3,360 research outputs found
Pressure-Induced Topological Phase Transitions in CdGeSb and CdSnSb
Topological quantum phase transitions (TQPTs) in a material induced by
external perturbations are often characterized by band touching points in the
Brillouin zone. The low-energy excitations near the degenerate band touching
points host different types of fermions while preserving the topological
protection of surface states. An interplay of different tunable topological
phases offers an insight into the evolution of the topological character. In
this paper, we study the occurrence of TQPTs as a function of hydrostatic
pressure in CdGeSb and CdSnSb chalcopyrites, using the first-principles
calculations. At ambient pressure, both materials are topological insulators
having a finite band gap with inverted order of Sb- and Sb-,
orbitals of valence bands at the point. On the application of
hydrostatic pressure the band gap reduces, and at the critical point of the
phase transition, these materials turn into Dirac semimetals. On further
increasing the pressure beyond the critical point, the band inversion is
reverted making them trivial insulators. The pressure-induced change in band
topology from non-trivial to trivial phase is also captured by L\"{u}ttinger
model Hamiltonian calculations. Our model demonstrates the critical role played
by a pressure-induced anisotropy in frontier bands in driving the phase
transitions. These theoretical findings of peculiar coexistence of multiple
topological phases in the same material provide a realistic and promising
platform for the experimental realization of the TQPT.Comment: 7 pages, 7 figure
Integer programming based solution approaches for the train dispatching problem
Railroads face the challenge of competing with the trucking industry in a fastpaced environment. In this respect, they are working toward running freight trains on schedule and reducing travel times. The planned train schedules consist of departure and arrival times at main stations on the rail network. A detailed timetable, on the other hand, consists of the departure and arrival times of each train in each track section of its route. The train dispatching problem aims to determine detailed timetables over a rail network in order to minimize deviations from the planned schedule. We provide a new integer programming formulation for this problem based on a spacetime network; we propose heuristic algorithms to solve it and present computational results of these algorithms. Our approach includes some realistic constraints that have not been previously considered as well as all the assumptions and practical issues considered by the earlier works
Multiple Triple-Point Fermions in Heusler Compounds
Using the density functional theoretical calculations, we report a new set of
topological semimetals XYZ (X = \{Cu, Rh, Pd, Ag, Au, Hg\}, Y = \{Li, Na,
Sc, Zn, Y, Zr, Hf, La, Pr, Pm, Sm, Tb, Dy, Ho, Tm\} and Z =\{Mg, Al, Zn, Ga, Y,
Ag, Cd, In, Sn, Ta, Sm\}), which show the existence of multiple topological
triple point fermions along four independent axes. These fermionic
quasiparticles have no analogues elementary particle in the standard model. The
angle-resolved photoemission spectroscopy is simulated to obtain the exotic
topological surface states and the characteristic Fermi arcs. The inclusion of
spin-orbit coupling splits the triple-point into two Dirac points. The
triple-point fermions are exhibited on the easily cleavable (111) surface and
are well separated from the surface point, allowing them to be
resolved in the surface spectroscopic techniques. This intermediate linearly
dispersive degeneracy between Weyl and Dirac points may offer prospective
candidates for quantum transport applications
Tunnelling Characteristics of Stone-Wales Defects in Monolayers of Sn and Group-V Elements
Topological defects in ultrathin layers are often formed during synthesis and
processing, thereby, strongly influencing their electronic properties . In this
paper, we investigate the role of Stone-Wales (SW) defects in modifying the
electronic properties of the monolayers of Sn and group-V elements. The
calculated results find the electronic properties of stanene (monolayer of Sn
atoms) to be strongly dependent on the concentration of SW-defects e.g.,
defective stanene has nearly zero band gap (~ 0.03 eV) for the defect
concentration of 2.2 x 10^13 cm^-2 which opens up to 0.2 eV for the defect
concentration of 3.7 x 10^13 cm^-2. In contrast, SW-defects appear to induce
conduction states in the semiconducting monolayers of group-V elements. These
conduction states act as channels for electron tunnelling, and the calculated
tunnelling characteristics show the highest differential conductance for the
negative bias with the asymmetric current-voltage characteristics. On the other
hand, the highest differential conductance was found for the positive bias in
stanene. Simulated STM topographical images of stanene and group-V monolayers
show distinctly different features in terms of their cross-sectional views and
distance-height profiles which can serve as fingerprints to identify the
topological defects in the monolayers of group-IV and group-V elements in
experiments.Comment: 18 pages, 5 figures, 1 tabl
Short-Term and Medium-Term Prospects of Agricultural Sector in Gujarat – Some Policy Recommendations
By mid-September, 2003 it is becoming clear that Gujarat is likely to experience a bumper crop in the year 2003-04. The State Ministry of Agriculture is looking for some concrete suggestions, advice and policy recommendations to better manage the situation likely to be created by the bumper crop this year. Falling prices in the face of bumper crop can considerably wipe out positive effect on agricultural incomes in the hands of farmers in the State. Short-term measures to avoid such a situation need to be integrated into medium term and long term development strategy for the State agricultural sector. The present paper provides some implementable policy recommendations in this context.
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