142,108 research outputs found
The measurement of science and technology in China.
This paper introduced the background about the measurement of science and technology in China and selectively introduced the most recent statistic results released by the Institute of Scientific and Technical Information of China.China; Science and technology; Measurement;
Conductivity of Dirac fermions with phonon induced topological crossover
We study the Hall conductivity in single layer gapped Dirac fermion materials
including coupling to a phonon field, which not only modifies the
quasi-particle dynamics through the usual self-energy term but also
renormalizes directly the gap. Consequently the Berry curvature is modified. As
the temperature is increased the sign of the renormalized gap can change and
the material can cross over from a band insulator to a topological insulator at
higher temperature (T). The effective Chern numbers defined for valley and spin
Hall conductivity show a rich phase diagram with increasing temperature. While
the spin and valley DC Hall conductivity is no longer quantized at elevated
temperature a change in sign with increasing T is a clear indication of a
topological crossover. The chirality of the circularly polarized light which is
dominantly absorbed by a particular valley can change with temperature as a
result of a topological crossover.Comment: 6 pages, 5 figures, to appear in Phys. Rev.
Electron-phonon correlations on spin texture of gapped helical Dirac Fermions
The metallic surface states of a topological insulator support helical Dirac
fermions protected by topology with their spin locked perpendicular to their
momentum. They can acquire mass through magnetic doping or through
hybridization of states on opposite faces of a thin sample. In this case there
can be a component of electron spin oriented perpendicular to the surface
plane. The electron-phonon interaction renormalizes the dynamics of the charge
carriers through their spectral density. It also modifies the gap channel and a
second spectral function enters which, not only determines the out of plane
spin component, but also comes into in-plane properties. While the out of plane
spin component is decreased below the Fermi momentum (), the in plane
component increases. There are also correlation tails extending well beyond
. The angular resolved photo-emission line shapes aquire Holstein side
bands. The effective gap in the density of states is reduced and the optical
conductivity aquires distinct measurable phonon structure even for modest value
of the electron-phonon coupling.Comment: 9pages, 9 figure
Early Turn-taking Prediction with Spiking Neural Networks for Human Robot Collaboration
Turn-taking is essential to the structure of human teamwork. Humans are
typically aware of team members' intention to keep or relinquish their turn
before a turn switch, where the responsibility of working on a shared task is
shifted. Future co-robots are also expected to provide such competence. To that
end, this paper proposes the Cognitive Turn-taking Model (CTTM), which
leverages cognitive models (i.e., Spiking Neural Network) to achieve early
turn-taking prediction. The CTTM framework can process multimodal human
communication cues (both implicit and explicit) and predict human turn-taking
intentions in an early stage. The proposed framework is tested on a simulated
surgical procedure, where a robotic scrub nurse predicts the surgeon's
turn-taking intention. It was found that the proposed CTTM framework
outperforms the state-of-the-art turn-taking prediction algorithms by a large
margin. It also outperforms humans when presented with partial observations of
communication cues (i.e., less than 40% of full actions). This early prediction
capability enables robots to initiate turn-taking actions at an early stage,
which facilitates collaboration and increases overall efficiency.Comment: Submitted to IEEE International Conference on Robotics and Automation
(ICRA) 201
Longitudinal and spin/valley Hall optical conductivity in single layer
A monolayer of has a non-centrosymmetric crystal structure, with
spin polarized bands. It is a two valley semiconductor with direct gap falling
in the visible range of the electromagnetic spectrum. Its optical properties
are of particular interest in relation to valleytronics and possible device
applications. We study the longitudinal and the transverse Hall dynamical
conductivity which is decomposed into charge, spin and valley contributions.
Circular polarized light associated with each of the two valleys separately is
considered and results are filtered according to spin polarization. Temperature
can greatly change the spin admixture seen in the frequency window where they
are not closely in balance.Comment: 8 pages, 5 figures, accepted by Phys. Rev.
Phonon structure in dispersion curves and density of states of massive Dirac Fermions
Dirac fermions exist in many solid state systems including graphene, silicene
and other two dimensional membranes such as are found in group VI
dichalcogenides, as well as on the surface of some insulators where such states
are protected by topology. Coupling of those fermions to phonons introduces new
structures in their dispersion curves and, in the case of massive Dirac
fermions, can shift and modify the gap. We show how these changes present in
angular-resolved photoemission spectroscopy of the dressed charge carrier
dispersion curves and scanning tunneling microscopy measurements of their
density of states. In particular we focus on the region around the band gap. In
this region the charge carrier spectral density no longer consists of a
dominant quasiparticle peak and a smaller incoherent phonon related background.
The quasiparticle picture has broken down and this leads to important
modification in both dispersion curves and density of states.Comment: 12 pages, 6 figures, to appear in PR
Hexagonal warping on spin texture, Hall conductivity and circular dichroism of Topological Insulator
The topological protected electronic states on the surface of a topological
insulator can progressively change their Fermi cross-section from circular to a
snowflake shape as the chemical potential is increased above the Dirac point
because of an hexagonal warping term in the Hamiltonian. Another effect of
warping is to change the spin texture which exists when a finite gap is
included by magnetic doping, although the in-plane spin component remains
locked perpendicular to momentum. It also changes the orbital magnetic moment,
the matrix element for optical absorption and the circular dichroism. We find
that the Fermi surface average of z-component of spin is closely related to the
value of the Berry phase. This holds even when the Hamiltonian includes a
subdominant non-relativistic quadratic in momentum term (which provides
particle-hole asymmetry) in addition to the dominant relativistic Dirac term.
There is also a qualitative correlation between and the dichroism. For the case when the chemical potential
falls inside the gap between valence and conduction band, the Hall conductivity
remains quantized and unaffected in value by the hexagonal warping term.Comment: 10 figures, accepted in PR
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