117 research outputs found
Exfoliated hexagonal BN as gate dielectric for InSb nanowire quantum dots with improved gate hysteresis and charge noise
We characterize InSb quantum dots induced by bottom finger gates within a
nanowire that is grown via the vapor-liquid-solid process. The gates are
separated from the nanowire by an exfoliated 35\,nm thin hexagonal BN flake. We
probe the Coulomb diamonds of the gate induced quantum dot exhibiting charging
energies of and orbital excitation energies up to
. The gate hysteresis for sweeps covering 5 Coulomb diamonds
reveals an energy hysteresis of only between upwards and
downwards sweeps. Charge noise is studied via long-term measurements at the
slope of a Coulomb peak revealing potential fluctuations of at 1\,Hz. This makes h-BN the dielectric with
the currently lowest gate hysteresis and lowest low-frequency potential
fluctuations reported for low-gap III-V nanowires. The extracted values are
similar to state-of-the art quantum dots within Si/SiGe and Si/SiO
systems
Andreev reflection at high magnetic fields: Evidence for electron and hole transport in edge states
We have studied magnetotransport in arrays of niobium filled grooves in an
InAs/AlGaSb heterostructure. The critical field of up to 2.6 T permits to enter
the quantum Hall regime. In the superconducting state, we observe strong
magnetoresistance oscillations, whose amplitude exceeds the Shubnikov-de Haas
oscillations by a factor of about two, when normalized to the background.
Additionally, we find that above a geometry-dependent magnetic field value the
sample in the superconducting state has a higher longitudinal resistance than
in the normal state. Both observations can be explained with edge channels
populated with electrons and Andreev reflected holes.Comment: accepted for Phys Rev Lett, some changes to tex
Superconducting proximity effects in metals with a repulsive pairing interaction
Studies of the superconducting proximity effect in normal
conductor/superconductor junctions almost universally assume no
effective electron-electron coupling in the region. While such an
approximation leads to a simple description of the proximity effect, it is
unclear how it could be rigorously justified. We reveal a much more complex
picture of the proximity effect in bilayers, where is a clean s-wave
BCS superconductor and is a simple metal with a repulsive effective
electron coupling. We elucidate the proximity effect behavior using a highly
accurate method to self-consistently solve the Bogoliubov-deGennes equations.
We present our results for a wide range of values of the interface scattering,
the Fermi wave vector mismatch, the temperature, and the ratio of the
effective interaction strengths in the and region. We find that the
repulsive interaction, represented by a negative , strongly alters the
signatures of the proximity effect as can be seen in the spatial dependence of
the Cooper pair amplitude and the pair potential, as well as in the local
density of states near the interface.Comment: 12 pages, including 10 figures. To appear in Phys. Rev.
Electrical current noise of a beam splitter as a test of spin-entanglement
We investigate the spin entanglement in the superconductor-quantum dot system
proposed by Recher, Sukhorukov and Loss, coupling it to an electronic
beam-splitter. The superconductor-quantum dot entangler and the beam-splitter
are treated within a unified framework and the entanglement is detected via
current correlations. The state emitted by the entangler is found to be a
linear superposition of non-local spin-singlets at different energies, a
spin-entangled two-particle wavepacket. Colliding the two electrons in the
beam-splitter, the singlet spin-state gives rise to a bunching behavior,
detectable via the current correlators. The amount of bunching depends on the
relative positions of the single particle levels in the quantum dots and the
scattering amplitudes of the beam-splitter. The singlet spin entanglement,
insensitive to orbital dephasing but suppressed by spin dephasing, is
conveniently quantified via the Fano factors. It is found that the
entanglement-dependent contribution to the Fano factor is of the same magnitude
as the non-entangled, making an experimental detection feasible. A detailed
comparison between the current correlations of the non-local spin-singlet state
and other states, possibly emitted by the entangler, is performed. This
provides conditions for an unambiguous identification of the non-local singlet
spin entanglement.Comment: 13 pages, 8 figures, section on quantification of entanglement adde
Coherent current transport in wide ballistic Josephson junctions
We present an experimental and theoretical investigation of coherent current
transport in wide ballistic superconductor-two dimensional electron
gas-superconductor junctions. It is found experimentally that upon increasing
the junction length, the subharmonic gap structure in the current-voltage
characteristics is shifted to lower voltages, and the excess current at
voltages much larger than the superconducting gap decreases. Applying a theory
of coherent multiple Andreev reflection, we show that these observations can be
explained in terms of transport through Andreev resonances.Comment: 4 pages, 4 figure
Nonequilibrium Josephson current in ballistic multiterminal SNS-junctions
We study the nonequilibrium Josephson current in a long two-dimensional
ballistic SNS-junction with a normal reservoir coupled to the normal part of
the junction. The current for a given superconducting phase difference
oscillates as a function of voltage applied between the normal reservoir and
the SNS-junction. The period of the oscillations is , with
the length of the junction, and the amplitude of the oscillations decays as
for and zero temperature. The critical
current shows a similar oscillating, decaying behavior as a function of
voltage, changing sign every oscillation. Normal specular or diffusive
scattering at the NS-interfaces does not qualitatively change the picture.Comment: Proceeding of MS2000, to appear in Physica
Control of Josephson current by Aharonov-Casher Phase in a Rashba Ring
We study the interference effect induced by the Aharonov-Casher phase on the
Josephson current through a semiconducting ring attached to superconducting
leads. Using a 1D model that incorporates spin-orbit coupling in the
semiconducting ring, we calculate the Andreev levels analytically and
numerically, and predict oscillations of the Josephson current due to the AC
phase. This result is valid from the point contact limit to the long channel
length limit, as defined by the ratio of the junction length and the BCS
healing length. We show in the long channel length limit that the impurity
scattering has no effect on the oscillation of the Josephson current, in
contrast to the case of conductivity oscillations in a spin-orbit coupled ring
system attached to normal leads where impurity scattering reduces the amplitude
of oscillations. Our results suggest a new scheme to measure the AC phase with,
in principle, higher sensitivity. In addition, this effect allows for control
of the Josephson current through the gate voltage tuned AC phase.Comment: 12pages, 8 figure
Spin relaxation: From 2D to 1D
In inversion asymmetric semiconductors, spin-orbit interactions give rise to
very effective relaxation mechanisms of the electron spin. Recent work, based
on the dimensionally constrained D'yakonov Perel' mechanism, describes
increasing electron-spin relaxation times for two-dimensional conducting layers
with decreasing channel width. The slow-down of the spin relaxation can be
understood as a precursor of the one-dimensional limit
Ballistic spin-polarized transport and Rashba spin precession in semiconductor nanowires
We present numerical calculations of the ballistic spin-transport properties
of quasi-one-dimensional wires in the presence of the spin-orbit (Rashba)
interaction. A tight-binding analog of the Rashba Hamiltonian which models the
Rashba effect is used. By varying the robustness of the Rashba coupling and the
width of the wire, weak and strong coupling regimes are identified. Perfect
electron spin-modulation is found for the former regime, regardless of the
incident Fermi energy and mode number. In the latter however, the
spin-conductance has a strong energy dependence due to a nontrivial subband
intermixing induced by the strong Rashba coupling. This would imply a strong
suppression of the spin-modulation at higher temperatures and source-drain
voltages. The results may be of relevance for the implementation of
quasi-one-dimensional spin transistor devices.Comment: 19 pages (incl. 9 figures). To be published in PR
Microscopic nonequilibrium theory of double-barrier Josephson junctions
We study nonequilibrium charge transport in a double-barrier Josephson
junction, including nonstationary phenomena, using the time-dependent
quasiclassical Keldysh Green's function formalism. We supplement the kinetic
equations by appropriate time-dependent boundary conditions and solve the
time-dependent problem in a number of regimes. From the solutions,
current-voltage characteristics are derived. It is understood why the
quasiparticle current can show excess current as well as deficit current and
how the subgap conductance behaves as function of junction parameters. A
time-dependent nonequilibrium contribution to the distribution function is
found to cause a non-zero averaged supercurrent even in the presence of an
applied voltage. Energy relaxation due to inelastic scattering in the
interlayer has a prominent role in determining the transport properties of
double-barrier junctions. Actual inelastic scattering parameters are derived
from experiments. It is shown as an application of the microscopic model, how
the nature of the intrinsic shunt in double-barrier junctions can be explained
in terms of energy relaxation and the opening of Andreev channels.Comment: Accepted for Phys. Rev.
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