Superconducting Proximity Effects in Ferromagnet/Superconductor Heterostructures

We consider in this paper superconducting proximity effects in clean Ferromagnet/Ferromagnet/Superconductor (F1/F2/S) heterostructures. We make use of a numerical self-consistent solution of the microscopic Bogoliubov?Cde Gennes equations to investigate the superconducting transition temperatures as a function of the angle ?? between the magnetizations in the ferromagnetic layers. Unlike in F1/S/F2 trilayers, we find that the superconducting transition temperature is a nonmonotonic function of ??.     

Multi-vortex State Induced by Proximity Effects in a Small Superconducting Square

The influence of the different negative values of the deGennes parameter \(b\) in the thermodynamic properties of a superconducting infinitely long prism of square cross section area \(S=9\xi ^{2}(0)\) in the presence of a magnetic field is investigated theoretically by solving numerically the nonlinear Ginzburg-Landau equations; \(\xi (0)\) is the coherent length at zero temperature. We obtain the vorticity, magnetic induction, Cooper pair density, magnetization and phase of the order parameter as functions of the external applied magnetic field and the \(b\) parameter. Our results show that a multi-vortex state appear in the sample choosing a convenient value of \(b<0\) parameter, even for such small system. Also, we study a superconducting parallelepiped of volume \(V=Sd\) by means of true \(3D\) numerical simulations; \(d\) is the height of the parallelepiped. We focused our analysis on the way which the magnetization curves approximate from \(d\) finite to the characteristic curve of \(d\rightarrow \infty \) . This is the case for which the magnetic field and the order parameter are invariant along \(z\) -direction. For a superconductor of size \(S=9\xi ^2(0)\) we find that the limit below which the system should be considered a real three-dimensional sample when is \(d=8\xi \) .     

Superconducting Proximity Effect and Charging Effect on the Normal-Metal(Semiconductor) Wire

We theoretically investigated superconducting proximitycorrections of the conductance of a mesoscopic metal wire interms of the Kubo-formula. In diagrammatic expression, theelectromagnetic response kernel is quite similar to that ofreproducible conductance fluctuation. However, the proximitycorrection modifies the average conductance of the wire eventhough the latter gives a mesoscopic fluctuation.This Kubo-formula expression is applicable to the analysis ofcharging effects on the proximity correction in asuperconductor(S)/Mesoscopic-normalmetal-wire(N) hybrid systemwith a very small S/N interface. Although the chargingeffect exponentially suppresses Andreev reflections at theS/N interface, the proximity correction survives.We propose an experiment to confirm this charging effect,where a degenerated semiconductor is used as the normal-metalwire. The proximity correction can be picked from the entireconductance as the magnetoconductance of an interferometer,and the charging effect is depicted by a gate control of thesuperconducting island. The semiconductor interferometer ismore promising than a metallic one because the conductancewithout the proximity effect is bigger.     

Instability of Nonequilibrium Superconducting State and the Proximity Effect in MoRe/Al Sandwiches

The proximity effect in MoRe/Al sandwiches with different critical temperatures T _c of MoRe films has been studied. The estimation of the proximity effect parameters has been made and their dependence on the Al thickness built. Features associated with instability of nonequilibrium superconducting state have been observed in the current-voltage characteristics of MoRe/Al–I–Pb tunnel junctions as a result of quasiparticle tunnel injection into the MoRe/Al sandwich. The estimation of the parameter of tunnel injection has been made and its impact on instabilities emergence studied. MoRe–I–Pb tunnel junctions have been produced and their current-voltage characteristics studied to be compared with the results obtained.     

使用无线电引信弹道监测技术进行弹丸转速测试

文章介绍了使用竖环天线型无线电引信和弹道监测接收设备进行外弹道弹丸转速测试的工作原理;并结合多次外场转速测试试验的实际经验,介绍了地面接收站的布站、无线电引信信号的接收以及转速数据的采集、分析和处理方法.对弹丸转速测试的应用技术进行了比较深入的研究.     

A Novel Particle/Photon Detector Based on a Superconducting Proximity Array of Nanodots

The current frontiers in the investigation of high-energy particles demand for new detection methods. Higher sensitivity to low-energy deposition, high-energy resolution to identify events and improve the background rejection, and large detector masses have to be developed to detect even an individual particle that weakly interacts with ordinary matter. Here, we will describe the concept and the layout of a novel superconducting proximity array which show dynamic vortex Mott insulator to metal transitions, as an ultra-sensitive compact radiation-particle detector.     

Proximity Effect on the Density of States and Majorana Bound States in a Thin Superconducting Ferromagnetic Nanowire

This article presents analysis on the identification of Majorana states in a model of a heterogeneous superconducting ferromagnetic nanowire. It is demonstrated the significant effect that the exchange field has on the energy of bound in-gap states and on the density of states (DOS) itself, which appear as a periodic dependence of the DOS on the magnitude of the field as well as the dislocation in energy of the Majorana state candidate, located at zero energy in absence of the exchange field. Furthermore, the proximity effect is demonstrated to have observable consequences in a superconductor even in the clean limit. The model is a unidimensional system of fermions subject to a p-wave order parameter. The quasiclassical method is employed through the Eilenberger equations resulting in explicit expressions for the DOS, readily testable experimentally.     

Dephasing of Bose-Einstein condensates

Abstract

We discuss some recent work on the dephasing of Bose-Einstein condensates of interacting atoms due to fluctuations in the chemical potential μ. At vanishing temperature such fluctuations are due to the uncertainty of the number of particles in the condensate caused by the depletion effect. The dephasing is in this case a non-diffusive (and in principle reversible) effect called ‘collapse’ (and ‘revival’). Above a certain cross-over temperature the dephasing proceeds mainly via a diffusive process which follows in time a short and ineffective phase collapse. The phase-diffusion constant is dominated by the temperature dependent condensate-number fluctuations caused by the scattering of low-energy excitations off the condensate. We also discuss the (in general subdominant) contribution of the occupation-number fluctuations in quasi-particle states in the thermal cloud. For simplicity we discuss here results for a box-like trapping potential.     

On Low-Temperature Dephasing by Electron-Electron Interaction

The quantum coherence of electrons can be probed by studying weak localizationcorrections to the conductivity. Interaction effects lead to dephasing, with electron-electron interactions being the important intrinsic mechanism. A controversy exists whether or not the dephasing rate, as measured in a weak localization experiment, vanishes at low temperatures. We review the non-perturbative analysis of this question and some of the arguments whichhave been raised against it. The compact form of the presentation shouldmake the derivation more transparent and accessible for discussions. Wealso compare with recent experiments.     

Low-Frequency Relaxation Rate in the Superconducting YBa2Cu3O6.7

The submillimeter-wave 3 cm ^–1 < < 40 cm ^–1 complex conductivity of the reduced YBa ₂ Cu ₃ O _6.7 film, (T _C =56.5 K) was investigated for temperatures 4 K < T < 300 K. The frequency dependence of the effective quasiparticle scattering rate 1/*() was extracted from the spectra. 1/* is shown to be frequency independent at low frequencies and high temperatures. On decreasing temperature the scattering rate increases with increasing frequencies. Finally, at 6 K it follows a power-law, 1/* ^1.75±0.3.     

Absence of Zero-Temperature Dephasing by Electron-Electron Interaction

The relation between the non-perturbative analysis of low temperature dephasing by Golubev and co-workers and the perturbation theory is discussed. The contributions missed in each order of perturbation theory by the non-perturbative analysis are listed. The main source of the mistake is pointed out.     

Dephasing of Coherent Echoes by Nuclear Quadrupoles

Recent experiments on tunneling systems in insulating glasses showed an unexpected magnetic-field dependence of spontaneous polarization echoes. Besides a strong overall increase with the magnetic field, the echo amplitude slowly oscillates with a frequency that is proportional to B. We develop a theory that couples the tunneling motion to nuclear quadrupoles and obtain the beat frequency and dephasing in terms of the nuclear magnetic and quadrupolar moments, the waiting time, and the magnetic field. Our results compare favorably with measurements on various amorphous solids. PACS numbers: 77.22Ch, 61.43.Fs, 64.90.+b     

Andreev Reflection and Proximity effect

The Andreev Reflection is the key mechanism for thesuperconducting proximity effect. It provides phasecorrelations in a system of non-interacting electrons atmesoscopic scales, i.e. over distances much larger than themicroscopic lengths: Fermi wavelength and elastic electronmean free path. This field of research has attracted anincreasing interest in the recent years in part because of thetremendous development of nanofabrication technologies, andalso because of the richness of the involved quantum effects.In this paper we review some recently achieved advances. Wealso discuss new open questions, in particular non-equilibriumeffects and proximity effect in systems with ferromagneticelements.     

Superconducting materials and applications

The field of superconductivity has witnessed tremendous excitement in recent years, starting with the discovery of what has come to be known as ‘high temperature superconductors’. This has led to extensive activity on many aspects concerning the mechanism of superconductivity, new materials and systems and their technological applications. Further impetus to research in this area has been provided by the discovery of superconductivity in doped fullerenes. The first identification of superconductivity in a quarternary borocarbide system Y-Ni-B-C which must be regarded as the foremost fundamental Indian contribution in recent times, has further stimulated interest in this field. Notwithstanding the new excitements, the conventional superconductors continue to be the workhorses for technological applications. This review selectively presents some of the aspects of the developments in the entire gamut of the known superconducting systems from the stand point of materials and their applications. An update of the article “Current trends in the development and applications of superconducting materials”, Sundaram and Radhakrishnan (1989)     

Superconducting cavities

Superconducting cavities are characterized by extremely low rf losses. The best results have been obtained with niobium cavities, namely high field Q₀ values of 10¹⁰ at rf flux densities up to 0.16 T (1 600 G) and low field Q₀ values of 10¹¹. Most important is the surface treatment, which is reported below. Hard superconductors with high critical temperatures, such as Nb₃Sn, have potentially even more favourable rf characteristics compared to niobium and a report is given on first measurements on Nb₃Sn cavities. As the rf losses of superconductor cavities are smaller by the factor of 10⁶ than those of copper resonators, various potential applications are available, in particular with linear accelerators.     

Superconducting instruments

Instruments utilizing superconducting quantum interference devices (SQUIDS) operating at liquid helium temperature offer the greatest available sensitivity for many different electromagnetic measurements. Their equivalent noise temperature of less than one mK and low drift allow low level measurements of ac and dc current, voltage and resistance. We describe SQUID arrays to measure small vector magnetic fields and spacial gradients associated with geophysical and biomedical phenomena and present information on the design, advantages and limitations of these systems.