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.     

超导体/铁磁体绝缘层-超导体隧道结的直流Josephson效应

在超导体铁磁体绝缘层超导体结(SFIS)中,运用BogoliubovdeGennes(BdG)方程和FurusakiTsukada(FT)电流公式,计算铁磁超导共存态的自洽方程和SFIS结中的直流Josephson电流.研究表明,铁磁超导态的磁交换能h对准粒子的Andreev反射有抑制作用,使得SFIS结中的直流Josephson电流随铁磁超导共存态的磁交换能h增大而减弱 关键词： S/F-I-S结 铁磁超导态 直流Josephson电流     

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.     

Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, current injection and electromagnetic radiation. In particular we will consider the ‘giant’ Josephson critical current in ‘long’ SIS tunnel junctions and the regular and anomalous nonequilibrium Josepson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).     

Nonequilibrium Josephson effect through helical edge states

We study Josephson junctions between superconductors connected through the helical edge states of a two-dimensional topological insulator in the presence of a magnetic barrier. As the equilibrium Andreev bound states of the junction are 4π periodic in the superconducting phase difference, it was speculated that, at finite dc bias voltage, the junction exhibits a fractional Josephson effect with half the Josephson frequency. Using the scattering matrix formalism, we show that his effect is absent in the average current. However, clear signatures can be seen in the finite-frequency current noise. Furthermore, we discuss other manifestations of the Majorana bound states forming at the edges of the superconductors.     

Resonant transport through midgap states in voltage-biased Josephson junctions of d-wave superconductors

We study theoretically the ac Josephson effect in voltage-biased planar junctions of d-wave superconductors. For some orientations of the superconductors a current peak is found at finite voltage in the current–voltage characteristics. We pick out the relevant physical processes and write down an analytical formula for the current which clearly shows how the midgap state acts as a resonance and produces the peak. We present a possible explanation for the zero-bias conductance peak, recently found in experiments on grain boundary junctions of high-temperature superconductors, in terms of resonant transmission through midgap state of quasiparticles undergoing multiple Andreev reflections. We note that within our framework the zero-bias conductance peak appears in rather transparent Josephson junctions of d-wave superconductors.     

Spin current due to spinlike Andreev reflection

We propose theoretically a spinlike Andreev reflection that may occur at the interface of a semimetal and a doped excitonic insulator. In contrast to the usual Andreev reflection, this process does not produce the charge current, but gives rise to a pure spin current. Further investigation reveals the existence of a spinlike Josephson phenomenon between two excitonic insulators with ferromagnetism, which results in a dissipationless spin current. We stress that the present spin current satisfies the conservation condition and is quite different from the spin Hall current induced from spin-orbital interactions.     

Two coupled Josephson junctions: dc voltage controlled by biharmonic current

We study transport properties of two Josephson junctions coupled by an external shunt resistance. One of the junctions (say, the first) is driven by an unbiased ac current consisting of two harmonics. The device can rectify the ac current yielding a dc voltage across the first junction. For some values of coupling strength, controlled by an external shunt resistance, a dc voltage across the second junction can be generated. By variation of system parameters such as the relative phase or frequency of two harmonics, one can conveniently manipulate both voltages with high efficiency, e.g. changing the dc voltages across the first and second junctions from positive to negative values and vice versa.     

Dissipative currents in superfluid 3He weak links

We calculate the current-pressure relation for pinholes connecting two volumes of bulk superfluid 3He-B. The theory of multiple Andreev reflections, adapted from superconducting weak links, leads to a nonlinear dependence of the dc current on pressure bias. In arrays of pinholes one has to take into account oscillations of the texture at the Josephson frequency. The associated radiation of spin waves from the junction leads to an additional dissipative current at small biases, in agreement with measurements.     

Nonlocal Andreev reflection and spin current in a three-terminal Aharonov--Bohm interferometer

This paper theoretically reports the nonlocal Andreev reflection and spin current in a normal metal-ferromagnetic metal-superconducting Aharonov--Bohm interferometer. It is found that the electronic current and spin current are sensitive to systematic parameters, such as the gate voltage of quantum dots and the external magnetic flux. The electronic current in the normal metal lead results from two competing processes: quasiparticle transmission and nonlocal Andreev reflection. The appearance of zero spin-up electronic current (or spin-down electronic current) signals the existence of nonlocal Andreev reflection, and the presence of zero electronic current results in the appearance of pure spin current.     

Incomplete Andreev reflection in a clean SFS junction

We study the stationary Josephson effect in a ballistic superconductor/ferromagnet/superconductor junction for arbitrarily large spin polarizations. Due to the exchange interaction in the ferromagnet, the Andreev reflection is incomplete. We describe how this effect modifies the Josephson current in the crossover from a superconductor/normal metal/superconductor junction to a superconductor/half metal/superconductor junction.     

Universal distribution of transparencies in highly conductive Nb/AlO(x)/Nb junctions

We report the observation of the universal distribution of transparencies, predicted by Schep and Bauer [Phys. Rev. Lett. 78, 3015 (1997)] for dirty sharp interfaces, in uniform Nb/AlO(x)/Nb junctions with high specific conductance (10(8) ohm(-1) cm(-2)). Experiments used the BCS density of states in superconducting niobium for transparency distribution probing. Experimental results for both the dc I-V curves at magnetic-field-suppressed supercurrent and the Josephson critical current in zero magnetic field coincide remarkably well with calculations based on the multimode theory of multiple Andreev reflections and the Schep-Bauer distribution.     

The Josephson effect between triplet superconductors through a finite ferromagnetic barrier

Charge and spin transport in a junction involving two triplet superconductors and a ferromagnetic barrier are studied. We use Bogoliubov-de Gennes wavefunctions to construct the Green's function, from which we obtain the Josephson currents in terms of the Andreev reflection coefficients. We focus on the consequences of a finite barrier width for the occurrence of 0-π transitions and for the spin currents, and examine the appropriateness of the common δ-function approximation for the tunneling region.     

Andreev tunneling and Josephson current in light irradiated graphene

We investigate the Andreev tunneling and Josephson current in graphene irradiated with high-frequency linearly polarized light. The corresponding stroboscopic dynamics can be solved using Floquet mechanism which results in an effective stationary theory to the problem exhibiting an anisotropic Dirac spectrum and modified pseudospin-momentum locking. When applied to an irradiated normal graphene - superconductor (NS) interface, such analysis reveal Andreev reflection (AR) to become an oscillatory function of the optical strength. Specifically we find that, by varying the polarization direction we can both suppress AR considerably or cause the Andreev transport to remain maximum at sub-gap excitation energies even in the presence of Fermi level mismatch. Furthermore, we study the optical effect on the Andreev bound states (ABS) within a short normal-graphene sheet, sandwiched between two s-wave superconductors. It shows redistribution of the low energy regime in the ABS spectrum, which in turn, has major effect in shaping the Josephson super-current. Subjected to efficient tuning, such current can be sufficiently altered even at the charge neutrality point. Our observations provide useful feedback in regulating the quantum transport in Dirac-like systems, achieved via controlled off-resonant optical irradiation on them.     

Current-induced nonequilibrium spin polarization in semiconductor-nanowire/s-wave superconductor junctions with strong spin–orbit coupling

We have studied the characteristics of current-induced nonequilibrium spin polarization in semiconductor-nanowire/swave superconductor junctions with strong spin–orbit coupling. It was found that within some parameter regions the magnitude of the current-induced nonequilibrium spin polarization density in such structures will increase(or decrease) with the decrease(or increase) of the charge current density, in contrast to that found in normal spin–orbit coupled semiconductor structures. It was also found that the unusual characteristics of the current-induced nonequilibrium spin polarization in such structures can be well explained by the effect of the Andreev reflection.     

Nondemolition measurements of a single quantum spin using Josephson oscillations

We consider a single localized spin-1/2 between the singlet superconducting leads of a Josephson junction (e.g., a superconducting STM). For the spin subject to a dc magnetic field B parallel z, we study the spin dynamics and the possibility to measure the spin state via transport through the junction embedded in a dissipative circuit. Turning on the tunneling or a voltage bias induces oscillations of the Josephson current, with an amplitude sensitive to the initial value of the z component of the spin, S(z)=+/-1/2. At low temperatures, when effects of quasiparticles are negligible, this procedure realizes a quantum nondemolition measurement of S(z).     

Multiple Andreev reflections in ballistic Nb-InGaAs/InP-Nb junctions

The current–voltage characteristics of ballistic Nb-InGaAs/InP-Nb Josephson junctions have been investigated. At temperatures below 1 K a negative differential conductance, which usually leads to a hysteresis in the current–voltage characteristics, was resolved by connecting an additional external shunt resistor to the junction. The negative differential conductance is explained by heating and conductance enhancement due to multiple Andreev reflections. The structures observed in the differential resistance measurements as a function of the bias voltage are explained by self-detection of Josephson radiation at low bias voltages and subharmonic gap structures at higher bias voltages.     

Subharmonic shapiro steps and assisted tunneling in superconducting point contacts

We analyze the current in a superconducting point contact of arbitrary transmission in the presence of a microwave radiation. The interplay between the ac Josephson current and the microwave signal gives rise to Shapiro steps at voltages V = (m/n)h(omega)(r)/2e, where n,m are integer numbers and omega(r) is the radiation frequency. The subharmonic steps (n not equal 1) are a consequence of multiple Andreev reflections (MAR) and provide a signature of the peculiar ac Josephson effect at high transmission. Moreover, the dc current exhibits a rich subgap structure due to photon-assisted MARs.     

Divergence at low bias and down-mixing of the current noise in a diffusive superconductor-normal-metal-superconductor junction

We present current noise measurements in a long diffusive superconductor-normal-metal-superconductor junction in the low voltage regime, in which transport can be partially described in terms of coherent multiple Andreev reflections. We show that, when decreasing voltage, the current noise exhibits a strong divergence together with a broad peak. We ascribe this peak to the mixing between the ac-Josephson current and the noise of the junction itself. We show that the junction noise corresponds to the thermal noise of a nonlinear resistor 4k B T/R with R=V/I(V) and no adjustable parameters.     

d-wave induced zero-field resonances in dc pi-superconducting quantum interference devices

A dc pi SQUID consists of a superconducting ring interrupted by two Josephson junctions, one of which carries in equilibrium a pi phase difference, caused, for example, by the d-wave pairing symmetry of the high- T(c) cuprates. If this phase shift is maintained in the voltage state, anomalous resonance currents are expected in the SQUIDs transport characteristics. Here we report the observation of such resonances for high- T(c) dc pi SQUIDs, providing evidence for the influence of the d-wave symmetry on the voltage state of a Josephson junction for frequencies of several tens of GHz.