9,281 research outputs found
Shadow epitaxy for in-situ growth of generic semiconductor/superconductor devices
Uniform, defect-free crystal interfaces and surfaces are crucial ingredients
for realizing high-performance nanoscale devices. A pertinent example is that
advances in gate-tunable and topological superconductivity using
semiconductor/superconductor electronic devices are currently built on the hard
proximity-induced superconducting gap obtained from epitaxial indium
arsenide/aluminium heterostructures. Fabrication of devices requires selective
etch processes; these exist only for InAs/Al hybrids, precluding the use of
other, potentially superior material combinations. We present a crystal growth
platform -- based on three-dimensional structuring of growth substrates --
which enables synthesis of semiconductor nanowire hybrids with in-situ
patterned superconductor shells. This platform eliminates the need for etching,
thereby enabling full freedom in choice of hybrid constituents. We realise and
characterise all the most frequently used architectures in superconducting
hybrid devices, finding increased yield and electrostatic stability compared to
etched devices, along with evidence of ballistic superconductivity. In addition
to aluminium, we present hybrid devices based on tantalum, niobium and
vanadium.
This is the submitted version of the manuscript. The accepted, peer reviewed
version is available from Advanced Materials:
http://doi.org/10.1002/adma.201908411
Previous title: Shadow lithography for in-situ growth of generic
semiconductor/superconductor device
Femtosecond electrons probing currents and atomic structure in nanomaterials
The investigation of ultrafast electronic and structural dynamics in
low-dimensional systems like nanowires and two-dimensional materials requires
femtosecond probes providing high spatial resolution and strong interaction
with small volume samples. Low-energy electrons exhibit large scattering cross
sections and high sensitivity to electric fields, but their pronounced
dispersion during propagation in vacuum so far prevented their use as
femtosecond probe pulses in time-resolved experiments. Employing a
laser-triggered point-like source of either divergent or collimated electron
wave packets, we developed a hybrid approach for femtosecond point projection
microscopy and femtosecond low-energy electron diffraction. We investigate
ultrafast electric currents in nanowires with sub-100 femtosecond temporal and
few 10 nm spatial resolutions and demonstrate the potential of our approach for
studying structural dynamics in crystalline single-layer materials.Comment: 18 pages, 4 figures, includes 8 pages supplementary informatio
From Andreev to Majorana bound states in hybrid superconductor-semiconductor nanowires
Electronic excitations above the ground state must overcome an energy gap in
superconductors with spatially-homogeneous s-wave pairing. In contrast,
inhomogeneous superconductors such as those with magnetic impurities or weak
links, or heterojunctions containing normal metals or quantum dots, can host
subgap electronic excitations that are generically known as Andreev bound
states (ABSs). With the advent of topological superconductivity, a new kind of
ABS with exotic qualities, known as Majorana bound state (MBS), has been
discovered. We review the main properties of ABSs and MBSs, and the
state-of-the-art techniques for their detection. We focus on hybrid
superconductor-semiconductor nanowires, possibly coupled to quantum dots, as
one of the most flexible and promising experimental platforms. We discuss how
the combined effect of spin-orbit coupling and Zeeman field in these wires
triggers the transition from ABSs into MBSs. We show theoretical progress
beyond minimal models in understanding experiments, including the possibility
of different types of robust zero modes that may emerge without a
band-topological transition. We examine the role of spatial non-locality, a
special property of MBS wavefunctions that, together with non-Abelian braiding,
is the key to realizing topological quantum computation.Comment: Review. 23 pages, 8 figures, 1 table. Shareable published version by
Springer Nature at https://rdcu.be/b7DWT (free to read but not to download
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
Effects of the electrostatic environment on superlattice Majorana nanowires
Finding ways of creating, measuring, and manipulating Majorana bound states (MBSs) in superconducting-semiconducting nanowires is a highly pursued goal in condensed matter physics. It was recently proposed that a periodic covering of the semiconducting nanowire with superconductor fingers would allow both gating and tuning the system into a topological phase while leaving room for a local detection of the MBS wave function. We perform a detailed, self-consistent numerical study of a three-dimensional (3D) model for a finite-length nanowire with a superconductor superlattice including the effect of the surrounding electrostatic environment, and taking into account the surface charge created at the semiconductor surface. We consider different experimental scenarios where the superlattice is on top or at the bottom of the nanowire with respect to a back gate. The analysis of the 3D electrostatic profile, the charge density, the low-energy spectrum, and the formation of MBSs reveals a rich phenomenology that depends on the nanowire parameters as well as on the superlattice dimensions and the external back-gate potential. The 3D environment turns out to be essential to correctly capture and understand the phase diagram of the system and the parameter regions where topological superconductivity is establishedWe thank E. J. H. Lee, H. Beidenkopf, E. G. Michel, N. Avraham, H. Shtrikman, and J. Nygård for valuable discussions. Research supported by the Spanish MINECO through Grants No. FIS2016-80434-P, No. BES-2017-080374, and No. FIS2017-84860-R (AEI/FEDER, EU), the European Union's Horizon 2020 research and innovation programme under the FETOPEN Grant Agreement No. 828948 and Grant Agreement LEGOTOP No. 788715, the Ramón y Cajal programme RYC-2011-09345, the María de Maeztu Programme for Units of Excellence in R&D (MDM-2014-0377), the DFG (CRC/Transregio 183, EI 519/7- 1), the Israel Science Foundation (ISF), and the Binational Science Foundation (BSF
Universal Vectorial and Ultrasensitive Nanomechanical Force Field Sensor
Miniaturization of force probes into nanomechanical oscillators enables
ultrasensitive investigations of forces on dimensions smaller than their
characteristic length scale. Meanwhile it also unravels the force field
vectorial character and how its topology impacts the measurement. Here we
expose an ultrasensitive method to image 2D vectorial force fields by
optomechanically following the bidimensional Brownian motion of a singly
clamped nanowire. This novel approach relies on angular and spectral tomography
of its quasi frequency-degenerated transverse mechanical polarizations:
immersing the nanoresonator in a vectorial force field does not only shift its
eigenfrequencies but also rotate eigenmodes orientation as a nano-compass. This
universal method is employed to map a tunable electrostatic force field whose
spatial gradients can even take precedence over the intrinsic nanowire
properties. Enabling vectorial force fields imaging with demonstrated
sensitivities of attonewton variations over the nanoprobe Brownian trajectory
will have strong impact on scientific exploration at the nanoscale
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