Direct Graphene Growth on Insulator

Fabrication of graphene devices is often hindered by incompatibility between the silicon technology and the methods of graphene growth. Exfoliation from graphite yields excellent films but is good mainly for research. Graphene grown on metal has a technological potential but requires mechanical transfer. Growth by SiC decomposition requires a temperature budget exceeding the technological limits. These issues could be circumvented by growing graphene directly on insulator, implying Van der Waals growth. During growth, the insulator acts as a support defining the growth plane. In the device, it insulates graphene from the Si substrate. We demonstrate planar growth of graphene on mica surface. This was achieved by molecular beam deposition above 600{\deg}C. High resolution Raman scans illustrate the effect of growth parameters and substrate topography on the film perfection. Ab initio calculations suggest a growth model. Data analysis highlights the competition between nucleation at surface steps and flat surface. As a proof of concept, we show the evidence of electric field effect in a transistor with a directly grown channel.Comment: 13 pages, 6 figure

Current-phase relations of few-mode InAs nanowire Josephson junctions

Gate-tunable semiconductor nanowires with superconducting leads have great potential for quantum computation and as model systems for mesoscopic Josephson junctions. The supercurrent, $I$, versus the phase, $\phi$, across the junction is called the current-phase relation (CPR). It can reveal not only the amplitude of the critical current, but also the number of modes and their transmission. We measured the CPR of many individual InAs nanowire Josephson junctions, one junction at a time. Both the amplitude and shape of the CPR varied between junctions, with small critical currents and skewed CPRs indicating few-mode junctions with high transmissions. In a gate-tunable junction, we found that the CPR varied with gate voltage: Near the onset of supercurrent, we observed behavior consistent with resonant tunneling through a single, highly transmitting mode. The gate dependence is consistent with modeled subband structure that includes an effective tunneling barrier due to an abrupt change in the Fermi level at the boundary of the gate-tuned region. These measurements of skewed, tunable, few-mode CPRs are promising both for applications that require anharmonic junctions and for Majorana readout proposals

Canal Seepage Reduction by Soil Compaction

Large-scale tests were conducted of in-place compaction of irrigation district earthen canal bottoms and sides. Five canal pools with sandy loam soils were compacted. Seepage reduction of about 86% was obtained when the sides and bottoms were compacted; reductions of 12 – 31% were obtained when only sides were compacted

Zero-Energy Modes from Coalescing Andreev States in a Two-Dimensional Semiconductor-Superconductor Hybrid Platform

We investigate zero-bias conductance peaks that arise from coalescing subgap Andreev states, consistent with emerging Majorana zero modes, in hybrid semiconductor-superconductor wires defined in a two-dimensional InAs/Al heterostructure using top-down lithography and gating. The measurements indicate a hard superconducting gap, ballistic tunneling contact, and in-plane critical fields up to $3$~T. Top-down lithography allows complex geometries, branched structures, and straightforward scaling to multicomponent devices compared to structures made from assembled nanowires.Comment: Includes Supplementary Materia

Suppressing quasiparticle poisoning with a voltage-controlled filter

We study single-electron charging events in an Al/InAs nanowire hybrid system with deliberately introduced gapless regions. The occupancy of a Coulomb island is detected using a nearby radio-frequency quantum dot as a charge sensor. We demonstrate that a 1 micron gapped segment of the wire can be used to efficiently suppress single electron poisoning of the gapless region and therefore protect the parity of the island while maintaining good electrical contact with a normal lead. In the absence of protection by charging energy, the 1e switching rate can be reduced below 200 per second. In the same configuration, we observe strong quantum charge fluctuations due to exchange of electron pairs between the island and the lead. The magnetic field dependence of the poisoning rate yields a zero-field superconducting coherence length of ~ 90 nm

Geometry of symmetry

p. 325-412, [2] p. of plates : ill. ; 27 cm.Includes bibliographical references (p. 408-412)

Social groupings in fishes

p. 397-481, [11] p. of plates : ill. ; 27 cm.Includes bibliographical references (p. 475-481)

Urostyle in Umbra pygmaea

5 p. : ill. ; 24 cm.Includes bibliographical references (p. 5)

Fish sounds

p. 327-378, [2] p. of plates : ill. ; 27 cm.Includes bibliographical references (p. 372-374)."Studies on fish-produced sounds were made around the clock and through the seasons in a small Florida bay. 2. The dominant sounds were produced by Galeichthys felis and Opsanus beta, and an as yet unidentified species. Less frequently occurring unidentified sounds of considerable variety seemed to be made by transients or strays. 3. Galeichthys produced its 'percolator choruses' from April through October, with a summer lull in July and August, never earlier than 5:00 P.M. and never continued later than 10:50 P.M., with a duration ranging from nine minutes to four hours. 4. There was a distinct tendency for Galeichthys to be more sonic during the period near the new moon, a feature less noticeable in Opsanus. 5. Choruses of Galeichthys formed only when the water temperature ranged between 74 F. and 89 F. and began only after the light intensity fell to values of from 1 to 1900 foot-candles. 6. Opsanus produced its 'boat-whistle' sound from March to October, with a summer lull from May through July, most vigorous about the time Galeichthys choruses were full, but heard irregularly at all times of day and night. 7. The frequency of the 'boat-whistle' of Opsanus varied with water temperature, ranging from about 0.93 sound per minute at 74 F. to about 1.92 sounds per minute at 83 F., with none below 73 F. or above 91 F. 8. The third, but unidentified, sound-producer made its peculiar repetitional soft tapping sound from October to June, with a lull from January through March. 9. This third sound occurred in a temperature range of from 64 F. to 83 F. and with light intensities ranging from zero to 150 foot-candles; no clear relationship to moon phases could be established. 10. An appendix describes the apparatus used and in some cases developed"--P. 372

Symmetric Operation of the Resonant Exchange Qubit

We operate a resonant exchange qubit in a highly symmetric triple-dot configuration using IQ-modulated RF pulses. At the resulting three-dimensional sweet spot the qubit splitting is an order of magnitude less sensitive to all relevant control voltages, compared to the conventional operating point, but we observe no significant improvement in the quality of Rabi oscillations. For weak driving this is consistent with Overhauser field fluctuations modulating the qubit splitting. For strong driving we infer that effective voltage noise modulates the coupling strength between RF drive and the qubit, thereby quickening Rabi decay. Application of CPMG dynamical decoupling sequences consisting of up to n = 32 {\pi} pulses significantly prolongs qubit coherence, leading to marginally longer dephasing times in the symmetric configuration. This is consistent with dynamical decoupling from low frequency noise, but quantitatively cannot be explained by effective gate voltage noise and Overhauser field fluctuations alone. Our results inform recent strategies for the utilization of partial sweet spots in the operation and long-distance coupling of triple-dot qubits.Comment: 6 pages, 5 figure