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.
We demonstrate concomitant measurement of phase-dependent critical current and Andreev bound state spectrum in a highly transmissive InAs Josephson junction embedded in a dc superconducting quantum ...interference device (SQUID). Tunneling spectroscopy reveals Andreev bound states with near unity transmission probability. A nonsinusoidal current-phase relation is derived from the Andreev spectrum, showing excellent agreement with the one extracted from the SQUID critical current. Both measurements are reconciled within a short junction model where multiple Andreev bound states, with various transmission probabilities, contribute to the entire supercurrent flowing in the junction.
Abstract
Majorana zero modes are leading candidates for topological quantum computation due to non-local qubit encoding and non-abelian exchange statistics. Spatially separated Majorana modes are ...expected to allow phase-coherent single-electron transport through a topological superconducting island via a mechanism referred to as teleportation. Here we experimentally investigate such a system by patterning an elongated epitaxial InAs-Al island embedded in an Aharonov-Bohm interferometer. With increasing parallel magnetic field, a discrete sub-gap state in the island is lowered to zero energy yielding persistent 1
e
-periodic Coulomb blockade conductance peaks (
e
is the elementary charge). In this condition, conductance through the interferometer is observed to oscillate in a perpendicular magnetic field with a flux period of
h
/
e
(
h
is Planck’s constant), indicating coherent transport of single electrons through the islands, a signature of electron teleportation via Majorana modes.
We present measurements of one-dimensional superconductor-semiconductor Coulomb islands, fabricated by gate confinement of a two-dimensional InAs heterostructure with an epitaxial Al layer. When ...tuned via electrostatic side gates to regimes without subgap states, Coulomb blockade reveals Cooper-pair mediated transport. When subgap states are present, Coulomb peak positions and heights oscillate in a correlated way with magnetic field and gate voltage, as predicted theoretically, with (anti)crossings in (parallel) transverse magnetic field indicating Rashba-type spin-orbit coupling. Overall results are consistent with a picture of overlapping Majorana zero modes in finite wires.
Majorana zero modes-quasiparticle states localized at the boundaries of topological superconductors-are expected to be ideal building blocks for fault-tolerant quantum computing
. Several ...observations of zero-bias conductance peaks measured by tunnelling spectroscopy above a critical magnetic field have been reported as experimental indications of Majorana zero modes in superconductor-semiconductor nanowires
. On the other hand, two-dimensional systems offer the alternative approach of confining Majorana channels within planar Josephson junctions, in which the phase difference φ between the superconducting leads represents an additional tuning knob that is predicted to drive the system into the topological phase at lower magnetic fields than for a system without phase bias
. Here we report the observation of phase-dependent zero-bias conductance peaks measured by tunnelling spectroscopy at the end of Josephson junctions realized on a heterostructure consisting of aluminium on indium arsenide. Biasing the junction to φ ≈ π reduces the critical field at which the zero-bias peak appears, with respect to φ = 0. The phase and magnetic-field dependence of the zero-energy states is consistent with a model of Majorana zero modes in finite-size Josephson junctions. As well as providing experimental evidence of phase-tuned topological superconductivity, our devices are compatible with superconducting quantum electrodynamics architectures
and are scalable to the complex geometries needed for topological quantum computing
.
Scaling of Majorana Zero-Bias Conductance Peaks Nichele, Fabrizio; Drachmann, Asbjørn C C; Whiticar, Alexander M ...
Physical review letters,
09/2017, Letnik:
119, Številka:
13
Journal Article
Recenzirano
Odprti dostop
We report an experimental study of the scaling of zero-bias conductance peaks compatible with Majorana zero modes as a function of magnetic field, tunnel coupling, and temperature in one-dimensional ...structures fabricated from an epitaxial semiconductor-superconductor heterostructure. Results are consistent with theory, including a peak conductance that is proportional to tunnel coupling, saturates at 2e^{2}/h, decreases as expected with field-dependent gap, and collapses onto a simple scaling function in the dimensionless ratio of temperature and tunnel coupling.
The Andreev spectrum of a quantum dot embedded in a hybrid semiconductor-superconductor interferometer can be modulated by electrostatic gating, magnetic flux through the interferometer, and Zeeman ...splitting from an in-plane magnetic field. We demonstrate parity transitions in the embedded quantum dot system and show that the Zeeman-driven transition is accompanied by a 0-π transition in the superconducting phase across the dot. We further demonstrate that flux through the interferometer modulates both dot parity and 0-π transitions.
Methane (CH4), often in substantial quantities, is reported for numerous surface manifestations (seeps, springs) and aquifers in continental serpentinized ultramafic rocks, in ophiolites, peridotite ...massifs and intrusions. Frequently, this gas is considered to have a dominant abiotic origin, with variable, though generally minor, components of biotic gas (thermogenic or microbial). Abiotic CH4 production through low temperature Fischer-Tropsch Type reactions (FTT) between a carbon (C) compound and H2 is endorsed by most of scholars, but direct derivation from olivine hydration (serpentinization), magmatic sources and fluid inclusions have also been suggested. Here, we review the application of FTT in geology, discussing the appropriateness of several C compounds (CO2, CO, formic acid, formate or elemental C) as CH4 precursors and of aqueous versus gas phase reactions. We examine published gas geochemical and flux data that provide clear constraints on the methane origins. In the analysis we add new isotopic and gas seepage data acquired in surface gas manifestations at Acquasanta, in the Voltri ophiolite (Genova, Italy). Multiple lines of evidence including (a) stable isotope compositions of CH4, CO2 and helium, (b) radiocarbon-free CH4, (c) isotopic disequilibrium between CH4 and H2O, (d) low temperatures of CH4 production based on clumped isotopes and heat flow data, and (e) methane seepage forms and intensities, suggest that CH4 is not formed directly in water or from magmatic sources or fluid inclusions. Rather, all the lines of evidence taken together are compatible with the hypothesis of low temperature (<140 °C) Sabatier reactions (CO2 hydrogenation) in gas-phase and within metal-rich (catalyst) ultramafic rocks. A similar abiotic origin could occur for methane observed in Precambrian shields. Chromitites can support considerable rates of gas generation, potentially higher than those in some shales. In particular, the gas flux intensity and seepage distribution suggest that gas-bearing ultramafic rock systems may be considered analogous to conventional, biotic natural gas systems where, after production in source rocks, CH4 could migrate, accumulate in reservoir rocks and seep to the surface. Microbial CH4 generated in water at lower temperatures, generally as minor secondary contributions, may also commingle in these environments.
•The application of Fischer-Tropsch Type reaction in geology is examined.•Laboratory and field data suggest abiotic CH4 generation via low T Sabatier reaction.•Sabatier reaction must be in gas-phase.•CO, formate, formic acid or elemental C are unlikely CH4 precursors.•Magmatic, high T processes and fluid inclusion hypotheses are unlikely.
The diagenetic cycling of carbon within recent unconsolidated sediments and soils generally can be followed more effectively by discerning changes in the dissolved constituents of the interstitial ...fluids, rather than by monitoring changes in the bulk or solid organic components. The major dissolved carbon species in diagenetic settings are represented by the two carbon redox end-members CH4 and CO2. Bacterial uptake by methanogens of either CO2 or “preformed” reduced carbon substrates such as acetate, methanol or methylated amines can be tracked with the aid of carbon (13C/12C) and hydrogen (D/H≡2H/1H) isotopes. The bacterial reduction of CO2 to CH4 is associated with a kinetic isotope effect (KIE) for carbon which discriminates against 13C. This leads to carbon isotope separation between CO2 and CH4 (εC) exceeding 95 and gives rise to δ13CCH4 values as negative as −110‰ vs. PDB. The carbon KIE associated with fermentation of methylated substrates is lower (εC is ca. 40 to 60, with δ13CCH4 values of −50‰ to −60‰). Hydrogen isotope effects during methanogenesis of methylated substrates can lead to deuterium depletions as large as δDCH4=−531‰ vs. SMOW, whereas, bacterial D/H discrimination for the CO2-reduction pathway is significantly less (δDCH4 ca. −170‰ to −250‰). These field observations have been confirmed by culture experiments with labeled isotopes, although hydrogen isotope exchange and other factors may influence the hydrogen distributions. Bacterial consumption of CH4, both aerobic and anaerobic, is also associated with KIEs for C and H isotopes that enrich the residual CH4 in the heavier isotopes. Carbon fractionation factors related to CH4 oxidation are generally less than εC=10, although values >20 are known. The KIE for hydrogen (εH) during aerobic and anaerobic CH4 oxidation is between 95 and 285. The differences in C and H isotope ratios of CH4, in combination with the isotope ratios of the coexisting H2O and CO2 pairs, differentiate the various bacterial CH4 generation and consumption pathways, and elucidate the cycling of labile sedimentary carbon.
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