1,089 research outputs found
Coupling of shells in a carbon nanotube quantum dot
We systematically study the coupling of longitudinal modes (shells) in a
carbon nanotube quantum dot. Inelastic cotunneling spectroscopy is used to
probe the excitation spectrum in parallel, perpendicular and rotating magnetic
fields. The data is compared to a theoretical model including coupling between
shells, induced by atomically sharp disorder in the nanotube. The calculated
excitation spectra show good correspondence with experimental data.Comment: 8 pages, 4 figure
Tunneling Spectroscopy of Quasiparticle Bound States in a Spinful Josephson Junction
The spectrum of a segment of InAs nanowire, confined between two
superconducting leads, was measured as function of gate voltage and
superconducting phase difference using a third normal-metal tunnel probe.
Sub-gap resonances for odd electron occupancy---interpreted as bound states
involving a confined electron and a quasiparticle from the superconducting
leads, reminiscent of Yu-Shiba-Rusinov states---evolve into Kondo-related
resonances at higher magnetic fields. An additional zero bias peak of unknown
origin is observed to coexist with the quasiparticle bound states.Comment: Supplementary information available here:
https://dl.dropbox.com/u/1742676/Chang_Sup.pd
A Semiconductor Nanowire-Based Superconducting Qubit
We introduce a hybrid qubit based on a semiconductor nanowire with an
epitaxially grown superconductor layer. Josephson energy of the transmon-like
device ("gatemon") is controlled by an electrostatic gate that depletes
carriers in a semiconducting weak link region. Strong coupling to an on-chip
microwave cavity and coherent qubit control via gate voltage pulses is
demonstrated, yielding reasonably long relaxation times (0.8 {\mu}s) and
dephasing times (1 {\mu}s), exceeding gate operation times by two orders of
magnitude, in these first-generation devices. Because qubit control relies on
voltages rather than fluxes, dissipation in resistive control lines is reduced,
screening reduces crosstalk, and the absence of flux control allows operation
in a magnetic field, relevant for topological quantum information
Nonequilibrium Cotunneling through a Three-Level Quantum Dot
We calculate the nonlinear cotunneling conductance through a quantum dot with
3 electrons occupying the three highest lying energy levels. Starting from a
3-orbital Anderson model, we apply a generalized Schrieffer-Wolff
transformation to derive an effective Kondo model for the system. Within this
model we calculate the nonequilibrium occupation numbers and the corresponding
cotunneling current to leading order in the exchange couplings. We identify the
inelastic cotunneling thresholds and their splittings with applied magnetic
field, and make a qualitative comparison to recent experimental data on carbon
nanotube and InAs quantum-wire quantum dots. Further predictions of the model
like cascade resonances and a magnetic-field dependence of the orbital level
splitting are not yet observed but within reach of recent experimental work on
carbon nanotube and InAs nanowire quantum dots.Comment: 12 pages, 13 figure
Parity lifetime of bound states in a proximitized semiconductor nanowire
Quasiparticle excitations can compromise the performance of superconducting
devices, causing high frequency dissipation, decoherence in Josephson qubits,
and braiding errors in proposed Majorana-based topological quantum computers.
Quasiparticle dynamics have been studied in detail in metallic superconductors
but remain relatively unexplored in semiconductor-superconductor structures,
which are now being intensely pursued in the context of topological
superconductivity. To this end, we introduce a new physical system comprised of
a gate-confined semiconductor nanowire with an epitaxially grown superconductor
layer, yielding an isolated, proximitized nanowire segment. We identify
Andreev-like bound states in the semiconductor via bias spectroscopy, determine
the characteristic temperatures and magnetic fields for quasiparticle
excitations, and extract a parity lifetime (poisoning time) of the bound state
in the semiconductor exceeding 10 ms.Comment: text and supplementary information combine
Transport signatures of quasiparticle poisoning in a Majorana island
We investigate effects of quasiparticle poisoning in a Majorana island with
strong tunnel coupling to normal-metal leads. In addition to the main Coulomb
blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage,
consistent with transport through an excited (poisoned) state of the island.
Comparison to a simple model yields an estimate of parity lifetime for the
strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled
island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks
at high field, reflecting Majorana hybridization, are enhanced by the reduced
lever arm at strong coupling. In energy units, fluctuations are consistent with
previous measurements.Comment: includes supplementary materia
Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots
We study low-temperature transport through carbon nanotube quantum dots in
the Coulomb blockade regime coupled to niobium-based superconducting leads. We
observe pronounced conductance peaks at finite source-drain bias, which we
ascribe to elastic and inelastic cotunneling processes enhanced by the
coherence peaks in the density of states of the superconducting leads. The
inelastic cotunneling lines display a marked dependence on the applied gate
voltage which we relate to different tunneling-renormalizations of the two
subbands in the nanotube. Finally, we discuss the origin of an especially
pronounced sub-gap structure observed in every fourth Coulomb diamond
Mesoscopic conductance fluctuations in InAs nanowire-based SNS junctions
We report a systematic experimental study of mesoscopic conductance
fluctuations in superconductor/normal/superconductor (SNS) devices
Nb/InAs-nanowire/Nb. These fluctuations far exceed their value in the normal
state and strongly depend on temperature even in the low-temperature regime.
This dependence is attributed to high sensitivity of perfectly conducting
channels to dephasing and the SNS fluctuations thus provide a sensitive probe
of dephasing in a regime where normal transport fails to detect it. Further,
the conductance fluctuations are strongly non-linear in bias voltage and reveal
sub-gap structure. The experimental findings are qualitatively explained in
terms of multiple Andreev reflections in chaotic quantum dots with imperfect
contacts.Comment: Manuscript and supplemen
Raman spectroscopy and electrical properties of InAs nanowires with local oxidation enabled by substrate micro-trenches and laser irradiation
The thermal gradient along indium-arsenide nanowires was engineered by a
combination of fabricated micro- trenches in the supporting substrate and
focused laser irradiation. This allowed local control of thermally activated
oxidation reactions of the nanowire on the scale of the diffraction limit. The
locality of the oxidation was detected by micro-Raman mapping, and the results
were found consistent with numerical simulations of the temperature profile.
Applying the technique to nanowires in electrical devices the locally oxidized
nanowires remained conducting with a lower conductance as expected for an
effectively thinner conducting core
Hard gap in epitaxial semiconductor-superconductor nanowires
Many present and future applications of superconductivity would benefit from
electrostatic control of carrier density and tunneling rates, the hallmark of
semiconductor devices. One particularly exciting application is the realization
of topological superconductivity as a basis for quantum information processing.
Proposals in this direction based on proximity effect in semiconductor
nanowires are appealing because the key ingredients are currently in hand.
However, previous instances of proximitized semiconductors show significant
tunneling conductance below the superconducting gap, suggesting a continuum of
subgap states---a situation that nullifies topological protection. Here, we
report a hard superconducting gap induced by proximity effect in a
semiconductor, using epitaxial Al-InAs superconductor-semiconductor nanowires.
The hard gap, along with favorable material properties and gate-tunability,
makes this new hybrid system attractive for a number of applications, as well
as fundamental studies of mesoscopic superconductivity.Comment: Combined text and supplementary information, Nature Nanotechnology
(2015
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