Scaling of Majorana Zero-Bias Conductance Peaks

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.Comment: Accepted in Physical Review Letter

Evidence of topological superconductivity in planar Josephson junctions

Majorana zero modes are quasiparticle states localized at the boundaries of topological superconductors that are expected to be ideal building blocks for fault-tolerant quantum computing. Several observations of zero-bias conductance peaks measured in tunneling 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 to confine Ma jorana channels within planar Josephson junctions, in which the phase difference {\phi} between the superconducting leads represents an additional tuning knob predicted to drive the system into the topological phase at lower magnetic fields. Here, we report the observation of phase-dependent zero-bias conductance peaks measured by tunneling spectroscopy at the end of Josephson junctions realized on a InAs/Al heterostructure. Biasing the junction to {\phi} ~ {\pi} significantly reduces the critical field at which the zero-bias peak appears, with respect to {\phi} = 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. Besides providing experimental evidence of phase-tuned topological superconductivity, our devices are compatible with superconducting quantum electrodynamics architectures and scalable to complex geometries needed for topological quantum computing.Comment: main text and extended dat

Thermogenic and bacterial hydrocarbon gases (free and sorbed) in Middle Valley, Juan De Fuca Ridge, Leg 139

The sorbed gases at all four Ocean Drilling Program Leg 139 sites (855, 856, 857, and 858) showed the unmistakable presence of thermogenic hydrocarbons. No indication of abiogenic gas was found. The evidence for the thermogenic hydrocarbons includes elevated contents of higher hydrocarbons (i.e., C₁/[C₂ + C₃] ca. 2-20), δ¹³CCH₄ between -30‰ and -45‰ (PDB), δ¹³CC₂H₆ between -18.7‰ and -26. l‰, and δ¹³CC₃H₈ from -20.5‰ to -25. l‰. The carbon isotope ratios of C₂ and C₃ indicate that the organic matter that generated these hydrocarbons is mature to overmature (1.8% to 4% Rο equivalent) reflecting the range in higher heat flow in the region. Considering the geologic setting, it is highly probable that the thermogenic gas was formed by hydrothermal processes. The presence of considerable amounts of ethene and propene (up to 7.1 hydrocarbon percent [h.c.%]) along with the accelerated maturation of organic matter support this conclusion. Some of the sorbed gases, especially at Sites 857 and 858, have a possible bacterial gas component admixed with the thermogenic, as is also seen in the free gases. The free gases are generally distinct from the sorbed gases and have C₁/(C₂+ C₃) ratios up to 200 and δ¹³CCH₄ as light as -56. l‰. The bacterial hydrocarbons in the free gas are most prominent at depths where bacterial sulfate reduction has removed most or all of the dissolved sulfate. It is uncertain from the geochemical information whether or not the hydrothermal gases are autothonous or have migrated into the sediments vertically or laterally. However, the general increase in hydrocarbon concentration with depth could indicate a limited upward diffusion or advection which is consistent with the heat and fluid flow information for these holes

Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism

Diagenetic relationships of methanogenesis, nutrients, acoustic turbidity, pockmarks and freshwater seepages in Eckernförde Bay

High organic loading (4-5 wt%) and sedimentation rates (1.4 mm yr(-1)) in Eckernforde Bay sediments lead to anaerobic conditions within the uppermost 15 cm. Intense bacteria] sulphate reduction (0.011-0.15 mM SO4red2- yr(-1)) exhausts dissolved sulphate around 150 cm sediment depth, resulting in methanogenesis at greater sediment depth by carbonate reduction (1.8-8.5 muM CH4 yr(-1)). Extensive regions of Eckernforde Bay are characterized by acoustically turbid sediments (> 300 cm sediment depth), resulting from deeper sediments supersaturated in methane, forming free gas accumulation zones. Methane migrating upward into the sulphate reduction zone is effectively consumed by sulphate reducing bacteria at rates 3-10 times greater than methanogenesis, i.e. 46-95 muM CH4 (ox) yr(-1). The pathways of methane formation and anaerobic oxidation are confirmed by stable carbon and hydrogen isotope evidence. Pockmarks, i.e. shallow surface depressions in some Eckernforde Bay sediments, are not associated with gas ebullition; rather, these physical features result from the expulsion of freshwater. These episodic springs or seepages of groundwater from the underlying, Holocene glacial lags and sands into the basin at pockmark sites have characteristic low chloride and methane concentrations. The geochemical evidence indicates that the commercial accumulation of petroleum in the lower Cretaceous, Dogger-Beta-Hauptsandstein Schwedeneck field in Eckernforde Bay (1500 mbsf) has no surface manifestation and does not influence either the occurrence of acoustic turbidity or pockmark