232 research outputs found
Infinite coherence time of edge spins in finite-length chains
Motivated by the recent observation that exponentially long coherence times
can be achieved for edge spins in models with strong zero modes, we study the
impact of level crossings in finite-length spin chains on the dynamics of the
edge spins. Focussing on the XY spin-1=2 chain with transverse or longitudinal
magnetic field, two models relevant to understand recent experimental results
on cobalt adatoms, we show that the edge spins can remain coherent for an
infinite time even for a finite-length chain if the magnetic field is tuned to
a value at which there is a level crossing. Furthermore, we show that the edge
spins remain coherent for any initial state for the integrable case of
transverse field because all states have level crossings at the same value of
the field, while the coherence time is increasingly large for lower
temperatures in the case of longitudinal field, which is non-integrable.Comment: 7 pages, 6 figure
Anticollinear magnetic order induced by impurities in the frustrated Heisenberg model of pnictides
We present Monte Carlo simulations for a classical antiferromagnetic
Heisenberg model with both nearest () and next-nearest () exchange
couplings on the square lattice in the presence of non-magnetic impurities. We
show that the order-by-disorder entropy selection, associated with the
Ising-like phase transition that appears for in the pure spin
model, is quenched at low temperature due to the presence of non-magnetic
impurities. Evidences that a new competing order is stabilized around the
impurities, and in turn induces a re-entrance phase transition are reported.
Implications for local magnetic measurement of the parent compound of iron
pnictides are briefly discussed
Entropy dependence of correlations in one-dimensional SU(N) antiferromagnets
Motivated by the possibility to load multi-color fermionic atoms in optical
lattices, we study the entropy dependence of the properties of the
one-dimensional antiferromagnetic SU(N) Heisenberg model, the effective model
of the SU(N) Hubbard model with one particle per site (filling 1/N). Using
continuous-time world line Monte Carlo simulations for N=2 to 5, we show that
characteristic short-range correlations develop at low temperature as a
precursor of the ground state algebraic correlations. We also calculate the
entropy as a function of temperature, and we show that the first sign of
short-range order appears at an entropy per particle that increases with N and
already reaches 0.8k_B at N=4, in the range of experimentally accessible
values.Comment: 5 pages, 3 figures, 2 table
Semiclassical approach to ground-state properties of hard-core bosons in two dimensions
Motivated by some inconsistencies in the way quantum fluctuations are
included beyond the classical treatment of hard-core bosons on a lattice in the
recent literature, we revisit the large-S semi-classical approach to hard-core
bosons on the square lattice at T=0. First of all, we show that, if one stays
at the purely harmonic level, the only correct way to get the 1/S correction to
the density is to extract it from the derivative of the ground state energy
with respect to the chemical potential, and that to extract it from a
calculation of the ground state expectation value of the particle number
operator, it is necessary to include 1/\sqrt{S} corrections to the harmonic
ground state. Building on this alternative approach to get 1/S corrections, we
provide the first semiclassical derivation of the momentum distribution, and we
revisit the calculation of the condensate density. The results of these as well
as other physically relevant quantities such as the superfluid density are
systematically compared to quantum Monte Carlo simulations. This comparison
shows that the logarithmic corrections in the dilute Bose gas limit are only
captured by the semi-classical approach if the 1/S corrections are properly
calculated, and that the semi-classical approach is able to reproduce the 1/k
divergence of the momentum distribution at k=0. Finally, the effect of 1/S^2
corrections is briefly discussed.Comment: 14 pages, 8 figure
Floating, critical and dimerized phases in a frustrated spin-3/2 chain
We study spontaneous dimerization and emergent criticality in a spin-3/2
chain with antiferromagnetic nearest-neighbor , next-nearest-neighbor
and three-site interactions. In the absence of three-site
interaction , we provide evidence that the model undergoes a remarkable
sequence of three phase transitions as a function of , going
successively through a critical commesurate phase, a partially dimerized gapped
phase, a critical floating phase with quasi-long-range incommensurate order, to
end up in a fully dimerized phase at very large . In the field theory
language, this implies that the coupling constant of the marginal operator
responsible for dimerization changes sign three times. For large enough ,
the fully dimerized phase is stabilized for all , and the phase
transitions between the critical phases and this phase are both
Wess-Zumino-Witten (WZW) SU(2) along part of the boundary and turn first
order at some point due to the presence of a marginal operator in the WZW
SU(2) model. By contrast, the transition between the two dimerized phase is
always first order, and the phase transitions between the partially dimerized
phase and the critical phases are Kosterlitz-Thouless. Finally, we discuss the
intriguing spin-1/2 edge states that emerge in the partially dimerized phase
for even chains. Unlike their counterparts in the spin-1 chain, they are not
confined and disappear upon increasing in favour of a reorganization of
the dimerization pattern.Comment: 14 pages, 23 figure
Variational Monte-Carlo investigation of SU() Heisenberg chains
Motivated by recent experimental progress in the context of ultra-cold
multi-color fermionic atoms in optical lattices, we have investigated the
properties of the SU() Heisenberg chain with totally antisymmetric
irreducible representations, the effective model of Mott phases with
particles per site. These models have been studied for arbitrary and
with non-abelian bosonization [I. Affleck, Nuclear Physics B 265, 409 (1986);
305, 582 (1988)], leading to predictions about the nature of the ground state
(gapped or critical) in most but not all cases. Using exact diagonalization and
variational Monte-Carlo based on Gutzwiller projected fermionic wave functions,
we have been able to verify these predictions for a representative number of
cases with and , and we have shown that the opening of
a gap is associated to a spontaneous dimerization or trimerization depending on
the value of m and N. We have also investigated the marginal cases where
abelian bosonization did not lead to any prediction. In these cases,
variational Monte-Carlo predicts that the ground state is critical with
exponents consistent with conformal field theory.Comment: 9 pages, 10 figures, 3 table
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