71,920 research outputs found
Four-Dimensional Topological Insulators with Nodal-Line Boundary States
Conventional topological insulators and superconductors have topologically
protected nodal points on their boundaries, and the recent interests in
nodal-line semimetals only concerned bulk band structures. Here, we present a
novel four-dimensional topological insulator protected by an anti-unitary
reflection symmetry, whose boundary band has a single -symmetric nodal line
with double topological charges. Inspired by the recent experimental
realization of the four-dimensional quantum Hall effect, we also propose a
cold-atom system which realizes the novel topological insulator with tunable
parameters as extra dimensions.Comment: 5 pages and 3 figures for main text, 10 pages and 2 figures for
supplemental materia
Machine Learning Phase Transition: An Iterative Proposal
We propose an iterative proposal to estimate critical points for statistical
models based on configurations by combing machine-learning tools. Firstly,
phase scenarios and preliminary boundaries of phases are obtained by
dimensionality-reduction techniques. Besides, this step not only provides
labelled samples for the subsequent step but also is necessary for its
application to novel statistical models. Secondly, making use of these samples
as training set, neural networks are employed to assign labels to those samples
between the phase boundaries in an iterative manner. Newly labelled samples
would be put in the training set used in subsequent training and the phase
boundaries would be updated as well. The average of the phase boundaries is
expected to converge to the critical temperature in this proposal. In concrete
examples, we implement this proposal to estimate the critical temperatures for
two q-state Potts models with continuous and first order phase transitions.
Linear and manifold dimensionality-reduction techniques are employed in the
first step. Both a convolutional neural network and a bidirectional recurrent
neural network with long short-term memory units perform well for two Potts
models in the second step. The convergent behaviors of the estimations reflect
the types of phase transitions. And the results indicate that our proposal may
be used to explore phase transitions for new general statistical models.Comment: We focus on the iterative strategy but not the concrete tools like
specific dimension-reduction techniques, CNN and BLSTM in this work. Other
machine-learning tools with similar functions may be applied to new
statistical models with this proposa
Effect of Dependent Scattering on Light Absorption in Highly Scattering Random Media
The approximate nature of radiative transfer equation (RTE) leads to a bunch
of considerations on the effect of "dependent scattering" in random media,
especially particulate media composed of discrete scatterers, in the last a few
decades, which usually indicates those deviations RTE (combined with ISA) lead
to from experimental and exact numerical results due to electromagnetic wave
interference. Here we theoretically and numerically demonstrate the effect of
dependent scattering on absorption in disordered media consisting of highly
scattering scatterers. By making comparison between the independent scattering
approximation-radiative transfer equation (ISA-RTE) and the full-wave coupled
dipole method (CDM), we find that deviations between the two methods increase
as scatterer density in the media increases. The discrepancy also grows with
optical thickness. To quantitatively take dependent scattering effect into
account, we develop a theoretical model using quasi-crystalline approximation
(QCA) to derive dependent-scattering corrected radiative properties, based on
the path-integral diagrammatic technique in multiple scattering theory. The
model results in a more reasonable agreement with numerical simulations. The
present work has profound implications for the coherent scattering physics in
random media with absorption, correctly modeling light absorptance in random
media and interpreting the experimental observations in various applications
for random media such as solar energy concentration, micro/nanofluids,
structural color generation, etc.Comment: 30 pages, 8 figures, submitte
Topological photonic states in one-dimensional dimerized ultracold atomic chains
We study the topological optical states in one-dimensional (1D) dimerized
ultracold atomic chains, as an extension of the Su-Schrieffer-Heeger (SSH)
model. By taking the fully retarded near-field and far-field dipole-dipole
interactions into account, we describe the system by an effective non-Hermitian
Hamiltonian, vastly different from the Hermitian Hamiltonian of the
conventional SSH model. We analytically calculate the complex bandstructures
for infinitely long chains, and show that the topological invariant, i.e., the
complex Zak phase, is still quantized and becomes nontrivial when the
dimerization parameter , despite the broken chiral symmetry and
non-Hermiticity. We have verified the validity of the bulk-boundary
correspondence for this non-Hermitian system by further analyzing the
eigenstate distributions along with their inverse participation ratios (IPRs)
for finite chains, where topologically protected edge states are unambiguously
identified. We also reveal that such topological edge states are robust under
symmetry-breaking disorders. For transverse eigenstates, we further discover
the increase of localization length of topological edge states with the
increase of lattice period due to the presence of strong far-field
dipole-dipole interactions. Moreover, the ultra-strong scattering cross section
and ultra-narrow linewidth of a single cold atom allow us to observe in more
detail about topological states than in conventional systems, such as the
frequency shift with respect to the single-atom resonance and the largely
tunable bandgap. We envisage these topological photonic states can provide an
efficient interface between light and matter.Comment: 13 pages 10 figures Comments are welcom
Role of Near-Field Interaction on Light Transport in Disordered Media
Understanding light-matter interaction in disordered photonic media allows
people to manipulate light scattering and achieve exciting applications using
seemingly scrambled media. As the concentration of scattering particles rises,
they are inclined to step into near fields of each other in deep subwavelength
scale. The fundamental physics involving the interplay between disorder and
near-field interaction (NFI) is still not fully understood. We theoretically
examine the role of NFI by analyzing the underlying multiple scattering
mechanism. We find NFI leads to a stronger collective behavior involving more
particles and widens the photonic pseudo-bandgap of disordered media. It also
excites more weakly decayed longitudinal modes and results in higher local
density of states. By introducing a sticky short-range order, we demonstrate
the possibility of enhancing off-momentum-shell NFI of multiple scattering
process. Our results have profound implications in understanding and harnessing
nanoscale light-matter interaction for novel disordered photonic devices.Comment: 7 pages, 5 figures, plus a 9-page supplementary material, comments
are welcom
Coexistence of Superconductivity and Ferromagnetism in Dilute Co-doped La_{1.89} Ce_{0.11} Cu O_{4\pm\d} System
Thin films of the optimally electron-doped -phase superconductor
LaCeCuO_{4\pm \d} are investigated by dilute Co doping,
formed as LaCe(CuCo)O_{4\pm\d} (LCCCO) with
= 0.01 -- 0.05. The following results are obtained for the first time: for
the whole dilute Co doping range, LCCCO thin films show long-range
ferromagnetic ordering at the temperature range from 5 K to 300 K, which is
likely due to the RKKY interaction; in the very dilute Co doping, = 0.01
and 0.02, the superconductivity is maitained, the system shows the coexistence
of superconductivity and ferromagnetism in the CuO plane. This may be
based on the nature of the charge carriers in electron-doped high-
cuprate superconductors.Comment: 5 pages, 4 figures, accepted for publication in Physical Review
Low-temperature thermal conductivity of antiferromagnetic S = 1/2 chain material CuCl_22((CH_3)_2SO)
We study the heat transport of S = 1/2 chain compound
CuCl_22((CH_3)_2SO) along the b axis (vertical to the chain direction)
at very low temperatures. The zero-field thermal conductivity (\kappa) shows a
distinct kink at about 0.9 K, which is related to the long-range
antiferromagnetic (AF) transition. With applying magnetic field along the c
axis, \kappa(H) curves also show distinct changes at the phase boundaries
between the AF and the high-field disordered states. These results indicate a
strong spin-phonon interaction and the magnetic excitations play a role in the
b-axis heat transport as phonon scatterers.Comment: 3 pages, 3 figures, accepted for publication in J. Appl. Phys.
(Proceedings of the 58th MMM Conference
A new approach for modelling mixed traffic flow with motorized vehicles and non-motorized vehicles based on cellular automaton model
In this study, we provide a novel approach for modelling the mixed traffic
flow. The basic idea is to integrate models for nonmotorized vehicles
(nm-vehicles) with models for motorized vehicles (m-vehicles). Based on the
idea, a model for mix traffic flow is realized in in the following two steps.
At a first step, the models that can be integrated should be chosen. The famous
NaSch cellular automata (NCA) model for m-vehicles and the Burgur cellular
automata (BCA) model for nm-vehicles are used in this paper, since the two
models are similar and comparable. At a second step, we should study coupling
rules between m-vehicles and nm-vehicles to represent their interaction.
Special lane changing rules are designed for the coupling process. The proposed
model is named as the combined cellular automata (CCA) model. The model is
applied to a typical mixed traffic scenario, where a bus stop without special
stop bay is set on nonmotorized lanes. The simulation results show that the
model can describe both the interaction between the flow of nm-vehicles and
m-vehicles and their characters.Comment: 20 pages, 5 figures, Submitted to Phys. Rev.
Low-temperature heat transport of Nd_{2-x}Ce_{x}CuO_{4} single crystals
We report a study of the Ce doping effect on the thermal conductivity
(\kappa) of Nd_{2-x}Ce_{x}CuO_{4} (NCCO) at low temperatures down to 0.3 K and
in magnetic fields up to 14 T. It is found that with Ce doping, the electronic
thermal conductivity increases; at the same time, the a-axis field induced
changes in \kappa(H), associated with the spin flop and spin polarization of
Nd^{3+} sublattice, and the spin flop of Cu^{2+} sublattice, gradually
disappear. These are clearly due to the electron doping and the destruction of
the antiferromagnetic orders. In the superconducting NCCO with x = 0.14 and
0.18, although the electronic thermal conductivity shows sizable field
dependencies with H // c, the paramagnetic scattering of phonons is still
playing the dominant role in the heat transport, which is different from many
other cuprates. In the lightly doped samples (x = 0.03 and 0.06), the low-T
\kappa(H) isotherms with H // c show a step-like anomaly and is likely related
to the spin/charge stripes.Comment: 10 pages, 7 figures, accepted for publication in Phys. Rev.
Low-temperature heat transport of the geometrically frustrated antiferromagnets R_2Ti_2O_7 (R = Gd and Er)
We report a systematic study on the low-temperature thermal conductivity
(\kappa) of R_2Ti_2O_7 (R = Gd and Er) single crystals with different
directions of magnetic field and heat current. It is found that the magnetic
excitations mainly act as phonon scatterers rather than heat carriers, although
these two materials have long-range magnetic orders at low temperatures. The
low-T \kappa(H) isotherms of both compounds show rather complicated behaviors
and have good correspondences with the magnetic transitions, where the
\kappa(H) curves show drastic dip- or step-like changes. In comparison, the
field dependencies of \kappa are more complicated in Gd_2Ti_2O_7, due to the
complexity of its low-T phase diagram and field-induced magnetic transitions.
These results demonstrate the significant coupling between spins and phonons in
these materials and the ability of heat-transport properties probing the
magnetic transitions.Comment: 9 pages, 6 figures, accepted for publication in Phys. Rev.
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