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