1,634 research outputs found
Spin Hall effect in spin-valley coupled monolayer transition-metal dichalcogenides
We study both the intrinsic and extrinsic spin Hall effect in spin-valley
coupled monolayers of transition metal dichalcogenides. We find that whereas
the skew-scattering contribution is suppressed by the large band gap, the
side-jump contribution is comparable to the intrinsic one with opposite sign in
the presence of scalar and magnetic scattering. Intervalley scattering tends to
suppress the side-jump contribution due to the loss of coherence. By tuning the
ratio of intra- to intervalley scattering, the spin Hall conductivity shows a
sign change in hole-doped samples. Multiband effect in other doping regime is
considered, and it is found that the sign change exists in the heavily
hole-doped regime, but not in the electron-doped regime
The edge engineering of topological Bi(111) bilayer
A topological insulator is a novel quantum state, characterized by
symmetry-protected non-trivial edge/surface states. Our first-principle
simulations show the significant effects of the chemical decoration on edge
states of topological Bi(111) bilayer nanoribbon, which remove the trivial edge
state and recover the Dirac linear dispersion of topological edge state. By
comparing the edge states with and without chemical decoration, the Bi(111)
bilayer nanoribbon offers a simple system for assessing conductance fluctuation
of edge states. The chemical decoration can also modify the penetration depth
and the spin texture of edge states. A low-energy effective model is proposed
to explain the distinctive spin texture of Bi(111) bilayer nanoribbon, which
breaks the spin-momentum orthogonality along the armchair edge.Comment: 5 pages, 5 figure
Two energy scales and close relationship between the pseudogap and superconductivity in underdoped cuprate superconductors
By measuring the low temperature specific heat, the low energy quasi-particle
excitation has been derived and analyzed in systematically doped
LaSrCuO single crystals. The Volovik's relation predicted
for a d-wave superconductor has been well demonstrated in wide doping regime,
showing a robust evidence for the d-wave pairing symmetry. Furthermore the
nodal gap slope of the superconducting gap is derived and is found
to follow the same doping dependence of the pseudogap obtained from ARPES and
tunnelling measurement. This strongly suggests a close relationship between the
pseudogap and superconductivity. Taking the entropy conservation into account,
we argue that the ground state of the pseudogap phase should have Fermi arcs
with finite density of states at zero K, and the transport data show that it
behaves like an insulator due to probably weak localization. A nodal metal
picture for the pseudogap phase cannot interpret the data. Based on the Fermi
arc picture for the pseudogap phase it is found that the superconducting energy
scale or in underdoped regime is governed by both the maximum gap and the
spectral weight from the Fermi arcs. This suggests that there are two energy
scales: superconducting energy scale and the pseudogap. The superconductivity
may be formed by the condensation of Fermi arc quasiparticles through pairing
by exchanging virtue bosons.Comment: 4 pages, 5 figure
Geometric curvatures of plane symmetry black hole
In this paper, we study the properties and thermodynamic stability of the
plane symmetry black hole from the viewpoint of geometry. Weinhold metric and
Ruppeiner metric are obtained, respectively. The Weinhold curvature gives phase
transition points, which correspond to the first-order phase transition only at
N=1, where is a parameter in the plane symmetry black hole. While the
Ruppeiner one shows first-order phase transition points for arbitrary . Both of which give no any information about the second-order phase
transition. Considering the Legendre invariant proposed by Quevedo et. al., we
obtain a unified geometry metric, which gives a correctly the behavior of the
thermodynamic interactions and phase transitions. The geometry is also found to
be curved and the scalar curvature goes to negative infinity at the Davies'
phase transition points when the logarithmic correction is included.Comment: 16 pages, 6 figure
Determining layer number of two dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrate
Transition-metal dichalcogenide (TMD) semiconductors have been widely studied
due to their distinctive electronic and optical properties. The property of TMD
flakes is a function of its thickness, or layer number (N). How to determine N
of ultrathin TMDs materials is of primary importance for fundamental study and
practical applications. Raman mode intensity from substrates has been used to
identify N of intrinsic and defective multilayer graphenes up to N=100.
However, such analysis is not applicable for ultrathin TMD flakes due to the
lack of a unified complex refractive index () from monolayer to bulk
TMDs. Here, we discuss the N identification of TMD flakes on the SiO/Si
substrate by the intensity ratio between the Si peak from 100-nm (or 89-nm)
SiO/Si substrates underneath TMD flakes and that from bare SiO/Si
substrates. We assume the real part of of TMD flakes as that of
monolayer TMD and treat the imaginary part of as a fitting
parameter to fit the experimental intensity ratio. An empirical ,
namely, , of ultrathin MoS, WS and WSe
flakes from monolayer to multilayer is obtained for typical laser excitations
(2.54 eV, 2.34 eV, or 2.09 eV). The fitted of MoS has
been used to identify N of MoS flakes deposited on 302-nm SiO/Si
substrate, which agrees well with that determined from their shear and
layer-breathing modes. This technique by measuring Raman intensity from the
substrate can be extended to identify N of ultrathin 2D flakes with N-dependent
. For the application purpose, the intensity ratio excited by
specific laser excitations has been provided for MoS, WS and
WSe flakes and multilayer graphene flakes deposited on Si substrates
covered by 80-110 nm or 280-310 nm SiO layer.Comment: 10 pages, 4 figures. Accepted by Nanotechnolog
A Low-Pass Filter of Wide Stopband with a Novel Multilayer Fractal Photonic Bandgap Structure
A novel, multilayer fractal PBG is presented here as the substrate for a microstrip line and the resulting configuration builds a low-pass filter of wide stopband. Experimental results in comparison with the corresponding two filters with monolayer PBG show that the proposed filter drastically enhances the width of the stopband
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