超导体/正常金属/半导体/正常金属/超导体结的研究

在约瑟夫逊效应的理论研究中,大体上有微观和宏观两种方法。采用的是宏观的方法。利用推广的Ja-cobson方法,讨论了超导体/正常金属/半导体/正常金属/超导体结的约瑟夫逊效应,并在小电流假设下,推导出了该结的电流密度与位相之间的关系。     

Magnetic flux control of chiral Majorana edge modes in topological superconductor

We study the transport of chiral Majorana edge modes (CMEMs) in a hybrid quantum anomalous Hall insulator-topological superconductor (QAHI-TSC) system in which the TSC region contains a Josephson junction and a cavity. The Josephson junction undergoes a topological transition when the magnetic flux through the cavity passes through half-integer multiples of magnetic flux quantum. For the trivial phase, the CMEMs transmit along the QAHI-TSC interface as without magnetic flux. However, for the nontrivial phase, a zero-energy Majorana state appears in the cavity, leading to that a CMEM can resonantly tunnel through the Majorana state to a different CMEM. These findings may provide a feasible scheme to control the transport of CMEMs by using the magnetic flux and the transport pattern can be customized by setting the size of the TSC.     

Josephson current in superconductor/ferromagnet/superconductor junctions

By using the Bogoliubov–de Gennes equation and the Nambu spinor Greens function approach, we have theoretically studied the dc Josephson current and the coupling phase state of superconductor/ferromagnet/superconductor (SC/FM/SC) junctions, where the FM is of weak ferromagnetism. From the behavior of the temperature-dependent dc Josephson current (I_c), we confirm that such SC/FM/SC junction may change from 0-phase to π-phase state with increasing the temperature (T), for particular parameters of the thickness and the strength of ferromagnetism of the FM interlayer. We attribute such changement to an extra phase difference between the two SCs. The results are qualitatively consistent with an experiment [Phys. Rev. Lett. 86 (2001) 2427], which shows a sharp cusp structure on the I_c−T curves of Nb/Cu_0.48Ni_0.52/Nb junction for specific thickness of the Cu_0.48Ni_0.52, indicating the junction changes from 0-phase state at high temperatures to π-phase state at low temperatures.     

Persistent current in a superconductor/quantum-dot ring/superconductor system

By means of the non-equilibrium Green's function technique, the inter-dot and Josephson currents in a superconductor/quantum-dot ring/superconductor (S/QDR/S) system are theoretically investigated. We found that a persistent current can coexist with the Josephson current in this hybrid QDR system when the inter-dot currents are all flowing in the clockwise (or anticlockwise) direction. The magnitude and direction of the persistent current can be controlled experimentally by the adjustment of some structure parameters, such as the quantum dot (QD) levels, the phase difference of the two external superconducting leads and the magnetic flux phase factor.     

Tunable supercurrent in superconductor/normal metal/superconductor Josephson junctions

When two superconductors are connected by a weak link a supercurrent flows determined by the difference in the macroscopic quantum phases of the superconductors. Originally, this phenomenon was discovered by Josephson for the case of a weak link formed by a thin tunnel barrier. The supercurrent I is related to the phase difference ϕ through the Josephson current–phase relation, I = I_csin ϕ, with I_c, the critical current, depending on the properties of the weak link. A similar relation holds for weak links consisting of a normal metal, a semiconductor or a constriction . In all cases, the phase differenceϕ =  0 when no supercurrent flows through the junction, and ϕ increases monotonically with increasing supercurrent until the critical current is reached. Using nanolithography techniques we have succeeded in making and studying a Josephson junction with a normal metal weak link, in which we have direct access to the microscopic current-carrying states inside the link. We find that the fundamental Josephson relation can be changed fromI = I_csin ϕ toI = I_csin(ϕ + π), i.e. to a π -junction, by suitably controlling the energy distribution of the current-carrying states in the normal metal. This fundamental change in the way these Josephson junctions behave has potential implications for their use in superconducting electronics as well as (quantum) logic circuits based on superconductors.     

Josephson current in ferromagnetic d-wave superconductor/ferromagnetic d-wave superconductor junction

We solve a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, study the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor (S)/insulating layer (I)/d-wave S junction are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction depend to a great extent on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field can under certain conditions enhance the Josephson critical current in a d-wave S/I/d-wave S junction.     

s波超导体绝缘层dx2-y2波超导体结的直流Josephson电流

在s波超导体绝缘层dx2-y2波超导体结(sId)中,考虑到结界面粗糙散射,运用BogoliubovdeGennes(BdG)方程和FurusakiTsukada(FT)电流公式,计算超导结中的准粒子传输系数和直流Josephson电流.结果表明:sId超导结的直流Josephson电流随温度以及结两侧的相位差变化的关系曲线强烈地依赖于d波超导体的晶轴方位;结界面的粗糙散射对Josephson电流有抑制作用 关键词： s/I/d超导结 dx2-y2波超导体 直流Josephson电流     

Zeeman effects on Josephson current in d-wave superconductor/d-wave superconductor junctions

This paper solves a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, and studies the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor(S)/insulating layer(I)/d-wave S junctions are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction to a great extent depend on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field under certain conditions can enhance the Josephson critical current in a d-wave S/I/d-wave S junction.     

高温超导混频器噪声系数测量

基于小型脉冲管制冷机,建立了以Josephson结为混频器的高温超导混频系统,并以Y-因子测量方法为基础,建立了与之配套的噪声系数测量系统。测量出以Josephson结为混频器的超导混频系统的噪声系数。在最好的情况下,测量的噪声系数值为5.50dB。     

嵌入Fabry-Perot谐振腔的高温超导双晶约瑟夫森结的毫米波辐照特性研究

采用嵌入Fabry-Perot谐振腔的方式,研究了Tl2Ba2CaCu2O8(Tl-2212)高温超导薄膜双晶约瑟夫森结的毫米波辐照特性.重点研究了双晶约瑟夫森结与Fabry-Perot谐振腔的耦合特性,发现放置在谐振腔中的结的位置、角度、结的图形以及Fabry-Perot谐振腔中两镜面间的距离等对耦合都有很大的影响.通过精细调节这些参数,可使双晶约瑟夫森结与外加毫米波达到最佳耦合.在最佳耦合情况下,能观察到9级明显的夏皮罗台阶.利用Fabry-Perot谐振腔技术,解决了约瑟夫森结与毫米波的耦合问题,为高温超导约瑟夫森结的毫米波及太赫兹波的辐射和检测研究奠定了基础.     

Influence of Rashba spin-orbit coupling on Josephson effect in triplet superconductor/two-dimensional semiconductor/triplet superconductor junctions

We study theoretically Josephson effect in a planar ballistic junction between two triplet superconductors with p-wave orbital symmetries and separated by a two-dimensional (2D) semiconductor channel with strong Rashba spin-orbit coupling. In triplet superconductors, three types of orbital symmetries are considered. We use Bogoliubov-de Gennes formalism to describe quasiparticle propagations through the junction and the supercurrents are calculated in terms of Andreev reflection coefficients. The features of the variation of the supercurrents with the change of the strength of Rashba spin-orbit coupling are investigated in some detail. It is found that for the three types of orbital symmetries considered, both the magnitudes of supercurrent and the current-phase relations can be manipulated effectively by tuning the strength of Rashba spin-orbit coupling. The interplay of Rashba spin-orbit coupling and Zeeman magnetic field on supercurrent is also investigated in some detail.     

Proximity effects in a superconductor/ferromagnet junction

A proximity effect in an s-wave superconductor/ferromagnet (SC/F) junction is theoretically studied using the second order perturbation theory for the tunneling Hamiltonian and Green's function method. We calculate a pair amplitude induced by the proximity effect in a weak ferromagnetic metal (FM) and a half-metal (HM). In the SC/FM junction, it is found that a spin-singlet pair amplitude (Ψ_s) and spin-triplet pair amplitude (Ψ_t) are induced in FM and both amplitudes depend on the frequency in the Matsubara representation. Ψ_s is an even function and Ψ_t is an odd function with respect to the Matsubara frequency (ω_n). In the SC/HM junction, we examine the proximity effects by taking account of magnon excitations in HM. It is found that the triplet-pair correlation is induced in HM. The induced pair amplitude in HM shows a damped oscillation as a function of the position and contains the terms of even and odd functions of ω_n as in the case of the SC/FM junction. We discuss that in our tunneling model the pair amplitude of even function of ω_n only contributes to a Josephson current.     

Josephson current in superconductor/ferromagnet/ superconductor junctions

The Bogoliubov-de Gennes equation and Nambu spinor Green's function approach are applied to studying the Josephson current in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions in the clean limit. It is found that the critical current exhibits a damped oscillation with the F thickness d, the oscillation period equal to 2πξ_F with ξ_F the coherence length of the F. The change of the critical current from positive to negative is determined by factor cosφ^′ with φ^′=d/ξ_F as the F-induced phase difference. The exponent decay of the critical current is close related to that of the superconductor order parameter in the F, both of them having the same decay length.     

高温超导约瑟夫森结阵列的相位锁定

根据交流约瑟夫森效应,研究了嵌入Fabry-Perot谐振腔里的约瑟夫森结阵列在微波辐照下的相位锁定问题。结阵列是由生长在双晶钇稳定氧化锆(YSZ)基片上的YB2Cu3O7(YBCO)超导薄膜光刻成微桥得到的。通过优化设计,在温度为79.2K辐照频率为77.465GHz的条件下,包含620个串联约瑟夫森双晶结的结阵列在外加微波辐照下实现了相位锁定,得到了陡峭的夏皮罗台阶。其第一级夏皮罗台阶的电压约为0.1V,台阶高度约为0.17mA。试验结果表明,这种结阵列结构在电压基准的应用上有极大的优势,而且这种准光学耦合方法在太赫兹信号发生和检测方面有很好的应用前景。     

Contribution of the triplet component of the supercurrent-carrying density of states to the Josephson current in a nonequilibrium superconductor/ferromagnet/superconductor junction

A weak link of two superconductors with s-type pairing through a ferromagnet has been theoretically investigated in the regime of a nonequilibrium spin-dependent distribution of electrons over energy levels in a ferromagnetic interlayer. It has been shown that, under the given conditions, the triplet component of the supercurrent-carrying density of states, which does not participate in the Josephson current transfer under equilibrium and spin-independent nonequilibrium conditions, is involved in the Josephson current transfer through the junction. In this case, the standard supercurrent transferred by the singlet component of the supercurrent-carrying density of states remains unchanged as compared to the case of the equilibrium distribution of electrons in the interlayer. An additional current transferred by the triplet component is controlled by a voltage that controls the specific shape and the degree of nonequilibrium of the electron distribution function in the interlayer. Depending on this controlling parameter, the additional current can substantially amplify or attenuate the standard supercurrent and also switch the junction between 0 and π states.     

Phase-charge duality of a josephson junction in a fluctuating electromagnetic environment

We have measured the current-voltage characteristics of a single Josephson junction placed in a high impedance environment. The transfer of Cooper pairs through the junction is governed by overdamped quasicharge dynamics, leading to Coulomb blockade and Bloch oscillations. Exact duality exists to the standard overdamped phase dynamics of a Josephson junction, resulting in a dual shape of the current-voltage characteristic, with current and voltage changing roles. We demonstrate this duality with experiments which allow for a quantitative comparison with a theory that includes the effect of fluctuations due to the finite temperature of the electromagnetic environment.     

Dynamical layer decoupling in a stripe-ordered high-T(c) superconductor

In the stripe-ordered state of a strongly correlated two-dimensional electronic system, under a set of special circumstances, the superconducting condensate, like the magnetic order, can occur at a nonzero wave vector corresponding to a spatial period double that of the charge order. In this case, the Josephson coupling between near neighbor planes, especially in a crystal with the special structure of La(2-x)Ba(x)CuO(4), vanishes identically. We propose that this is the underlying cause of the dynamical decoupling of the layers recently observed in transport measurements at x = 1/8.     

Tunneling spectroscopy of two-level systems inside a Josephson junction

We consider a two-level system (TLS) with energy level separation plankvOmega0 inside a Josephson junction. The junction is shunted by a resistor R and is voltage V biased. If the TLS modulates the Josephson energy and/or is optically active, it is Rabi driven by the Josephson oscillations in the running phase regime near the resonance 2eV=plankvOmega0. The Rabi oscillations, in turn, translate into oscillations of current and voltage that can be detected in noise measurements. This effect provides an option to fully characterize the TLS inside Josephson junction and to find the TLS's contribution to the decoherence when the junction is used as a qubit.     

Phenomenological theory of the underdoped phase of a high-Tc superconductor

We model the Fermi surface of the cuprates by one-dimensional nested parts near (0, pi) and (pi, 0) and unnested parts near the zone diagonals. Fermions in the nested regions form 1D spin liquids and develop spectral gaps below some approximately T*, but superconducting order is prevented by 1D phase fluctuations. We show that the Josephson coupling between these order parameters locks their relative phase at pi at the crossover scale T**相似文献   

On possibility of the spontaneous magnetic flux in a Josephson junction containing magnetic impurities

The Josephson junction containing localized magnetic moments in a dielectric layer between two superconductors is considered. Conditions are studied under which the phase difference between superconductors in the state with energy minimum is equal to π (such a junction we call π-junction). In addition we consider “one-dimensional” Josephson junction one part (2) of which is π-junction, the other (1) being the usual Josephson junction ( 0 - junction). Conditions are found under which in such a system there is a spontaneous vortex with the centre at the boundary between the parts 1 and 2 and magnetic flux associated with this vortex. The vortex appears by second order phase transition as temperature decreases for T_c.