990 research outputs found
Magnetic properties of nanoscale compass-Heisenberg planar clusters
We study a model of spins 1/2 on a square lattice, generalizing the quantum
compass model via the addition of perturbing Heisenberg interactions between
nearest neighbors, and investigate its phase diagram and magnetic excitations.
This model has motivations both from the field of strongly correlated systems
with orbital degeneracy and from that of solid-state based devices proposed for
quantum computing. We find that the high degeneracy of ground states of the
compass model is fragile and changes into twofold degenerate ground states for
any finite amplitude of Heisenberg coupling. By computing the spin structure
factors of finite clusters with Lanczos diagonalization, we evidence a rich
variety of phases characterized by Z2 symmetry, that are either ferromagnetic,
C-type antiferromagnetic, or of Neel type, and analyze the effects of quantum
fluctuations on phase boundaries. In the ordered phases the anisotropy of
compass interactions leads to a finite excitation gap to spin waves. We show
that for small nanoscale clusters with large anisotropy gap the lowest
excitations are column-flip excitations that emerge due to Heisenberg
perturbations from the manifold of degenerate ground states of the compass
model. We derive an effective one-dimensional XYZ model which faithfully
reproduces the exact structure of these excited states and elucidates their
microscopic origin. The low energy column-flip or compass-type excitations are
robust against decoherence processes and are therefore well designed for
storing information in quantum computing. We also point out that the dipolar
interactions between nitrogen-vacancy centers forming a rectangular lattice in
a diamond matrix may permit a solid-state realization of the anisotropic
compass-Heisenberg model.Comment: 24 pages, 18 figure
Dimerization versus Orbital Moment Ordering in the Mott insulator YVO
We use exact diagonalization combined with mean-field theory to investigate
the phase diagram of the spin-orbital model for cubic vanadates. The spin-orbit
coupling competes with Hund's exchange and triggers a novel phase, with the
ordering of orbital magnetic moments stabilized by the tilting of
VO octahedra. It explains qualitatively spin canting and reduction of
magnetization observed in YVO. At finite temperature an orbital Peierls
instability in the -type antiferromagnetic phase induces modulation of
magnetic exchange constants even in absence of lattice distortions. The
calculated spin structure factor shows a magnon splitting due to the orbital
Peierls dimerization.Comment: 4 pages, 5 figures, Revte
Investigating prenatal perceived support as protective factor against adverse birth outcomes: a community cohort study.
Studies show that prenatal maternal anxiety may act as a risk factor for adverse birth outcomes, whilst prenatal social support may rather act as a protective factor. However, studies examining prenatal anxiety symptoms, prenatal perceived support, and neonatal and/or obstetric outcomes are lacking.
This study investigated whether, in a community sample, prenatal perceived support: (1) had a protective influence on birth outcomes (gestational age (GA), birthweight (BW), 5-minute Apgar score, and mode of delivery); (2) acted as a protective factor, moderating the relationship between anxiety symptoms and the aforementioned birth outcomes.
During their third trimester of pregnancy, 182 nulliparous child-bearers completed standardized questionnaires of anxiety (HADS-A) and perceived support (MOS-SSS). Birth outcomes data was extracted from medical records.
(1) Perceived support did not significantly predict any birth outcomes. However, perceived tangible support - MOS-SSS subscale assessing perceived material/financial aid - significantly positively predicted the 5-minute Apgar score. (2) Perceived support did not significantly moderate the relationship between anxiety symptoms and birth outcomes. However, perceived tangible support significantly moderated the relationship between anxiety symptoms and the 5-minute Apgar score.
When experienced within non-clinical thresholds, prenatal anxiety symptoms do not increase the risk of adverse neonatal and obstetric outcomes when perceived support is present
Icosahedral packing of polymer-tethered nanospheres and stabilization of the gyroid phase
We present results of molecular simulations that predict the phases formed by
the self-assembly of model nanospheres functionalized with a single polymer
"tether", including double gyroid, perforated lamella and crystalline bilayer
phases. We show that microphase separation of the immiscible tethers and
nanospheres causes confinement of the nanoparticles, which promotes local
icosahedral packing that stabilizes the gyroid and perforated lamella phases.
We present a new metric for determining the local arrangement of particles
based on spherical harmonic "fingerprints", which we use to quantify the extent
of icosahedral ordering.Comment: 8 pages, 4 figure
Dissipationless Spin Current between Two Coupled Ferromagnets
We demonstrate the general principle which states that a dissipationless spin
current flows between two coupled ferromagnets if their magnetic orders are
misaligned. This principle applies regardless the two ferromagnets are metallic
or insulating, and also generally applies to bulk magnetic insulators. On a
phenomenological level, this principle is analogous to Josephson effect, and
yields a dissipationless spin current that is independent from scattering. The
microscopic mechanisms for the dissipationless spin current depend on the
systems, which are elaborated in details. A uniform, static magnetic field is
further proposed to be an efficient handle to create the misaligned
configuration and stabilize the dissipationless spin current.Comment: 10 pages, 5 figure
Cluster Property and Robustness of Ground States of Interacting Many Bosons
We study spatial correlation functions of local operators of interacting many
bosons confined in a box of a large, but volume V, for various `ground states'
whose energy densities are almost degenerate. The ground states include the
coherent state of interacting bosons (CSIB), the number state of interacting
bosons (NSIB), and the number-phase squeezed state of interacting bosons, which
interpolates between the CSIB and NSIB. It was shown previously that only the
CSIB is robust (i.e., does not decohere for a macroscopically long time)
against the leakage of bosons into an environment. We show that for the CSIB
the spatial correlation of any local operators A(r) and B(r') (which are
localized around r and r', respectively) vanishes as |r - r' | \sim V^{1/3} \to
\infty, i.e., the CSIB has the `cluster property.' In contrast, the other
ground states do not possess the cluster property. Therefore, we have
successfully shown that the robust state has the cluster property. This ensures
the consistency of the field theory of bosons with macroscopic theories.Comment: We have replaced the manuscript in order to update the reference list
and to fix typos. (5 pages, no figures) In the final manuscript, a few
sentences have added for more detailed explanation. Journal PDF at
http://jpsj.jps.or.jp/journal/JPSJ-71-1.htm
Wind and Tide-Induced Hydrodynamics and Sedimentation of Two Tidal Inlets in Western Greece
To quantify the detailed mechanisms that cause sedimentation at the tidal inlets in the Messolonghi-Aetoliko lagoonal system, the MIKE 21 FM (HD & ST) numerical simulation models were applied. The study focuses on the hydrodynamic circulation in the vicinity of the tidal inlets and the associated wind and tide-induced currents, as well as sediment transport, bed level evolution and total sediment load accumulation, with emphasis on the vicinity of two tidal inlets, where the problem is more acute. Based on the numerical predictions, which have been qualitatively corroborated via satellite images from Google Earth, it is shown that under the prevailing winds and tidal action, sandbars are formed in front or behind the tidal inlets depending on the wind direction, resulting in a gradual decrease of the mean water level at the mouths. Under the combined action of the tide and the wind the characteristic horizontal structure of the flow includes cyclonic and anti-cyclonic eddies that form at the near tip area of the jetties, which are associated with sediment transport mechanisms
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