222 research outputs found
Hybridization at superconductor-semiconductor interfaces
Hybrid superconductor-semiconductor devices are currently one of the most
promising platforms for realizing Majorana zero modes. Their topological
properties are controlled by the band alignment of the two materials, as well
as the electrostatic environment, which are currently not well understood.
Here, we pursue to fill in this gap and address the role of band bending and
superconductor-semiconductor hybridization in such devices by analyzing a gated
single Al-InAs interface using a self-consistent Schrodinger-Poisson approach.
Our numerical analysis shows that the band bending leads to an interface
quantum well, which localizes the charge in the system near the
superconductor-semiconductor interface. We investigate the hybrid band
structure and analyze its response to varying the gate voltage and thickness of
the Al layer. This is done by studying the hybridization degrees of the
individual subbands, which determine the induced pairing and effective
-factors. The numerical results are backed by approximate analytical
expressions which further clarify key aspects of the band structure. We find
that one can obtain states with strong superconductor-semiconductor
hybridization at the Fermi energy, but this requires a fine balance of
parameters, with the most important constraint being on the width of the Al
layer. In fact, in the regime of interest, we find an almost periodic
dependence of the hybridization degree on the Al width, with a period roughly
equal to the thickness of an Al monolayer. This implies that disorder and shape
irregularities, present in realistic devices, may play an important role for
averaging out this sensitivity and, thus, may be necessary for stabilizing the
topological phase.Comment: 10 Figures. 16 pages. Published versio
Direct observation of interface and nanoscale compositional modulation in ternary III-As heterostructure nanowires
Straight, axial InAs nanowire with multiple segments of GaInAs were grown.
High resolution X-ray energy-dispersive spectroscopy (EDS) mapping reveal the
distribution of group III atoms at the axial interfaces and at the sidewalls.
Significant Ga enrichment, accompanied by a structural change is observed at
the GaInAs/InAs interfaces and a higher Ga concentration for the early grown
GaInAs segments. The elemental map and EDS line profile infer Ga enrichment at
the facet junctions between the sidewalls. The relative chemical potentials of
ternary alloys and the thermodynamic driving force for liquid to solid
transition explains the growth mechanisms behind the enrichment.Comment: 12 Pages, 4 figure
Current-phase relations of few-mode InAs nanowire Josephson junctions
Gate-tunable semiconductor nanowires with superconducting leads have great
potential for quantum computation and as model systems for mesoscopic Josephson
junctions. The supercurrent, , versus the phase, , across the junction
is called the current-phase relation (CPR). It can reveal not only the
amplitude of the critical current, but also the number of modes and their
transmission. We measured the CPR of many individual InAs nanowire Josephson
junctions, one junction at a time. Both the amplitude and shape of the CPR
varied between junctions, with small critical currents and skewed CPRs
indicating few-mode junctions with high transmissions. In a gate-tunable
junction, we found that the CPR varied with gate voltage: Near the onset of
supercurrent, we observed behavior consistent with resonant tunneling through a
single, highly transmitting mode. The gate dependence is consistent with
modeled subband structure that includes an effective tunneling barrier due to
an abrupt change in the Fermi level at the boundary of the gate-tuned region.
These measurements of skewed, tunable, few-mode CPRs are promising both for
applications that require anharmonic junctions and for Majorana readout
proposals
Revealing charge-tunneling processes between a quantum dot and a superconducting island through gate sensing
We report direct detection of charge-tunneling between a quantum dot and a
superconducting island through radio-frequency gate sensing. We are able to
resolve spin-dependent quasiparticle tunneling as well as two-particle
tunneling involving Cooper pairs. The quantum dot can act as an RF-only sensor
to characterize the superconductor addition spectrum, enabling us to access
subgap states without transport. Our results provide guidance for future
dispersive parity measurements of Majorana modes, which can be realized by
detecting the parity-dependent tunneling between dots and islands.Comment: 6 pages, 4 figures, supplemental material included as ancillary fil
Single nanowire solar cells beyond the Shockley-Queisser limit
Light management is of great importance to photovoltaic cells, as it
determines the fraction of incident light entering the device. An optimal
pn-junction combined with an optimal light absorption can lead to a solar cell
efficiency above the Shockley-Queisser limit. Here, we show how this is
possible by studying photocurrent generation for a single core-shell p-i-n
junction GaAs nanowire solar cell grown on a silicon substrate. At one sun
illumination a short circuit current of 180 mA/cm^2 is obtained, which is more
than one order of magnitude higher than what would be predicted from
Lambert-Beer law. The enhanced light absorption is shown to be due to a light
concentrating property of the standing nanowire as shown by photocurrent maps
of the device. The results imply new limits for the maximum efficiency
obtainable with III-V based nanowire solar cells under one sun illumination.Comment: 19 pages, 3 figure
Suppressing quasiparticle poisoning with a voltage-controlled filter
We study single-electron charging events in an Al/InAs nanowire hybrid system
with deliberately introduced gapless regions. The occupancy of a Coulomb island
is detected using a nearby radio-frequency quantum dot as a charge sensor. We
demonstrate that a 1 micron gapped segment of the wire can be used to
efficiently suppress single electron poisoning of the gapless region and
therefore protect the parity of the island while maintaining good electrical
contact with a normal lead. In the absence of protection by charging energy,
the 1e switching rate can be reduced below 200 per second. In the same
configuration, we observe strong quantum charge fluctuations due to exchange of
electron pairs between the island and the lead. The magnetic field dependence
of the poisoning rate yields a zero-field superconducting coherence length of ~
90 nm
Advances in the theory of III-V Nanowire Growth Dynamics
Nanowire (NW) crystal growth via the vapour_liquid_solid mechanism is a
complex dynamic process involving interactions between many atoms of various
thermodynamic states. With increasing speed over the last few decades many
works have reported on various aspects of the growth mechanisms, both
experimentally and theoretically. We will here propose a general continuum
formalism for growth kinetics based on thermodynamic parameters and transition
state kinetics. We use the formalism together with key elements of recent
research to present a more overall treatment of III_V NW growth, which can
serve as a basis to model and understand the dynamical mechanisms in terms of
the basic control parameters, temperature and pressures/beam fluxes.
Self-catalysed GaAs NW growth on Si substrates by molecular beam epitaxy is
used as a model system.Comment: 63 pages, 25 figures and 4 tables. Some details are explained more
carefully in this version aswell as a new figure is added illustrating
various facets of a WZ crysta
Doping incorporation paths in catalyst-free Be-doped GaAs nanowires
The incorporation paths of Be in GaAs nanowires grown by the Ga-assisted
method in molecular beam epitaxy has been investigated by electrical
measurements of nanowires with different doping profiles. We find that Be atoms
incorporate preferentially via the nanowire side facets, while the
incorporation path through the Ga droplet is negligible. We also demonstrate
that Be can diffuse into the volume of the nanowire giving an alternative
incorporation path. This work is an important step towards controlled doping of
nanowires and will serve as a help for designing future devices based on
nanowires.Comment: 4 pages, 4 figure
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