78,009 research outputs found
Time-Reversal Symmetry Breaking and Spontaneous Anomalous Hall Effect in Fermi Fluids
We study the spontaneous non-magnetic time-reversal symmetry breaking in a
two-dimensional Fermi liquid without breaking either the translation symmetry
or the U(1) charge symmetry. Assuming that the low-energy physics is described
by fermionic quasiparticle excitations, we identified an "emergent" local
symmetry in momentum space for an -band model. For a large class of
models, including all one-band and two-band models, we found that the
time-reversal and chiral symmetry breaking can be described by the
gauge theory associated with this emergent local symmetry. This
conclusion enables the classification of the time-reversal symmetry-breaking
states as types I and II, depending on the type of accompanying spatial
symmetry breaking. The properties of each class are studied. In particular, we
show that the states breaking both time-reversal and chiral symmetries are
described by spontaneously generated Berry phases. We also show examples of the
time-reversal symmetry-breaking phases in several different microscopically
motivated models and calculate their associated Hall conductance within a
mean-field approximation. The fermionic nematic phase with time-reversal
symmetry breaking is also presented and the possible realizations in strongly
correlated models such as the Emery model are discussed.Comment: 18 pages, 8 figure
Improved Compact Visibility Representation of Planar Graph via Schnyder's Realizer
Let be an -node planar graph. In a visibility representation of ,
each node of is represented by a horizontal line segment such that the line
segments representing any two adjacent nodes of are vertically visible to
each other. In the present paper we give the best known compact visibility
representation of . Given a canonical ordering of the triangulated , our
algorithm draws the graph incrementally in a greedy manner. We show that one of
three canonical orderings obtained from Schnyder's realizer for the
triangulated yields a visibility representation of no wider than
. Our easy-to-implement O(n)-time algorithm bypasses the
complicated subroutines for four-connected components and four-block trees
required by the best previously known algorithm of Kant. Our result provides a
negative answer to Kant's open question about whether is a
worst-case lower bound on the required width. Also, if has no degree-three
(respectively, degree-five) internal node, then our visibility representation
for is no wider than (respectively, ).
Moreover, if is four-connected, then our visibility representation for
is no wider than , matching the best known result of Kant and He. As a
by-product, we obtain a much simpler proof for a corollary of Wagner's Theorem
on realizers, due to Bonichon, Sa\"{e}c, and Mosbah.Comment: 11 pages, 6 figures, the preliminary version of this paper is to
appear in Proceedings of the 20th Annual Symposium on Theoretical Aspects of
Computer Science (STACS), Berlin, Germany, 200
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Surface constraints on the depth of the Atlantic meridional overturning circulation: Southern Ocean versus North Atlantic
Paleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean
Persistent spin current in spin-orbit coupling systems in the absence of an external magnetic field
The spin-orbit coupling systems with a zero magnetic field is studied under
the equilibrium situation, {\it i.e.}, without a voltage bias. A persistent
spin current is predicted to exist under most circumstances, although the
persistent charge current and the spin accumulation are identically zero. In
particular, a two-dimensional quantum wire is investigated in detail.
Surprisingly, a persistent spin current is found to flow along the confined
direction, due to the spin precession in accompany with the particle motion.
This provides an interesting example of constant spin flowing without inducing
a spin accumulation, contrary to common intuition.Comment: 4 pages, 5 figure
Nature of Spin Hall Effect in a finite Ballistic Two-Dimensional System with Rashba and Dresselhaus spin-orbit interaction
The spin Hall effect in a finite ballistic two-dimensional system with Rashba
and Dresselhaus spin-orbit interaction is studied numerically. We find that the
spin Hall conductance is very sensitive to the transverse measuring location,
the shape and size of the device, and the strength of the spin-orbit
interaction. Not only the amplitude of spin Hall conductance but also its sign
can change. This non-universal behavior of the spin Hall effect is essentially
different from that of the charge Hall effect, in which the Hall voltage is
almost invariant with the transverse measuring site and is a monotonic function
of the strength of the magnetic field. These surprise behavior of the spin Hall
conductance are attributed to the fact that the eigenstates of the spin Hall
system is extended in the transverse direction and do not form the edge states.Comment: 5 pages, 5 figure
Bias-controllable intrinsic spin polarization in a quantum dot
We propose a novel scheme to efficiently polarize and manipulate the electron
spin in a quantum dot. This scheme is based on the spin-orbit interaction and
it possesses following advantages: (1) The direction and the strength of the
spin polarization is well controllable and manipulatable by simply varying the
bias or the gate voltage. (2) The spin polarization is quite large even with a
weak spin-orbit interaction. (3) Both electron-electron interaction and
multi-energy levels do not weaken but strengthen the spin polarization. (4) It
has the short spin flip time. (5) The device is free of a magnetic field or a
ferromagnetic material. (6) It can be easily realized with present technology.Comment: 9 pages, 5 figure
Spin relaxation in -type (111) GaAs quantum wells
We investigate the spin relaxation limited by the D'yakonov-Perel' mechanism
in -type (111) GaAs quantum wells, by means of the kinetic spin Bloch
equation approach. In (111) GaAs quantum wells, the in-plane effective magnetic
field from the D'yakonov-Perel' term can be suppressed to zero on a special
momentum circle under the proper gate voltage, by the cancellation between the
Dresselhaus and Rashba spin-orbit coupling terms. When the spin-polarized
electrons mainly distribute around this special circle, the in-plane
inhomogeneous broadening is small and the spin relaxation can be suppressed,
especially for that along the growth direction of quantum well. This
cancellation effect may cause a peak (the cancellation peak) in the density or
temperature dependence of the spin relaxation time. In the density
(temperature) dependence, the interplay between the cancellation peak and the
ordinary density (Coulomb) peak leads to rich features of the density
(temperature) dependence of the spin relaxation time. The effect of impurities,
with its different weights on the cancellation peak and the Coulomb peak in the
temperature dependence of the spin relaxation, is revealed. We also show the
anisotropy of the spin relaxation with respect to the spin-polarization
direction.Comment: 8 pages, 6 figure
Fourier transform and rigidity of certain distributions
Let be a finite dimensional vector space over a local field, and be
its dual. For a closed subset of , and of , consider the space
of tempered distributions on whose support are contained
in and support of whose Fourier transform are contained in . We show
that possesses a certain rigidity property, for ,
which are some finite unions of affine subspaces.Comment: 10 page
Objective and efficient terahertz signal denoising by transfer function reconstruction
As an essential processing step in many disciplines, signal denoising efficiently improves data quality without extra cost. However, it is relatively under-utilized for terahertz spectroscopy. The major technique reported uses wavelet denoising in the time-domain, which has a fuzzy physical meaning and limited performance in low-frequency and water-vapor regions. Here, we work from a new perspective by reconstructing the transfer function to remove noise-induced oscillations. The method is fully objective without a need for defining a threshold. Both reflection imaging and transmission imaging were conducted. The experimental results show that both low- and high-frequency noise and the water-vapor influence were efficiently removed. The spectrum accuracy was also improved, and the image contrast was significantly enhanced. The signal-to-noise ratio of the leaf image was increased up to 10 dB, with the 6 dB bandwidth being extended by over 0.5 THz
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