271 research outputs found

### Thermalization of a pump-excited Mott insulator

We use nonequilibrium dynamical mean-field theory in combination with a
recently implemented strong-coupling impurity solver to investigate the
relaxation of a Mott insulator after a laser excitation with frequency
comparable to the Hubbard gap. The time evolution of the double occupancy
exhibits a crossover from a strongly damped transient at short times towards an
exponential thermalization at long times. In the limit of strong interactions,
the thermalization time is consistent with the exponentially small decay rate
for artificially created doublons, which was measured in ultracold atomic
gases. When the interaction is comparable to the bandwidth, on the other hand,
the double occupancy thermalizes within a few times the inverse bandwidth along
a rapid thermalization path in which the exponential tail is absent. Similar
behavior can be observed in time-resolved photoemission spectroscopy. Our
results show that a simple quasi-equilibrium description of the electronic
state breaks down for pump-excited Mott insulators characterized by strong
interactions.Comment: 8 pages, 4 figure

### Photo-induced states in a Mott insulator

We investigate the properties of the metallic state obtained by photo-doping
carriers into a Mott insulator. In a strongly interacting system, these
carriers have a long life-time, so that they can dissipate their kinetic energy
to a phonon bath. In the relaxed state, the scattering rate saturates at a
non-zero temperature-independent value, and the momentum-resolved spectral
function features broad bands which differ from the well-defined quasi-particle
bands of a chemically doped system. Our results indicate that a photo-doped
Mott insulator behaves as a bad metal, in which strong scattering between
doublons and holes inhibits Fermi-liquid behavior down to low temperature.Comment: 5 page

### Decoupling method for dynamical mean field theory calculations

In this paper we explore the use of an equation of motion decoupling method
as an impurity solver to be used in conjunction with the dynamical mean field
self-consistency condition for the solution of lattice models. We benchmark the
impurity solver against exact diagonalization, and apply the method to study
the infinite $U$ Hubbard model, the periodic Anderson model and the $pd$ model.
This simple and numerically efficient approach yields the spectra expected for
strongly correlated materials, with a quasiparticle peak and a Hubbard band. It
works in a large range of parameters, and therefore can be used for the
exploration of real materials using LDA+DMFT.Comment: 30 pages, 7 figure

### Nonequilibrium dynamical mean-field calculations based on the non-crossing approximation and its generalizations

We solve the impurity problem which arises within nonequilibrium dynamical
mean-field theory for the Hubbard model by means of a self-consistent
perturbation expansion around the atomic limit. While the lowest order, known
as the non-crossing approximation (NCA), is reliable only when the interaction
U is much larger than the bandwidth, low-order corrections to the NCA turn out
to be sufficient to reproduce numerically exact Monte Carlo results in a wide
parameter range that covers the insulating phase and the metal-insulator
crossover regime at not too low temperatures. As an application of the
perturbative strong-coupling impurity solver we investigate the response of the
double occupancy in the Mott insulating phase of the Hubbard model to a
dynamical change of the interaction or the hopping, a technique which has been
used as a probe of the Mott insulating state in ultracold fermionic gases.Comment: 14 pages, 9 figure

### Nonthermal symmetry broken states in the strongly interacting Hubbard model

We study the time evolution of the antiferromagnetic order parameter after
interaction quenches in the Hubbard model. Using the nonequilibrium dynamical
mean field formalism, we show that the system, after a quench from intermediate
to strong interaction, is trapped in a nonthermal state which is reminiscent of
a photo-doped state and protected by the slow decay of doublons. If the
effective doping of this state is low enough, it exhibits robust
antiferromagnetic order, even if the system is highly excited and the thermal
state thus expected to be paramagnetic. We comment on the implication of our
findings for the stability of nonthermal superconducting states

### Transport Properties of the Infinite Dimensional Hubbard Model

Results for the optical conductivity and resistivity of the Hubbard model in
infinite spatial dimensions are presented. At half filling we observe a gradual
crossover from a normal Fermi-liquid with a Drude peak at $\omega=0$ in the
optical conductivity to an insulator as a function of $U$ for temperatures
above the antiferromagnetic phase transition. When doped, the ``insulator''
becomes a Fermi-liquid with a corresponding temperature dependence of the
optical conductivity and resistivity. We find a $T^2$-coefficient in the low
temperature resistivity which suggests that the carriers in the system acquire
a considerable mass-enhancement due to the strong local correlations. At high
temperatures, a crossover into a semi-metallic regime takes place.Comment: 14 page

### pi-Junction behavior and Andreev bound states in Kondo quantum dots with superconducting leads

We investigate the temperature- and coupling-dependent transport through
Kondo dot contacts with symmetric superconducting s-wave leads. For finite
temperature T we use a superconducting extension of a selfconsistent auxiliary
boson scheme, termed SNCA, while at T=0 a perturbative renormalization group
treatment is applied. The finite-temperature phase diagram for the 0--pi
transition of the Josephson current in the junction is established and related
to the phase-dependent position of the subgap Kondo resonance with respect to
the Fermi energy. The conductance of the contact is evaluated in the zero-bias
limit. It approaches zero in the low-temperature regime, however, at finite T
its characteristics are changed through the coupling- and temperature-dependent
0--pi transition.Comment: 12 pages, 12 figure

### Kondo effect in a magnetic field and the magnetoresistivity of Kondo alloys

The effect of a magnetic field on the spectral density of a $\rm{S=1/2}$
Kondo impurity is investigated at zero and finite temperatures by using
Wilson's numerical renormalization group method. A splitting of the total
spectral density is found for fields larger than a critical value
$H_{c}(T=0)\approx 0.5 T_{K}$, where $T_{K}$ is the Kondo scale. The splitting
correlates with a peak in the magnetoresistivity of dilute magnetic alloys
which we calculate and compare with the experiments on
$\rm{Ce_{x}La_{1-x}Al_{2}}, x=0.0063$. The linear magnetoconductance of quantum
dots exhibiting the Kondo effect is also calculated.Comment: 4 pages, 4 eps figure

### Self-Consistent Perturbation Theory for Thermodynamics of Magnetic Impurity Systems

Integral equations for thermodynamic quantities are derived in the framework
of the non-crossing approximation (NCA). Entropy and specific heat of 4f
contribution are calculated without numerical differentiations of thermodynamic
potential. The formulation is applied to systems such as PrFe4P12 with
singlet-triplet crystalline electric field (CEF) levels.Comment: 3 pages, 2 figures, proc. ASR-WYP-2005 (JAERI

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