Relaxation oscillators made of bridge-type Josephson contacts

Nb, NbN, and Nb₃Ge nanobridges on sapphire substrates with hysteretic I–V-characteristics are used as active elements in relaxation oscillators. Amplitudes of up to 350 mV and frequencies of up to 500 MHz could be generated, which make these oscillators useful for many applications. Linewidth and amplitude of the relaxation oscillations in dependence of nanobridge geometry are studied in some detail.Dedicated to Prof. Dr. Dres. h.c. A. Scharmann on the occasion of his 60^th birthday     

On the conductance oscillations in a mesoscopic proximity system

A fork-shapedN-branch Andreev interferometer is considered and the conductance of the system is calculated as a function of the phase difference in the superconducting order parameters across the interferometer. The results obtained from the quasiclassical, and the scattering-matrix theories are quantitatively compared, which were in good qualitative agreement showing a typical 2π-periodic interference pattern with certain fine structures.     

Tunneling conductance of a Bi2Sr2CaCu2O8-SnO2 junction along the c-axis

fine structure was observed in the conductance curve of a tunneling junction composed of a single crystalline Bi2212 and an evaporated SnO₂ film. It is similar to those of Bi2212-GaAs mechanical junctions and there is a certain correspondence between the structure and the phonon density of states. Thus the previous conclusion that the structure is due to phonons has been complemented by this work. The energy gap 2 was 57 meV at 13 K and T _c was 78 K. 2(0)/k _B T _c is then 8.3. (T) showed the BCS-like temperature dependence.     

Thermal conductance of ballistic point contacts

We study the thermal conductance of ballistic point contacts. These contacts are realized as few nanometer long pillars in so-called air-gap heterostructures (AGHs). The pillar length is orders of magnitude smaller than the mean free path of the phonons up to room temperature. Because of the small dimension and the low density of the pillars, the thermal conductance of the AGHs is several orders of magnitude reduced in comparison to bulk structures. The measurement results are in quantitative agreement with a simple model that is based on the Boltzmann transport equation.     

Static characteristics of Josephson interferometers

This investigation deals with the range in operating currents for which a Josephson interferometer, sometimes also referred to as Superconducting QUantum Interference Device (SQUID), may remain in the zero-voltage Josephson condition. An interferometer consists of one or more inductive loops each of which contains two Josephson junctions or other weak links. Two types of current are considered. Gate currentI _gpasses the junctions in parallel. Control currentI _cgenerates magnetic flux via inductive coupling in the loops. Zero-voltage operation is possible within certain areas of theI _g,I _cplane. These areas are manifestations of flux-quantum states and their boundary lines are referred to as static characteristics. In view of the nonlinearity of the constituting equations, not all their formal solutions are physically realizable. A stability analysis yields criteria which permit the identification of realizable operating conditions. The static characteristics comprise operating conditions where the limit of stability is reached. To obtain the static characteristics, linearized equations may be utilized if theLI _o product, a measure for the size of an interferometer, is large compared to the flux quantumΦ ₀, whereL is the inductance per loop, andI _o the maximum Josephson current per junction. As a general method of solving system of transcendental equations, continuation is discussed. The utilization of continuation for obtaining interferometer characteristics is explained. It is shown that some changes in the gate-current feed arrangement are equivalent to shearing the characteristics in theI _g,I _cplane. Analytical results are given on extrema, inflexion points, and singularities in the shape of cusps which conceptually relate to the existence and connectivity of flux-quantum states. Experimental static characteristics are presented on two-and four-junction interferometers. They are in agreement with characteristics computed on the basis of simple lumped circuit models. Relevant circuit parameters are obtained from the experimental characteristics.     

Noise properties of SQUIDs with Superconductor-Semiconductor-Superconductor proximity effect Josephson junctions

SQUIDs using Superconductor-Semiconductor-Superconductor (SSmS) proximity effect Josephson junctions were prepared and noise measurements were carried out. Since SSmS junctions are basic elements of Josephson field effect transistors (JoFETs), information about dynamic properties of JoFETs can be gained in this way. A planar geometry was used for the SSmS junctions, with a single crystalline silicon wafer acting as both, substrate and proximity layer. Rf- and dc-SQUIDs could be realized. When the SQUIDs were operated in a flux locked loop, flux noise values comparable to conventional tunnel junction SQUIDs were measured.     

Temperature dependence of conductance and thermopower anomalies of quantum point contacts

It has been shown within the Landauer approach that the presence of the 0.7 anomaly in the conductance of a ballistic microcontact and the respective plateau in the thermopower implies pinning of the potential barrier height at a depth of k _B T below the Fermi level. A simple way of taking into account the effect of electron-electron interaction on the profile and temperature dependence of a smooth one-dimensional potential barrier in the lower subband of the microcontact has been proposed. The calculated temperature dependences of the conductance and Seebeck coefficient agree with the experimental gate-voltage dependences, including the emergence of anomalous plateaus with an increase in temperature.     

Spin conductance and spin filtering in ferromagnetic semiconductor quantum point contacts

By combining the spin dependent transport properties of the ferromagnetic semiconductors with the basic physics of the quantum point contacts, we investigate the spin polarized transport through ferromagnetic semiconductor quantum point contacts. We find that the spin conductance strongly depends on the spin orientation, the magnitude of the spin splitting energy and the shape of the cross sections of the point contacts.     

Weak links in the superconducting transition of multiphase Bi-Ca-Sr-Cu-O compounds

The ac resistivities and dc V-I characteristics have been studied in a multiphase Bi-Ca-Sr-Cu-O sample in the temperature region where the transition from the normal to the superconducting state takes place. The resistivity drops sharply between 112K and 107K and follows a tail before zero resistivity is reached near 94K. The resistivity in the tail region is current dependent, this current dependence is easily suppressed by a magnetic field. We argue that this phenomenon is due to weak links between regions of the 110K-phase material.     

Resonantly enhanced nonlinear conductance in long quantum point contacts near pinch-off

We report on a remarkable resonance in the differential conductance of long quantum point contacts (QPCs) that is observed as a precursor to regular quantized transport. This effect is increasingly pronounced in longer QPCs, in which the differential conductance may resonantly exceed 2e2/h. From a study of the experimental characteristics of this feature, we suggest that it may be associated with the formation of a well-resolved energy gap that opens dynamically as a result of enhanced many-body interactions in long QPCs.     

基于自洽输运理论的介观体系动态电导的研究

基于介观体系电子动态输运的自洽理论,讨论了介观结构的动态电导.作为该理论的应用,采用一介观相干平行板电容器模型来进行研究. 结果表明:体系的动态电导与外场频率和体系费米能有关,为一复数且有有限虚部. 当外场频率较小时,动态电导随费米能的变化所呈现的特性和直流情形非常相似,但是随着外场频率的增加,两者差异就变得非常明显,体系动态电导随外场频率的变化呈现一些峰值结构. 在给定体系费米能时,动态电导随着外场频率的变化而产生振荡,并且出现了负的电导虚部,电导虚部的正负表明了体系的电容特性和电感特性. 关键词： 自洽输运理论 相干平行板电容器 电导 介观体系     

THz Cherenkov radiation of Josephson vortex

It is shown that Josephson vortices travelling in sandwich embedded in dielectric media radiate electromagnetic waves with THz frequencies. This phenomenon is caused by the Cherenkov effect and takes place if vortex velocity exceeds the speed of light in dielectric.