9,347 research outputs found
Electron Band Structure in a Two Dimensional Periodic Magnetic Field
In this paper we study the energy spectrum of a two dimensional electron gas
(2DEG) in a two dimensional periodic magnetic field. Both a square magnetic
lattice and a triangular one are considered. We consider the general case where
the magnetic field in a cell can be of any shape. A general feature of the band
structure is bandwidth oscillation as a function of the Landau index. A
triangular magnetic lattice on a 2DEG can be realized by the vortex lattice of
a superconductor film coated on top of a heterojunction. Our calculation
indicates a way of relating the energy spectrum of the 2DEG to the vortex
structure. We have also derived conditions under which the effects of a weak
magnetic modulation, periodic or not, may be reproduced by an electric
potential modulation, and vice versa.Comment: 16 pages in TeX and 5 uuencoded figure
Bose-Einstein Condensation with Entangled Order Parameter
We propose a practically accessible non-mean-field ground state of
Bose-Einstein condensation (BEC), which occurs in an interspecies two-particle
entangled state, and is thus described by an entangled order parameter. A
suitably defined entanglement entropy is used as the characterization of the
non-mean-field nature, and is found to persist in a wide parameter regime. The
interspecies entanglement leads to novel interference terms in the dynamical
equations governing the single particle orbital wavefunctions. Experimental
feasibility and several methods of probe are discussed. We urge the study of
multi-channel scattering between different species of atoms.Comment: V1: 5 pages, 4 figures. Accepted by Phys. Rev. Lett.; V2: A couple of
very minor typos corrected, publishe
Berry Phase Effects on Electronic Properties
Ever since its discovery, the Berry phase has permeated through all branches
of physics. Over the last three decades, it was gradually realized that the
Berry phase of the electronic wave function can have a profound effect on
material properties and is responsible for a spectrum of phenomena, such as
ferroelectricity, orbital magnetism, various (quantum/anomalous/spin) Hall
effects, and quantum charge pumping. This progress is summarized in a
pedagogical manner in this review. We start with a brief summary of necessary
background, followed by a detailed discussion of the Berry phase effect in a
variety of solid state applications. A common thread of the review is the
semiclassical formulation of electron dynamics, which is a versatile tool in
the study of electron dynamics in the presence of electromagnetic fields and
more general perturbations. Finally, we demonstrate a re-quantization method
that converts a semiclassical theory to an effective quantum theory. It is
clear that the Berry phase should be added as a basic ingredient to our
understanding of basic material properties.Comment: 48 pages, 16 figures, submitted to RM
Chiral anomaly and anomalous finite-size conductivity in graphene
Graphene is a monolayer of carbon atoms packed into a hexagon lattice to host
two pairs of massless two-dimensional Dirac fermions in the absence of or with
negligible spin-orbit coupling. It is known that the existence of non-zero
electric polarization in reduced momentum space which is associated with a
hidden chiral symmetry will lead to the zero-energy flat band of zigzag
nanoribbon. The Adler-Bell-Jackiw chiral anomaly or non-conservation of chiral
charges at different valleys can be realized in a confined ribbon of finite
width. In the laterally diffusive regime, the finite-size correction to
conductivity is always positive and goes inversely with the square of the
lateral dimension W, which is different from the finite-size correction
inversely with W from boundary modes. This anomalous finite-size conductivity
reveals the signature of the chiral anomaly in graphene, and is measurable
experimentally.Comment: 5 pages, 2 figure
Multi-Modal Multi-Scale Deep Learning for Large-Scale Image Annotation
Image annotation aims to annotate a given image with a variable number of
class labels corresponding to diverse visual concepts. In this paper, we
address two main issues in large-scale image annotation: 1) how to learn a rich
feature representation suitable for predicting a diverse set of visual concepts
ranging from object, scene to abstract concept; 2) how to annotate an image
with the optimal number of class labels. To address the first issue, we propose
a novel multi-scale deep model for extracting rich and discriminative features
capable of representing a wide range of visual concepts. Specifically, a novel
two-branch deep neural network architecture is proposed which comprises a very
deep main network branch and a companion feature fusion network branch designed
for fusing the multi-scale features computed from the main branch. The deep
model is also made multi-modal by taking noisy user-provided tags as model
input to complement the image input. For tackling the second issue, we
introduce a label quantity prediction auxiliary task to the main label
prediction task to explicitly estimate the optimal label number for a given
image. Extensive experiments are carried out on two large-scale image
annotation benchmark datasets and the results show that our method
significantly outperforms the state-of-the-art.Comment: Submited to IEEE TI
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