79 research outputs found
Measurement of the spin polarization of the magnetic semiconductor EuS with zero-field and Zeeman-split Andreev reflection spectroscopy
We report measurements of the spin polarization (\textbf{\textit{P}}) of the
concentrated magnetic semiconductor EuS using both zero-field and Zeeman-split
Andreev reflection spectroscopy (ARS) with EuS/Al planar junctions. The
zero-field ARS spectra are well described by the modified (spin-polarized) BTK
model with expected superconducting energy gap and actual measurement
temperature (no additional spectral broadening). The fittings consistently
yield \textbf{\textit{P}} close to 80% regardless of the barrier strength.
Moreover, we performed ARS in the presence of a Zeeman-splitting of the
quasiparticle density of states in Al. To describe the Zeeman-split ARS
spectra, we develop a theoretical model which incorporates the solution to the
Maki-Fulde equations into the modified BTK analysis. The method enables the
determination of the magnitude as well as the sign of \textbf{\textit{P}} with
ARS, and the results are consistent with those from the zero-field ARS. The
experiments extend the utility of field-split superconducting spectroscopy from
tunnel junctions to Andreev junctions of arbitrary barrier strengths.Comment: 6 pages, 4 figure
Evidence of topological superconductivity in planar Josephson junctions
Majorana zero modes are quasiparticle states localized at the boundaries of
topological superconductors that are expected to be ideal building blocks for
fault-tolerant quantum computing. Several observations of zero-bias conductance
peaks measured in tunneling spectroscopy above a critical magnetic field have
been reported as experimental indications of Majorana zero modes in
superconductor/semiconductor nanowires. On the other hand, two dimensional
systems offer the alternative approach to confine Ma jorana channels within
planar Josephson junctions, in which the phase difference {\phi} between the
superconducting leads represents an additional tuning knob predicted to drive
the system into the topological phase at lower magnetic fields. Here, we report
the observation of phase-dependent zero-bias conductance peaks measured by
tunneling spectroscopy at the end of Josephson junctions realized on a InAs/Al
heterostructure. Biasing the junction to {\phi} ~ {\pi} significantly reduces
the critical field at which the zero-bias peak appears, with respect to {\phi}
= 0. The phase and magnetic field dependence of the zero-energy states is
consistent with a model of Majorana zero modes in finite-size Josephson
junctions. Besides providing experimental evidence of phase-tuned topological
superconductivity, our devices are compatible with superconducting quantum
electrodynamics architectures and scalable to complex geometries needed for
topological quantum computing.Comment: main text and extended dat
Closing of the Induced Gap in a Hybrid Superconductor-Semiconductor Nanowire
Hybrid superconductor-semiconductor nanowires are predicted to undergo a
field-induced phase transition from a trivial to a topological superconductor,
marked by the closure and re-opening of the excitation gap, followed by the
emergence of Majorana bound states at the nanowire ends. Many local
density-of-states measurements have reported signatures of the topological
phase, however this interpretation has been challenged by alternative
explanations. Here, by measuring nonlocal conductance, we identify the closure
of the excitation gap in the bulk of the semiconductor before the emergence of
zero-bias peaks. This observation is inconsistent with scenarios where
zero-bias peaks occur due to end-states with a trivially gapped bulk, which
have been extensively considered in the theoretical and experimental
literature. We observe that after the gap closes, nonlocal signals fluctuate
strongly and persist irrespective of the presence of local-conductance
zero-bias peaks. Thus, our observations are also incompatible with a simple
picture of clean topological superconductivity. This work presents a new
experimental approach for probing the spatial extent of states in Majorana
wires, and reveals the presence of a regime with a continuum of spatially
extended states and uncorrelated zero-bias peaks
Coherent transport through a Majorana island in an Aharonov-Bohm interferometer
Majorana zero modes are leading candidates for topological quantum
computation due to non-local qubit encoding and non-abelian exchange
statistics. Spatially separated Majorana modes are expected to allow
phase-coherent single-electron transport through a topological superconducting
island via a mechanism referred to as teleportation. Here we experimentally
investigate such a system by patterning an elongated epitaxial InAs-Al island
embedded in an Aharonov-Bohm interferometer. With increasing parallel magnetic
field, a discrete sub-gap state in the island is lowered to zero energy
yielding persistent 1e-periodic Coulomb blockade conductance peaks (e is the
elementary charge). In this condition, conductance through the interferometer
is observed to oscillate in a perpendicular magnetic field with a flux period
of h/e (h is Planck's constant), indicating coherent transport of single
electrons through the islands, a signature of electron teleportation via
Majorana modes, could also be observed, suggesting additional non-Majorana
mechanisms for 1e transport through these moderately short wires
Разработка модели продвижения услуг Smart Академии в IT сфере
Работа раскрывает основные тенденции корпоративного обучения IT-специалистов и их адаптации у работодателя. Практическая и научная значимость работы заключается в разработанной модели продвижения корпоративной практики обучения на примере Smart Академии, являющейся продуктом томской IT-компании Smart World.The work reveals the main trends of corporate training of IT-specialists and the adaptation in the companies. The practical and scientific significance is in developed promotion model in corporate training practices on the example of Smart Academy. Smart Academy is the product of Tomsk IT company Smart World
Spin dynamics in semiconductors
This article reviews the current status of spin dynamics in semiconductors
which has achieved a lot of progress in the past years due to the fast growing
field of semiconductor spintronics. The primary focus is the theoretical and
experimental developments of spin relaxation and dephasing in both spin
precession in time domain and spin diffusion and transport in spacial domain. A
fully microscopic many-body investigation on spin dynamics based on the kinetic
spin Bloch equation approach is reviewed comprehensively.Comment: a review article with 193 pages and 1103 references. To be published
in Physics Reports
Measuring thermoelectric transport properties of materials
In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, zT. Advice for improving the data quality in Seebeck coefficient, electrical resistivity, and thermal conductivity (from flash diffusivity and heat capacity) measurements are given together with methods for identifying possible erroneous data. Measurement of the Hall coefficient and calculation of the charge carrier concentration and mobility is also included due to its importance for understanding materials. It is not intended to be a complete record or comparison of all the different techniques employed in thermoelectrics. Rather, by providing an overview of common techniques and their inherent difficulties it is an aid to new researchers or students in the field. The focus is mainly on high temperature measurements but low temperature techniques are also briefly discussed
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