202 research outputs found
Detection Mechanism in SNSPD: Numerical Results of a Conceptually Simple, Yet Powerful Detection Model
In a recent publication we have proposed a numerical model that describes the
detection process of optical photons in superconducting nanowire single-photon
detectors (SNSPD). Here, we review this model and present a significant
improvement that allows us to calculate more accurate current distributions for
the inhomogeneous quasi-particle densities occurring after photon absorption.
With this new algorithm we explore the detector response in standard NbN SNSPD
for photons absorbed off-center and for 2-photon processes. We also discuss the
outstanding performance of SNSPD based on WSi. Our numerical results indicate a
different detection mechanism in WSi than in NbN or similar materials.Comment: Presented at ASC 2014 (invited) and submitted to IEEE Transaction on
Applied Superconductivity (Special Issue
Sequential superconductor-Bose insulator-Fermi insulator phase transitions in two-dimensional a-WSi
A zero-temperature magnetic-field-driven superconductor to insulator
transition (SIT) in quasi-two-dimensional superconductors is expected to occur
when the applied magnetic-field crosses a certain critical value. A fundamental
question is whether this transition is due to the localization of Cooper pairs
or due to the destruction of them. Here we address this question by studying
the SIT in amorphous WSi. Transport measurements reveal the localization of
Cooper pairs at a quantum critical field B_c^1 (Bose-insulator), with a product
of the correlation length and dynamical exponents zv~4/3 near the quantum
critical point (QCP). Beyond B_c^1, superconducting fluctuations still persist
at finite temperatures. Above a second critical field B_c^2>B_c^1, the Cooper
pairs are destroyed and the film becomes a Fermi-insulator. The different
phases all merge at a tricritical point at finite temperatures with zv=2/3. Our
results suggest a sequential superconductor to Bose insulator to Fermi
insulator phase transition, which differs from the conventional scenario
involving a single quantum critical point
Energy-Dissipation Performance of Combined Low Yield Point Steel Plate Damper Based on Topology Optimization and Its Application in Structural Control
In view of the disadvantages such as higher yield stress and inadequate adjustability, a combined low yield point steel plate damper involving low yield point steel plates and common steel plates is proposed. Three types of combined plate dampers with new hollow shapes are proposed, and the specific forms include interior hollow, boundary hollow, and ellipse hollow. The “maximum stiffness” and “full stress state” are used as the optimization objectives, and the topology optimization of different hollow forms by alternating optimization method is to obtain the optimal shape. Various combined steel plate dampers are calculated by finite element simulation, the results indicate that the initial stiffness of the boundary optimized damper and interior optimized damper is lager, the hysteresis curves are full, and there is no stress concentration. These two types of optimization models made in different materials rations are studied by numerical simulation, and the adjustability of yield stress of these combined dampers is verified. The nonlinear dynamic responses, seismic capacity, and damping effect of steel frame structures with different combined dampers are analyzed. The results show that the boundary optimized damper has better energy-dissipation capacity and is suitable for engineering application
Superconducting fluctuations and characteristic time scales in amorphous WSi
We study magnitudes and temperature dependences of the electron-electron and
electron-phonon interaction times which play the dominant role in the formation
and relaxation of photon induced hotspot in two dimensional amorphous WSi
films. The time constants are obtained through magnetoconductance measurements
in perpendicular magnetic field in the superconducting fluctuation regime and
through time-resolved photoresponse to optical pulses. The excess
magnetoconductivity is interpreted in terms of the weak-localization effect and
superconducting fluctuations. Aslamazov-Larkin, and Maki-Thompson
superconducting fluctuation alone fail to reproduce the magnetic field
dependence in the relatively high magnetic field range when the temperature is
rather close to Tc because the suppression of the electronic density of states
due to the formation of short lifetime Cooper pairs needs to be considered. The
time scale {\tau}_i of inelastic scattering is ascribed to a combination of
electron-electron ({\tau}_(e-e)) and electron-phonon ({\tau}_(e-ph))
interaction times, and a characteristic electron-fluctuation time
({\tau}_(e-fl)), which makes it possible to extract their magnitudes and
temperature dependences from the measured {\tau}_i. The ratio of
phonon-electron ({\tau}_(ph-e)) and electron-phonon interaction times is
obtained via measurements of the optical photoresponse of WSi microbridges.
Relatively large {\tau}_(e-ph)/{\tau}_(ph-e) and {\tau}_(e-ph)/{\tau}_(e-e)
ratios ensure that in WSi the photon energy is more efficiently confined in the
electron subsystem than in other materials commonly used in the technology of
superconducting nanowire single-photon detectors (SNSPDs). We discuss the
impact of interaction times on the hotspot dynamics and compare relevant
metrics of SNSPDs from different materials
Short-range magnetic interactions and spin-glass behavior in the quasi-2D nickelate Pr4Ni3O8
The nickelate Pr4Ni3O8 features quasi-two-dimensional layers consisting of
three stacked square-planar NiO2 planes, in a similar way to the well-known
cuprate superconductors. The mixed-valent nature of Ni and its metallic
properties makes it a candidate for potentially unconventional
superconductivity. We have synthesized Pr4Ni3O8 by topotactic reduction of
Pr4Ni3O10 in 10 percent hydrogen gas, and report on measurements of
powder-neutron diffraction, magnetization and muon-spin rotation (uSR). We find
that Pr4Ni3O8 shows complicated spin-glass behavior with a distinct magnetic
memory effect in the temperature range from 2 to 300 K and a freezing
temperature T_s ~ 68 K. Moreover, the analysis of uSR spectra indicates two
magnetic processes characterized by remarkably different relaxation rates: a
slowly-relaxing signal, resulting from paramagnetic fluctuations of Pr/Ni ions,
and a fast-relaxing signal, whose relaxation rate increases substantially below
~ 70 K which can be ascribed to the presence of short-range correlated regions.
We conclude that the complex spin-freezing process in Pr4Ni3O8 is governed by
these multiple magnetic interactions. It is possible that the complex magnetism
in Pr4Ni3O8 is detrimental to the occurrence of superconductivity
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