular-lattice Heisenberg antiferromagnets, and our best estimates give values for the sublattice magnetization which are 62% and 51% of the classical results for the square and triangular lattices, respectively. We furthermore make a conjecture as to why previous series expansion calculations have not indicated Néel-like long-range order for the triangular-lattice Heisenberg antiferromagnet. We investigate the critical behavior of the anisotropic systems by obtaining approximate values for the positions of phase transition points.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1023220222019
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 6
    Electronic Resource
    Electronic Resource
    A coupled-cluster study of the ground-state energy and properties of an anisotropic quantum spin lattice model exhibiting antiferromagnetism in various phases (1991)
    Springer
    Theoretical chemistry accounts 80 (1991), S. 181-205 
    Details
    ISSN: 1432-2234
    Keywords: Coupled cluster method ; Quantum spin lattice models ; XXZ-model Hamiltonian ; Low-dimensional anisotropic quantum antiferromagnets ; Phase transitions ; Calculation of staggered magnetization and correlation functions ; Ground states and excited states
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary We extend the domain of applicability of the coupled-cluster method (CCM) to include quantum-mechanical spin-1/2 systems on discrete lattices. We study the specific case of anisotropic antiferromagnetic interactions described by the nearest-neighbour XXZ-model Hamiltonian. The isotropic version of this model on a two-dimensional (2-d) square lattice is of great current interest as a possible description of the interactions between the electrons in the singly-occupied $$d_{x^2 - y^2 } $$ orbitals on the copper atoms in the ceramic copper oxide materials displaying high-temperature superconductivity. Although very few exact results are known for the 2-d XXZ-model, its 1-d counterpart has been exactly solved by Bethe-ansatz techniques, and we therefore use it here as a benchmark for our new CCM techniques. Even starting with the classical Néel state as the model reference state, we find that the CCM is capable at relatively low levels of truncation of giving accurate values for the ground-state energies. In this regard, we discuss several new CCM truncation hierarchies which have not previously been applied to either atoms and molecules or continuous extended systems. Furthermore, the method gives a good qualitative description of most of the known or anticipated behaviour of the correlation functions, order parameters, and elementary excitations over an entire (zero-temperature) phase, right up to the transition point, as the anisotropy is varied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01119619
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 7
    Electronic Resource
    Electronic Resource
    An overview of coupled cluster theory and its applications in physics (1991)
    Springer
    Theoretical chemistry accounts 80 (1991), S. 95-148 
    Details
    ISSN: 1432-2234
    Keywords: Coupled cluster theory ; Normal coupled cluster method (NCCM) ; Extended coupled cluster method (ECCM) ; Quantum many-body techniques ; Electron gas ; Atomic nuclei ; Nuclear matter ; Quantum field theory ; Quantum hydrodynamics ; Positron annihilation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary What has since become known as the normal coupled cluster method (NCCM) was invented about thirty years ago to calculate ground-state energies of closed-shell atomic nuclei. Coupled cluster (CC) techniques have since been developed to calculate excited states, energies of open-shell systems, density matrices and hence other properties, sum rules, and the sub-sum-rules that follow from imbedding linear response theory within the NCCM. Further extensions deal both with systems at nonzero temperature and with general dynamical behaviour. More recently, a new version of CC theory, the so-called extended coupled cluster method (ECCM) has been introduced. It has the potential to describe such global phenomena as phase transitions, spontaneous symmetry breaking, states of topological excitation, and nonequilibrium behaviour. CC techniques are now widely recognized as providing one of the most universally applicable, most powerful, and most accurate of all microscopicab initio methods in quantum many-body theory. The number of successful applications within physics is now impressively large. In most such cases the numerical results are either the best or among the best available. A typical case is the electron gas, where the CC results for the correlation energy agree over the entire metallic density range to within less than 1 millihartree (or 〈1%) with the essentially exact Green's function Monte Carlo results. The role of CC theory within modern quantum many-body theory is first surveyed, by a comparison with other techniques. Its full range of applications in physics is then reviewed. These include problems in nuclear physics, both for finite nuclei and infinite nuclear matter; the electron gas; various integrable and nonintegrable models; various relativistic quantum field theories; and quantum spin chain and lattice models. Particular applications of the ECCM include the quantum hydrodynamics of a zero-temperature, strongly-interacting condensed Bose fluid; a charged impurity in a polarizable medium (e.g., positron annihilation in metals); and various anharmonic oscillator and spin systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01119617
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 8
    Electronic Resource
    Electronic Resource
    A study of the Galitskii-Feynman T matrix for liquid 3He (1976)
    Springer
    Journal of low temperature physics 23 (1976), S. 393-410 
    Details
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Galitskii-Feynman T matrix, which sums the infinite ladder series in a many-fermion system for both particle-particle and hole-hole scattering, is studied in detail for a family of realistic He-He interactions. The structure of the S-wave bound-state singularity, reported previously, and its dependence on the bare interaction are documented at length. Special attention is devoted to the T matrix in the scattering region, where the c.m. energy of the interacting pair is positive. In particular, the on-energy-shell T matrix in this region is parametrized in terms of real “effective” phase shifts incorporating many-body effects. The critical behavior discussed previously in the bound-state region manifests itself clearly in the zero-energy limit of these phase shifts for the S wave. Below (above) a certain critical density, which is a function of both temperature and c.m. momentum, this limit approaches the value 0(−π) radians. A generalized Levinson's theorem relates this behavior to the existence of fermion-fermion pairing. An especially striking feature of these many-body phase shifts is the cusp behavior exhibited at the Fermi surface in the lowtemperature limit, which turns out to arise essentially from the structure of the particle and hole occupation probabilities. Throughout this study the temperature dependence of the T matrix is particularly emphasized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00116928
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 9
    Electronic Resource
    Electronic Resource
    Bound-state pairing singularities in the 3He Galitskii-Feynman T-matrix: Temperature dependence (1975)
    Springer
    Journal of low temperature physics 20 (1975), S. 573-584 
    Details
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: The temperature-dependent Galitskii-Feynman T-matrix, which sums the two-body scattering series, allowing any number of pairs of either particles outside or holes inside the Fermi sea in intermediate states, for a two-body potential appropriate to liquid 3He, is shown to exhibit a bound-state singularity. The two-body binding energy within the 3He background is shown to be highly sensitive to both the temperature and the density of the system. A critical density below which the binding disappears is observed to be a function of the temperature and of the total momentum of the interacting pair. Detailed numerical computations of this structure are presented using the modified Frost-Musulin potential. To our knowledge, these computations represent the first attempt at a temperature-dependent many-body calculation based on a realistic 3He potential. Possible consequences are drawn for more detailed and realistic calculations of the properties of liquid 3He.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00120869
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 10
    Electronic Resource
    Electronic Resource
    Low-energy He-He interactions with phenomenological potentials (1977)
    Springer
    Journal of low temperature physics 26 (1977), S. 669-690 
    Details
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The low-energy atom-atom scattering properties are studied for various combinations of He isotopes interacting via a family of semiphenomenological potentials. Some of the potentials bind the (4He)2 molecule and some do not, and it is observed that molecular beam scattering measurements at energies currently accessible cannot resolve the difference between the two types of potentials. Our results are discussed within the framework of possible measurements to resolve this discrepancy. We present a method for solving the Schrödinger equation which is uniformly applicable for both bound and scattering state solutions and which is particularly suited to the types of potentials describing interactions for the rare gas atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00654874
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...