13 research outputs found
Effective g-factor in Majorana Wires
We use the effective g-factor of subgap states, g*, in hybrid InAs nanowires
with an epitaxial Al shell to investigate how the superconducting density of
states is distributed between the semiconductor core and the metallic shell. We
find a step-like reduction of g* and improved hard gap with reduced carrier
density in the nanowire, controlled by gate voltage. These observations are
relevant for Majorana devices, which require tunable carrier density and g*
exceeding the g-factor of the proximitizing superconductor. Additionally, we
observe the closing and reopening of a gap in the subgap spectrum coincident
with the appearance of a zero-bias conductance peak
Theory of Caroli-de Gennes-Matricon analogs in full-shell nanowires
Full-shell nanowires are hybrid nanostructures consisting of a semiconducting
core encapsulated in an epitaxial superconducting shell. When subject to an
external magnetic flux, they exhibit the Little-Parks (LP) phenomenon of
flux-modulated superconductivity, an effect connected to the physics of
Abrikosov vortex lines in type-II superconductors. We show that full-shell
nanowires can host subgap states that are a variant of the Caroli-de
Gennes-Matricon (CdGM) states in vortices. These CdGM analogs are in fact
shell-induced Van Hove singularities in propagating core subbands. We elucidate
their structure, parameter dependence and behavior in tunneling spectroscopy
through a series of models of growing complexity. We show through microscopic
numerical simulations that they exhibit a characteristic skewness towards high
magnetic fields inside non-zero LP lobes resulting from the interplay of three
ingredients. First, core subbands exhibit a diamagnetic response, so that they
disperse with flux depending on their generalized angular momentum. Second, the
band bending at the core/shell interface induces a ring-like profile on the
CdGM analog state wavefunctions with average radius smaller than the core
radius. And last, degeneracy points emerge where all the CdGM Van Hove
singularities coalesce. This happens when the flux threading each wavefunction
is equal to an integer multiple of the flux quantum, a condition that shifts
the degeneracy points away from the center of the LP lobes, skewing the CdGM
analogs. Our analysis unlocks a transparent analytical description that allows
to extract precise microscopic information about the nanowire by measuring the
energy and skewness of CdGM analogs.Comment: 15 pages, 9 figure
Selective Area Grown Semiconductor-Superconductor Hybrids: A Basis for Topological Networks
We introduce selective area grown hybrid InAs/Al nanowires based on molecular
beam epitaxy, allowing arbitrary semiconductor-superconductor networks
containing loops and branches. Transport reveals a hard induced gap and
unpoisoned 2e-periodic Coulomb blockade, with temperature dependent 1e features
in agreement with theory. Coulomb peak spacing in parallel magnetic field
displays overshoot, indicating an oscillating discrete near-zero subgap state
consistent with device length. Finally, we investigate a loop network, finding
strong spin-orbit coupling and a coherence length of several microns. These
results demonstrate the potential of this platform for scalable topological
networks among other applications.Comment: NBI QDEV 201
Spin-polarized bound states in semiconductor-superconductor-ferromagnetic insulator islands
We report Coulomb blockade transport studies of InAs nanowires grown with
epitaxial superconducting Al and ferromagnetic insulator EuS on overlapping
facets. By comparing experimental results to a theoretical model, we associate
cotunneling features in even-odd bias spectra with spin-polarized Andreev
levels, indicating that spin splitting exceeding the induced superconducting
gap at zero applied magnetic field. Energies of the polarized subgap states can
be tuned on either side of zero by electrostatic gates
Supercurrent transport through 1-periodic full-shell Coulomb islands
We experimentally investigate supercurrent through Coulomb islands, where
island and leads are fabricated from semiconducting nanowires with fully
surrounding superconducting shells. Applying flux along the wire yields a
series of destructive Little-Parks lobes with reentrant supercurrent. We find
Coulomb blockade with 2 peak spacing in the zeroth lobe and 1 average
spacing, with regions of significant even-odd modulation, in the first lobe.
Evolution of Coulomb-peak amplitude through the first lobe is consistent with a
theoretical model of supercurrent carried predominantly by zero-energy states
in the leads and the island.Comment: 11 pages, 5+5 figure
Supercurrent reversal in ferromagnetic hybrid nanowire Josephson junctions
We report supercurrent transport measurements in hybrid Josephson junctions
comprised of semiconducting InAs nanowires with epitaxial ferromagnetic
insulator EuS and superconducting Al coatings. The wires display a hysteretic
superconducting window close to the coercivity, away from zero external
magnetic field. Using a multi-interferometer setup, we measure the
current-phase relation of multiple magnetic junctions and find an abrupt switch
between and 0 phases within the superconducting window. We attribute the
0- transition to the discrete flipping of the EuS domains and provide a
qualitative theory showing that a sizable exchange field can polarize the
junction and lead to the supercurrent reversal. Both and phases can
be realized at zero external field by demagnetizing the wire
Selective-Area-Grown Semiconductor-Superconductor Hybrids: A Basis for Topological Networks
In-plane selective area InSb–Al nanowire quantum networks
Strong spin-orbit semiconductor nanowires coupled to a superconductor are
predicted to host Majorana zero modes. Exchange (braiding) operations of
Majorana modes form the logical gates of a topological quantum computer and
require a network of nanowires. Here, we develop an in-plane selective-area
growth technique for InSb-Al semiconductor-superconductor nanowire networks
with excellent quantum transport properties. Defect-free transport channels in
InSb nanowire networks are realized on insulating, but heavily mismatched InP
substrates by 1) full relaxation of the lattice mismatch at the
nanowire/substrate interface on a (111)B substrate orientation, 2) nucleation
of a complete network from a single nucleation site, which is accomplished by
optimizing the surface diffusion length of the adatoms. Essential quantum
transport phenomena for topological quantum computing are demonstrated in these
structures including phase-coherent transport up to 10 m and a hard
superconducting gap accompanied by 2-periodic Coulomb oscillations with an
Al-based Cooper pair island integrated in the nanowire network.Comment: Data repository is available at
https://doi.org/10.5281/zenodo.4589484 . Author version of the text before
peer review, while see DOI for the published versio