37 research outputs found
Magnetic and Transport Properties of a Coupled Hubbard Bilayer with Electron and Hole Doping
The single band, two dimensional Hubbard Hamiltonian has been extensively
studied as a model for high temperature superconductivity. While Quantum Monte
Carlo simulations within the dynamic cluster approximation are now providing
considerable evidence for a d-wave superconducting state at low temperature,
such a transition remains well out of reach of finite lattice simulations
because of the "sign problem". We show here that a bilayer Hubbard model, in
which one layer is electron doped and one layer is hole doped, can be studied
to lower temperatures and exhibits an interesting signal of d-wave pairing. The
results of our simulations bear resemblance to a recent report on the magnetic
and superconducting properties of BaCaCuOF which contains
both electron and hole doped CuO planes. We also explore the phase diagram
of bilayer models in which each sheet is at half-filling
Quantum Monte Carlo Study of an Interaction-Driven Band Insulator to Metal Transition
We study the transitions from band insulator to metal to Mott insulator in
the ionic Hubbard model on a two dimensional square lattice using determinant
Quantum Monte Carlo. Evaluation of the temperature dependence of the
conductivity demonstrates that the metallic region extends for a finite range
of interaction values. The Mott phase at strong coupling is accompanied by
antiferromagnetic (AF) order. Inclusion of these intersite correlations changes
the phase diagram qualitatively compared to dynamical mean field theory.Comment: 4 pages, 6 figure
Metallic phase in the two-dimensional ionic Hubbard model
We investigate the phases of the ionic Hubbard model in a two-dimensional
square lattice using determinant quantum Monte Carlo (DQMC). At half-filling,
when the interaction strength or the staggered potential dominate we find Mott
and band insulators, respectively. When these two energies are of the same
order we find a metallic region. Charge and magnetic structure factors
demonstrate the presence of antiferromagnetism only in the Mott region,
although the externally imposed density modulation is present everywhere in the
phase diagram. Away from half-filling, other insulating phases are found.
Kinetic energy correlations do not give clear signals for the existence of a
bond-ordered phase
Attractive Hubbard Model on a Honeycomb Lattice
We study the attractive fermionic Hubbard model on a honeycomb lattice using
determinantal quantum Monte Carlo simulations. By increasing the interaction
strength U (relative to the hopping parameter t) at half-filling and zero
temperature, the system undergoes a quantum phase transition at 5.0 < U_c/t <
5.1 from a semi-metal to a phase displaying simultaneously superfluid behavior
and density order. Doping away from half-filling, and increasing the
interaction strength at finite but low temperature T, the system always appears
to be a superfluid exhibiting a crossover between a BCS and a molecular regime.
These different regimes are analyzed by studying the spectral function. The
formation of pairs and the emergence of phase coherence throughout the sample
are studied as U is increased and T is lowered
Superconductivity in striped and multi-Fermi-surface Hubbard models: From the cuprates to the pnictides
Single- and multi-band Hubbard models have been found to describe many of the
complex phenomena that are observed in the cuprate and iron-based
high-temperature superconductors. Simulations of these models therefore provide
an ideal framework to study and understand the superconducting properties of
these systems and the mechanisms responsible for them. Here we review recent
dynamic cluster quantum Monte Carlo simulations of these models, which provide
an unbiased view of the leading correlations in the system. In particular, we
discuss what these simulations tell us about superconductivity in the
homogeneous 2D single-orbital Hubbard model, and how charge stripes affect this
behavior. We then describe recent simulations of a bilayer Hubbard model, which
provides a simple model to study the type and nature of pairing in systems with
multiple Fermi surfaces such as the iron-based superconductors.Comment: Published as part of Superstripes 2011 (Rome) conference proceeding
Competition of crystal field splitting and Hund's rule coupling in two-orbital magnetic metal-insulator transitions
Competition of crystal field splitting and Hund's rule coupling in magnetic
metal-insulator transitions of half-filled two-orbital Hubbard model is
investigated by multi-orbital slave-boson mean field theory. We show that with
the increase of Coulomb correlation, the system firstly transits from a
paramagnetic (PM) metal to a {\it N\'{e}el} antiferromagnetic (AFM) Mott
insulator, or a nonmagnetic orbital insulator, depending on the competition of
crystal field splitting and the Hund's rule coupling. The different AFM Mott
insulator, PM metal and orbital insulating phase are none, partially and fully
orbital polarized, respectively. For a small and a finite crystal
field, the orbital insulator is robust. Although the system is nonmagnetic, the
phase boundary of the orbital insulator transition obviously shifts to the
small regime after the magnetic correlations is taken into account. These
results demonstrate that large crystal field splitting favors the formation of
the orbital insulating phase, while large Hund's rule coupling tends to destroy
it, driving the low-spin to high-spin transition.Comment: 4 pages, 4 figure
Hatano-Nelson model with a periodic potential
We study a generalisation of the Hatano-Nelson Hamiltonian in which a
periodic modulation of the site energies is present in addition to the usual
random distribution. The system can then become localized by disorder or
develop a band gap, and the eigenspectrum shows a wide variety of topologies.
We determine the phase diagram, and perform a finite size scaling analysis of
the localization transition.Comment: 7 pages, 10 figure
Excitation Spectrum of One-dimensional Extended Ionic Hubbard Model
We use Perturbative Continuous Unitary Transformations (PCUT) to study the
one dimensional Extended Ionic Hubbard Model (EIHM) at half-filling in the band
insulator region. The extended ionic Hubbard model, in addition to the usual
ionic Hubbard model, includes an inter-site nearest-neighbor (n.n.) repulsion,
. We consider the ionic potential as unperturbed part of the Hamiltonian,
while the hopping and interaction (quartic) terms are treated as perturbation.
We calculate total energy and ionicity in the ground state. Above the ground
state, (i) we calculate the single particle excitation spectrum by adding an
electron or a hole to the system. (ii) the coherence-length and spectrum of
electron-hole excitation are obtained. Our calculations reveal that for V=0,
there are two triplet bound state modes and three singlet modes, two anti-bound
states and one bound state, while for finite values of there are four
excitonic bound states corresponding to two singlet and two triplet modes. The
major role of on-site Coulomb repulsion is to split singlet and triplet
collective excitation branches, while tends to pull the singlet branches
below the continuum to make them bound states.Comment: 10 eps figure
Single and two-particle energy gaps across the disorder-driven superconductor-insulator transition
The competition between superconductivity and localization raises profound
questions in condensed matter physics. In spite of decades of research, the
mechanism of the superconductor-insulator transition (SIT) and the nature of
the insulator are not understood. We use quantum Monte Carlo simulations that
treat, on an equal footing, inhomogeneous amplitude variations and phase
fluctuations, a major advance over previous theories. We gain new microscopic
insights and make testable predictions for local spectroscopic probes. The
energy gap in the density of states survives across the transition, but
coherence peaks exist only in the superconductor. A characteristic pseudogap
persists above the critical disorder and critical temperature, in contrast to
conventional theories. Surprisingly, the insulator has a two-particle gap scale
that vanishes at the SIT, despite a robust single-particle gap.Comment: 7 pages, 5 figures (plus supplement with 4 pages, 5 figures