263,322 research outputs found
Quantum Entanglement as a Diagnostic of Phase Transitions in Disordered Fractional Quantum Hall Liquids
We investigate the disorder-driven phase transition from a fractional quantum
Hall state to an Anderson insulator using quantum entanglement methods. We find
that the transition is signaled by a sharp increase in the sensitivity of a
suitably averaged entanglement entropy with respect to disorder -- the
magnitude of its disorder derivative appears to diverge in the thermodynamic
limit. We also study the level statistics of the entanglement spectrum as a
function of disorder. However, unlike the dramatic phase-transition signal in
the entanglement entropy derivative, we find a gradual reduction of level
repulsion only deep in the Anderson insulating phase.Comment: 8 pages, 8 figures, including the supplemental material, published in
PRL as an Editors' Suggestio
A Model Approximation Scheme for Planning in Partially Observable Stochastic Domains
Partially observable Markov decision processes (POMDPs) are a natural model
for planning problems where effects of actions are nondeterministic and the
state of the world is not completely observable. It is difficult to solve
POMDPs exactly. This paper proposes a new approximation scheme. The basic idea
is to transform a POMDP into another one where additional information is
provided by an oracle. The oracle informs the planning agent that the current
state of the world is in a certain region. The transformed POMDP is
consequently said to be region observable. It is easier to solve than the
original POMDP. We propose to solve the transformed POMDP and use its optimal
policy to construct an approximate policy for the original POMDP. By
controlling the amount of additional information that the oracle provides, it
is possible to find a proper tradeoff between computational time and
approximation quality. In terms of algorithmic contributions, we study in
details how to exploit region observability in solving the transformed POMDP.
To facilitate the study, we also propose a new exact algorithm for general
POMDPs. The algorithm is conceptually simple and yet is significantly more
efficient than all previous exact algorithms.Comment: See http://www.jair.org/ for any accompanying file
Overall properties of the Gaia DR1 reference frame
We compare quasar positions of the auxiliary quasar solution with ICRF2
sources using different samples and evaluate the influence on the {\it Gaia}
DR1 reference frame owing to the Galactic aberration effect over the
J2000.0-J20015.0 period. Then we estimate the global rotation between TGAS with
{\it Tycho}-2 proper motion systems to investigate the property of the {\it
Gaia} DR1 reference frame. Finally, the Galactic kinematics analysis using the
K-M giant proper motions is performed to understand the property of {\it Gaia}
DR1 reference frame. The positional comparison between the auxiliary quasar
solution and ICRF2 shows negligible orientation and validates the declination
bias of \mas~in {\it Gaia} quasar positions with respect to ICRF2.
Galactic aberration effect is thought to cause an offset \mas~of
the axis direction of {\it Gaia} DR1 reference frame. The global rotation
between TGAS and {\it Tycho}-2 proper motion systems, obtained by different
samples, shows a much smaller value than the claimed value \masyr. For
the Galactic kinematics analysis of the TGAS K-M giants, we find possible
non-zero Galactic rotation components beyond the classical Oort constants: the
rigid part \masyr~and the differential part
\masyr~around the axis of Galactic
coordinates, which indicates possible residual rotation in {\it Gaia} DR1
reference frame or problems in the current Galactic kinematical model.Comment: 6 pages, 1 figure. Accepted for publication in A&
Quantum Manifestation of Elastic Constants in Nanostructures
Generally, there are two distinct effects in modifying the properties of
low-dimensional nanostructures: surface effect (SS) due to increased
surface-volume ratio and quantum size effect (QSE) due to quantum confinement
in reduced dimension. The SS has been widely shown to affect the elastic
constants and mechanical properties of nanostructures. Here, using Pb nanofilm
and graphene nanoribbon as model systems, we demonstrate the QSE on the elastic
constants of nanostructures by first-principles calculations. We show that
generally QSE is dominant in affecting the elastic constants of metallic
nanostructures while SS is more pronounced in semiconductor and insulator
nanostructures. Our findings have broad implications in quantum aspects of
nanomechanics
Roll waves in mud
The stability of a viscoplastic fluid film falling down an inclined plane is explored, with the aim of determining the critical Reynolds number for the onset of roll waves. The Herschel–Bulkley constitutive law is adopted and the fluid is assumed two-dimensional and incompressible. The linear stability problem is described for an equilibrium in the form of a uniform sheet flow, when perturbed by introducing an infinitesimal stress perturbation. This flow is stable for very high Reynolds numbers because the rigid plug riding atop the fluid layer cannot be deformed and the free surface remains flat. If the flow is perturbed by allowing arbitrarily small strain rates, on the other hand, the plug is immediately replaced by a weakly yielded ‘pseudo-plug’ that can deform and reshape the free surface. This situation is modelled by lubrication theory at zero Reynolds number, and it is shown how the fluid exhibits free-surface instabilities at order-one Reynolds numbers. Simpler models based on vertical averages of the fluid equations are evaluated, and one particular model is identified that correctly predicts the onset of instability. That model is used to describe nonlinear roll waves
A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs
We present a near-infrared (0.9-2.4 microns) spectroscopic study of 73 field
ultracool dwarfs having spectroscopic and/or kinematic evidence of youth
(~10-300 Myr). Our sample is composed of 48 low-resolution (R~100) spectra and
41 moderate-resolution spectra (R>~750-2000). First, we establish a method for
spectral typing M5-L7 dwarfs at near-IR wavelengths that is independent of
gravity. We find that both visual and index-based classification in the near-IR
provide consistent spectral types with optical spectral types, though with a
small systematic offset in the case of visual classification at J and K band.
Second, we examine features in the spectra of ~10 Myr ultracool dwarfs to
define a set of gravity-sensitive indices based on FeH, VO, K, Na and H-band
continuum shape. We then create an index-based method for classifying the
gravities of M6-L5 dwarfs that provides consistent results with gravity
classifications from optical spectroscopy. Our index-based classification can
distinguish between young and dusty objects. Guided by the resulting
classifications, we propose a set of low-gravity spectral standards for the
near-IR. Finally, we estimate the ages corresponding to our gravity
classifications.Comment: Published in ApJ. IDL program for calculating indices
(allers13_index.pro) included in the source gzipped ta
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