2,340 research outputs found
A topological spin glass in diluted spin ice
It is a salient experimental fact that a large fraction of candidate spin
liquid materials freeze as the temperature is lowered. The question naturally
arises whether such freezing is intrinsic to the spin liquid ("disorder-free
glassiness") or extrinsic, in the sense that a topological phase simply
coexists with standard freezing of impurities. Here, we demonstrate a
surprising third alternative, namely that freezing and topological liquidity
are inseparably linked. The topological phase reacts to the introduction of
disorder by generating degrees of freedom of a new type (along with
interactions between them), which in turn undergo a freezing transition while
the topological phase supporting them remains intact.Comment: 4 pages + supplementary materia
Random Coulomb antiferromagnets: from diluted spin liquids to Euclidean random matrices
We study a disordered classical Heisenberg magnet with uniformly
antiferromagnetic interactions which are frustrated on account of their
long-range Coulomb form, {\em i.e.} in and in . This arises naturally as the limit of the
emergent interactions between vacancy-induced degrees of freedom in a class of
diluted Coulomb spin liquids (including the classical Heisenberg
antiferromagnets on checkerboard, SCGO and pyrochlore lattices) and presents a
novel variant of a disordered long-range spin Hamiltonian. Using detailed
analytical and numerical studies we establish that this model exhibits a very
broad paramagnetic regime that extends to very large values of in both
and . In , using the lattice-Green function based finite-size
regularization of the Coulomb potential (which corresponds naturally to the
underlying low-temperature limit of the emergent interactions between
orphan-spins), we only find evidence that freezing into a glassy state occurs
in the limit of strong coupling, , while no such transition seems to
exist at all in . We also demonstrate the presence and importance of
screening for such a magnet. We analyse the spectrum of the Euclidean random
matrices describing a Gaussian version of this problem, and identify a
corresponding quantum mechanical scattering problem.Comment: two-column PRB format; 17 pages; 24 .eps figure
Semiclassical spin liquid state of easy axis Kagome antiferromagnets
Motivated by recent experiments on Nd-langasite, we consider the effect of
strong easy axis single-ion anisotropy on spins interacting with
antiferromagnetic exchange on the Kagome lattice. When , the
collinear low energy states selected by the anisotropy map on to configurations
of the classical Kagome lattice Ising antiferromagnet. However, the low
temperature limit is quite different from the cooperative Ising paramagnet that
obtains classically for . We find that sub-leading multi-spin interactions arising from the transverse quantum
dynamics result in a crossover from an intermediate temperature classical
cooperative Ising paramagnet to a semiclassical spin liquid with distinct
short-ranged correlations for .Comment: 4 pages, 3 eps figure
Characterizing Driving Context from Driver Behavior
Because of the increasing availability of spatiotemporal data, a variety of
data-analytic applications have become possible. Characterizing driving
context, where context may be thought of as a combination of location and time,
is a new challenging application. An example of such a characterization is
finding the correlation between driving behavior and traffic conditions. This
contextual information enables analysts to validate observation-based
hypotheses about the driving of an individual. In this paper, we present
DriveContext, a novel framework to find the characteristics of a context, by
extracting significant driving patterns (e.g., a slow-down), and then
identifying the set of potential causes behind patterns (e.g., traffic
congestion). Our experimental results confirm the feasibility of the framework
in identifying meaningful driving patterns, with improvements in comparison
with the state-of-the-art. We also demonstrate how the framework derives
interesting characteristics for different contexts, through real-world
examples.Comment: Accepted to be published at The 25th ACM SIGSPATIAL International
Conference on Advances in Geographic Information Systems (ACM SIGSPATIAL
2017
Growth kinetics effects on self-assembled InAs/InP quantum dots
A systematic manipulation of the morphology and the optical emission
properties of MOVPE grown ensembles of InAs/InP quantum dots is demonstrated by
changing the growth kinetics parameters. Under non-equilibrium conditions of a
comparatively higher growth rate and low growth temperature, the quantum dot
density, their average size and hence the peak emission wavelength can be tuned
by changing efficiency of the surface diffusion (determined by the growth
temperature) relative to the growth flux. We further observe that the
distribution of quantum dot heights, for samples grown under varying
conditions, if normalized to the mean height, can be nearly collapsed onto a
single Gaussian curve.Comment: 2 figure
Distorted wurtzite unit cells: Determination of lattice parameters of non-polar a-plane AlGaN and estimation of solid phase Al content
Unlike c-plane nitrides, ``non-polar" nitrides grown in e.g. the a-plane or
m-plane orientation encounter anisotropic in-plane strain due to the anisotropy
in the lattice and thermal mismatch with the substrate or buffer layer. Such
anisotropic strain results in a distortion of the wurtzite unit cell and
creates difficulty in accurate determination of lattice parameters and solid
phase group-III content (x_solid) in ternary alloys. In this paper we show that
the lattice distortion is orthorhombic, and outline a relatively simple
procedure for measurement of lattice parameters of non-polar group III-nitrides
epilayers from high resolution x-ray diffraction measurements. We derive an
approximate expression for x_solid taking into account the anisotropic strain.
We illustrate this using data for a-plane AlGaN, where we measure the lattice
parameters and estimate the solid phase Al content, and also show that this
method is applicable for m-plane structures as well
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