291 research outputs found
Weak Topological Insulators in PbTe/SnTe Superlattices
It is desirable to realize topological phases in artificial structures by
engineering electronic band structures. In this paper, we investigate
superlattices along [001] direction and find a robust
weak topological insulator phase for a large variety of layer numbers m and
2n-m. We confirm this topologically non-trivial phase by calculating Z2
topological invariants and topological surface states based on the
first-principles calculations. We show that the folding of Brillouin zone due
to the superlattice structure plays an essential role in inducing topologically
non-trivial phases in this system. This mechanism can be generalized to other
systems in which band inversion occurs at multiple momenta, and gives us a
brand-new way to engineer topological materials in artificial structures.Comment: 6 pages, 4 figures, another author adde
Treating Coulomb exchange contributions in relativistic mean field calculations: why and how
The energy density functional (EDF) method is very widely used in nuclear
physics, and among the various existing functionals those based on the
relativistic Hartree (RH) approximation are very popular because the exchange
contributions (Fock terms) are numerically rather onerous to calculate.
Although it is possible to somehow 'mock up' the effects of meson-induced
exchange terms by adjusting the meson-nucleon couplings, the lack of Coulomb
exchange contributions hampers the accuracy of predictions. In this note, we
show that the Coulomb exchange effects can be easily included with a good
accuracy in a perturbative approach. Therefore, it would be desirable for
future relativistic EDF models to incorporate Coulomb exchange effects, at
least to some order of perturbation.Comment: 3 pages, 1 figure, Proceedings of the 20th Nuclear Physics Workshop
"Marie & Pierre Curie", Kazimierz, Poland, 25-29 September, 201
Spin-filtered Edge States with an Electrically Tunable Gap in a Two-Dimensional Topological Crystalline Insulator
Three-dimensional topological crystalline insulators were recently predicted
and observed in the SnTe class of IV-VI semiconductors, which host metallic
surface states protected by crystal symmetries. In this work, we study thin
films of these materials and expose their potential for device applications. We
demonstrate that thin films of SnTe and Pb(1-x)Sn(x)Se(Te) grown along the
(001) direction are topologically nontrivial in a wide range of film thickness
and carry conducting spin-filtered edge states that are protected by the (001)
mirror symmetry via a topological invariant. Application of an electric field
perpendicular to the film will break the mirror symmetry and generate a band
gap in these edge states. This functionality motivates us to propose a novel
topological transistor device, in which charge and spin transport are maximally
entangled and simultaneously controlled by an electric field. The high on/off
operation speed and coupling of spin and charge in such a device may lead to
electronic and spintronic applications for topological crystalline insulators.Comment: 6 pages, 5 figures, minor changes made, accepted to Nature Material
Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding
We report on a strong photoluminescence (PL) enhancement of monolayer MoS2
through defect engineering and oxygen bonding. Micro- PL and Raman images
clearly reveal that the PL enhancement occurs at cracks/defects formed during
high temperature vacuum annealing. The PL enhancement at crack/defect sites
could be as high as thousands of times after considering the laser spot size.
The main reasons of such huge PL enhancement include: (1) the oxygen chemical
adsorption induced heavy p doping and the conversion from trion to exciton; (2)
the suppression of non-radiative recombination of excitons at defect sites as
verified by low temperature PL measurements. First principle calculations
reveal a strong binding energy of ~2.395 eV for oxygen molecule adsorbed on an
S vacancy of MoS2. The chemical adsorbed oxygen also provides a much more
effective charge transfer (0.997 electrons per O2) compared to physical
adsorbed oxygen on ideal MoS2 surface. We also demonstrate that the defect
engineering and oxygen bonding could be easily realized by oxygen plasma
irradiation. X-ray photoelectron spectroscopy further confirms the formation of
Mo-O bonding. Our results provide a new route for modulating the optical
properties of two dimensional semiconductors. The strong and stable PL from
defects sites of MoS2 may have promising applications in optoelectronic
devices.Comment: 23 pages, 9 figures, to appear in ACS Nan
Sequence Design for Cognitive CDMA Communications under Arbitrary Spectrum Hole Constraint
To support interference-free quasi-synchronous code-division multiple-access
(QS-CDMA) communication with low spectral density profile in a cognitive radio
(CR) network, it is desirable to design a set of CDMA spreading sequences with
zero-correlation zone (ZCZ) property. However, traditional ZCZ sequences (which
assume the availability of the entire spectral band) cannot be used because
their orthogonality will be destroyed by the spectrum hole constraint in a CR
channel. To date, analytical construction of ZCZ CR sequences remains open.
Taking advantage of the Kronecker sequence property, a novel family of
sequences (called "quasi-ZCZ" CR sequences) which displays zero
cross-correlation and near-zero auto-correlation zone property under arbitrary
spectrum hole constraint is presented in this paper. Furthermore, a novel
algorithm is proposed to jointly optimize the peak-to-average power ratio
(PAPR) and the periodic auto-correlations of the proposed quasi-ZCZ CR
sequences. Simulations show that they give rise to single-user bit-error-rate
performance in CR-CDMA systems which outperform traditional non-contiguous
multicarrier CDMA and transform domain communication systems; they also lead to
CR-CDMA systems which are more resilient than non-contiguous OFDM systems to
spectrum sensing mismatch, due to the wideband spreading.Comment: 13 pages,10 figures,Accepted by IEEE Journal on Selected Areas in
Communications (JSAC)--Special Issue:Cognitive Radio Nov, 201
Experimental observation of Dirac-like surface states and topological phase transition in PbSnTe(111) films
The surface of a topological crystalline insulator (TCI) carries an even
number of Dirac cones protected by crystalline symmetry. We epitaxially grew
high quality PbSnTe(111) films and investigated the TCI phase by
in-situ angle-resolved photoemission spectroscopy. PbSnTe(111)
films undergo a topological phase transition from trivial insulator to TCI via
increasing the Sn/Pb ratio, accompanied by a crossover from n-type to p-type
doping. In addition, a hybridization gap is opened in the surface states when
the thickness of film is reduced to the two-dimensional limit. The work
demonstrates an approach to manipulating the topological properties of TCI,
which is of importance for future fundamental research and applications based
on TCI
Compressive performance of fiber reinforced polymer encased recycled concrete with nanoparticles
Nanomaterials have been used in improving the performance of construction materials due to their compacting micro-structure effect and accelerating cement hydration reaction. Considering the brittle characteristic of fiber reinforced polymer (termed as FRP) tube encased concrete and inferior properties of recycled concrete, nanoparticles were used in FRP tube encased recycled aggregate concrete. The axial compressive performance of FRP tube used in recycled concrete treated with nanoparticles strengthening, termed as FRP-NPRC, were investigated by axial compression experiments and theoretical analysis. Five experimental variables were considered including (1) the dosages and (2) varieties of nanoparticles (i.e. 1% and 2% of nanoSiO2, 1% and 2% of nanoCaCO3), (3) replacement ratios of recycled coarse aggregates (termed as RCAs) (0%, 50%, 70% and 100%) the RCAs were mainly produced from the waste cracked bricks, (4) the number of glass FRP (GFRP) tube layers (2, 4 and 6-layer) and (5) the mixing methods of concrete. Results indicate that the combination of FRP confinement and nanoparticle modification in recycled concrete exhibited up to 76.2% increase in compressive strength and 7.62 times ductility improvement. Furthermore, a design-oriented stress–strain model on the basis of the ultimate condition analysis were executed to evaluate the stress–strain property of this strengthened component
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