52 research outputs found
Odd-frequency superconducting pairing in Kitaev-based junctions
We investigate odd-frequency superconducting correlations in
normal-superconductor (NS) and short superconductor-normal-superconductor (SNS)
junctions with the S region described by the Kitaev model of spinless fermions
in one dimension. We demonstrate that, in both the trivial and topological
phases, Andreev reflection is responsible for the coexistence of even- and
odd-frequency pair amplitudes at interfaces, while normal reflections solely
contribute to odd-frequency pairing. At NS interfaces we find that the
odd-frequency pair amplitude exhibits large, but finite, values in the
topological phase at low frequencies. This enhancement is due to the emergence
of a Majorana zero mode at the interface, but notably there is no divergence
and a finite odd-frequency pair amplitude also exists outside the topological
phase. We also show that the local density of states and local odd-frequency
pairing can be characterized solely by Andreev reflections deep in the
topological phase. Moreover, in the topological phase of short SNS junctions,
we find that both even- and odd-frequency amplitudes capture the emergence of
topological Andreev bound states. For a superconducting phase difference
the odd-frequency magnitude exhibits a linear frequency () dependence at low-frequencies, while at it develops a
resonance peak () due to the protected Majorana zero modes.Comment: 12 pages, 7 figures + 7 pages of supplemental material. Published
versio
SNS junctions in nanowires with spin-orbit coupling: role of confinement and helicity on the sub-gap spectrum
We study normal transport and the sub-gap spectrum of
superconductor-normal-superconductor (SNS) junctions made of semiconducting
nanowires with strong Rashba spin-orbit coupling. We focus, in particular, on
the role of confinement effects in long ballistic junctions. In the normal
regime, scattering at the two contacts gives rise to two distinct features in
conductance, Fabry-Perot resonances and Fano dips. The latter arise in the
presence of a strong Zeeman field that removes a spin sector in the leads
(\emph{helical} leads), but not in the central region. Conversely, a helical
central region between non-helical leads exhibits helical gaps of half-quantum
conductance, with superimposed helical Fabry-Perot oscillations. These normal
features translate into distinct subgap states when the leads become
superconducting. In particular, Fabry-Perot resonances within the helical gap
become parity-protected zero-energy states (parity crossings), well below the
critical field at which the superconducting leads become topological. As
a function of Zeeman field or Fermi energy, these zero-modes oscillate around
zero energy, forming characteristic loops, which evolve continuously into
Majorana bound states as exceeds . The relation with the physics of
parity crossings of Yu-Shiba-Rusinov bound states is discussed.Comment: 12 pages main article, 14 figures + 5 pages supplementary material, 5
figures. Added new appendix. Other minor changes. Published versio
Non-Hermitian phase-biased Josephson junctions
We study non-Hermitian Josephson junctions formed by conventional
superconductors with a finite phase difference under non-Hermiticity naturally
appearing due to coupling to normal reservoirs. Depending on the structure of
non-Hermiticity, captured here in terms of retarded self-energies, the
low-energy spectrum hosts topologically stable exceptional points either at
zero or finite real energies as a function of the superconducting phase
difference. Interestingly, we find that the corresponding phase-biased
supercurrents acquire divergent profiles at such exceptional points, an
instance that turns out to be a natural and unique non-Hermitian effect that
signals a possible way to enhance the sensitivity of Josephson junctions. Our
work thus opens the way for realizing unique non-Hermitian phenomena due to the
interplay between non-Hermitian topology and the Josephson effect.Comment: 8 pages, 4 figures + 2 pages supplemental materia
Multiple Andreev reflection and critical current in topological superconducting nanowire junctions
We study transport in a voltage biased superconductor-normal-superconductor
(SNS) junction made of semiconducting nanowires with strong spin-orbit
coupling, as it transitions into a topological superconducting phase for
increasing Zeeman field. Despite the absence of a fractional steady-state ac
Josephson current in the topological phase, the dissipative multiple Andreev
reflection (MAR) current I_dc at different junction transparencies is
particularly revealing. It exhibits unique features related to topology, such
as the gap inversion, the formation of Majorana bound states, and
fermion-parity conservation. Moreover, the critical current I_c, which
remarkably does not vanish at the critical point where the system becomes
gapless, provides direct evidence of the topological transition.Comment: Published version, 21 pages, 7 figures, 3 appendice
Confinement-induced zero-bias peaks in conventional superconductor hybrids
Majorana bound states in topological superconductors have been predicted to appear in the form of zero-bias conductance peaks of height 2e2/h, which represents one of the most studied signatures so far. Here, we show that quasi-zero-energy states, similar to Majorana bound states, can naturally form in any superconducting hybrid junction due to confinement effects, in the absence of spin fields and, thus, without relation to topology. Remarkably, these topologically trivial quasi-zero-energy states produce zero-bias conductance peaks, that could be similar to Majorana signatures, but develop a different peak height ((4e2/h) and are less stable under gating or depletion of the confined region. Our results put forward confinement as an alternative mechanism to explain the ubiquitous presence of trivial zero-bias peaks and quasi-zero-energy states in superconductor hybrid
Exceptional odd-frequency pairing in non-Hermitian superconducting systems
We first show the realization of exceptional points in a non-Hermitian
superconducting system based on a conventional superconductor and then
demonstrate that, surprisingly, the system hosts odd-frequency pairing, solely
generated by the non-Hermiticity. While there is a coexistence of even- and
odd-frequency pairs under general conditions, we find that the even-frequency
term vanishes at the exceptional degeneracies, leaving only odd-frequency
pairing. This exceptional odd-frequency pairing is directly given by the
imaginary part of the eigenvalues at the exceptional points and can be measured
from the spectral function. Our results thus put forward non-Hermitian systems
as a powerful platform to realize odd-frequency superconducting pairing.Comment: 6 pages, 4 figures + 4 pages supplemental materia
Hybrid superconductor-semiconductor nanowire junctions as useful platforms to study Majorana bound states
Tesis Doctoral inédita leÃda en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de FÃsica de la Materia Condensada. Fecha de lectura: 20-05-2016Esta tesis tiene embargado el acceso al texto completo hasta el 20-11-201
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