203 research outputs found
Interplay of charge and spin correlations in nickel perovskites
Analyzing the motion of low--spin holes in a high--spin
background, we derive a sort of generalized t--J Hamiltonian for the planes of Sr--doped nickelates. In addition to the rather complex
carrier--spin and spin--spin couplings we take into account the coupling of the
doped holes to in--plane oxygen breathing modes by a Holstein--type interaction
term. Because of strong magnetic confinement effects the holes are nearly
entirely prelocalized and the electron--phonon coupling becomes much more
effective in forming polarons than in the isostructural cuprates. In the light
of recent experiments on we discuss how the variety of
the observed transport and charge/spin--ordering phenomena can be qualitatively
understood in terms of our model Hamiltonian.Comment: 2 pages, LTpaper.sty, Proc. XXI Int. Conf. on Low Temp. Phys. Prague
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Chebyshev approach to quantum systems coupled to a bath
We propose a new concept for the dynamics of a quantum bath, the Chebyshev
space, and a new method based on this concept, the Chebyshev space method. The
Chebyshev space is an abstract vector space that exactly represents the
fermionic or bosonic bath degrees of freedom, without a discretization of the
bath density of states. Relying on Chebyshev expansions the Chebyshev space
representation of a bath has very favorable properties with respect to
extremely precise and efficient calculations of groundstate properties, static
and dynamical correlations, and time-evolution for a great variety of quantum
systems. The aim of the present work is to introduce the Chebyshev space in
detail and to demonstrate the capabilities of the Chebyshev space method.
Although the central idea is derived in full generality the focus is on model
systems coupled to fermionic baths. In particular we address quantum impurity
problems, such as an impurity in a host or a bosonic impurity with a static
barrier, and the motion of a wave packet on a chain coupled to leads. For the
bosonic impurity, the phase transition from a delocalized electron to a
localized polaron in arbitrary dimension is detected. For the wave packet on a
chain, we show how the Chebyshev space method implements different boundary
conditions, including transparent boundary conditions replacing infinite leads.
Furthermore the self-consistent solution of the Holstein model in infinite
dimension is calculated. With the examples we demonstrate how highly accurate
results for system energies, correlation and spectral functions, and
time-dependence of observables are obtained with modest computational effort.Comment: 18 pages, 13 figures, to appear in Phys. Rev.
Spatiotemporal evolution of polaronic states in finite quantum systems
We study the quantum dynamics of small polaron formation and polaron
transport through finite quantum structures in the framework of the
one-dimensional Holstein model with site-dependent potentials and interactions.
Combining Lanczos diagonalization with Chebyshev moment expansion of the time
evolution operator, we determine how different initial states, representing
stationary ground states or injected wave packets, after an electron-phonon
interaction quench, develop in real space and time. Thereby, the full quantum
nature and dynamics of electrons and phonons is preserved. We find that the
decay out of the initial state sensitively depends on the energy and momentum
of the incoming particle, the electron-phonon coupling strength, and the phonon
frequency, whereupon bound polaron-phonon excited states may emerge in the
strong-coupling regime. The tunneling of a Holstein polaron through a quantum
wall/dot is generally accompanied by strong phonon number fluctuations due to
phonon emission and re-absorption processes.Comment: 13 pages, 15 figures, final versio
Sparse polynomial space approach to dissipative quantum systems: Application to the sub-ohmic spin-boson model
We propose a general numerical approach to open quantum systems with a
coupling to bath degrees of freedom. The technique combines the methodology of
polynomial expansions of spectral functions with the sparse grid concept from
interpolation theory. Thereby we construct a Hilbert space of moderate
dimension to represent the bath degrees of freedom, which allows us to perform
highly accurate and efficient calculations of static, spectral and dynamic
quantities using standard exact diagonalization algorithms. The strength of the
approach is demonstrated for the phase transition, critical behaviour, and
dissipative spin dynamics in the spin boson modelComment: 4 pages, 4 figures, revised version accepted for publication in PR
DMRG analysis of the SDW-CDW crossover region in the 1D half-filled Hubbard-Holstein model
In order to clarify the physics of the crossover from a spin-density-wave
(SDW) Mott insulator to a charge-density-wave (CDW) Peierls insulator in
one-dimensional (1D) systems, we investigate the Hubbard-Holstein Hamiltonian
at half filling within a density matrix renormalisation group (DMRG) approach.
Determining the spin and charge correlation exponents, the momentum
distribution function, and various excitation gaps, we confirm that an
intervening metallic phase expands the SDW-CDW transition in the weak-coupling
regime.Comment: revised versio
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