1,101 research outputs found
Ground state of the random-bond spin-1 Heisenberg chain
Stochastic series expansion quantum Monte Carlo is used to study the ground
state of the antiferromagnetic spin-1 Heisenberg chain with bond disorder.
Typical spin- and string-correlations functions behave in accordance with
real-space renormalization group predictions for the random-singlet phase. The
average string-correlation function decays algebraically with an exponent of
-0.378(6), in very good agreement with the prediction of , while the average spin-correlation function is found to decay with an
exponent of about -1, quite different from the expected value of -2. By
implementing the concept of directed loops for the spin-1 chain we show that
autocorrelation times can be reduced by up to two orders of magnitude.Comment: 9 pages, 10 figure
Criticality in coupled quantum spin-chains with competing ladder-like and two-dimensional couplings
Motivated by the geometry of spins in the material CaCuO, we study a
two-layer, spin-half Heisenberg model, with nearest-neighbor exchange couplings
J and \alpha*J along the two axes in the plane and a coupling J_\perp
perpendicular to the planes. We study these class of models using the
Stochastic Series Expansion (SSE) Quantum Monte Carlo simulations at finite
temperatures and series expansion methods at T=0. The critical value of the
interlayer coupling, J_\perp^c, separating the N{\'e}el ordered and disordered
ground states, is found to follow very closely a square root dependence on
. Both T=0 and finite-temperature properties of the model are
presented.Comment: 9 pages, 11 figs., 1 tabl
Metal-insulator transition in the one-dimensional Holstein model at half filling
We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure
A Study of the S=1/2 Alternating Chain using Multiprecision Methods
In this paper we present results for the ground state and low-lying
excitations of the alternating Heisenberg antiferromagnetic chain. Our
more conventional techniques include perturbation theory about the dimer limit
and numerical diagonalization of systems of up to 28 spins. A novel application
of multiple precision numerical diagonalization allows us to determine
analytical perturbation series to high order; the results found using this
approach include ninth-order perturbation series for the ground state energy
and one magnon gap, which were previously known only to third order. We also
give the fifth-order dispersion relation and third-order exclusive neutron
scattering structure factor for one-magnon modes and numerical and analytical
binding energies of S=0 and S=1 two-magnon bound states.Comment: 16 pages, 9 figures. for submission to Phys.Rev.B. PICT files of figs
available at http://csep2.phy.ornl.gov/theory_group/people/barnes/barnes.htm
Friedel oscillations in a two-band Hubbard model for CuO chains
Friedel oscillations induced by open boundary conditions in a two-band
Hubbard model for CuO chains are numerically studied. We find that for
physically realistic parameters and close to quarter filling, these
oscillations have a 2k_F modulation according with experimental results on
YBa_2Cu_3O_{7-delta}. In addition, we predict that, for the same parameters, as
hole doping is reduced from quarter filling to half filling, Friedel
oscillations would acquire a 4k_F modulation, typical of a strongly correlated
electrons regime. The 4k_F modulation dominates also in the electron doped
region. The range of parameters varied is very broad, and hence the results
reported could apply to other cuprates and other strongly correlated compounds
with quasi-one dimensional structures. On a more theoretical side, we stress
the fact that the copper and oxygen subsystems should be described by two
different Luttinger liquid exponents.Comment: 7 pages, 7 eps figure
From antiferromagnetism to d-wave superconductivity in the 2D t-J model
We have found that the two dimensional t-J model, for the physical parameter
range J/t = 0.4 reproduces the main experimental qualitative features of
High-Tc copper oxide superconductors: d-wave superconducting correlations are
strongly enhanced upon small doping and clear evidence of off diagonal long
range order is found at the optimal doping \delta ~ 0.15. On the other hand
antiferromagnetic long range order, clearly present at zero hole doping, is
suppressed at small hole density with clear absence of antiferromagnetism at
\delta >~ 0.1.Comment: 4 pages, 5 figure
Incorporation of Density Matrix Wavefunctions in Monte Carlo Simulations: Application to the Frustrated Heisenberg Model
We combine the Density Matrix Technique (DMRG) with Green Function Monte
Carlo (GFMC) simulations. The DMRG is most successful in 1-dimensional systems
and can only be extended to 2-dimensional systems for strips of limited width.
GFMC is not restricted to low dimensions but is limited by the efficiency of
the sampling. This limitation is crucial when the system exhibits a so-called
sign problem, which on the other hand is not a particular obstacle for the
DMRG. We show how to combine the virtues of both methods by using a DMRG
wavefunction as guiding wave function for the GFMC. This requires a special
representation of the DMRG wavefunction to make the simulations possible within
reasonable computational time. As a test case we apply the method to the
2-dimensional frustrated Heisenberg antiferromagnet. By supplementing the
branching in GFMC with Stochastic Reconfiguration (SR) we get a stable
simulation with a small variance also in the region where the fluctuations due
to minus sign problem are maximal. The sensitivity of the results to the choice
of the guiding wavefunction is extensively investigated. We analyse the model
as a function of the ratio of the next-nearest to nearest neighbor coupling
strength. We observe in the frustrated regime a pattern of the spin
correlations which is in-between dimerlike and plaquette type ordering, states
that have recently been suggested. It is a state with strong dimerization in
one direction and weaker dimerization in the perpendicular direction.Comment: slightly revised version with added reference
Magnetization process for a quasi-one-dimensional S=1 antiferromagnet
We investigate the magnetization process for a quasi-one-dimensional S=1
antiferromagnet with bond alternation. By combining the density matrix
renormalization group method with the interchain mean-field theory, we discuss
how the interchain coupling affects the magnetization curve. It is found that
the width of the magnetization plateau is considerably reduced upon introducing
the interchain coupling. We obtain the phase diagram in a magnetic field. The
effect of single-ion anisotropy is also addressed.Comment: 6 pages, 7 eps figure
Antineutrinos from Earth: A reference model and its uncertainties
We predict geoneutrino fluxes in a reference model based on a detailed
description of Earth's crust and mantle and using the best available
information on the abundances of uranium, thorium, and potassium inside Earth's
layers. We estimate the uncertainties of fluxes corresponding to the
uncertainties of the element abundances. In addition to distance integrated
fluxes, we also provide the differential fluxes as a function of distance from
several sites of experimental interest. Event yields at several locations are
estimated and their dependence on the neutrino oscillation parameters is
discussed. At Kamioka we predict N(U+Th)=35 +- 6 events for 10^{32} proton yr
and 100% efficiency assuming sin^2(2theta)=0.863 and delta m^2 = 7.3 X 10^{-5}
eV^2. The maximal prediction is 55 events, obtained in a model with fully
radiogenic production of the terrestrial heat flow.Comment: 24 pages, ReVTeX4, plus 7 postscript figures; minor formal changes to
match version to be published in PR
Spin-1/2 frustrated antiferromagnet on a spatially anisotopic square lattice: contribution of exact diagonalizations
The phase diagram of a spin-1/2 model is investigated by means of
exact diagonalizations on finite samples. This model is a generalization of the
model on the square lattice with two different nearest-neighbor
couplings and may be also viewed as an array of coupled Heisenberg
chains. The results suggest that the resonnating valence bond state predicted
by Nersesyan and Tsvelik [Phys. Rev. B {\bf 67}, 024422 (2003)] for is realized and extends beyond the limit of small interchain coupling
along a curve nearly coincident with the line where the energy per spin is
maximum. This line is likely bordered on both side by a columnar dimer long
range order. This columnar order could extends for which correspond
to the model.Comment: 14 pages, 21 figures, final versio
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