14,783 research outputs found
Valley contrasting physics in graphene: magnetic moment and topological transport
We investigate physical properties that can be used to distinguish the valley
degree of freedom in systems where inversion symmetry is broken, using graphene
systems as examples. We show that the pseudospin associated with the valley
index of carriers has an intrinsic magnetic moment, in close analogy with the
Bohr magneton for the electron spin. There is also a valley dependent Berry
phase effect that can result in a valley contrasting Hall transport, with
carriers in different valleys turning into opposite directions transverse to an
in-plane electric field. These effects can be used to generate and detect
valley polarization by magnetic and electric means, forming the basis for the
so-called valley-tronics applications
Intrinsic spin Hall effect in monolayers of group-VI dichalcogenides: A first-principles study
Using first-principles calculations within density functional theory, we
investigate the intrinsic spin Hall effect in monolayers of group-VI
transition-metal dichalcogenides MX2 (M = Mo, W and X = S, Se). MX2 monolayers
are direct band-gap semiconductors with two degenerate valleys located at the
corners of the hexagonal Brillouin zone. Because of the inversion symmetry
breaking and the strong spin-orbit coupling, charge carriers in opposite
valleys carry opposite Berry curvature and spin moment, giving rise to both a
valley- and a spin-Hall effect. The intrinsic spin Hall conductivity (ISHC) in
p-doped samples is found to be much larger than the ISHC in n-doped samples due
to the large spin-splitting at the valence band maximum. We also show that the
ISHC in inversion-symmetric bulk dichalcogenides is an order of magnitude
smaller compared to monolayers. Our result demonstrates monolayer
dichalcogenides as an ideal platform for the integration of valleytronics and
spintronics.Comment: published version (7 pages, 6 figures
Tunable Intrinsic Plasmons due to Band Inversion in Topological Materials
The band inversion has led to rich physical effects in both topological
insulators and topological semimetals. It has been found that the inverted band
structure with the Mexican-hat dispersion could enhance the interband
correlation leading to a strong intrinsic plasmon excitation. Its frequency
ranges from several to tens of and can be
effectively tuned by the external fields. The electron-hole asymmetric term
splits the peak of the plasmon excitation into double peaks. The fate and
properties of this plasmon excitation can also act as a probe to characterize
the topological phases even in the lightly doped systems. We numerically
demonstrate the impact of the band inversion on plasmon excitations in
magnetically doped thin films of three-dimensional strong topological
insulators, V- or Cr-doped (Bi, Sb)Te, which support the quantum
anomalous Hall states. Our work thus sheds some new light on the potential
applications of topological materials in plasmonics.Comment: 6 pages, 5 figures, Accepted in PR
Berry phase modification to the energy spectrum of excitons
By quantizing the semiclassical motion of excitons, we show that the Berry
curvature can cause an energy splitting between exciton states with opposite
angular momentum. This splitting is determined by the Berry curvature flux
through the -space area spanned by the relative motion of the
electron-hole pair in the exciton wave function. Using the gapped
two-dimensional Dirac equation as a model, we show that this splitting can be
understood as an effective spin-orbit coupling effect. In addition, there is
also an energy shift caused by other "relativistic" terms. Our result reveals
the limitation of the venerable hydrogenic model of excitons, and highlights
the importance of the Berry curvature in the effective mass approximation.Comment: 4.5 pages, 2 figures, reference updated and minor change
Forced Oscillation Source Location via Multivariate Time Series Classification
Precisely locating low-frequency oscillation sources is the prerequisite of
suppressing sustained oscillation, which is an essential guarantee for the
secure and stable operation of power grids. Using synchrophasor measurements, a
machine learning method is proposed to locate the source of forced oscillation
in power systems. Rotor angle and active power of each power plant are utilized
to construct multivariate time series (MTS). Applying Mahalanobis distance
metric and dynamic time warping, the distance between MTS with different phases
or lengths can be appropriately measured. The obtained distance metric,
representing characteristics during the transient phase of forced oscillation
under different disturbance sources, is used for offline classifier training
and online matching to locate the disturbance source. Simulation results using
the four-machine two-area system and IEEE 39-bus system indicate that the
proposed location method can identify the power system forced oscillation
source online with high accuracy.Comment: 5 pages, 3 figures. Accepted by 2018 IEEE/PES Transmission and
Distribution Conferenc
Intervalley Scattering and Localization Behaviors of Spin-Valley Coupled Dirac Fermions
We study the quantum diffusive transport of multivalley massive Dirac cones,
where time-reversal symmetry requires opposite spin orientations in
inequivalent valleys. We show that the intervalley scattering and intravalley
scattering can be distinguished from the quantum conductivity that corrects the
semiclassical Drude conductivity, due to their distinct symmetries and
localization trends. In immediate practice, it allows transport measurements to
estimate the intervalley scattering rate in hole-doped monolayers of group-VI
transition metal dichalcogenides (e.g., molybdenum dichalcogenides and tungsten
dichalcogenides), an ideal class of materials for valleytronics applications.
The results can be generalized to a large class of multivalley massive Dirac
systems with spin-valley coupling and time-reversal symmetry.Comment: 5 pages+4 pages of supplemental materials, 4 figure
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