804 research outputs found
Mott transition of fermionic atoms in a three-dimensional optical trap
We study theoretically the Mott metal-insulator transition for a system of
fermionic atoms confined in a three-dimensional optical lattice and a harmonic
trap. We describe an inhomogeneous system of several thousand sites using an
adaptation of dynamical mean field theory solved efficiently with the numerical
renormalization group method. Above a critical value of the on-site
interaction, a Mott-insulating phase appears in the system. We investigate
signatures of the Mott phase in the density profile and in time-of-flight
experiments.Comment: 4 pages and 5 figure
Magnetotransport through a strongly interacting quantum dot
We study the effect of a magnetic field on the conductance through a strongly
interacting quantum dot by using the finite temperature extension of Wilson's
numerical renormalization group method to dynamical quantities. The quantum dot
has one active level for transport and is modelled by an Anderson impurity
attached to left and right electron reservoirs. Detailed predictions are made
for the linear conductance and the spin-resolved conductance as a function of
gate voltage, temperature and magnetic field strength. A strongly coupled
quantum dot in a magnetic field acts as a spin filter which can be tuned by
varying the gate voltage. The largest spin-filtering effect is found in the
range of gate voltages corresponding to the mixed valence regime of the
Anderson impurity model.Comment: Revised version, to appear in PRB, 4 pages, 4 figure
Transport Coefficients of the Anderson Model via the Numerical Renormalization Group
The transport coefficients of the Anderson model are calculated by extending
Wilson's NRG method to finite temperature Green's functions. Accurate results
for the frequency and temperature dependence of the single--particle spectral
densities and transport time are obtained and used to extract
the temperature dependence of the transport coefficients in the strong
correlation limit. The low temperature anomalies in the resistivity, ,
thermopower, , thermal conductivity and Hall coefficient,
, are discussed. All quantities exhibit the expected Fermi liquid
behaviour at low temperature with power law dependecies on in very
good agreement with analytic results based on Fermi liquid theory. Scattering
of conduction electrons in higher, , angular momentum channels is also
considered and an expression is derived for the corresponding transport time
and used to discuss the influence of non--resonant scattering on the transport
properties.Comment: 45 pages, RevTeX, 28 figures, available on reques
Real-Time-RG Analysis of the Dynamics of the Spin-Boson Model
Using a real-time renormalization group method we determine the complete
dynamics of the spin-boson model with ohmic dissipation for coupling strengths
. We calculate the relaxation and dephasing time, the
static susceptibility and correlation functions. Our results are consistent
with quantum Monte Carlo simulations and the Shiba relation. We present for the
first time reliable results for finite cutoff and finite bias in a regime where
perturbation theory in or in tunneling breaks down. Furthermore, an
unambigious comparism to results from the Kondo model is achieved.Comment: 4 pages, 5 figures, 1 tabl
The numerical renormalization group method for quantum impurity systems
In the beginning of the 1970's, Wilson developed the concept of a fully
non-perturbative renormalization group transformation. Applied to the Kondo
problem, this numerical renormalization group method (NRG) gave for the first
time the full crossover from the high-temperature phase of a free spin to the
low-temperature phase of a completely screened spin. The NRG has been later
generalized to a variety of quantum impurity problems. The purpose of this
review is to give a brief introduction to the NRG method including some
guidelines of how to calculate physical quantities, and to survey the
development of the NRG method and its various applications over the last 30
years. These applications include variants of the original Kondo problem such
as the non-Fermi liquid behavior in the two-channel Kondo model, dissipative
quantum systems such as the spin-boson model, and lattice systems in the
framework of the dynamical mean field theory.Comment: 55 pages, 27 figures, submitted to Rev. Mod. Phy
Kondo proximity effect: How does a metal penetrate into a Mott insulator?
We consider a heterostructure of a metal and a paramagnetic Mott insulator
using an adaptation of dynamical mean field theory to describe inhomogeneous
systems. The metal can penetrate into the insulator via the Kondo effect. We
investigate the scaling properties of the metal-insulator interface close to
the critical point of the Mott insulator. At criticality, the quasiparticle
weight decays as 1/x^2 with distance x from the metal within our mean field
theory. Our numerical results (using the numerical renormalization group as an
impurity solver) show that the prefactor of this power law is extremely small.Comment: 4 pages, 3 figure
Scaling and universality in the anisotropic Kondo model and the dissipative two-state system
Scaling and universality in the Ohmic two-state system is investigated by
exploiting the equivalence of this model to the anisotropic Kondo model. For
the Ohmic two-state system, we find universal scaling functions for the
specific heat, , static susceptibility, , and
spin relaxation function depending on the reduced
temperature (frequency ), with
the renormalized tunneling frequency, and uniquely specified by the dissipation
strength (). The scaling functions can be used to extract
and in experimental realizations.Comment: 5 pages (LaTeX), 4 EPS figures. Minor changes, typos corrected,
journal reference adde
Anderson impurity model at finite Coulomb interaction U: generalized Non-crossing Approximation
We present an extension of the non-crossing approximation (NCA), which is
widely used to calculate properties of Anderson impurity models in the limit of
infinite Coulomb repulsion , to the case of finite . A
self-consistent conserving pseudo-particle representation is derived by
symmetrizing the usual NCA diagrams with respect to empty and doubly occupied
local states. This requires an infinite summation of skeleton diagrams in the
generating functional thus defining the ``Symmetrized finite-U NCA'' (SUNCA).
We show that within SUNCA the low energy scale (Kondo temperature) is
correctly obtained, in contrast to other simpler approximations discussed in
the literature.Comment: 7 pages, 6 figure
Renormalization Group Approach to Non-equilibrium Green Functions in Correlated Impurity Systems
We present a technique for calculating non-equilibrium Green functions for
impurity systems with local interactions. We use an analogy to the calculation
of response functions in the x-ray problem.The initial state and the final
state problems, which correspond to the situations before and after the
disturbance (an electric or magnetic field, for example) is suddenly switched
on, are solved with the aid of Wilson's momentum shell renormalization group.
The method is illustrated by calculating the non-equilibrium dynamics of the
ohmic two-state problem.Comment: 7 pages, 2 figure
Unified description of Fermi and non-Fermi liquid behavior in a conserving slave boson approximation for strongly correlated impurity models
We show that the presence of Fermi or non-Fermi liquid behavior in the SU(N)
x SU(M) Anderson impurity models may be read off the infrared threshold
exponents governing the spinon and holon dynamics in a slave boson
representation of these models. We construct a conserving T-matrix
approximation which recovers the exact exponents with good numerical accuracy.
Our approximation includes both coherent spin flip scattering and charge
fluctuation processes. For the single-channel case the tendency to form bound
states drastically modifies the low energy behavior. For the multi-channel case
in the Kondo limit the bound state contributions are unimportant.Comment: 4 pages, Latex, 3 postscript figures included Final version with
minor changes in wording, to appear in Phys.Rev.Let
- …