245 research outputs found
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.
Magnetization plateaux and jumps in a frustrated four-leg spin tube under a magnetic field
We study the ground state phase diagram of a frustrated spin-1/2 four-leg
spin tube in an external magnetic field. We explore the parameter space of this
model in the regime of all-antiferromagnetic exchange couplings by means of
three different approaches: analysis of low-energy effective Hamiltonian (LEH),
a Hartree variational approach (HVA) and density matrix renormalization group
(DMRG) for finite clusters. We find that in the limit of weakly interacting
plaquettes, low-energy singlet, triplet and quintuplet states play an important
role in the formation of fractional magnetization plateaux. We study the
transition regions numerically and analytically, and find that they are
described, at first order in a strong- coupling expansion, by an XXZ spin-1/2
chain in a magnetic field; the second-order terms give corrections to the XXZ
model. All techniques provide consistent results which allow us to predict the
existence of fractional plateaux in an important region in the space of
parameters of the model.Comment: 10 pages, 7 figures. Accepted for publication in Physical Review
Spin-phonon induced magnetic order in Kagome ice
We study the effects of lattice deformations on the Kagome spin ice, with
Ising spins coupled by nearest neighbor exchange and long range dipolar
interactions, in the presence of in-plane magnetic fields. We describe the
lattice energy according to the Einstein model, where each site distortion is
treated independently. Upon integration of lattice degrees of freedom,
effective quadratic spin interactions arise. Classical MonteCarlo simulations
are performed on the resulting model, retaining up to third neighbor
interactions, under different directions of the magnetic field. We find that,
as the effect of the deformation is increased, a rich plateau structure appears
in the magnetization curves.Comment: 7 pages, 8 figure
Machine learning techniques to construct detailed phase diagrams for skyrmion systems
Recently, there has been an increased interest in the application of machine
learning (ML) techniques to a variety of problems in condensed matter physics.
In this regard, of particular significance is the characterization of simple
and complex phases of matter. Here, we use a ML approach to construct the full
phase diagram of a well known spin model combining ferromagnetic exchange and
Dzyaloshinskii-Moriya (DM) interactions where topological phases emerge. At low
temperatures, the system is tuned from a spiral phase to a skyrmion crystal by
a magnetic field. However, thermal fluctuations induce two types of
intermediate phases, bimerons and skyrmion gas, which are not as easily
determined as spirals or skyrmion crystals. We resort to large scale Monte
Carlo simulations to obtain low temperature spin configurations, and train a
convolutional neural network (CNN), taking only snapshots at specific values of
the DM couplings, to classify between the different phases, focusing on the
intermediate and intricate topological textures. We then apply the CNN to
higher temperature configurations and to other DM values, to construct a
detailed magnetic field-temperature phase diagram, achieving outstanding
results. We discuss the importance of including the disordered paramagnetic
phases in order to get the phase boundaries, and finally, we compare our
approach with other ML algorithms.Comment: 9 pages, 10 figures; accepted for publication in Physical Review
Influences of Social Power and Normative Support on Condom Use Decisions: A Research Synthesis
A meta-analysis of 58 studies involving 30,270 participants examined how study population and methodological characteristics influence the associations among norms, control perceptions, attitudes, intentions and behaviour in the area of condom use. Findings indicated that control perceptions generally correlated more strongly among members of societal groups that lack power, including female, younger individuals, ethnic-minorities and people with lower educational levels. Furthermore, norms generally had stronger influences among younger individuals and among people who have greater access to informational social support, including males, ethnic majorities and people with higher levels of education. These findings are discussed in the context of HIV prevention efforts
Chiral phase transition and thermal Hall effect in an anisotropic spin model on the kagome lattice
We present a study of the thermal Hall effect in the extended Heisenberg
model with anisotropy in the kagome lattice. This model has the
particularity that, in the classical case, and for a broad region in parameter
space, an external magnetic field induces a chiral symmetry breaking: the
ground state is a doubly degenerate order with either positive or
negative net chirality. Here, we focus on the effect of this chiral phase
transition in the thermal Hall conductivity using Linear-Spin-Waves theory. We
explore the topology and calculate the Chern numbers of the magnonic bands,
obtaining a variety of topological phase transitions. We also compute the
magnonic effect to the critical temperature associated with the chiral phase
transition (). Our main result is that, the thermal Hall
conductivity, which is null for , becomes non-zero as a consequence
of the spontaneous chiral symmetry breaking at low temperatures. Therefore, we
present a simple model where it is possible to "switch" on/off the thermal
transport properties introducing a magnetic field and heating or cooling the
system.Comment: 9 pages, 6 figures, Accepted for publication in Phys. Rev.
Texture discrimination and multi-unit recording in the rat vibrissal nerve
BACKGROUND: Rats distinguish objects differing in surface texture by actively moving their vibrissae. In this paper we characterized some aspects of texture sensing in anesthetized rats during active touch. We analyzed the multifiber discharge from a deep vibrissal nerve when the vibrissa sweeps materials (wood, metal, acrylic, sandpaper) having different textures. We polished these surfaces with sandpaper (P1000) to obtain close degrees of roughness and we induced vibrissal movement with two-branch facial nerve stimulation. We also consider the change in pressure against the vibrissa as a way to improve the tactile information acquisition. The signals were compared with a reference signal (control) – vibrissa sweeping the air – and were analyzed with the Root Mean Square (RMS) and the Power Spectrum Density (PSD). RESULTS: We extracted the information about texture discrimination hidden in the population activity of one vibrissa innervation, using the RMS values and the PSD. The pressure level 3 produced the best differentiation for RMS values and it could represent the "optimum" vibrissal pressure for texture discrimination. The frequency analysis (PSD) provided information only at low-pressure levels and showed that the differences are not related to the roughness of the materials but could be related to other texture parameters. CONCLUSION: Our results suggest that the physical properties of different materials could be transduced by the trigeminal sensory system of rats, as are shown by amplitude and frequency changes. Likewise, varying the pressure could represent a behavioral strategy that improves the information acquisition for texture discrimination
From chiral spin liquids to skyrmion fluids and crystals, and their interplay with itinerant electrons
We present an in-depth study of the competition between skyrmions and a
chiral spin liquid in a model on the kagome lattice that was recently proposed
by some of the authors [H. D. Rosales, et al. Phys. Rev. Lett. 130, 106703
(2023)]. We present an analytical overview of the low-energy states using the
Luttinger-Tisza approximation. Then we add thermal fluctuations thanks to
large-scale Monte-Carlo simulations, and explore the entire parameter space
with a magnetic field , in-plane and out-of-plane
Dzyaloshinskii-Moriya interactions, using the ferromagnetic strength as unit of
energy. While skyrmions and the chiral spin liquid live in different regions of
parameter space, we show how to bring them together, stabilizing a skyrmion
fluid in between; a region where the density of well-defined skyrmions can be
tuned from quasi-zero (gas) to saturated (liquid) before ordering of the
skyrmions (solid). In particular, we investigate the two-dimensional melting of
the skyrmion solid. Our analysis also brings to light a long-range ordered
phase with Z symmetry. To conclude, when conduction electrons are coupled
to the local spins, different chiral magnetic textures stabilized in this model
(skyrmion solid, liquid and gas \& chiral spin liquid) induce anomalous Quantum
Hall effect in the magnetically disordered skyrmion liquid for specific
band-filling fractions. Landau levels persist even in the skyrmion-liquid
regime in absence of broken translational symmetry and gradually disappear as
the skyrmion density decreases to form a gas.Comment: 18 pages, 16 figure
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