3,732 research outputs found
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, , versus the phase, , 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 ~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
Social groupings in fishes
p. 397-481, [11] p. of plates : ill. ; 27 cm.Includes bibliographical references (p. 475-481)
Geometry of symmetry
p. 325-412, [2] p. of plates : ill. ; 27 cm.Includes bibliographical references (p. 408-412)
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
Pentagonal symmetry
p. 177-219, 2 p. of plates : ill. ; 27 cm.Includes bibliographical references (p. 217)
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