Josephson current carried by Andreev levels in superconducting quantum point contacts

The dc Josephson effect in a superconducting quantum point contact, where supercurrent flows through a small number of channels, is reviewed. The central role of Andreev levels is emphasized which carry the whole supercurrent in short symmetric Josephson junctions including tunnel junctions. A simple intuitive view of the dc Josephson effect in a quantum point contact is given in terms of multiple Andreev reflections. The quantization of the critical current in superconducting quantum point contacts is briefly discussed.     

Spontaneous spin polarization in quantum point contacts

We use spatial spin separation by a magnetic focusing technique to probe the polarization of quantum point contacts. The point contacts are fabricated from p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in the low-density regime, when the conductance of a point contact is tuned to < 2e2/h. Polarization is stronger in samples with a well-defined "0.7 structure."     

Critical current in superconducting quantum point contacts

The quantization of the critical current I _c in a quantum point contact is studied on varying the number of free channels (varying the constriction width d ₀ in 2DEG). It is shown that the shape of the quantum I _c is not universal and depends on the parameters of the contact, in particular, on the properties of the 2DEG-S boundaries. Because of the effect of normal reflection from the S-2DEG boundaries, the quantum critical current nonmonotonically depends on d ₀ and may have a resonance structure. If E _F coincides with a quasi-stationary level of the square potential well formed by the contact walls, the critical current has a local maximum (resonance) equal (in the first channel) to the ratio of the electron charge to its time of travel through the contact. The critical current at the maximum is determined by the lowest positive Andreev level (for the phase difference π).     

Anomalous Josephson vortex solutions in Josephson junctions with multiple tunneling channels

We construct a theory for Josephson junction with multiple tunneling channels. We focus on two situations, i.e., a heterotic junction composed of two-gap-superconductor, insulator, and one-gap-superconductor, and a grain-boundary junction formed by two identical multi-gap superconductors. Then, we show that the magnetic field distribution of the Josephson vortex for ±s-wave superconductivity is much more enlarged than that for s-wave without sign change between the order parameters. We display such anomalous vortices and suggest how to evaluate the enlargement.     

Anomalous Josephson effect in p-wave dirty junctions

The Josephson effect in p-wave superconductor/diffusive normal metal/p-wave superconductor junctions is studied theoretically. Amplitudes of Josephson currents are several orders of magnitude larger than those in s-wave junctions. Current-phase (J-phi) relations in low temperatures are close to those in ballistic junctions such as J proportional to sin(phi/2) and J proportional to phi even in the presence of random impurity potentials. A cooperative effect between the midgap Andreev resonant states and the proximity effect causes such anomalous properties and is a character of the spin-triplet superconductor junctions.     

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.     

Spontaneous Josephson spin current in triplet superconductor/ferromagnet/triplet superconductor junctions

This paper theoretically studies Josephson spin current through triplet superconductor/ferromagnet/triplet superconductor junctions. At the ferromagnet/superconductor interfaces, the ferromagnetic scattering potential gives rise to coupling between the Andreev bound states and lifts their spin degeneracy. These spin-split Andreev states carry the Josephson spin current through the junctions. The generated spin supercurrent can be controlled by the magnetization of a ferromagnetic thin layer and bias voltage across the junctions.     

Josephson current in Luttinger liquid-superconductor junctions

We study the Josephson current through a Luttinger liquid in contact with two superconductors. We show that it can be deduced from the Casimir energy in a two-boundary version of the sine-Gordon model. We develop a new thermodynamic Bethe ansatz, which, combined with a subtle analytic continuation procedure, allows us to calculate this energy in closed form, and obtain the complete current-crossover function from the case of complete normal to complete Andreev reflection.     

Dissipative current in SIFS Josephson junctions

We investigate superconductor/insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We consider the case of a strong tunnel barrier such that the left S layer and the right FS bilayer are decoupled. We calculate quantitatively the density of states (DOS) in the FS bilayer for arbitrary length of the ferromagnetic layer, using a self-consistent numerical method. We compare these results with a known analytical DOS approximation, which is valid when the ferromagnetic layer is long enough. Finally we calculate quantitatively the current–voltage characteristics of a SIFS junction.     

Dynamics of spin transport in voltage-biased Josephson junctions

We investigate spin transport in voltage-biased spin-active Josephson junctions. The interplay of spin filtering, spin mixing, and multiple Andreev reflection leads to nonlinear voltage dependence of the dc and ac spin current. We compute the voltage characteristics of the spin current (IS) for superconductor-ferromagnet-superconductor Josephson junctions. The subharmonic gap structure of IS(V) is shown to be sensitive to the degree of spin mixing generated by the ferromagnetic interface, and exhibits a pronounced even-odd effect associated with spin transport during multiple Andreev reflection processes. For strong spin mixing both the magnitude and the direction of the dc spin current can be sensitively controlled by the bias voltage.     

Critical current in planar SNS Josephson junctions

Specific features of the proximity effect and Josephson behavior of submicron planar SNS junctions fabricated by electron beam lithography and shadow evaporation have been studied experimentally and theoretically. The critical current of the junctions has been found to drastically increase with a decrease in temperature, which is associated with a change in the effective size of the weak link owing to the additional SN interface.     

Superconducting quantum point contacts

We review our experiments on the electronic transport properties of atomic contacts between metallic electrodes, in particular superconducting ones. Despite ignorance of the exact atomic configuration, these ultimate quantum point contacts can be manipulated and well characterized in-situ. They allow performing fundamental tests of the scattering theory of quantum transport. In particular, we discuss the case of the Josephson effect.     

The dc Josephson current in cylindrical junctions

The dc Josephson current between co-axial cylinders is calculated for arbitrary Josephson penetration depth. The results support the interpretation of recent experiments which determined the magnetic-field dependence of the Josephson critical current density. The interpretation centers on the conclusion that the Josephson current vanishes when the fluxoid quantum number differ.     

Superconducting quantum interference device without Josephson junctions

A new type of a superconducting quantum interference device (SQUID) based on a single superconducting loop without Josephson junctions and with asymmetric contacts has been proposed. This SQUID offers advantages in simplicity of fabrication and a steeper dependence of measured quantities on the magnetic flux. To confirm the possibility of making this type of SQUID, the magnetic field dependence of the critical current in an aluminum ring with asymmetric contacts has been experimentally investigated.     

Anomalous rf induced steps in long resonant Josephson tunnel junctions

Anomalous phase locked current steps are experimentally observed in the I–V characteristic of a long Josephson tunnel junction driven by microwaves at the frequency of its geometrical resonance.     

高温超导量子干涉器中的约瑟夫森结

超导量子干涉器（SQUID）是以约瑟夫森结为核心元件的极其灵敏的磁通-电压转换器.在最近20年间,超导电子学界利用多种铜氧化物高温超导材料,制造出了高温超导约瑟夫森结乃至高温超导量子干涉器（高Tc SQUID）;特别是,利用YBa2Cu3O7-δ（YBCO）制出了多种高温超导约瑟夫森结,并进而制成了具有一定实用性的高Tc SQUID.文章主要是对高Tc SQUID中所使用的高温超导约瑟夫森结进行评述.     

Macroscopic quantum tunneling in differently loaded Josephson junctions

In relation to the problem of quantum measurement, macroscopic quantum tunneling (MQT) in Josephson junctions is one of the most investigated topics. Although many theoretical studies have been devoted to this argument, the agreement on a single theoretical model has not yet been reached. The purpose of the present work is to analyze the load “seen” by the junction, in order to evaluate the contribution of the dissipation that modifies the decay-rate of the metastable state of the junction. The present work reports some theoretical results in relation to different kinds of loading systems, namely, the cases of capacitive, inductive and purely resistive load. The resistive load is considered also in relation to an experimental test.     

A model of quantum measurement in Josephson junctions

A model for the quantum measurement of the electronic current in a Josephson junction is presented and analyzed. The model is similar to a Stern-Gerlach apparatus, relying on the deflection of a spin-polarized particle beam by the magnetic field created by the Josephson current. The aim is (1) to explore, with the help of a simple model, some general ideas about the nature of the information which can be obtained by measurements upon a quantum system and (2) to find new approaches for obtaining information about the nature of the states of a macroscopic quantum system. In the case of sufficiently strong coupling between the system and the apparatus, we find that the model provides in principle a standard ideal measurement of the value of the instantaneous Josephson current. In the case of weak coupling, where the measurement is not ideal, we show that the scattering of neutrons from a junction can in principle be used to measure the average value of the Josephson current, thereby allowing an experimental distinction to be made between an eigenstate of relative phase and one of relative Cooper pair number. The possibility of the latter type of measurement suggests an experimental approach to answer a question of fundamental interest, namely whether two isolated superconductors (or superfluids) possess a definite relative phase or a definite relative number of superconducting (or super/lowing) particles.     

The current phase relation in Josephson tunnel junctions

The J(?) relation in SFIFS, SNINS, and SIS tunnel junctions is studied. A method for the analytical solution of linearized Usadel equations has been developed and applied to these structures. It is shown that the Josephson current across the structure has a sum of sin? and sin2? components. Two different physical mechanisms are responsible for the sign of sin2?. The first one is the depairing by current, which contributes positively to the sin2? term, while the second one is the finite transparency of SF or SN interfaces, which provides the negative contribution. In SFIFS junctions, where the first harmonic vanishes at the 0-π transition, the calculated second harmonic fully determines the J(?) curve.