1,105 research outputs found
Pairing and Excitation Spectrum in doped t-J Ladders
Exact diagonalization studies for a doped t-J ladder (or double chain) show
hole pairing in the ground state. The excitation spectrum separates into a
limited number of quasiparticles which carry charge and spin and a triplet mode. At half-filling the former vanish but the latter
evolves continuously into the triplet band of the spin liquid. At low doping
the quasiparticles form a dilute Fermi gas with a strong attraction but
simultaneously the Fermi wavevector, as would be measured in photoemission, is
large.Comment: 10 pages and 4 PostScript figures, RevTeX 3.0, ETH-TH/94-0
Competing states in the t-J model: uniform d-wave state versus stripe state
Variational studies of the t-J model on the square lattice based on infinite
projected-entangled pair states (iPEPS) confirm an extremely close competition
between a uniform d-wave superconducting state and different stripe states. The
site-centered stripe with an in-phase d-wave order has an equal or only
slightly lower energy than the stripe with anti-phase d-wave order. The optimal
stripe filling is not constant but increases with J/t. A nematic anisotropy
reduces the pairing amplitude and the energies of stripe phases are lowered
relative to the uniform state with increasing nematicity.Comment: 6 pages, 4 figures, 4 pages of supplemental materia
Quantum spin chains in a magnetic field
We demonstrate that the ``worm'' algorithm allows very effective and precise
quantum Monte Carlo (QMC) simulations of spin systems in a magnetic field, and
its auto-correlation time is rather insensitive to the value of H at low
temperature. Magnetization curves for the and chains are
presented and compared with existing Bethe ansatz and exact diagonalization
results. From the Green function analysis we deduce the magnon spectra in the
s=1 system, and directly establish the "relativistic" form E(p)=(\Delta ^2 +v^2
p^2)^{1/2} of the dispersion law.Comment: 6 pages, 8 figures; removed discussion of spin-2 case - will be
published later in a separate pape
Thermodynamics of the 3D Hubbard model on approach to the Neel transition
We study the thermodynamic properties of the 3D Hubbard model for
temperatures down to the Neel temperature using cluster dynamical mean-field
theory. In particular we calculate the energy, entropy, density, double
occupancy and nearest-neighbor spin correlations as a function of chemical
potential, temperature and repulsion strength. To make contact with cold-gas
experiments, we also compute properties of the system subject to an external
trap in the local density approximation. We find that an entropy per particle
at is sufficient to achieve a Neel state in the
center of the trap, substantially higher than the entropy required in a
homogeneous system. Precursors to antiferromagnetism can clearly be observed in
nearest-neighbor spin correlators.Comment: 4 pages, 6 figure
Quantum Monte Carlo Simulation of the Trellis Lattice Heisenberg Model for SrCuO and CaVO
We study the spin-1/2 trellis lattice Heisenberg model, a coupled spin ladder
system, both by perturbation around the dimer limit and by quantum Monte Carlo
simulations. We discuss the influence of the inter-ladder coupling on the spin
gap and the dispersion, and present results for the temperature dependence of
the uniform susceptibility. The latter was found to be parameterized well by a
mean-field type scaling ansatz. Finally we discuss fits of experimental
measurements on SrCuO and CaVO to our results.Comment: 7 pages, 8 figure
Critical temperature of interacting Bose gases in periodic potentials
The superfluid transition of a repulsive Bose gas in the presence of a sinusoidal potential which represents a simple-cubic optical lattice is investigated using quantum Monte Carlo simulations. At the average filling of one particle per well the critical temperature has a nonmonotonic dependence on the interaction strength, with an initial sharp increase and a rapid suppression at strong interactions in the vicinity of the Mott transition. In an optical lattice the positive shift of the transition is strongly enhanced compared to the homogenous gas. By varying the lattice filling we find a crossover from a regime where the optical lattice has the dominant effect to a regime where interactions dominate and the presence of the lattice potential becomes almost irrelevant
Mott Domains of Bosons Confined on Optical Lattices
In the absence of a confining potential, the boson Hubbard model in its
ground state is known to exhibit a superfluid to Mott insulator quantum phase
transition at commensurate fillings and strong on-site repulsion. In this
paper, we use quantum Monte Carlo simulations to study the ground state of the
one dimensional bosonic Hubbard model in a trap. We show that some, but not
all, aspects of the Mott insulating phase persist when a confining potential is
present. The Mott behavior is present for a continuous range of incommensurate
fillings, a very different situation from the unconfined case. Furthermore the
establishment of the Mott phase does not proceed via a quantum phase transition
in the traditional sense. These observations have important implications for
the interpretation of experimental results for atoms trapped on optical
lattices. Initial results show that, qualitatively, the same results persist in
higher dimensions.Comment: Revtex file, five figures, include
Orbital Order Effect of Two-Dimensional Spin Gap System for CaV4O9
Effects of possible orbital order in magnetic properties of two-dimensional
spin gap system for CaVO are investigated theoretically. After
analyzing experimental data, we show that single orbital models assumed in the
literature are insufficient to reproduce the data. To understand the origin of
the discrepancy, we assume that in state of V, and
orbitals have substantial contributions in the lowest-energy atomic level which
leads to a double-degeneracy. We study possible configurations of the orbital
order. By exact diagonalization and perturbation expansion, we calculate the
susceptibility, wavenumber dependence of low-lying excitations and equal-time
spin-spin correlations which is related to integrated intensity of the neutron
inelastic scattering. These quantities sensitively depend on the configuration
of the orbital order. The calculated results for some configurations of the
orbital order reproduce many experimental results much better than the previous
single-orbital models. However some discrepancy still remains to completely
reproduce all of the reported experimental results. To understand the origin of
these discrepancies, we point out the possible importance of the partially
occupied orbital in addition to orbital order of partially filled
and orbitals.Comment: 19 pages LATEX, 15 postscript figures, using jpsj.sty,to be published
in J.Phys.Soc.Jpn. Vol.67 No.2 (1998
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