146 research outputs found
Reply to the Comment on the 'Hole-digging' in ensembles of tunneling molecular magnets
Reply to the Comment of J.J. Alonso and J.F. Fernandez on the paper
"'Hole-digging' in ensembles of tunneling molecular magnets" of I.S. Tupitsyn,
P.C.E. Stamp and N.V. Prokof'ev (Phys. Rev. B 69, 132406, (2004)).Comment: 1 LaTeX page, 1 PS figure; submitted to PR
Effective Hamiltonian in the Problem of a "Central Spin" Coupled to a Spin Environment
We consider here the problem of a "giant spin", with spin quantum number
S>>1, interacting with a set of microscopic spins. Interactions between the
microscopic spins are ignored. This model describes the low-energy properties
of magnetic grains or magnetic macromolecules interacting with a surrounding
spin environment, such as nuclear spins. We describe a general method for
truncating the model to another one, valid at low energies, in which a
two-level system interacts with the environmental spins, and higher energy
terms are absorbed into a new set of couplings. This is done using an instanton
technique. We then verify the accuracy of this technique, by comparing the
results for the low energy effective Hamiltonian, with results derived for the
original giant spin, coupled to a microscopic spin, using exact diagonalisation
techniques.Comment: 15 pages, Latex, with 9 ps figure
Continuous-Time Quantum Monte Carlo Algorithm for the Lattice Polaron
An efficient continuous-time path-integral Quantum Monte Carlo algorithm for
the lattice polaron is presented. It is based on Feynman's integration of
phonons and subsequent simulation of the resulting single-particle
self-interacting system. The method is free from the finite-size and
finite-time-step errors and works in any dimensionality and for any range of
electron-phonon interaction. The ground-state energy and effective mass of the
polaron are calculated for several models. The polaron spectrum can be measured
directly by Monte Carlo, which is of general interest.Comment: 5 pages, 4 figures, published versio
Low-Temperature Quantum Relaxation in a System of Magnetic Nanomolecules
We argue that to explain recent resonant tunneling experiments on crystals of
Mn and Fe, particularly in the low-T limit, one must invoke dynamic
nuclear spin and dipolar interactions. We show the low-, short-time
relaxation will then have a form, where depends on the
nuclear , on the tunneling matrix element between the two
lowest levels, and on the initial distribution of internal fields in the
sample, which depends very strongly on sample shape. The results are directly
applicable to the system. We also give some results for the long-time
relaxation.Comment: 4 pages, 3 PostScript figures, LaTe
Band structure of the Jahn-Teller polaron from Quantum Monte Carlo
A path-integral representation is constructed for the Jahn-Teller polaron
(JTP). It leads to a perturbation series that can be summed exactly by the
diagrammatic Quantum Monte Carlo technique. The ground-state energy, effective
mass, spectrum and density of states of the three-dimensional JTP are
calculated with no systematic errors. The band structure of JTP interacting
with dispersionless phonons, is found to be similar to that of the Holstein
polaron. The mass of JTP increases exponentially with the coupling constant. At
small phonon frequencies, the spectrum of JTP is flat at large momenta, which
leads to a strongly distorted density of states with a massive peak at the top
of the band.Comment: 5 pages of REVTeX, 3 figure
Diagrammatic Monte Carlo for Correlated Fermions
We show that Monte Carlo sampling of the Feynman diagrammatic series (DiagMC)
can be used for tackling hard fermionic quantum many-body problems in the
thermodynamic limit by presenting accurate results for the repulsive Hubbard
model in the correlated Fermi liquid regime. Sampling Feynman's diagrammatic
series for the single-particle self-energy we can study moderate values of the
on-site repulsion () and temperatures down to . We
compare our results with high temperature series expansion and with single-site
and cluster dynamical mean-field theory.Comment: 4 pages, 5 figures, stylistic change
Topological multicritical point in the Toric Code and 3D gauge Higgs Models
We report a new type of multicritical point that arises from competition
between the Higgs and confinement transitions in a Z_2 gauge system. The phase
diagram of the 3d gauge Higgs model has been obtained by Monte-Carlo simulation
on large (up to 60^3) lattices. We find the transition lines continue as
2nd-order until merging into a 1st-order line. These findings pose the question
of an effective field theory for a multicritical point involving noncommuting
order parameters. A similar phase diagram is predicted for the 2-dimensional
quantum toric code model with two external fields, h_z and h_x; this problem
can be mapped onto an anisotropic 3D gauge Higgs model.Comment: 4 pages, 3 figure
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