3,769 research outputs found
Three-sublattice Skyrmion crystal in the antiferromagnetic triangular lattice
The frustrated classical antiferromagnetic Heisenberg model with
Dzyaloshinskii-Moriya (DM) interactions on the triangular lattice is studied
under a magnetic field by means of semiclassical calculations and large-scale
Monte Carlo simulations. We show that even a small DM interaction induces the
formation of an Antiferromagnetic Skyrmion crystal (AF-SkX) state. Unlike what
is observed in ferromagnetic materials, we show that the AF-SkX state consists
of three interpenetrating Skyrmion crystals (one by sublattice), and most
importantly, the AF-SkX state seems to survive in the limit of zero
temperature. To characterize the phase diagram we compute the average of the
topological order parameter which can be associated to the number of
topological charges or Skyrmions. As the magnetic field increases this
parameter presents a clear jump, indicating a discontinuous transition from a
spiral phase into the AF-SkX phase, where multiple Bragg peaks coexist in the
spin structure factor. For higher fields, a second (probably continuous)
transition occurs into a featureless paramagnetic phase.Comment: 8 pages, 8 figure
Quantum disordered phase on the frustrated honeycomb lattice
In the present paper we study the phase diagram of the Heisenberg model on
the honeycomb lattice with antiferromagnetic interactions up to third neighbors
along the line that include the point , corresponding
to the highly frustrated point where the classical ground state has macroscopic
degeneracy. Using the Linear Spin-Wave, Schwinger boson technique followed by a
mean field decoupling and exact diagonalization for small systems we find an
intermediate phase with a spin gap and short range N\'eel correlations in the
strong quantum limit (S=1/2). All techniques provide consistent results which
allow us to predict the existence of a quantum disordered phase, which may have
been observed in recent high-field ESR measurements in manganites.Comment: 4 figure
Field induced multiple order-by-disorder state selection in antiferromagnetic honeycomb bilayer lattice
In this paper we present a detailed study of the antiferromagnetic classical
Heisenberg model on a bilayer honeycomb lattice in a highly frustrated regime
in presence of a magnetic field. This study shows strong evidence of entropic
order-by-disorder selection in different sectors of the magnetization curve.
For antiferromagnetic couplings , we find that at low
temperatures there are two different regions in the magnetization curve
selected by this mechanism with different number of soft and zero modes. These
regions present broken symmetry and are separated by a not fully
collinear classical plateau at . At higher temperatures, there is a
crossover from the conventional paramagnet to a cooperative magnet. Finally, we
also discuss the low temperature behavior of the system for a less frustrated
region, .Comment: revised version - accepted for publication in Physical Review B - 12
pages, 11 figure
Metastable and scaling regimes of a one-dimensional Kawasaki dynamics
We investigate the large-time scaling regimes arising from a variety of
metastable structures in a chain of Ising spins with both first- and
second-neighbor couplings while subject to a Kawasaki dynamics. Depending on
the ratio and sign of these former, different dynamic exponents are suggested
by finite-size scaling analyses of relaxation times. At low but
nonzero-temperatures these are calculated via exact diagonalizations of the
evolution operator in finite chains under several activation barriers. In the
absence of metastability the dynamics is always diffusive.Comment: 18 pages, 8 figures. Brief additions. To appear in Phys. Rev.
Critical fluctuations in an optical parametric oscillator: when light behaves like magnetism
We study the nondegenerate optical parametric oscillator in a planar
interferometer near threshold, where critical phenomena are expected. These
phenomena are associated with nonequilibrium quantum dynamics that are known to
lead to quadrature entanglement and squeezing in the oscillator field modes. We
obtain a universal form for the equation describing this system, which allows a
comparison with other phase transitions. We find that the unsqueezed
quadratures of this system correspond to a two-dimensional XY-type model with a
tricritical Lifshitz point. This leaves open the possibility of a controlled
experimental investigation into this unusual class of statistical models. We
evaluate the correlations of the unsqueezed quadrature using both an exact
numerical simulation and a Gaussian approximation, and obtain an accurate
numerical calculation of the non-Gaussian correlations.Comment: Title changed. New figures adde
Probabilistic quantum phase-space simulation of Bell violations and their dynamical evolution
Quantum simulations of Bell inequality violations are numerically obtained
using probabilistic phase space methods, namely the positive P-representation.
In this approach the moments of quantum observables are evaluated as moments of
variables that have values outside the normal eigenvalue range. There is thus a
parallel with quantum weak measurements and weak values. Nevertheless, the
representation is exactly equivalent to quantum mechanics. A number of states
violating Bell inequalities are sampled, demonstrating that these quantum
paradoxes can be treated with probabilistic methods. We treat quantum dynamics
by simulating the time evolution of the Bell state formed via parametric
down-conversion, and discuss multi-mode generalizations
Probabilistic simulation of mesoscopic "Schr\"odinger cat" states
We carry out probabilistic phase-space sampling of mesoscopic Schr\"odinger
cat quantum states, demonstrating multipartite Bell violations for up to 60
qubits. We use states similar to those generated in photonic and ion-trap
experiments. These results show that mesoscopic quantum superpositions are
directly accessible to probabilistic sampling, and we analyze the properties of
sampling errors. We also demonstrate dynamical simulation of super-decoherence
in ion traps. Our computer simulations can be either exponentially faster or
slower than experiment, depending on the correlations measured
Quantum probabilistic sampling of multipartite 60-qubit Bell inequality violations
We show that violation of genuine multipartite Bell inequalities can be
obtained with sampled, probabilistic phase space methods. These genuine Bell
violations cannot be replicated if any part of the system is described by a
local hidden variable theory. The Bell violations are simulated
probabilistically using quantum phase-space representations. We treat
mesoscopically large Greenberger-Horne-Zeilinger (GHZ) states having up to 60
qubits, using both a multipartite SU(2) Q-representation and the positive
P-representation. Surprisingly, we find that sampling with phase-space
distributions can be exponentially faster than experiment. This is due to the
classical parallelism inherent in the simulation of quantum measurements using
phase-space methods. Our probabilistic sampling method predicts a contradiction
with local realism of "Schr\"odinger-cat" states that can be realized as a GHZ
spin state, either in ion traps or with photonic qubits. We also present a
quantum simulation of the observed super-decoherence of the ion-trap "cat"
state, using a phenomenological noise model
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