3,350 research outputs found
The Landau-Zener transition and the surface hopping method for the 2D Dirac equation for graphene
A Lagrangian surface hopping algorithm is implemented to study the two
dimensional massless Dirac equation for Graphene with an electrostatic
potential, in the semiclassical regime. In this problem, the crossing of the
energy levels of the system at Dirac points requires a particular treatment in
the algorithm in order to describe the quantum transition-- characterized by
the Landau-Zener probability-- between different energy levels. We first derive
the Landau-Zener probability for the underlying problem, then incorporate it
into the surface hopping algorithm. We also show that different asymptotic
models for this problem derived in [O. Morandi, F. Sch{\"u}rrer, J. Phys. A:
Math. Theor. 44 (2011)] may give different transition probabilities. We conduct
numerical experiments to compare the solutions to the Dirac equation, the
surface hopping algorithm, and the asymptotic models of [O. Morandi, F.
Sch{\"u}rrer, J. Phys. A: Math. Theor. 44 (2011)]
What are spin currents in Heisenberg magnets?
We discuss the proper definition of the spin current operator in Heisenberg
magnets subject to inhomogeneous magnetic fields. We argue that only the
component of the naive "current operator" J_ij S_i x S_j in the plane spanned
by the local order parameters and is related to real transport of
magnetization. Within a mean field approximation or in the classical ground
state the spin current therefore vanishes. Thus, finite spin currents are a
direct manifestation of quantum correlations in the system.Comment: 4 pages, 1 figure, published versio
Instability of rotating black holes: large D analysis
We study the stability of odd-dimensional rotating black holes with equal
angular momenta by performing an expansion in the inverse of the number of
dimensions D. Universality at large allows us to calculate analytically the
complex frequency of quasinormal modes to next-to-leading order in the
expansion. We identify the onset of non-axisymmetric, bar-mode instabilities at
a specific finite rotation, and axisymmetric instabilities at larger rotation.
The former occur at the threshold where the modes become superradiant, and
before the ultraspinning regime is reached. Our results fully confirm the
picture found in numerical studies, with very good quantitative agreement. We
extend the analysis to the same class of black holes in Anti-deSitter space,
and find the same qualitative features. We also discuss the appearance at high
frequencies of the universal set of (stable) quasinormal modes.Comment: 38 pages, 14 figures. v3: NLO results included so the instability is
shown to occur before the ultraspinning regime of rotation. Significant
improvements in accuracy. Ancillary Mathematica notebook contains details of
NLO result
Loss tolerant device-independent quantum key distribution: a proof of principle
We here present the rate analysis and a proof of principle realization of a
device-independent quantum key distribution (QKD) protocol requiring the lowest
detection efficiency necessary to achieve a secure key compared to
device-independent protocols known so far. The protocol is based on
non-maximally entangled state and its experimental realization has been
performed by two-photon bipartite entangled states. The improvement with
respect to protocols involving maximally entangled states has been estimated.Comment: 8 pages, 4 figure + appendi
Semantic Part Segmentation using Compositional Model combining Shape and Appearance
In this paper, we study the problem of semantic part segmentation for
animals. This is more challenging than standard object detection, object
segmentation and pose estimation tasks because semantic parts of animals often
have similar appearance and highly varying shapes. To tackle these challenges,
we build a mixture of compositional models to represent the object boundary and
the boundaries of semantic parts. And we incorporate edge, appearance, and
semantic part cues into the compositional model. Given part-level segmentation
annotation, we develop a novel algorithm to learn a mixture of compositional
models under various poses and viewpoints for certain animal classes.
Furthermore, a linear complexity algorithm is offered for efficient inference
of the compositional model using dynamic programming. We evaluate our method
for horse and cow using a newly annotated dataset on Pascal VOC 2010 which has
pixelwise part labels. Experimental results demonstrate the effectiveness of
our method
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