298 research outputs found
Hybrid Superconductor-Quantum Point Contact Devices using InSb Nanowires
Proposals for studying topological superconductivity and Majorana bound
states in nanowires proximity coupled to superconductors require that transport
in the nanowire is ballistic. Previous work on hybrid nanowire-superconductor
systems has shown evidence for Majorana bound states, but these experiments
were also marked by disorder, which disrupts ballistic transport. In this
letter, we demonstrate ballistic transport in InSb nanowires interfaced
directly with superconducting Al by observing quantized conductance at
zero-magnetic field. Additionally, we demonstrate that the nanowire is
proximity coupled to the superconducting contacts by observing Andreev
reflection. These results are important steps for robustly establishing
topological superconductivity in InSb nanowires
Single-electron tunneling in InP nanowires
We report on the fabrication and electrical characterization of field-effect
devices based on wire-shaped InP crystals grown from Au catalyst particles by a
vapor-liquid-solid process. Our InP wires are n-type doped with diameters in
the 40-55 nm range and lengths of several microns. After being deposited on an
oxidized Si substrate, wires are contacted individually via e-beam fabricated
Ti/Al electrodes. We obtain contact resistances as low as ~10 kOhm, with minor
temperature dependence. The distance between the electrodes varies between 0.2
and 2 micron. The electron density in the wires is changed with a back gate.
Low-temperature transport measurements show Coulomb-blockade behavior with
single-electron charging energies of ~1 meV. We also demonstrate energy
quantization resulting from the confinement in the wire.Comment: 4 pages, 3 figure
Measurement of g-factor tensor in a quantum dot and disentanglement of exciton spins
We perform polarization-resolved magneto-optical measurements on single InAsP
quantum dots embedded in an InP nanowire. In order to determine all elements of
the electron and hole -factor tensors, we measure in magnetic field with
different orientations. The results of these measurements are in good agreement
with a model based on exchange terms and Zeeman interaction. In our experiment,
polarization analysis delivers a powerful tool that not only significantly
increases the precision of the measurements, but also enables us to probe the
exciton spin state evolution in magnetic fields. We propose a disentangling
scheme of heavy-hole exciton spins enabling a measurement of the electron spin
time
Disentangling the effects of spin-orbit and hyperfine interactions on spin blockade
We have achieved the few-electron regime in InAs nanowire double quantum
dots. Spin blockade is observed for the first two half-filled orbitals, where
the transport cycle is interrupted by forbidden transitions between triplet and
singlet states. Partial lifting of spin blockade is explained by spin-orbit and
hyperfine mechanisms that enable triplet to singlet transitions. The
measurements over a wide range of interdot coupling and tunneling rates to the
leads are well reproduced by a simple transport model. This allows us to
separate and quantify the contributions of the spin-orbit and hyperfine
interactions.Comment: 5 pages, 4 figure
Realization of microwave quantum circuits using hybrid superconducting-semiconducting nanowire Josephson elements
We report the realization of quantum microwave circuits using hybrid
superconductor-semiconductor Josephson elements comprised of InAs nanowires
contacted by NbTiN. Capacitively-shunted single elements behave as transmon
qubits with electrically tunable transition frequencies. Two-element circuits
also exhibit transmon-like behavior near zero applied flux, but behave as flux
qubits at half the flux quantum, where non-sinusoidal current-phase relations
in the elements produce a double-well Josephson potential. These hybrid
Josephson elements are promising for applications requiring microwave
superconducting circuits operating in magnetic field.Comment: Main text: 4 pages, 4 figures; Supplement: 10 pages, 8 figures, 1
tabl
Spin and Orbital Spectroscopy in the Absence of Coulomb Blockade in Lead Telluride Nanowire Quantum Dots
We investigate quantum dots in semiconductor PbTe nanowire devices. Due to
the accessibility of ambipolar transport in PbTe, quantum dots can be occupied
both with electrons and holes. Owing to a very large dielectric constant in
PbTe of order 1000, we do not observe Coulomb blockade which typically
obfuscates the orbital and spin spectra. We extract large and highly
anisotropic effective Lande g-factors, in the range 20-44. The absence of
Coulomb blockade allows direct readout, at zero source-drain bias, of
spin-orbit hybridization energies of up to 600 microelectronvolt. These spin
properties make PbTe nanowires, the recently synthesized members of group IV-VI
materials family, attractive as a materials platform for quantum technology,
such as spin and topological qubits.Comment: Full transport data available via Zenodo at
https://zenodo.org/communities/frolovlab
Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires
Double quantum dot in the few-electron regime is achieved using local gating
in an InSb nanowire. The spectrum of two-electron eigenstates is investigated
using electric dipole spin resonance. Singlet-triplet level repulsion caused by
spin-orbit interaction is observed. The size and the anisotropy of
singlet-triplet repulsion are used to determine the magnitude and the
orientation of the spin-orbit effective field in an InSb nanowire double dot.
The obtained results are confirmed using spin blockade leakage current
anisotropy and transport spectroscopy of individual quantum dots.Comment: 5 pages, supplementary material available at
http://link.aps.org/supplemental/10.1103/PhysRevLett.108.16680
Suppression of Zeeman gradients by nuclear polarization in double quantum dots
We use electric dipole spin resonance to measure dynamic nuclear polarization
in InAs nanowire quantum dots. The resonance shifts in frequency when the
system transitions between metastable high and low current states, indicating
the presence of nuclear polarization. We propose that the low and the high
current states correspond to different total Zeeman energy gradients between
the two quantum dots. In the low current state, dynamic nuclear polarization
efficiently compensates the Zeeman gradient due to the -factor mismatch,
resulting in a suppressed total Zeeman gradient. We present a theoretical model
of electron-nuclear feedback that demonstrates a fixed point in nuclear
polarization for nearly equal Zeeman splittings in the two dots and predicts a
narrowed hyperfine gradient distribution
Delocalized states in three-terminal superconductor-semiconductor nanowire devices
We fabricate three-terminal hybrid devices with a nanowire segment
proximitized by a superconductor, and with two tunnel probe contacts on either
side of that segment. We perform simultaneous tunneling measurements on both
sides. We identify some states as delocalized above-gap states observed on both
ends, and some states as localized near one of the tunnel barriers. Delocalized
states can be traced from zero to finite magnetic fields beyond 0.5 T. In the
parameter regime of delocalized states, we search for correlated subgap
resonances required by the Majorana zero mode hypothesis. While both sides
exhibit ubiquitous low-energy features at high fields, no correlation is
inferred. Simulations using a one-dimensional effective model suggest that
delocalized states may belong to lower one-dimensional subbands, while the
localized states originate from higher subbands. To avoid localization in
higher subbands, disorder may need to be further reduced to realize Majorana
zero modes.Comment: Original data available at https://zenodo.org/record/395824
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