5,593 research outputs found
Reversal of particle-hole scattering-rate asymmetry in Anderson impurity model
We study the particle-hole asymmetry of the scattering rate in strongly
correlated electron systems by examining the cubic and
terms in the imaginary part of the self-energy of the Anderson impurity model.
We show that the sign is opposite in the weak-coupling and strong-coupling
limits, explaining the differences found in theoretical approaches taking the
respective limits as the starting points. The sign change in fact precisely
delineates the cross-over between the weak and strong correlation regimes of
the model. For weak interaction the sign reversal occurs for small values
of the doping , while for interaction of order , being the hybridization strength, the cross-over curve
rapidly shifts to the large-doping range. This curve based on the impurity
dynamics is genuinely different from other cross-over curves defined through
impurity thermodynamic and static properties.Comment: 4 pages, 5 figure
Doping a correlated band insulator: A new route to half metallic behaviour
We demonstrate in a simple model the surprising result that turning on an
on-site Coulomb interaction U in a doped band insulator leads to the formation
of a half-metallic state. In the undoped system, we show that increasing U
leads to a first order transition between a paramagnetic, band insulator and an
antiferomagnetic Mott insulator at a finite value U_{AF}. Upon doping, the
system exhibits half metallic ferrimagnetism over a wide range of doping and
interaction strengths on either side of U_{AF}. Our results, based on dynamical
mean field theory, suggest a novel route to half-metallic behavior and provide
motivation for experiments on new materials for spintronics.Comment: 5 pages, 7 figure
Variational Monte Carlo and Configurational Interaction Studies of and its Fragments
The molecule and its fragments are studied using Configuration
Interaction (CI) and Variational Monte Carlo (VMC) techniques, within the
Hubbard model. Using benzene as a test case, we compare the results of the
approximate calculations with exact calculations. The fragments of
studied are pyracylene, fluoranthene and corannulene. The energies, bond
orders, spin-spin and charge-correlation functions of these systems are
obtained for various values of the Hubbard parameter, . The analysis of bond
orders and correlation functions of these individual molecules allow us to
visualise pyracylene as a naphthalene unit with two ethylenic moieties and
fluoranthene as weakly bridged benzene and naphthalene units. Corannulene is
the largest fragment of that we have studied. The hexagon-hexagon(h-h)
bond orders are slightly larger than those of the hexagon-pentagon bonds(h-p),
a feature also found in other fragments. We also find bonds between two
co-ordinated carbon sites to be stronger than bonds involving three coordinated
carbon sites. In , the h-h bonds are stronger than in corannulene and
the h-p bonds weaker than in corannulene for all correlation strengths.
Introducing bond alternation in the buckyball enhances this difference.Comment: 42 pages, 5 figures available on request, to appear in J. Phys. Che
Phase Diagram of the Half-Filled Ionic Hubbard Model
We study the phase diagram of the ionic Hubbard model (IHM) at half-filling
using dynamical mean field theory (DMFT), with two impurity solvers, namely,
iterated perturbation theory (IPT) and continuous time quantum Monte Carlo
(CTQMC). The physics of the IHM is governed by the competition between the
staggered potential and the on-site Hubbard U. In both the methods we
find that for a finite and at zero temperature, anti-ferromagnetic
(AFM) order sets in beyond a threshold via a first order phase
transition below which the system is a paramagnetic band insulator. Both the
methods show a clear evidence for a transition to a half-metal phase just after
the AFM order is turned on, followed by the formation of an AFM insulator on
further increasing U. We show that the results obtained within both the methods
have good qualitative and quantitative consistency in the intermediate to
strong coupling regime. On increasing the temperature, the AFM order is lost
via a first order phase transition at a transition temperature within both the methods, for weak to intermediate values of U/t. But
in the strongly correlated regime, where the effective low energy Hamiltonian
is the Heisenberg model, IPT is unable to capture the thermal (Neel) transition
from the AFM phase to the paramagnetic phase, but the CTQMC does. As a result,
at any finite temperature T, DMFT+CTQMC shows a second phase transition (not
seen within DMFT+IPT) on increasing U beyond . At , when
the Neel temperature for the effective Heisenberg model becomes lower
than T, the AFM order is lost via a second order transition. In the
3-dimensonal parameter space of , there is a line of
tricritical points that separates the surfaces of first and second order phase
transitions.Comment: Revised versio
Quantum phase transition in capacitively coupled double quantum dots
We investigate two equivalent, capacitively coupled semiconducting quantum
dots, each coupled to its own lead, in a regime where there are two electrons
on the double dot. With increasing interdot coupling a rich range of behavior
is uncovered: first a crossover from spin- to charge-Kondo physics, via an
intermediate SU(4) state with entangled spin and charge degrees of freedom;
followed by a quantum phase transition of Kosterlitz-Thouless type to a
non-Fermi liquid `charge-ordered' phase with finite residual entropy and
anomalous transport properties. Physical arguments and numerical
renormalization group methods are employed to obtain a detailed understanding
of the problem.Comment: 4 pages, 3 figure
Synchronous and Asynchronous Mott Transitions in Topological Insulator Ribbons
We address how the nature of linearly dispersing edge states of two
dimensional (2D) topological insulators evolves with increasing
electron-electron correlation engendered by a Hubbard like on-site repulsion
in finite ribbons of two models of topological band insulators. Using an
inhomogeneous cluster slave rotor mean-field method developed here, we show
that electronic correlations drive the topologically nontrivial phase into a
Mott insulating phase via two different routes. In a synchronous transition,
the entire ribbon attains a Mott insulating state at one critical that
depends weakly on the width of the ribbon. In the second, asynchronous route,
Mott localization first occurs on the edge layers at a smaller critical value
of electronic interaction which then propagates into the bulk as is further
increased until all layers of the ribbon become Mott localized. We show that
the kind of Mott transition that takes place is determined by certain
properties of the linearly dispersing edge states which characterize the
topological resilience to Mott localization.Comment: 4+ pages, 5 figure
Quasi-universal transient behavior of a nonequilibrium Mott insulator driven by an electric field
We use a self-consistent strong-coupling expansion for the self-energy
(perturbation theory in the hopping) to describe the nonequilibrium dynamics of
strongly correlated lattice fermions. We study the three-dimensional
homogeneous Fermi-Hubbard model driven by an external electric field showing
that the damping of the ensuing Bloch oscillations depends on the direction of
the field, and that for a broad range of field strengths, a long-lived
transient prethermalized state emerges. This long-lived transient regime
implies that thermal equilibrium may be out of reach of the time scales
accessible in present cold atom experiments, but shows that an interesting new
quasi-universal transient state exists in nonequilibrium governed by a
thermalized kinetic energy but not a thermalized potential energy. In addition,
when the field strength is equal in magnitude to the interaction between atoms,
the system undergoes a rapid thermalization, characterized by a different
quasi-universal behavior of the current and spectral function for different
values of the hopping.Comment: (5 pages, 5 figures, ReVTeX
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