62 research outputs found
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
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
Single-particle excitations across the many-body localization transition in quasi-periodic systems
We study many-body localization transition in one dimensional systems in the
presence of a deterministic quasi-periodic potential. We focus on
single-particle excitations produced in highly excited many-body eigenstates
obtained through single-particle Green's function in real space. A finite-size
scaling analysis of the ratio of the typical to average value of the local
density of states of single particle excitations is performed assuming that the
correlation length diverges at the transition point with a power-law . Both for the Aubry-Andre (AA) model and the generalized
AA model, the finite size scaling of the local density of states obeys the
single parameter scaling. A good quality scaling collapse is obtained for which satisfies the generalized Luck's criterion for quasiperiodic
systems. This analysis supports the continuous nature of the many-body
localization transition in systems with AA and generalized AA potentials.Comment: 10 pages, 11 figure
Can correlations drive a band insulator metallic?
We analyze the effects of the on-site Coulomb repulsion U on a band insulator
using dynamical mean field theory (DMFT). We find the surprising result that
the gap is suppressed to zero at a critical Uc1 and remains zero within a
metallic phase. At a larger Uc2 there is a second transition from the metal to
a Mott insulator, in which the gap increases with increasing U. These results
are qualitatively different from Hartree-Fock theory which gives a
monotonically decreasing but non-zero insulating gap for all finite U.Comment: 4 pages, 5 figure
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