19 research outputs found
Proximity Enhanced Quantum Spin Hall State in Graphene
Graphene is the first model system of two-dimensional topological insulator
(TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in
graphene, however, has eluded direct experimental detection because of its
extremely small energy gap due to the weak spin-orbit coupling. Here we predict
by ab initio calculations a giant (three orders of magnitude) proximity induced
enhancement of the TI energy gap in the graphene layer that is sandwiched
between thin slabs of Sb2Te3 (or MoTe2). This gap (1.5 meV) is accessible by
existing experimental techniques, and it can be further enhanced by tuning the
interlayer distance via compression. We reveal by a tight-binding study that
the QSH state in graphene is driven by the Kane-Mele interaction in competition
with Kekul\'e deformation and symmetry breaking. The present work identifies a
new family of graphene-based TIs with an observable and controllable bulk
energy gap in the graphene layer, thus opening a new avenue for direct
verification and exploration of the long-sought QSH effect in graphene.Comment: 4 figures in Carbon, 201
Giant spin-vorticity coupling excited by shear-horizontal surface acoustic waves
A non-magnetic layer can inject spin-polarized currents into an adjacent
ferromagnetic layer via spin vorticity coupling (SVC), inducing spin wave
resonance (SWR). In this work, we present the theoretical model of SWR
generated by shear-horizontal surface acoustic wave (SH-SAW) via SVC, which
contains distinct vorticities from well-studied Rayleigh SAW. Both Rayleigh-
and SH-SAW delay lines have been designed and fabricated with a Ni81Fe19/Cu
bilayer integrated on ST-cut quartz. Given the same wavelength, the measured
power absorption of SH-SAW is four orders of magnitudes higher than that of the
Rayleigh SAW. In addition, a high-order frequency dependence of the SWR is
observed in the SH-SAW, indicating SVC can be strong enough to compare with
magnetoelastic coupling
Pairing in graphene: A quantum Monte Carlo study
To address the issue of electron correlation driven superconductivity in
graphene, we perform a systematic quantum Monte Carlo study of the pairing
correlation in the t-U-V Hubbard model on a honeycomb lattice. For V=0 and
close to half filling, we find that pairing with d+id symmetry dominates
pairing with extended-s symmetry. However, as the system size or the on-site
Coulomb interaction increases, the long-range part of the d+id pairing
correlation decreases and tends to vanish in the thermodynamic limit. An
inclusion of nearest-neighbor interaction V, either repulsive or attractive,
has a small effect on the extended-s pairing correlation, but strongly
suppresses the d+id pairing correlation.Comment: 5 pages, 5 figure
Graphene-based topological insulator with an intrinsic bulk band gap above room temperature
Topological insulators (TIs) represent a new quantum state of matter
characterized by robust gapless states inside the insulating bulk gap. The
metallic edge states of a two-dimensional (2D) TI, known as quantum spin Hall
(QSH) effect, are immune to backscattering and carry fully spin-polarized
dissipationless currents. However, existing 2D TIs realized in HgTe and
InAs/GaSb suffer from small bulk gaps (<10 meV) well below room temperature,
thus limiting their application in electronic and spintronic devices. Here, we
report a new 2D TI comprising a graphene layer sandwiched between two Bi2Se3
slabs that exhibits a large intrinsic bulk band gap of 30 to 50 meV, making it
viable for room-temperature applications. Distinct from previous strategies for
enhancing the intrinsic spin-orbit coupling effect of the graphene lattice, the
present graphene-based TI operates on a new mechanism of strong inversion
between graphene Dirac bands and Bi2Se3 conduction bands. Strain engineering
leads to effective control and substantial enhancement of the bulk gap.
Recently reported synthesis of smooth graphene/Bi2Se3 interfaces demonstrates
feasibility of experimental realization of this new 2D TI structure, which
holds great promise for nanoscale device applications.Comment: 3 figures, 1 tabl
Opening Band Gap without Breaking Lattice Symmetry: A New Route toward Robust Graphene-Based Nanoelectronics
Developing graphene-based nanoelectronics hinges on opening a band gap in the
electronic structure of graphene, which is commonly achieved by breaking the
inversion symmetry of the graphene lattice via an electric field (gate bias) or
asymmetric doping of graphene layers. Here we introduce a new design strategy
that places a bilayer graphene sheet sandwiched between two cladding layers of
materials that possess strong spin-orbit coupling (e.g., Bi2Te3). Our ab initio
and tight-binding calculations show that proximity enhanced spin-orbit coupling
effect opens a large (44 meV) band gap in bilayer graphene without breaking its
lattice symmetry, and the band gap can be effectively tuned by interlayer
stacking pattern and significantly enhanced by interlayer compression. The
feasibility of this quantum-well structure is demonstrated by recent
experimental realization of high-quality heterojunctions between graphene and
Bi2Te3, and this design also conforms to existing fabrication techniques in the
semiconductor industry. The proposed quantum-well structure is expected to be
especially robust since it does not require an external power supply to open
and maintain a band gap, and the cladding layers provide protection against
environmental degradation of the graphene layer in its device applications
Intermediate ferroelectric orthorhombic and monoclinic MB phases in [110]- electric field cooled Pb(Mg1/3Nb2/3)O3-30%PbTiO3 crystals
Structural phase transformations of [110] electric field cooled
Pb(Mg1/3Nb2/3)O3-30%PbTiO3 (PMN-30%PT) crystals have been performed by x-ray
diffraction in a field-cooled (FC) condition. A phase sequence of
cubic(C)-tetragonal(T)-orthorhombic (O)-monoclinic (MB) was found on
field-cooling (FC); and a R-MB-O one was observed with increasing field
beginning from the zero field-cooled (ZFC) condition at room temperature. The
application of the [110] electric field induced a dramatic change in the phase
sequence in the FC condition, compared to the corresponding data for PMN-30%PT
crystals in a [001] field, which shows that the phase sequence in the FC
condition is altered by the crystallographic direction along which a modest
electric field (E) is applied. Only when E is applied along [110] are
intermediate O and MB phases observed.Comment: 10 pages, 8 figure
Identification of Rice Transcription Factors Associated with Drought Tolerance Using the Ecotilling Method
The drought tolerance (DT) of plants is a complex quantitative trait. Under natural and artificial selection, drought tolerance represents the crop survival ability and production capacity under drought conditions (Luo, 2010). To understand the regulation mechanism of varied drought tolerance among rice genotypes, 95 diverse rice landraces or varieties were evaluated within a field screen facility based on the ‘line–source soil moisture gradient’, and their resistance varied from extremely resistant to sensitive. The method of Ecotype Targeting Induced Local Lesions in Genomes (Ecotilling) was used to analyze the diversity in the promoters of 24 transcription factor families. The bands separated by electrophoresis using Ecotilling were converted into molecular markers. STRUCTURE analysis revealed a value of K = 2, namely, the population with two subgroups (i.e., indica and japonica), which coincided very well with the UPGMA clusters (NTSYS-pc software) using distance-based analysis and InDel markers. Then the association analysis between the promoter diversity of these transcription factors and the DT index/level of each variety was performed. The results showed that three genes were associated with the DT index and that five genes were associated with the DT level. The sequences of these associated genes are complex and variable, especially at approximately 1000 bp upstream of the transcription initiation sites. The study illuminated that association analysis aimed at Ecotilling diversity of natural groups could facilitate the isolation of rice genes related to complex quantitative traits
Laser-induced ultrasonic measurements for the detection and reconstruction of surface defects
Laser-induced ultrasonic measurement is a non-contact non-destructive technology that can be employed for the testing and assessment of surface defects. In order to improve the correct identification of defects, the full matrix capture (FMC) and total focusing method (TFM) are applied on the imaging process. FMC data includes A-scans resulting from the combination of all measurement axes defined by the sequential generation and detection of utilized laser beams in the system. In this paper, an aluminium block with four holes whose diameters range from 1Â mm to 2.5Â mm is assessed through B-scans, the synthetic aperture focusing technique (SAFT) and FMC/TFM. The results demonstrate that the FMC/TFM technology can significantly improve the imaging quality and signal-to-noise ratio (SNR). In addition, this method has higher lateral resolution and larger imaging range compared with traditional B-scans
Polar EuO(111) on Ir(111): A two-dimensional oxide
Through reactive molecular-beam epitaxy or Eu postoxidation and postannealing, large EuO(111) bilayer islands of high quality and exceptional stability are grown on Ir(111). We use scanning tunneling microscopy, low-energy electron diffraction, magneto-optical Kerr effect measurements, and density functional theory to characterize the properties of this ultrathin polar rare-earth metal oxide in atomistic detail. We analyze the crystallographic properties and their growth dependence, the film morphology including the atomic structure of defects, the mechanisms for reduction in the electrostatic potential related to the film polarity, as well as the work function, thermal stability, and magnetic properties, resulting in a comprehensive picture of this new two-dimensional material