982 research outputs found
ENCORE: An Extended Contractor Renormalization algorithm
Contractor renormalization (CORE) is a real-space renormalization-group
method to derive effective Hamiltionians for microscopic models. The original
CORE method is based on a real-space decomposition of the lattice into small
blocks and the effective degrees of freedom on the lattice are tensor products
of those on the small blocks. We present an extension of the CORE method that
overcomes this restriction. Our generalization allows the application of CORE
to derive arbitrary effective models whose Hilbert space is not just a tensor
product of local degrees of freedom. The method is especially well suited to
search for microscopic models to emulate low-energy exotic models and can guide
the design of quantum devices.Comment: 5 pages, 4 figure
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
Supersolid phase induced by correlated hopping in spin-1/2 frustrated quantum magnets
We show that correlated hopping of triplets, which is often the dominant
source of kinetic energy in dimer-based frustrated quantum magnets, produces a
remarkably strong tendency to form supersolid phases in a magnetic field. These
phases are characterized by simultaneous modulation and ordering of the
longitudinal and transverse magnetization respectively. Using Quantum Monte
Carlo and a semiclassical approach for an effective hard-core boson model with
nearest-neighbor repulsion on a square lattice, we prove in particular that a
supersolid phase can exist even if the repulsion is not strong enough to
stabilize an insulating phase at half-filling. Experimental implications for
frustrated quantum antiferromagnets in a magnetic field at zero and finite
temperature are discussed.Comment: 4 pages; 4 figures; published versio
From the Cooper problem to canted supersolids in Bose-Fermi mixtures
We calculate the phase diagram of the Bose-Fermi Hubbard model on the 3d
cubic lattice at fermionic half filling and bosonic unit filling by means of
single-site dynamical mean-field theory. For fast bosons, this is equivalent to
the Cooper problem in which the bosons can induce s-wave pairing between the
fermions. We also find miscible superfluid and canted supersolid phases
depending on the interspecies coupling strength. In contrast, slow bosons favor
fermionic charge density wave structures for attractive fermionic interactions.
These competing instabilities lead to a rich phase diagram within reach of cold
gas experiments.Comment: 5 pages, 4 figures; replaced with published versio
Diagrammatic Quantum Monte Carlo solution of the two-dimensional Cooperon-Fermion model
We investigate the two-dimensional cooperon-fermion model in the correlated
regime with a new continuous-time diagrammatic determinant quantum Monte Carlo
(DDQMC) algorithm. We estimate the transition temperature , examine the
effectively reduced band gap and cooperon mass, and find that delocalization of
the cooperons enhances the diamagnetism. When applied to diamagnetism of the
pseudogap phase in high- cuprates, we obtain results in a qualitative
agreement with recent torque magnetization measurements.Comment: 8 pages, 11 figure
Interacting classical dimers on the square lattice
We study a model of close-packed dimers on the square lattice with a nearest
neighbor interaction between parallel dimers. This model corresponds to the
classical limit of quantum dimer models [D.S. Rokhsar and S.A. Kivelson, Phys.
Rev. Lett.{\bf 61}, 2376 (1988)]. By means of Monte Carlo and Transfer Matrix
calculations, we show that this system undergoes a Kosterlitz-Thouless
transition separating a low temperature ordered phase where dimers are aligned
in columns from a high temperature critical phase with continuously varying
exponents. This is understood by constructing the corresponding Coulomb gas,
whose coupling constant is computed numerically. We also discuss doped models
and implications on the finite-temperature phase diagram of quantum dimer
models.Comment: 4 pages, 4 figures; v2 : Added results on doped models; published
versio
Dynamical mean field solution of the Bose-Hubbard model
We present the effective action and self-consistency equations for the
bosonic dynamical mean field (B-DMFT) approximation to the bosonic Hubbard
model and show that it provides remarkably accurate phase diagrams and
correlation functions. To solve the bosonic dynamical mean field equations we
use a continuous-time Monte Carlo method for bosonic impurity models based on a
diagrammatic expansion in the hybridization and condensate coupling. This
method is readily generalized to bosonic mixtures, spinful bosons, and
Bose-Fermi mixtures.Comment: 10 pages, 3 figures. includes supplementary materia
Universal critical temperature for Kosterlitz-Thouless transitions in bilayer quantum magnets
Recent experiments show that double layer quantum Hall systems may have a
ground state with canted antiferromagnetic order. In the experimentally
accessible vicinity of a quantum critical point, the order vanishes at a
temperature T_{KT} = \kappa H, where H is the magnetic field and \kappa is a
universal number determined by the interactions and Berry phases of the thermal
excitations. We present quantum Monte Carlo simulations on a model spin system
which support the universality of \kappa and determine its numerical value.
This allows experimental tests of an intrinsically quantum-mechanical universal
quantity, which is not also a property of a higher dimensional classical
critical point.Comment: 5 pages, 4 figure
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