189 research outputs found
X-ray variability of AGNs in the soft and the hard X-ray bands
We investigate the X-ray variability characteristics of hard X-ray selected
AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data.
The uncertainties involved in the individual dwell measurements of ASM are
critically examined and a method is developed to combine a large number of
dwells with appropriate error propagation to derive long duration flux
measurements (greater than 10 days). We also provide a general prescription to
estimate the errors in variability derived from rms values from unequally
spaced data. Though the derived variability for individual sources are not of
very high significance, we find that, in general, the soft X-ray variability is
higher than those in hard X-rays and the variability strengths decrease with
energy for the diverse classes of AGN. We also examine the strength of
variability as a function of the break time scale in the power density spectrum
(derived from the estimated mass and bolometric luminosity of the sources) and
find that the data are consistent with the idea of higher variability at time
scales longer than the break time scale.Comment: 17 pages, 15 Postscript figures, 3 tables, accepted for publication
in Ap
Supercurrent through a single transverse mode in nanowire Josephson junctions
Hybrid superconductor-semiconductor materials are fueling research in
mesoscopic physics and quantum technology. Recently demonstrated smooth
-Sn superconductor shells, due to the increased induced gap, are
expanding the available parameter space to new regimes. Fabricated on
quasiballistic InSb nanowires, with careful control over the hybrid interface,
Sn shells yield critical current-normal resistance products exceeding
temperature by at least an order of magnitude even when nanowire resistance is
of order 10k. In this regime Cooper pairs travel through a purely 1D
quantum wire for at least part of their trajectory. Here, we focus on the
evolution of supercurrent in magnetic field parallel to the nanowire. Long
decay up to fields of 1T is observed. At the same time, the decay for higher
occupied subbands is notably faster in some devices but not in others. We
analyze this using a tight-binding numerical model that includes the Zeeman,
orbital and spin-orbit effects. When the first subband is spin polarized, we
observe a dramatic suppression of supercurrent, which is also confirmed by the
model and suggests an absence of significant triplet supercurrent generation
Electric field tunable superconductor-semiconductor coupling in Majorana nanowires
We study the effect of external electric fields on
superconductor-semiconductor coupling by measuring the electron transport in
InSb semiconductor nanowires coupled to an epitaxially grown Al superconductor.
We find that the gate voltage induced electric fields can greatly modify the
coupling strength, which has consequences for the proximity induced
superconducting gap, effective g-factor, and spin-orbit coupling, which all
play a key role in understanding Majorana physics. We further show that level
repulsion due to spin-orbit coupling in a finite size system can lead to
seemingly stable zero bias conductance peaks, which mimic the behavior of
Majorana zero modes. Our results improve the understanding of realistic
Majorana nanowire systems.Comment: 10 pages, 5 figures, supplemental information as ancillary fil
Quantized Majorana conductance
Majorana zero-modes hold great promise for topological quantum computing.
Tunnelling spectroscopy in electrical transport is the primary tool to identify
the presence of Majorana zero-modes, for instance as a zero-bias peak (ZBP) in
differential-conductance. The Majorana ZBP-height is predicted to be quantized
at the universal conductance value of 2e2/h at zero temperature. Interestingly,
this quantization is a direct consequence of the famous Majorana symmetry,
'particle equals antiparticle'. The Majorana symmetry protects the quantization
against disorder, interactions, and variations in the tunnel coupling. Previous
experiments, however, have shown ZBPs much smaller than 2e2/h, with a recent
observation of a peak-height close to 2e2/h. Here, we report a quantized
conductance plateau at 2e2/h in the zero-bias conductance measured in InSb
semiconductor nanowires covered with an Al superconducting shell. Our
ZBP-height remains constant despite changing parameters such as the magnetic
field and tunnel coupling, i.e. a quantized conductance plateau. We distinguish
this quantized Majorana peak from possible non-Majorana origins, by
investigating its robustness on electric and magnetic fields as well as its
temperature dependence. The observation of a quantized conductance plateau
strongly supports the existence of non-Abelian Majorana zero-modes in the
system, consequently paving the way for future braiding experiments.Comment: 5 figure
On the limits to mobility in InAs quantum wells with nearly lattice-matched barriers
The growth and the density dependence of the low temperature mobility of a
series of two-dimensional electron systems confined to un-intentionally doped,
low extended defect density InAs quantum wells with AlGaSb
barriers are reported. The electron mobility limiting scattering mechanisms
were determined by utilizing dual-gated devices to study the dependence of
mobility on carrier density and electric field independently. Analysis of the
possible scattering mechanisms indicate the mobility was limited primarily by
rough interfaces in narrow quantum wells and a combination of alloy disorder
and interface roughness in wide wells at high carrier density within the first
occupied electronic sub-band. At low carrier density the functional dependence
of the mobility on carrier density provided evidence of coulombic scattering
from charged defects. A gate-tuned electron mobility exceeding 750,000
cm/Vs was achieved at a sample temperature of 2 K.Comment: 23 pages, 7 figures, 1 tabl
Teaching Medical Students to Help Patients Manage Their Weight: Outcomes of an Eight-School Randomized Controlled Trial
BACKGROUND: Given the rising rates of obesity there is a pressing need for medical schools to better prepare students for intervening with patients who have overweight or obesity and for prevention efforts.
OBJECTIVE: To assess the effect of a multi-modal weight management curriculum on counseling skills for health behavior change.
DESIGN: A pair-matched, group-randomized controlled trial (2015-2020) included students enrolled in eight U.S. medical schools randomized to receive either multi-modal weight management education (MME) or traditional weight management education (TE).
SETTING/PARTICIPANTS: Students from the class of 2020 (N=1305) were asked to participate in an objective structured clinical examination (OSCE) focused on weight management counseling and complete pre and post surveys. A total of 70.1% of eligible students (N=915) completed the OSCE and 69.3% (N=904) completed both surveys.
INTERVENTIONS: The MME implemented over three years included a web-based course, a role-play classroom exercise, a web-patient encounter with feedback, and an enhanced clerkship experience with preceptors trained in weight management counseling (WMC). Counseling focused on the 5As (Ask, Advise, Assess, Assist, Arrange) and patient-centeredness.
MEASUREMENTS: The outcome was student 5As WMC skills assessed using an objective measure, an OSCE, scored using a behavior checklist, and a subjective measure, student self-reported skills for performing the 5As.
RESULTS: Among MME students who completed two of three WMC components compared to those who completed none, exposure was significantly associated with higher OSCE scores and self-reported 5A skills.
LIMITATIONS: Variability in medical schools requiring participation in the WMC curriculum.
CONCLUSIONS: This trial revealed that medical students struggle with delivering weight management counseling to their patients who have overweight or obesity. Medical schools, though restrained in adding curricula, should incorporate should incorporate multiple WMC curricula components early in medical student education to provide knowledge and build confidence for supporting patients in developing individualized plans for weight management.
NIH TRIAL REGISTRY NUMBER: R01-194787
Planar Josephson Junctions Templated by Nanowire Shadowing
More and more materials, with a growing variety of properties, are built into
electronic devices. This is motivated both by increased device performance and
by the studies of materials themselves. An important type of device is a
Josephson junction based on the proximity effect between a quantum material and
a superconductor, useful for fundamental research as well as for quantum and
other technologies. When both junction contacts are placed on the same surface,
such as a two-dimensional material, the junction is called ``planar". One
outstanding challenge is that not all materials are amenable to the standard
planar junction fabrication. The device quality, rather than the intrinsic
characteristics, may be defining the results. Here, we introduce a technique in
which nanowires are placed on the surface and act as a shadow mask for the
superconductor. The advantages are that the smallest dimension is determined by
the nanowire diameter and does not require lithography, and that the junction
is not exposed to chemicals such as etchants. We demonstrate this method with
an InAs quantum well, using two superconductors - Al and Sn, and two
semiconductor nanowires - InAs and InSb. The junctions exhibit critical current
levels consistent with transparent interfaces and uniform width. We show that
the template nanowire can be operated as a self-aligned electrostatic gate.
Beyond single junctions, we create SQUIDs with two gate-tunable junctions. We
suggest that our method can be used for a large variety of quantum materials
including van der Waals layers, topological insulators, Weyl semimetals and
future materials for which proximity effect devices is a promising research
avenue.Comment: Written using The Block Method. Data on Zenodo DOI:
https://doi.org/10.5281/zenodo.641608
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