8,957 research outputs found
Raman fingerprint of semi-metal WTe2 from bulk to monolayer
Tungsten ditelluride (WTe2), a layered transition-metal dichalcogenide (TMD),
has recently demonstrated an extremely large magnetoresistance effect, which is
unique among TMDs. This fascinating feature seems to be correlated with its
special electronic structure. Here, we report the observation of 6 Raman peaks
corresponding to the A_2^4, A_1^9, A_1^8, A_1^6, A_1^5 and A_1^2 phonons, from
the 33 Raman-active modes predicted for WTe2. This provides direct evidence to
distinguish the space group of WTe2 from that of other TMDs. Moreover, the
Raman evolution of WTe2 from bulk to monolayer is clearly revealed. It is
interesting to find that the A_2^4 mode, centered at ~109.8 cm-1, is forbidden
in a monolayer, which may be attributable to the transition of the point group
from C2v (bulk) to C2h (monolayer). Our work characterizes all observed Raman
peaks in the bulk and few-layer samples and provides a route to study the
physical properties of two-dimensional WTe2.Comment: 19 pages, 4 figures and 2 table
Conductance modulation in spin field-efect transistors under finite bias voltages
The conductance modulations in spin field-effect transistors under finite
bias voltages were studied. It was shown that when a finite bias voltage is
applied between two terminals of a spin field-effect transistor, the spin
precession states of injected spin-polarized electrons in the semiconductor
channel of the device will depend not only the gate-voltage controlled Rashba
spin-orbit coupling but also depend on the bias voltage and, hence, the
conductance modulation in the device due to Rashba spin-orbit coupling may also
depend sensitively on the bias voltage.Comment: 7 pages, 3 figures, to appear in Physical Review B, (April, 2004
Electron Wave Spin in a Quantum Well
The particle-wave duality of the electron poses a principle question of
whether the spin is a property of the particle or the wave. In this paper, the
wave nature of the spin is studied for an electron inside a two-dimensional
quantum well. By solving the exact spinor eigen solution to the Dirac
equation, we show that a stable circulating total current density exists inside
the well with a donut shaped topography. A spin value is modified by the
confining geometry of the well. Our analysis also shows that a free electron
Gaussian wavepacket is unstable and experiences quick decoherence.Comment: 5 pages, 4 figure
Combining Spot and Futures Markets: A Hybrid Market Approach to Dynamic Spectrum Access
Dynamic spectrum access is a new paradigm of secondary spectrum utilization
and sharing. It allows unlicensed secondary users (SUs) to exploit
opportunistically the under-utilized licensed spectrum. Market mechanism is a
widely-used promising means to regulate the consuming behaviours of users and,
hence, achieves the efficient allocation and consumption of limited resources.
In this paper, we propose and study a hybrid secondary spectrum market
consisting of both the futures market and the spot market, in which SUs
(buyers) purchase under-utilized licensed spectrum from a spectrum regulator,
either through predefined contracts via the futures market, or through spot
transactions via the spot market. We focus on the optimal spectrum allocation
among SUs in an exogenous hybrid market that maximizes the secondary spectrum
utilization efficiency. The problem is challenging due to the stochasticity and
asymmetry of network information. To solve this problem, we first derive an
off-line optimal allocation policy that maximizes the ex-ante expected spectrum
utilization efficiency based on the stochastic distribution of network
information. We then propose an on-line VickreyCClarkeCGroves (VCG) auction
that determines the real-time allocation and pricing of every spectrum based on
the realized network information and the pre-derived off-line policy. We
further show that with the spatial frequency reuse, the proposed VCG auction is
NP-hard; hence, it is not suitable for on-line implementation, especially in a
large-scale market. To this end, we propose a heuristics approach based on an
on-line VCG-like mechanism with polynomial-time complexity, and further
characterize the corresponding performance loss bound analytically. We finally
provide extensive numerical results to evaluate the performance of the proposed
solutions.Comment: This manuscript is the complete technical report for the journal
version published in INFORMS Operations Researc
Effects of spin imbalance on the electric-field driven quantum dissipationless spin current in -doped Semiconductors
It was proposed recently by Murakami et al. [Science \textbf{301},
1348(2003)] that in a large class of -doped semiconductors, an applied
electric field can drive a quantum dissipationless spin current in the
direction perpendicular to the electric field. In this paper we investigate the
effects of spin imbalance on this intrinsic Hall effect. We show that in
a real sample with boundaries, due to the presence of spin imbalance near the
edges of the sample, the spin Hall conductivity is not a constant but a
sensitively - quantity, and due to this fact, in order to
take the effects of spin imbalance properly into account, a microscopic
calculation of both the quantum dissipationless spin Hall current and the spin
accumulation on an equal footing is thus required. Based on such a microscopic
calculation, a detailed discussion of the effects of spin imbalance on the
intrinsic spin Hall effect in thin slabs of -doped semiconductors are
presented.Comment: 8 pages, 2 figures, An extended version with detailed calculations To
appear in Phys. Rev.
Mixture of Bilateral-Projection Two-dimensional Probabilistic Principal Component Analysis
The probabilistic principal component analysis (PPCA) is built upon a global
linear mapping, with which it is insufficient to model complex data variation.
This paper proposes a mixture of bilateral-projection probabilistic principal
component analysis model (mixB2DPPCA) on 2D data. With multi-components in the
mixture, this model can be seen as a soft cluster algorithm and has capability
of modeling data with complex structures. A Bayesian inference scheme has been
proposed based on the variational EM (Expectation-Maximization) approach for
learning model parameters. Experiments on some publicly available databases
show that the performance of mixB2DPPCA has been largely improved, resulting in
more accurate reconstruction errors and recognition rates than the existing
PCA-based algorithms
Thrust distribution in Higgs decays at the next-to-leading order and beyond
We present predictions for the thrust distribution in hadronic decays of the
Higgs boson at the next-to-leading order and the approximate
next-to-next-to-leading order. The approximate NNLO corrections are derived
from a factorization formula in the soft/collinear phase-space regions. We find
large corrections, especially for the gluon channel. The scale variations at
the lowest orders tend to underestimate the genuine higher order contributions.
The results of this paper is therefore necessary to control the perturbative
uncertainties of the theoretical predictions. We also discuss on possible
improvements to our results, such as a soft-gluon resummation for the 2-jets
limit, and an exact next-to-next-to-leading order calculation for the
multi-jets region
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