357 research outputs found
Paramagnetism in the kagome compounds (Zn,Mg,Cd)Cu(OH)Cl
Frustrated magnetism on the kagome lattice has been a fertile ground for rich
and fascinating physics, ranging from experimental evidence of a spin liquid to
theoretical predictions of exotic superconductivity. Among experimentally
realized spin- kagome magnets, herbertsmithite, kapellasite, and
haydeeite [(Zn,Mg)Cu(OH)Cl] are all well described by a
three-parameter Heisenberg model, but they exhibit distinctly different
physics. We address the problem using a pseudofermion functional
renormalization-group approach and analyze the low-energy physics in the
experimentally accessible parameter range. Our analysis places kapellasite and
haydeeite near the boundaries between magnetically ordered and disordered
phases, implying that slight modifications could dramatically affect their
magnetic properties. Inspired by this, we perform \textit{ab initio} density
functional theory calculations of (Zn,Mg,Cd)Cu (OH)Cl at
various pressures. Our results suggest that by varying pressure and composition
one can traverse a paramagnetic regime between different magnetically ordered
phases.Comment: Published version. Main paper (7 pages, 5 figures) and Supplemental
material (7 pages, 4 figures, 3 tables). Change in titl
Kinetic transition in the order–disorder transformation at a solid/liquid interface
Phase-field analysis for the kinetic transition in an ordered crystal structure growing from an undercooled liquid is carried out. The results are interpreted on the basis of analytical and numerical solutions of equations describing the dynamics of the phase field, the long-range order parameter as well as the atomic diffusion within the crystal/liquid interface and in the bulk crystal. As an example, the growth of a binary A50B50 crystal is described, and critical undercoolings at characteristic changes of growth velocity and the long-range order parameter are defined. For rapidly growing crystals, analogies and qualitative differences are found in comparison with known non-equilibrium effects, particularly solute trapping and disorder trapping. The results and model predictions are compared qualitatively with results of the theory of kinetic phase transitions (Chernov 1968 Sov. Phys. JETP 26, 1182–1190) and with experimental data obtained for rapid dendritic solidification of congruently melting alloy with order–disorder transition (Hartmann et al. 2009 Europhys. Lett. 87, 40007 (doi:10.1209/0295-5075/87/40007)). This article is part of the theme issue ‘From atomistic interfaces to dendritic patterns’. © 2018 The Author(s) Published by the Royal Society. All rights reserved.Russian Science Foundation, RSF: 16-11-1009550WM1541Deutsche Forschungsgemeinschaft, DFGData accessibility. This article has no additional data. Authors’ contributions. All the authors contributed equally to the present research paper. Competing interests. The authors declare that they have no competing interests. Funding. This work was supported by the Russian Science Foundation (grant no. 16-11-10095), the German Space Center Space Management (under contract number 50WM1541) and the Deutsche Forschungsgemeinschaft (DFG) (under grant no. Re1261/8-2)
Paramagnetism in the kagome compouds (Zn, Mg, Cd)Cu3(OH)6Cl2
Frustrated magnetism on the kagome lattice has been a fertile ground for rich
and fascinating physics, ranging from experimental evidence of a spin liquid
to theoretical predictions of exotic superconductivity. Among experimentally
realized spin-12 kagome magnets, herbertsmithite, kapellasite, and haydeeite
[(Zn,Mg)Cu3(OH)6Cl2] are all well described by a three-parameter Heisenberg
model, but they exhibit distinctly different physics. We address the problem
using a pseudofermion functional renormalization-group approach and analyze
the low-energy physics in the experimentally accessible parameter range. Our
analysis places kapellasite and haydeeite near the boundaries between
magnetically ordered and disordered phases, implying that slight modifications
could dramatically affect their magnetic properties. Inspired by this, we
perform \textit{ab initio} density functional theory calculations of
(Zn,Mg,Cd)Cu3 (OH)6Cl2 at various pressures. Our results suggest that by
varying pressure and composition one can traverse a paramagnetic regime
between different magnetically ordered phases
Ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model on the triangular lattice: A variational study based on entangled-plaquette states
We study, on the basis of the general entangled-plaquette variational ansatz,
the ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model
on the triangular lattice. Our numerical estimates are in good agreement with
available exact results and comparable, for large system sizes, to those
computed via the best alternative numerical approaches, or by means of
variational schemes based on specific (i.e., incorporating problem dependent
terms) trial wave functions. The extrapolation to the thermodynamic limit of
our results for lattices comprising up to N=324 spins yields an upper bound of
the ground-state energy per site (in units of the exchange coupling) of
[ for the XX model], while the estimated
infinite-lattice order parameter is (i.e., approximately 64% of the
classical value).Comment: 8 pages, 3 tables, 2 figure
Phase diagram of a distorted kagome antiferromagnet and application to Y-kapellasite
We investigate the magnetism of a previously unexplored distorted spin-1/2 kagome model consisting of three symmetry-inequivalent nearest-neighbor antiferromagnetic Heisenberg couplings J⬡, J, and J′, and uncover a rich ground state phase diagram even at the classical level. Using analytical arguments and numerical techniques we identify a collinear Q→=0 magnetic phase, two unusual non-collinear coplanar Q→=(1/3,1/3) phases and a classical spin liquid phase with a degenerate manifold of non-coplanar ground states, resembling the jammed spin liquid phase found in the context of a bond-disordered kagome antiferromagnet. We further show with density functional theory calculations that the recently synthesized Y-kapellasite Y3Cu9(OH)19Cl8 is a realization of this model and predict its ground state to lie in the region of Q→=(1/3,1/3) order, which remains stable even after the inclusion of quantum fluctuation effects within variational Monte Carlo and pseudofermion functional renormalization group. The presented model opens a new direction in the study of kagome antiferromagnets
Discrete exterior calculus (DEC) for the surface Navier-Stokes equation
We consider a numerical approach for the incompressible surface Navier-Stokes
equation. The approach is based on the covariant form and uses discrete
exterior calculus (DEC) in space and a semi-implicit discretization in time.
The discretization is described in detail and related to finite difference
schemes on staggered grids in flat space for which we demonstrate second order
convergence. We compare computational results with a vorticity-stream function
approach for surfaces with genus 0 and demonstrate the interplay between
topology, geometry and flow properties. Our discretization also allows to
handle harmonic vector fields, which we demonstrate on a torus.Comment: 21 pages, 9 figure
Global Pattern Search at Scale
In recent years, data collection has far outpaced the tools for data analysis in the area of non-traditional GEOINT analysis. Traditional tools are designed to analyze small-scale numerical data, but there are few good interactive tools for processing large amounts of unstructured data such as raw text. In addition to the complexities of data processing, presenting the data in a way that is meaningful to the end user poses another challenge. In our work, we focused on analyzing a corpus of 35,000 news articles and creating an interactive geovisualization tool to reveal patterns to human analysts. Our comprehensive tool, Global Pattern Search at Scale (GPSS), addresses three major problems in data analysis: free text analysis, high volumes of data, and interactive visualization. GPSS uses an Accumulo database for high-volume data storage, and a matrix of word counts and event detection algorithms to process the free text. For visualization, the tool displays an interactive web application to the user, featuring a map overlaid with document clusters and events, search and filtering options, a timeline, and a word cloud. In addition, the GPSS tool can be easily adapted to process and understand other large free-text datasets
Direct Evidence for Dominant Bond-directional Interactions in a Honeycomb Lattice Iridate Na2IrO3
Heisenberg interactions are ubiquitous in magnetic materials and have been
prevailing in modeling and designing quantum magnets. Bond-directional
interactions offer a novel alternative to Heisenberg exchange and provide the
building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as
its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential
realizations of the Kitaev model, and their reported magnetic behaviors may be
interpreted within the Kitaev framework. However, the extent of their relevance
to the Kitaev model remains unclear, as evidence for bond-directional
interactions remains indirect or conjectural. Here, we present direct evidence
for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and
show that they lead to strong magnetic frustration. Diffuse magnetic x-ray
scattering reveals broken spin-rotational symmetry even above Neel temperature,
with the three spin components exhibiting nano-scale correlations along
distinct crystallographic directions. This spin-space and real-space
entanglement directly manifests the bond-directional interactions, provides the
missing link to Kitaev physics in honeycomb iridates, and establishes a new
design strategy toward frustrated magnetism.Comment: Nature Physics, accepted (2015
Creation and manipulation of entanglement in spin chains far from equilibrium
We investigate creation, manipulation, and steering of entanglement in spin
chains from the viewpoint of quantum communication between distant parties. We
demonstrate how global parametric driving of the spin-spin coupling and/or
local time-dependent Zeeman fields produce a large amount of entanglement
between the first and the last spin of the chain. This occurs whenever the
driving frequency meets a resonance condition, identified as "entanglement
resonance". Our approach marks a promising step towards an efficient quantum
state transfer or teleportation in solid state system. Following the reasoning
of Zueco et al. [1], we propose generation and routing of multipartite
entangled states by use of symmetric tree-like structures of spin chains.
Furthermore, we study the effect of decoherence on the resulting spin
entanglement between the corresponding terminal spins.Comment: 10 pages, 8 figure
Changes in cytokine production in healthy subjects practicing Guolin Qigong : a pilot study
BACKGROUND: Guolin Qigong is a combination of meditation, controlled breathing and physical movement designed to control the vital energy (qi) of the body and consequently to improve spiritual, physical and mental health. Practice of Qigong has been reported to alter immunological function, but there have been few studies of its effects on cytokines, the key regulators of immunity. METHODS: Numbers of peripheral blood cytokine-secreting cells were determined by ELISPOT in 19 healthy volunteers aged 27 – 55, before they were taught the practice of Qigong and after 3, 7 and 14 weeks of daily practice. The effect of Qigong on blood cortisol was also examined. RESULTS: Numbers of IL4 and IL12-secreting cells remained stable. IL6 increased at 7 weeks and TNFα increased in unstimulated cultures at 3 and 7 weeks but decreased at these times in LPS and SAC-stimulated cultures. Of particular interest, IFNγ-secreting cells increased and IL10-secreting cells decreased in PHA-stimulated cultures, resulting in significant increases in the IFNγ:IL10 ratio. Cortisol, a known inhibitor of type 1 cytokine production, was reduced by practicing Qigong. CONCLUSION: These preliminary studies in healthy subjects, although not necessarily representative of a randomized healthy population and not including a separate control group, have indicated that blood levels of the stress-related hormone cortisol may be lowered by short-term practice of Qigong and that there are concomitant changes in numbers of cytokine-secreting cells. Further studies of the effect of Qigong in patients with clinical diseases known to be associated with type 2 cytokine predominance are merited
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