33,831 research outputs found
Quasiparticle Lifetimes and the Conductivity Scattering Rate
We compute the single-particle inverse lifetime, along with the
conductivity-derived scattering rate, for a metallic system in an s-wave
superconducting state. When both electron-phonon and electron-impurity
scattering are included, we find that while these scattering rates are in
qualitative agreement, in general quantitative agreement is lacking. We also
derive results for the quasiparticle lifetime within the BCS framework with
impurity scattering, which makes it clear that impurity scattering is
suppressed for electrons near the Fermi surface in the superconducting state.Comment: 46 pages, Postscript figures (to be published in Aust. J. Phys.
Decay properties of spectral projectors with applications to electronic structure
Motivated by applications in quantum chemistry and solid state physics, we
apply general results from approximation theory and matrix analysis to the
study of the decay properties of spectral projectors associated with large and
sparse Hermitian matrices. Our theory leads to a rigorous proof of the
exponential off-diagonal decay ("nearsightedness") for the density matrix of
gapped systems at zero electronic temperature in both orthogonal and
non-orthogonal representations, thus providing a firm theoretical basis for the
possibility of linear scaling methods in electronic structure calculations for
non-metallic systems. We further discuss the case of density matrices for
metallic systems at positive electronic temperature. A few other possible
applications are also discussed.Comment: 63 pages, 13 figure
Holographic estimate of heavy quark diffusion in a magnetic field
We study the influence of a background magnetic field on the vector
meson in a DBI-extension of the soft wall model, building upon our earlier work
Phys. Rev. D91, 086002 (2015). In this specific holographic QCD model, we
discuss the heavy quark number susceptibility and diffusion constants of charm
quarks and their dependence on the magnetic field by either a hydrodynamic
expansion or by numerically solving the differential equation. This allows us
to determine the response of these transport coefficients to the magnetic
field. The effects of the latter are considered both from a direct as indirect
(medium) viewpoint. As expected, we find a magnetic field induced anisotropic
diffusion, with a stronger diffusion in the longitudinal direction compared to
the transversal one. We backup, at least qualitatively, our findings with a
hanging string analysis of heavy quark diffusion in a magnetic field. From the
quark number susceptibility we can extract an estimate for the effective
deconfinement temperature in the heavy quark sector, reporting consistency with
the phenomenon of inverse magnetic catalysis.Comment: 27 pages. v2: extra discussions and references, compatible with
version accepted by Phys.Rev.
Diagonalization- and Numerical Renormalization-Group-Based Methods for Interacting Quantum Systems
In these lecture notes, we present a pedagogical review of a number of
related {\it numerically exact} approaches to quantum many-body problems. In
particular, we focus on methods based on the exact diagonalization of the
Hamiltonian matrix and on methods extending exact diagonalization using
renormalization group ideas, i.e., Wilson's Numerical Renormalization Group
(NRG) and White's Density Matrix Renormalization Group (DMRG). These methods
are standard tools for the investigation of a variety of interacting quantum
systems, especially low-dimensional quantum lattice models. We also survey
extensions to the methods to calculate properties such as dynamical quantities
and behavior at finite temperature, and discuss generalizations of the DMRG
method to a wider variety of systems, such as classical models and quantum
chemical problems. Finally, we briefly review some recent developments for
obtaining a more general formulation of the DMRG in the context of matrix
product states as well as recent progress in calculating the time evolution of
quantum systems using the DMRG and the relationship of the foundations of the
method with quantum information theory.Comment: 51 pages; lecture notes on numerically exact methods. Pedagogical
review appearing in the proceedings of the "IX. Training Course in the
Physics of Correlated Electron Systems and High-Tc Superconductors", Vietri
sul Mare (Salerno, Italy, October 2004
On the Fermi Liquid to Polaron Crossover I: General Results
We use analytic techniques and the dynamical mean field method to study the
crossover from fermi liquid to polaron behavior in models of electrons
interacting with dispersionless classical phonons.Comment: 42 pages, 13 figure
Covalency, double-counting and the metal-insulator phase diagram in transition metal oxides
Dynamical mean field theory calculations are used to show that for late
transition-metal-oxides a critical variable for the Mott/charge-transfer
transition is the number of d-electrons, which is determined by charge transfer
from oxygen ions. Insulating behavior is found only for a narrow range of
d-occupancy, irrespective of the size of the intra-d Coulomb repulsion. The
result is useful in interpreting 'density functional +U' and 'density
functional plus dynamical mean field' methods in which additional correlations
are applied to a specific set of orbitals and an important role is played by
the 'double counting correction' which dictates the occupancy of these
correlated orbitals. General considerations are presented and are illustrated
by calculations for two representative transition metal oxide systems: layered
perovskite Cu-based "high-Tc" materials, an orbitally non-degenerate
electronically quasi-two dimensional systems, and pseudocubic rare earch
nickelates, an orbitally degenerate electronically three dimensional system.
Density functional calculations yield d-occupancies very far from the Mott
metal-insulator phase boundary in the nickelate materials, but closer to it in
the cuprates, indicating the sensitivity of theoretical models of the cuprates
to the choice of double counting correction and corroborating the critical role
of lattice distortions in attaining the experimentally observed insulating
phase in the nickelates.Comment: 10+ pages, 5 figure
- …