374 research outputs found
Magnetoresistence engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates
We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot
device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The
wire segment can be electrically tuned to a single dot or to a double dot
regime using the FSGs and a backgate. In both regimes we find a strong MR and a
sharp MR switching of up to 25\% at the field at which the magnetizations of
the FSGs are inverted by the external field. The sign and amplitude of the MR
and the MR switching can both be tuned electrically by the FSGs. In a double
dot regime close to pinch-off we find {\it two} sharp transitions in the
conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic
contacts, with one transition near zero and one at the FSG switching fields.
These surprisingly rich characteristics we explain in several simple resonant
tunneling models. For example, the TMR-like MR can be understood as a
stray-field controlled transition between singlet and a triplet double dot
states. Such local magnetic fields are the key elements in various proposals to
engineer novel states of matter and may be used for testing electron spin-based
Bell inequalities.Comment: 7 pages, 6 figure
Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts
Advanced synthesis of semiconductor nanowires (NWs) enables their application
in diverse fields, notably in chemical and electrical sensing, photovoltaics,
or quantum electronic devices. In particular, Indium Arsenide (InAs) NWs are an
ideal platform for quantum devices, e.g. they may host topological Majorana
states. While the synthesis has been continously perfected, only few techniques
were developed to tailor individual NWs after growth. Here we present three wet
chemical etch methods for the post-growth morphological engineering of InAs NWs
on the sub-100 nm scale. The first two methods allow the formation of
self-aligned electrical contacts to etched NWs, while the third method results
in conical shaped NW profiles ideal for creating smooth electrical potential
gradients and shallow barriers. Low temperature experiments show that NWs with
etched segments have stable transport characteristics and can serve as building
blocks of quantum electronic devices. As an example we report the formation of
a single electrically stable quantum dot between two etched NW segments.Comment: 9 pages, 5 figure
Identification of everyday objects on the basis of kinetic contours
Using kinetic contours derived from everyday objects, we investigated how motion affects object identification. In order not to be distinguishable when static, kinetic contours were made from random dot displays consisting of two regions, inside and outside the object contour. In Experiment 1, the dots were moving in only one of two regions. The objects were identified nearly equally well as soon as the dots either in the figure or in the background started to move. RTs decreased with increasing motion coherence levels and were shorter for complex, less compact objects than for simple, more compact objects. In Experiment 2, objects could be identified when the dots were moving both in the figure and in the background with speed and direction differences between the two. A linear increase in either the speed difference or the direction difference caused a linear decrease in RT for correct identification. In addition, the combination of speed and motion differences appeared to be super-additive
Towards low-dimensional hole systems in Be-doped GaAs nanowires
GaAs was central to the development of quantum devices but is rarely used for
nanowire-based quantum devices with InAs, InSb and SiGe instead taking the
leading role. p-type GaAs nanowires offer a path to studying strongly-confined
0D and 1D hole systems with strong spin-orbit effects, motivating our
development of nanowire transistors featuring Be-doped p-type GaAs nanowires,
AuBe alloy contacts and patterned local gate electrodes towards making
nanowire-based quantum hole devices. We report on nanowire transistors with
traditional substrate back-gates and EBL-defined metal/oxide top-gates produced
using GaAs nanowires with three different Be-doping densities and various AuBe
contact processing recipes. We show that contact annealing only brings small
improvements for the moderately-doped devices under conditions of lower anneal
temperature and short anneal time. We only obtain good transistor performance
for moderate doping, with conduction freezing out at low temperature for
lowly-doped nanowires and inability to reach a clear off-state under gating for
the highly-doped nanowires. Our best devices give on-state conductivity 95 nS,
off-state conductivity 2 pS, on-off ratio ~, and sub-threshold slope 50
mV/dec at T = 4 K. Lastly, we made a device featuring a moderately-doped
nanowire with annealed contacts and multiple top-gates. Top-gate sweeps show a
plateau in the sub-threshold region that is reproducible in separate cool-downs
and indicative of possible conductance quantization highlighting the potential
for future quantum device studies in this material system
Quantum transport in carbon nanotubes
Carbon nanotubes are a versatile material in which many aspects of condensed
matter physics come together. Recent discoveries, enabled by sophisticated
fabrication, have uncovered new phenomena that completely change our
understanding of transport in these devices, especially the role of the spin
and valley degrees of freedom. This review describes the modern understanding
of transport through nanotube devices.
Unlike conventional semiconductors, electrons in nanotubes have two angular
momentum quantum numbers, arising from spin and from valley freedom. We focus
on the interplay between the two. In single quantum dots defined in short
lengths of nanotube, the energy levels associated with each degree of freedom,
and the spin-orbit coupling between them, are revealed by Coulomb blockade
spectroscopy. In double quantum dots, the combination of quantum numbers
modifies the selection rules of Pauli blockade. This can be exploited to read
out spin and valley qubits, and to measure the decay of these states through
coupling to nuclear spins and phonons. A second unique property of carbon
nanotubes is that the combination of valley freedom and electron-electron
interactions in one dimension strongly modifies their transport behaviour.
Interaction between electrons inside and outside a quantum dot is manifested in
SU(4) Kondo behavior and level renormalization. Interaction within a dot leads
to Wigner molecules and more complex correlated states.
This review takes an experimental perspective informed by recent advances in
theory. As well as the well-understood overall picture, we also state clearly
open questions for the field. These advances position nanotubes as a leading
system for the study of spin and valley physics in one dimension where
electronic disorder and hyperfine interaction can both be reduced to a very low
level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure
Local electrical tuning of the nonlocal signals in a Cooper pair splitter
A Cooper pair splitter consists of a central superconducting contact, S, from
which electrons are injected into two parallel, spatially separated quantum
dots (QDs). This geometry and electron interactions can lead to correlated
electrical currents due to the spatial separation of spin-singlet Cooper pairs
from S. We present experiments on such a device with a series of bottom gates,
which allows for spatially resolved tuning of the tunnel couplings between the
QDs and the electrical contacts and between the QDs. Our main findings are
gate-induced transitions between positive conductance correlation in the QDs
due to Cooper pair splitting and negative correlations due to QD dynamics.
Using a semi-classical rate equation model we show that the experimental
findings are consistent with in-situ electrical tuning of the local and
nonlocal quantum transport processes. In particular, we illustrate how the
competition between Cooper pair splitting and local processes can be optimized
in such hybrid nanostructures.Comment: 9 pages, 6 figures, 2 table
Direct microwave measurement of Andreev-bound-state dynamics in a proximitized semiconducting nanowire
The modern understanding of the Josephson effect in mesosopic devices derives
from the physics of Andreev bound states, fermionic modes that are localized in
a superconducting weak link. Recently, Josephson junctions constructed using
semiconducting nanowires have led to the realization of superconducting qubits
with gate-tunable Josephson energies. We have used a microwave circuit QED
architecture to detect Andreev bound states in such a gate-tunable junction
based on an aluminum-proximitized InAs nanowire. We demonstrate coherent
manipulation of these bound states, and track the bound-state fermion parity in
real time. Individual parity-switching events due to non-equilibrium
quasiparticles are observed with a characteristic timescale . The of a topological nanowire
junction sets a lower bound on the bandwidth required for control of Majorana
bound states
Magnetic field tuning and quantum interference in a Cooper pair splitter
Cooper pair splitting (CPS) is a process in which the electrons of naturally
occurring spin-singlet pairs in a superconductor are spatially separated using
two quantum dots. Here we investigate the evolution of the conductance
correlations in an InAs CPS device in the presence of an external magnetic
field. In our experiments the gate dependence of the signal that depends on
both quantum dots continuously evolves from a slightly asymmetric Lorentzian to
a strongly asymmetric Fano-type resonance with increasing field. These
experiments can be understood in a simple three - site model, which shows that
the nonlocal CPS leads to symmetric line shapes, while the local transport
processes can exhibit an asymmetric shape due to quantum interference. These
findings demonstrate that the electrons from a Cooper pair splitter can
propagate coherently after their emission from the superconductor and how a
magnetic field can be used to optimize the performance of a CPS device. In
addition, the model calculations suggest that the estimate of the CPS
efficiency in the experiments is a lower bound for the actual efficiency.Comment: 5 pages + 4 pages supplementary informatio
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