53 research outputs found
Dresselhaus spin-orbit coupling in [111]-oriented semiconductor nanowires
The contribution of bulk inversion asymmetry to the total spin-orbit coupling
is commonly neglected for group III-V nanowires grown in the generic [111]
direction. We have solved the complete Hamiltonian of the circular nanowire
accounting for bulk inversion asymmetry via exact numerical diagonalization.
Three different symmetry classes of angular momentum states exist, which
reflects the threefold rotation symmetry of the crystal lattice about the [111]
axis. A particular group of angular momentum states contains degenerate modes
which are strongly coupled via the Dresselhaus Hamiltonian, which results in a
significant energy splitting with increasing momentum. Hence, under certain
conditions Dresselhaus spin-orbit coupling is relevant for [111] InAs and [111]
InSb nanowires. We demonstrate momentum-dependent energy splittings and the
impact of Dresselhaus spin-orbit coupling on the dispersion relation. In view
of possible spintronics applications relying on bulk inversion asymmetry we
calculate the spin expectation values and the spin texture as a function of the
Fermi energy. Finally, we investigate the effect of an axial magnetic field on
the energy spectrum and on the corresponding spin polarization.Comment: 11 Pages, 7 figure
Weak (anti)localization in tubular semiconductor nanowires with spin-orbit coupling
We compute analytically the weak (anti)localization correction to the Drude
conductivity for electrons in tubular semiconductor systems of zinc blende
type. We include linear Rashba and Dresselhaus spin-orbit coupling (SOC) and
compare wires of standard growth directions ,
, and . The motion on the
quasi-two-dimensional surface is considered diffusive in both directions:
transversal as well as along the cylinder axis. It is shown that Dresselhaus
and Rashba SOC similarly affect the spin relaxation rates. For the
growth direction, the long-lived spin states are of helical
nature. We detect a crossover from weak localization to weak anti-localization
depending on spin-orbit coupling strength as well as dephasing and scattering
rate. The theory is fitted to experimental data of an undoped
InAs nanowire device which exhibits a top-gate-controlled
crossover from positive to negative magnetoconductivity. Thereby, we extract
transport parameters where we quantify the distinct types of SOC individually.Comment: 17 pages, 9 figure
Impact of tunnel barrier strength on magnetoresistance in carbon nanotubes
We investigate magnetoresistance in spin valves involving CoPd-contacted
carbon nanotubes. Both temperature and bias voltage dependence clearly indicate
tunneling magnetoresistance as the origin. We show that this effect is
significantly affected by the tunnel barrier strength, which appears to be one
reason for the variation between devices previously detected in similar
structures. Modeling the data by means of the scattering matrix approach, we
find a non-trivial dependence of the magnetoresistance on the barrier strength.
Furthermore, analysis of the spin precession observed in a nonlocal Hanle
measurement yields a spin lifetime of ns, a value comparable
with those found in silicon- or graphene-based spin valve devices.Comment: 10 pages, 5 figures, 1 tabl
Exfoliated hexagonal BN as gate dielectric for InSb nanowire quantum dots with improved gate hysteresis and charge noise
We characterize InSb quantum dots induced by bottom finger gates within a
nanowire that is grown via the vapor-liquid-solid process. The gates are
separated from the nanowire by an exfoliated 35\,nm thin hexagonal BN flake. We
probe the Coulomb diamonds of the gate induced quantum dot exhibiting charging
energies of and orbital excitation energies up to
. The gate hysteresis for sweeps covering 5 Coulomb diamonds
reveals an energy hysteresis of only between upwards and
downwards sweeps. Charge noise is studied via long-term measurements at the
slope of a Coulomb peak revealing potential fluctuations of at 1\,Hz. This makes h-BN the dielectric with
the currently lowest gate hysteresis and lowest low-frequency potential
fluctuations reported for low-gap III-V nanowires. The extracted values are
similar to state-of-the art quantum dots within Si/SiGe and Si/SiO
systems
Parity transitions in the superconducting ground state of hybrid InSb-Al Coulomb islands
The number of electrons in small metallic or semiconducting islands is
quantized. When tunnelling is enabled via opaque barriers this number can
change by an integer. In superconductors the addition is in units of two
electron charges (2e), reflecting that the Cooper pair condensate must have an
even parity. This ground state (GS) is foundational for all superconducting
qubit devices. Here, we study a hybrid superconducting-semiconducting island
and find three typical GS evolutions in a parallel magnetic field: a robust
2e-periodic even-parity GS, a transition to a 2e-periodic odd-parity GS,and a
transition from a 2e- to a 1e-periodic GS. The 2e-periodic odd-parity GS
persistent in gate-voltage occurs when a spin-resolved subgap state crosses
zero energy. For our 1e-periodic GSs we explicitly show the origin being a
single zero-energy state gapped from the continuum, i.e. compatible with an
Andreev bound states stabilized at zero energy or the presence of Majorana zero
modes
Impact of junction length on supercurrent resilience against magnetic field in InSb-Al nanowire Josephson junctions
Semiconducting nanowire Josephson junctions represent an attractive platform
to investigate the anomalous Josephson effect and detect topological
superconductivity by studying Josephson supercurrent. However, an external
magnetic field generally suppresses the supercurrent through hybrid nanowire
junctions and significantly limits the field range in which the supercurrent
phenomena can be studied. In this work, we investigate the impact of the length
of InSb-Al nanowire Josephson junctions on the supercurrent resilience against
magnetic fields. We find that the critical parallel field of the supercurrent
can be considerably enhanced by reducing the junction length. Particularly, in
30 nm-long junctions supercurrent can persist up to 1.3 T parallel field -
approaching the critical field of the superconducting film. Furthermore, we
embed such short junctions into a superconducting loop and obtain the
supercurrent interference at a parallel field of 1 T. Our findings are highly
relevant for multiple experiments on hybrid nanowires requiring a magnetic
field-resilient supercurrent.Comment: 17 pages, 5 figures in main text. 22 pages, 10 figures in supporting
informatio
Selectivity Map for Molecular Beam Epitaxy of Advanced III-V Quantum Nanowire Networks
This is an open access article published under an ACS AuthorChoice License. See Standard ACS AuthorChoice/Editors' Choice Usage Agreement - https://pubs.acs.org/page/policy/authorchoice_termsofuse.htmlSelective-area growth is a promising technique for enabling of the fabrication of the scalable III-V nanowire networks required to test proposals for Majorana-based quantum computing devices. However, the contours of the growth parameter window resulting in selective growth remain undefined. Herein, we present a set of experimental techniques that unambiguously establish the parameter space window resulting in selective III-V nanowire networks growth by molecular beam epitaxy. Selectivity maps are constructed for both GaAs and InAs compounds based on in situ characterization of growth kinetics on GaAs(001) substrates, where the difference in group III adatom desorption rates between the III-V surface and the amorphous mask area is identified as the primary mechanism governing selectivity. The broad applicability of this method is demonstrated by the successful realization of high-quality InAs and GaAs nanowire networks on GaAs, InP, and InAs substrates of both (001) and (111)B orientations as well as homoepitaxial InSb nanowire networks. Finally, phase coherence in Aharonov-Bohm ring experiments validates the potential of these crystals for nanoelectronics and quantum transport applications. This work should enable faster and better nanoscale crystal engineering over a range of compound semiconductors for improved device performance
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