116 research outputs found
The Field-Tuned Superconductor-Insulator Transition with and without Current Bias
The magnetic-field-tuned superconductor-insulator transition has been studied
in ultrathin Beryllium films quench-condensed near 20 K. In the zero-current
limit, a finite-size scaling analysis yields the scaling exponent product vz =
1.35 +/- 0.10 and a critical sheet resistance R_{c} of about 1.2R_{Q}, with
R_{Q} = h/4e^{2}. However, in the presence of dc bias currents that are smaller
than the zero-field critical currents, vz becomes 0.75 +/- 0.10. This new set
of exponents suggests that the field-tuned transitions with and without dc bias
currents belong to different universality classes.Comment: RevTex 4 pages, 4 figures, and 1 table minor change
A bright nanowire single photon source based on SiV centers in diamond
The practical implementation of many quantum technologies relies on the
development of robust and bright single photon sources that operate at room
temperature. The negatively charged silicon-vacancy (SiV-) color center in
diamond is a possible candidate for such a single photon source. However, due
to the high refraction index mismatch to air, color centers in diamond
typically exhibit low photon out-coupling. An additional shortcoming is due to
the random localization of native defects in the diamond sample. Here we
demonstrate deterministic implantation of Si ions with high conversion
efficiency to single SiV- centers, targeted to fabricated nanowires. The
co-localization of single SiV- centers with the nanostructures yields a ten
times higher light coupling efficiency than for single SiV- centers in bulk
diamond. This enhanced photon out-coupling, together with the intrinsic
scalability of the SiV- creation method, enables a new class of devices for
integrated photonics and quantum science.Comment: 15 pages, 5 figure
Effect of in-plane line defects on field-tuned superconductor-insulator transition behavior in homogeneous thin film
Field-tuned superconductor-insulator transition (FSIT) behavior in 2D
isotropic and homogeneous thin films is usually accompanied by a nonvanishing
critical resistance at low . It is shown that, in a 2D film including line
defects paralle to each other but with random positions perpendicular to them,
the (apparent) critical resistance in low limit vanishes, as in the 1D
quantum superconducting (SC) transition, under a current parallel to the line
defects. This 1D-like critical resistive behavior is more clearly seen in
systems with weaker point disorder and may be useful in clarifying whether the
true origin of FSIT behavior in the parent superconductor is the glass
fluctuation or the quantum SC fluctuation. As a by-product of the present
calculation, it is also pointed out that, in 2D films with line-like defects
with a long but {\it finite} correlation length parallel to the lines, a
quantum metallic behavior intervening the insulating and SC ones appears in the
resistivity curves.Comment: 16 pages, 14 figure
Single ion implantation for single donor devices using Geiger mode detectors
Electronic devices that are designed to use the properties of single atoms
such as donors or defects have become a reality with recent demonstrations of
donor spectroscopy, single photon emission sources, and magnetic imaging using
defect centers in diamond. Improving single ion detector sensitivity is linked
to improving control over the straggle of the ion as well as providing more
flexibility in lay-out integration with the active region of the single donor
device construction zone by allowing ion sensing at potentially greater
distances. Using a remotely located passively gated single ion Geiger mode
avalanche diode (SIGMA) detector we have demonstrated 100% detection efficiency
at a distance of >75 um from the center of the collecting junction. This
detection efficiency is achieved with sensitivity to ~600 or fewer
electron-hole pairs produced by the implanted ion. Ion detectors with this
sensitivity and integrated with a thin dielectric, for example 5 nm gate oxide,
using low energy Sb implantation would have an end of range straggle of <2.5
nm. Significant reduction in false count probability is achieved by modifying
the ion beam set-up to allow for cryogenic operation of the SIGMA detector.
Using a detection window of 230 ns at 1 Hz, the probability of a false count
was measured as 1E-1 and 1E-4 for operation temperatures of 300K and 77K,
respectively. Low temperature operation and reduced false, dark, counts are
critical to achieving high confidence in single ion arrival. For the device
performance in this work, the confidence is calculated as a probability of >98%
for counting one and only one ion for a false count probability of 1E-4 at an
average ion number per gated window of 0.015.Comment: 10 pages, 5 figures, submitted to Nanotechnolog
Density of states and magnetoconductance of disordered Au point contacts
We report the first low temperature magnetotransport measurements on
electrochemically fabricated atomic scale gold nanojunctions. As , the
junctions exhibit nonperturbatively large zero bias anomalies (ZBAs) in their
differential conductance. We consider several explanations and find that the
ZBAs are consistent with a reduced local density of states (LDOS) in the
disordered metal. We suggest that this is a result of Coulomb interactions in a
granular metal with moderate intergrain coupling. Magnetoconductance of atomic
scale junctions also differs significantly from that of less geometrically
constrained devices, and supports this explanation.Comment: 5 pages, 5 figures. Accepted to PRB as Brief Repor
Anisotropic Magnetoconductance in Quench-Condensed Ultrathin Beryllium Films
Near the superconductor-insulator (S-I) transition, quench-condensed
ultrathin Be films show a large magnetoconductance which is highly anisotropic
in the direction of the applied field. Film conductance can drop as much as
seven orders of magnitude in a weak perpendicular field (< 1 T), but is
insensitive to a parallel field in the same field range. We believe that this
negative magnetoconductance is due to the field de-phasing of the
superconducting pair wavefunction. This idea enables us to extract the finite
superconducting phase coherence length in nearly superconducting films. Our
data indicate that this local phase coherence persists even in highly
insulating films in the vicinity of the S-I transition.Comment: 4 pages, 4 figure RevTex, Typos Correcte
Deterministic nanoscale quantum spin-defect implantation and diffraction strain imaging
Local crystallographic features negatively affect quantum spin defects by changing the local electrostatic environment, often resulting in degraded or varied qubit optical and coherence properties. Few tools exist that enable the deterministic synthesis and study of such intricate systems on the nano-scale, making defect-to-defect strain environment quantification difficult. In this paper, we highlight state-of-the-art capabilities from the U.S. Department of Energy's Nanoscale Science Research Centers that directly address these shortcomings. Specifically, we demonstrate how complementary capabilities of nano-implantation and nano-diffraction can be used to demonstrate the quantum relevant, spatially deterministic creation of neutral divacancy centers in 4H silicon carbide, while investigating and characterizing these systems on the scale with strain sensitivities on the order of relevant to defect formation dynamics. This work lays the foundation for ongoing studies into the dynamics and deterministic formation of low strain homogeneous quantum relevant spin defects in the solid state
Dynamics of short time--scale energy relaxation of optical excitations due to electron--electron scattering in the presence of arbitrary disorder
A non--equilibrium occupation distribution relaxes towards the Fermi--Dirac
distribution due to electron--electron scattering even in finite Fermi systems.
The dynamic evolution of this thermalization process assumed to result from an
optical excitation is investigated numerically by solving a Boltzmann equation
for the carrier populations using a one--dimensional disordered system. We
focus on the short time--scale behavior. The logarithmically long time--scale
associated with the glassy behavior of interacting electrons in disordered
systems is not treated in our investigation.
For weak disorder and short range interaction we recover the expected result
that disorder enhances the relaxation rate as compared to the case without
disorder. For sufficiently strong disorder, however, we find an opposite trend
due to the reduction of scattering probabilities originating from the strong
localization of the single--particle states. Long--range interaction in this
regime produces a similar effect. The relaxation rate is found to scale with
the interaction strength, however, the interplay between the implicit and the
explicit character of the interaction produces an anomalous exponent.Comment: 4 pages, 3 EPS figure
Accommodating individual travel history and unsampled diversity in Bayesian phylogeographic inference of SARS-CoV-2
Spatiotemporal bias in genome sampling can severely confound discrete trait phylogeographic inference. This has impeded our ability to accurately track the spread of SARS-CoV-2, the virus responsible for the COVID-19 pandemic, despite the availability of unprecedented numbers of SARS-CoV-2 genomes. Here, we present an approach to integrate individual travel history data in Bayesian phylogeographic inference and apply it to the early spread of SARS-CoV-2. We demonstrate that including travel history data yields i) more realistic hypotheses of virus spread and ii) higher posterior predictive accuracy compared to including only sampling location. We further explore methods to ameliorate the impact of sampling bias by augmenting the phylogeographic analysis with lineages from undersampled locations. Our reconstructions reinforce specific transmission hypotheses suggested by the inclusion of travel history data, but also suggest alternative routes of virus migration that are plausible within the epidemiological context but are not apparent with current sampling efforts.status: publishe
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