49 research outputs found
Phase Composition of Mo-Si-V Hypoeutectic Alloys
Thermodynamic modeling (TDM) of phase formation was performed with vanadium doping of the hypoeutectic Mo-Si alloy. It was found that the thermochemical properties of vanadium silicides (presented in the HSC Chemistry 6.12 database), when modeling Mo-Si(14.5-12.2)-V(5.0-20.0) alloys, lead to inadequate results regarding Mo-Si-V diagram state indicators. The simulation results agree satisfactorily with the Mo-Si-V diagram with the following values of ΔH0 298: for V3Si = - 180.4 kJ / mol, for V5Si3 = -433.6 kJ / mol, for VSi2 = -124.5 kJ / mol. According to the results of TDM and X-ray phase analysis (XRD) of the obtained alloys, it was found that vanadium in Mo-Si-V ternary alloys can be found both in the form of silicides, (Mo,V)3Si, and in the composition of the solid solution (Mo,V
Topological properties of superconducting junctions
Motivated by recent developments in the field of one-dimensional topological
superconductors, we investigate the topological properties of s-matrix of
generic superconducting junctions where dimension should not play any role. We
argue that for a finite junction the s-matrix is always topologically trivial.
We resolve an apparent contradiction with the previous results by taking into
account the low-energy resonant poles of s-matrix. Thus no common topological
transition occur in a finite junction. We reveal a transition of a different
kind that concerns the configuration of the resonant poles
Penetration of hot electrons through a cold disordered wire
We study a penetration of an electron with high energy E<<T through strongly
disordered wire of length L<<a (a being the localization length). Such an
electron can loose, but not gain the energy, when hopping from one localized
state to another. We have found a distribution function for the transmission
coefficient t. The typical t remains exponentially small in L/a, but with the
decrement, reduced compared to the case of direct elastic tunnelling. The
distribution function has a relatively strong tail in the domain of anomalously
high t; the average ~(a/L)^2 is controlled by rare configurations of
disorder, corresponding to this tail.Comment: 4 pages, 5 figure
Zero-voltage conductance peak from weak antilocalization in a Majorana nanowire
We show that weak antilocalization by disorder competes with resonant Andreev
reflection from a Majorana zero-mode to produce a zero-voltage conductance peak
of order e^2/h in a superconducting nanowire. The phase conjugation needed for
quantum interference to survive a disorder average is provided by particle-hole
symmetry - in the absence of time-reversal symmetry and without requiring a
topologically nontrivial phase. We identify methods to distinguish the Majorana
resonance from the weak antilocalization effect.Comment: 13 pages, 8 figures. Addendum, February 2014: Appendix B shows
results for weak antilocalization in the circular ensemble. (This appendix is
not in the published version.
Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions
The rise of topological phases of matter is strongly connected to their
potential to host Majorana bound states, a powerful ingredient in the search
for a robust, topologically protected, quantum information processing. In order
to produce such states, a method of choice is to induce superconductivity in
topological insulators. The engineering of the interplay between
superconductivity and the electronic properties of a topological insulator is a
challenging task and it is consequently very important to understand the
physics of simple superconducting devices such as Josephson junctions, in which
new topological properties are expected to emerge. In this article, we review
recent experiments investigating topological superconductivity in topological
insulators, using microwave excitation and detection techniques. More
precisely, we have fabricated and studied topological Josephson junctions made
of HgTe weak links in contact with two Al or Nb contacts. In such devices, we
have observed two signatures of the fractional Josephson effect, which is
expected to emerge from topologically-protected gapless Andreev bound states.
We first recall the theoretical background on topological Josephson junctions,
then move to the experimental observations. Then, we assess the topological
origin of the observed features and conclude with an outlook towards more
advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017,
published in "Topological Matter. Springer Series in Solid-State Sciences,
vol 190. Springer
Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device
We report on the observation of excitation of Majorana fermions in a Nb-InSb
nanowire quantum dot-Nb hybrid system. The InSb nanowire quantum dot is formed
between the two Nb contacts by weak Schottky barriers and is thus in the regime
of strong couplings to the contacts. Due to the proximity effect, the InSb
nanowire segments covered by superconductor Nb contacts turn to superconductors
with a superconducting energy gap . Under an applied magnetic field
larger than a critical value for which the Zeeman energy in the InSb nanowire
is , the entire InSb nanowire is found to be in a nontrivial
topological superconductor phase, supporting a pair of Majorana fermions, and
Cooper pairs can transport between the superconductor Nb contacts via the
Majorana fermion states. This transport process will be suppressed when the
applied magnetic field becomes larger than a second critical value at which the
transition to a trivial topological superconductor phase occurs in the system.
This physical scenario has been observed in our experiment. We have found that
the measured zero-bias conductance for our hybrid device shows a conductance
plateau in a range of the applied magnetic field in quasi-particle Coulomb
blockade regions.Comment: 7 pages, 4 figures, supplementary materials of 3 pages and 1 figur