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.     

磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森效应

通过求解磁性d波超导中的能隙和磁交换能的自洽方程, 研究磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森电流. 计算结果表明: 1)临界电流随中间的铁磁层厚度呈现出两种不同周期的振荡混合, 通过增强铁磁层中的磁交换能q₀和铁磁/磁性d波超导界面处的势垒强度z₀, 短周期分量可从长周期中分离出来, 反之, 通过降低q₀和z₀, 长周期分量可从短周期中分离出来; 2)在两边磁性d波超导的磁化方向取平行时, 在取一些特定的铁磁层厚度下, 磁性d波超导中的磁交换能可增强系统的临界电流. 关键词： 磁性d波超导体 铁磁体 约瑟夫森电流     

铁磁超导态/绝缘层/自旋三重态p波超导体结的直流Josephson电流

由Bogoliubov-de Gennes方程得到铁磁超导共存态(FS)的自洽方程，利用推广的Furusaki-Tsukada的电流公式计算了铁磁超导态/绝缘层/自旋三重态p波超导体(FS/I/p)结的直流Josephson电流随结的温度、相位差以及FS中磁交换能、结界面的势垒散射强度的变化关系.研究表明：FS中磁交换能、结界面的势垒散射均抑制FS/I/p结的直流Josephson电流.当自旋三重态超导体具有p_x波配对势时，自旋三重态超导体结的直流Josephson电流随结两侧相位差的振荡周期是π. 关键词： 铁磁超导态 自旋三重态超导体 p波超导体 直流Josephson电流     

Ferromagnetic features of zero-bias conductance peaks in a ferromagnet/insulator/superconductor junction

We present a general formula for tunneling conductance in ballistic ferromagnet/ferromagnetic insulator/superconductor junctions where the superconducting state has the opposite spin pairing symmetry. The formula shows, correctly, that ferromagnetism has been induced by the effective mass difference between up- and down-spin electrons. This effectively mass mismatched ferromagnet and a standard Stoner ferromagnet have been employed in this paper. As an application of the formulation, we have studied the tunneling effect for junctions including a spin-triplet p-wave superconductor, where we choose a normal insulator for the insulating region, although our formula can be used for a ferromagnetic insulator. Then, we have been able to devote our attention to features of a ferromagnetic metal. The conductance spectra show a clear difference between the two ferromagnets depending upon the method of normalization of the conductance. In particular, an essential difference is seen in the zero-bias conductance peaks, reflecting the characteristics of each ferromagnet. From the obtained results, we suggest that the measurements of the tunneling conductance in the junction provide us with useful information about the mechanism of itinerant ferromagnetism in metals.     

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.     

Distribution of pairing functions in superconducting spin valve SF1F2

The distribution of the spin-singlet component, the short-range spin-triplet component with zero projection, and the long-range spin-triplet component with projection ±1 of the superconducting pairing function has been obtained for different regimes of switching of a spin valve with a three-layer heterostructure (superconductor/ferromagnet/ferromagnet). The distribution of the components is discussed as the main reason for the behavior of the superconducting transition temperature as a function of the angle between the magnetic moments of the ferromagnetic layers in these regimes.     

铁磁-超导隧道结中粗糙界面散射对直流Josephson电流的影响

通过求解Bogoliubov—de Gennes(BdG)方程得到铁磁超导共存态(FS)的自洽方程，在考虑结界面的粗糙情形下，由推广的Furusaki—Tsukada(FT)的电流公式计算了铁磁超导态／绝缘层／s波超导体(FS／I／S)结的直流Josephson电流，讨论了FS/I／S结的直流Josephson临界电流随磁交换能、温度的变化情况．研究表明：铁磁超导态中磁交换能和结界面的粗糙势垒散射均对FS／I／S结的直流Josephson电流有抑制作用．     

Supercurrent pumping in Josephson junctions with a half-metallic ferromagnet

A Josephson current through a half-metallic ferromagnet between two conventional superconductors is theoretically studied. The spin dynamics such as magnon excitation plays a crucial role not only for the conversion between spin-singlet and spin-triplet pairs but also for the formation of the composite state of a triplet Cooper pair and magnon, by which the Josephson current flows between the superconductors. We propose the supercurrent pumping driven by the coherent precession of the magnetization by tuning the microwave frequency to the ferromagnetic resonance frequency in a ferromagnetic Josephson junction.     

Critical current oscillations in S/F heterostructures in the presence of s–d scattering

Josephson current is investigated in the superconductor/ferromagnet/superconductor junction. It was shown that the current exhibited damping oscillations as a function of the ferromagnetic layer thickness. Previous theories based on Usadel or Eilenberger equations have predicted that the damping length and oscillation period divided by 2π were the same for weak ferromagnetic spacer. This contradicts past experiments. A new calculation of the Josephson current is proposed. The Gorkov equations are solved taking into account s–d scattering in ferromagnet. It is shown that the oscillation period depends only on the exchange magnetic field in the spacer, whereas the damping length is connected to the ferromagnetic mean free path. The concordance with the former experiment allows one to conclude that s–d scattering as a pair-breaking mechanism plays a significant role in the proximity effect in S/F heterostructures.     

dc Josephson Effect in s-Wave Superconductor/Ferromagnet Insulator/p-Wave Superconductor Junctions

The Josephson currents in s-wave superconductor/ferromagnet insulator/p-wave superconductor（s/FI/p） junctions are calculated as a function of temperature and the phase taking into account the roughness scattering effect at interface. The phase dependence of the Josephson current I （φ） between s-wave and px-wave superconductor is predicted to be sin（2φ）. The ferromagnet scattering effect, the barrier strength, and the roughness strength at interface suppress the dc currents in s/FI/p junction.     

Microscopic theory of equilibrium properties of F/S/F trilayers with weak ferromagnets

The aim of this paper is to explain the non monotonic temperature dependence of the self-consistent superconducting gap of ferromagnet/superconductor/ferromagnet (F/S/F) trilayers with weak ferromagnets in the parallel alignment (equivalent to F/S bilayers). We show that this is due to Andreev bound states that compete with the formation of a minigap. Using a recursive algorithm we discuss in detail the roles of various parameters (thicknesses of the superconductor and ferromagnets, relative spin orientation of the ferromagnets, exchange field, temperature, disorder, interface transparencies).Received: 23 January 2004, Published online: 29 June 2004PACS: 74.78.Na Mesoscopic and nanoscale systems - 74.45. + c Proximity effects; Andreev effect; SN and SNS junctions - 74.50. + r Tunneling phenomena; point contacts, weak links, Josephson effects     

Josephson current through a half metal at low temperatures

We study the Josephson effect in the superconductor/diffusive half metal/superconductor junctions by using the recursive Green function method. In the presence of spin-flip scatterings at the interface, odd-frequency spin-triplet Cooper pairs penetrate deeply into a half metal and carry Josephson current. The critical Josephson current increases with decreasing temperatures near the transition temperature. At low temperatures, however, the critical current decreases with decreasing temperatures. Such reentrant behavior is unusual in the case of s-wave superconductor junctions. The penetration of odd-frequency pairs modifies quasiparticle density of states in a half metal near the Fermi energy, which is responsible for the nonmonotonic temperature dependence of critical Josephson current.     

拓扑绝缘基铁磁/铁磁超导隧道结的磁效应和塞曼效应(英文)

运用拓展的BTK理论研究了拓扑绝缘层上铁磁/铁磁超导隧道结的磁效应和塞曼效应,同时考虑了铁磁体和铁磁超导体之间的费米能级错配效应.研究发现:在该系统中塞曼效应和邻近效应可以共存;铁磁体和铁磁超导体之间的费米能级错配效应能够增强系统中发生在eV=Δ处的Andreev谐振散射过程和邻近效应.     

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.     

Proximity effect in multiterminal hybrid structures

We consider the proximity effect in multiterminal ferromagnet/superconductor (FSF) hybrid structures in which two or three electrodes are connected to a superconductor. We show that two competing effects take place in these systems: (i) pair breaking effects due to the response to the exchange field induced in the superconductor; (ii) a reduction of the superconducting order parameter at the interface that takes place already in NS junctions. We focus on this second effect that dominates if the thickness of the S layer is small enough. We consider several single-channel electrodes connected to the same site. We calculate the superconducting order parameter and the local density of state (LDOS). With two ferromagnetic electrodes connected to a superconductor we find that the superconducting order parameter in the ferromagnetic alignment is larger than the superconducting order parameter in the antiferromagnetic alignment ( > ), in agreement with [Eur. Phys. J. B 25, 373 (2002)]. If a third spin polarized electrode is connected to a superconductor we find that - can change sign as the transparency of the third electrode increases. This can be understood from the fact that the superconducting order parameter is reduced if pair correlations among the ferromagnetic electrodes increase. If the two ferromagnetic electrodes are within a finite distance we find Friedel oscillations in the Gorkov function but we still obtain > .     

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.     

双层结构铁磁-超导隧道结的直流Josephson电流

通过求解Bogoliubov_de Gennes(BdG)方程得到铁磁超导共存态(FS)的自洽方程，在考虑结 界面的粗糙情形下，由推广的Furusaki_Tsukada(FT)的电流公式计算了铁磁超导态/ 绝缘层 / 铁磁超导态(FS/I/FS)结的直流Josephson电流，讨论了FS/I/FS结的直流Josephson临界 电流随磁交换能、温度的变化情况.研究表明：当结界面势垒散射强度和粗糙势垒散射强度 比较弱时磁交换能总是抑制FS/I/FS结的直流Josephson临界电流，而当结左右两边FS中铁 关键词： FS/I/FS超导隧道结 铁磁超导共存态 直流Josephson电流 粗糙势垒散射     

Josephson effect due to odd-frequency pairs in diffusive half metals

Motivated by a recent experiment [Keizer et al., Nature (London) 439, 825 (2006)], we study the Josephson effect in superconductor/diffusive half metal/superconductor junctions using the recursive Green function method. The spin-flip scattering at the junction interfaces opens the Josephson channel of the odd-frequency spin-triplet Cooper pairs. As a consequence, the local density of states in a half metal has a large peak at the Fermi energy. Therefore the odd-frequency pairs can be detected experimentally by using the scanning tunneling spectroscopy.     

Coherence effects in S/I/N/I/FS tunnel junctions

The dc Josephson effect in superconductor / insulator / normal metal / insulator/ferromagnetic superconductor junctions has been studied. We calculate the de Josephson current based on the Bogoliubov de Gennes equation. The Josephson current is derived as a function of exchange field in ferromagnetic superconductor, normal metal thickness and insulating barrier strength. It is found that there exists an oscillation relation between the critical Josephson current and the normal metal thickness. The oscillation amplitude decreases as the thickness of the normal metal increases or the exchange field augments.[第一段]     

Josephson Effect in FS/I/N/I/FS Tunnel Junctions

The Bogoliubov de Gennes equation is applied to the study ofcoherence effects in the ferromagnetic superconductor/insulator/normalmetal/insulator/ferromagnetic/superconductor (FS/I/N/I/FS) junction. We calculated the Josephson current in FS/I/N/I/FS as a function of exchange field in ferromagnetic superconductor, temperature, and normal metal thickness. It is found that the Josephson critical current in FS/I/N/I/FS exhibits oscillations as a function of the length of normal metal. The exchange field always suppresses the Josephson critical current I_p for a parallel configuration of the magnetic moments of two ferromagnetic superconductor (FS) electrodes. In the antiparallel configuration, the Josephson critical current I_Ap at the minimum values of oscillation increases with the exchange field for strong barrier strength and at low temperatures.