153 research outputs found
AC Josephson effect in finite-length nanowire junctions with Majorana modes
It has been predicted that superconducting junctions made with topological
nanowires hosting Majorana bound states (MBS) exhibit an anomalous
4\pi-periodic Josephson effect. Finding an experimental setup with these
unconventional properties poses, however, a serious challenge: for
finite-length wires, the equilibrium supercurrents are always 2\pi-periodic as
anticrossings of states with the same fermionic parity are possible. We show,
however, that the anomaly survives in the transient regime of the ac Josephson
effect. Transients are moreover protected against decay by quasiparticle
poisoning as a consequence of the quantum Zeno effect, which fixes the parity
of Majorana qubits. The resulting long-lived ac Josephson transients may be
effectively used to detect MBS.Comment: 9 pages, 4 figures, published version (with supplementary material
Quantum Hall effect in graphene with twisted bilayer stripe defects
We analyze the quantum Hall effect in single layer graphene with bilayer
stripe defects. Such defects are often encountered at steps in the substrate of
graphene grown on silicon carbide. We show that AB or AA stacked bilayer
stripes result in large Hall conductivity fluctuations that destroy the quantum
Hall plateaux. The fluctuations are a result of the coupling of edge states at
opposite edges through currents traversing the stripe. Upon rotation of the
second layer with respect to the continuous monolayer (a twisted-bilayer stripe
defect), such currents decouple from the extended edge states and develop into
long-lived discrete quasi bound states circulating around the perimeter of the
stripe. Backscattering of edge modes then occurs only at precise resonant
energies, and hence the quantum Hall plateaux are recovered as twist angle
grows.Comment: 8 pages, 7 figures, published versio
Transport spectroscopy of NS nanowire junctions with Majorana fermions
We investigate transport though normal-superconductor nanowire junctions in
the presence of spin-orbit coupling and magnetic field. As the Zeeman field
crosses the critical bulk value B_c of the topological transition, a Majorana
bound state (MBS) is formed, giving rise to a sharp zero-bias anomaly (ZBA) in
the tunneling differential conductance. We identify novel features beyond this
picture in wires with inhomogeneous depletion, like the appearance of two MBSs
inside a long depleted region for B<B_c. The resulting ZBA is in most cases
weakly split and may coexist with Andreev bound states near zero energy. The
ZBA may appear without evidence of a topological gap closing. This latter
aspect is more evident in the multiband case and stems from a smooth pinch-off
barrier. Most of these features are in qualitative agreement with recent
experiments [Mourik et al, Science 336, 1003 (2012)]. We also discuss the rich
phenomenology of the problem in other regimes which remain experimentally
unexplored.Comment: 10 pages, 12 figures. Published version, supplementary material
include
Mapping the Topological Phase Diagram of Multiband Semiconductors with Supercurrents
We show that Josephson junctions made of multiband semiconductors with strong
spin-orbit coupling carry a critical supercurrent that contains
information about the non-trivial topology of the system. In particular, we
find that the emergence and annihilation of Majorana bound states in the
junction is reflected in strong even-odd effects in at small junction
transparency. This effect allows for a mapping between and the
topological phase diagram of the junction, thus providing a dc measurement of
its topology.Comment: 5 pages 3 figures. Published versio
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
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
Quantifying wave-function overlaps in inhomogeneous Majorana nanowires
A key property of Majorana zero modes is their protection against local perturbations. In the standard picture, this protection is guaranteed by a high degree of spatial nonlocality of the Majoranas, namely a suppressed wave-function overlap, in the topological phase. However, a careful characterization of resilience to local noise goes beyond mere spatial separation and must also take into account the projection of wave-function spin. By considering the susceptibility of a given zero mode to different local perturbations, we find the relevant forms of spin-resolved wave-function overlaps that measure its resilience. We quantify these overlaps and study their dependence with nanowire parameters in several classes of experimentally relevant configurations. These include nanowires with inhomogeneous depletion and induced pairing, barriers, and quantum dots. Smooth inhomogeneities have been shown to produce near-zero modes, so-called pseudo-Majoranas, below the critical Zeeman field in the bulk. Surprisingly, their resilience is found to be comparable or better than that of topological Majoranas in realistic systems. We further study how accurately their overlaps can be estimated using a purely local measurement on one end of the nanowire, accessible through conventional transport experiments. In uniform nanowires, this local estimator is remarkably accurate. In inhomogeneous cases, it is less accurate but can still provide reasonable estimates for potential inhomogeneities of the order of the superconducting gap. We further analyze the zero-mode wave-function structure, spin texture, and spectral features associated with each type of inhomogeneity. All our results highlight the strong connection between internal wave-function degrees of freedom, nonlocality, and protection in smoothly inhomogeneous nanowiresWe acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through Grants No. FIS2015-65706-P, No. FIS2015-64654-P, and No. FIS2016-80434-P (AEI/FEDER, EU), the Ramón y Cajal programme, Grants No. RYC-2011-09345 and No. RYC-2013-14645, and the “María de Maeztu” Programme for Units of Excellence in Research and Development (MDM-2014-0377
Laser-induced quantum pumping in graphene
We investigate non-adiabatic electron pumping in graphene generated by laser
irradiation with linear polarization parallel or perpendicular to the transport
direction. Transport is dominated by the spatially asymmetric excitation of
electrons from evanescent into propagating modes. For a laser with parallel
polarization, the pumping response exhibits a subharmonic resonant enhancement
which directly probes the Fermi energy; no such enhancement occurs for
perpendicular polarization. The resonance mechanism relies on the chirality of
charge carriers in graphene.Comment: 3 pages, 3 figure
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