Proximity effects in normal mental/spin-splitting material/superconductor junctions

We extend the Blonder, Tinkham and Klapwijk (BTK) theory to the study of the coexistence between ferromagnetism and s-wave superconductivity in ferromagnet/superconductor (F/S) structures. It is found that the ferromagnetism and s-wave superconductivity can coexist near the F/S interface, which is induced by proximity effects. On the F side, the density of states (DOS) exhibits some superconducting-like properties, and it displays a damped oscillation from `0' to `$\piE-mail： hlya7505@yahoo.com.cn/qk/85823A/200710/25696438.htmlproximity effects, `\pi' state, Rashba spin--orbit coupling7210, 74503/3/2007 12:00:00 AMWe extend the Blonder, Tinkham and Klapwijk (BTK) theory to the study of the coexistence between ferromagnetism and s-wave superconductivity in ferromagnet/superconductor (F/S) structures. It is found that the ferromagnetism and s-wave superconductivity can coexist near the F/S interface, which is induced by proximity effects. On the F side, the density of states (DOS) exhibits some superconducting-like properties, and it displays a damped oscillation from `0' to `$\pi$' states with increasing either the thickness of F film or the exchange energy. We also study the influences of the spin-polarized exchange splitting in the F and the spin-degeneracy by Rashba spin--orbit coupling (RSOC) in the two-dimensional electron gas (2DGE) on the proximity effects. It is shown that the case of Rashba spin-degeneracy is very different from that of the spin-polarized exchange splitting.http://cpb.iphy.ac.cn/CN/10.1088/1009-1963/16/10/042https://cpb.iphy.ac.cn/CN/article/downloadArticleFile.do?attachType=PDF&id=1088232007-10-08' states with increasing either the thickness of F film or the exchange energy. We also study the influences of the spin-polarized exchange splitting in the F and the spin-degeneracy by Rashba spin--orbit coupling (RSOC) in the two-dimensional electron gas (2DGE) on the proximity effects. It is shown that the case of Rashba spin-degeneracy is very different from that of the spin-polarized exchange splitting.     

All-oxide superconductor/ferroic hybrids

This short review focuses on interface interactions in hybrid heterostructures which combine high-temperature superconductors, ferromagnets and ferroelectrics. We specifically examine two different phenomena. One of them is electric-field effects at superconductor/ferroelectric interfaces, which allow a nonvolatile, reconfigurable nanoscale modulation of superconductivity. The second phenomenon is the proximity effect at superconductor/ferromagnet interfaces. We focus in particular on the penetration of superconducting correlations into half-metal ferromagnets. For both phenomena, an overview is given on materials’ issues, fabrication techniques, physical mechanisms and related devices.     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

Orientation dependence of proximity effect in ferromagnet/$\mathsf{d}$-wave superconductor junctions

The Nambu spinor Greens function approach is applied to studying the proximity effect in ferromagnet/d-wave superconductor (FM/d-wave SC) junctions. It is found that the magnitude of the proximity effect depends to a great extent on the orientation of the SC crystal with respect to the interface normal. On the FM side, near the interface there are two different types of density of states (DOS) with superconducting features. On the SC side, the DOS near the interface is spin dependent, indicating a local coexistence of weak ferromagnetism and d-wave superconductivity.Received: 10 February 2004, Published online: 12 July 2004PACS: 74.45. + c Proximity effects; Andreev effect; SN and SNS junctions - 74.50. + r Tunneling phenomena; point contacts, weak links, Josephson effects - 74.20.Rp Pairing symmetries (other than s-wave)     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al₂O₃/PdNi/Nb tunnel junctions. Received 1st July 2002 Published online 19 November 2002     

Possible Evidence for Spin-Transfer Torque Induced by Spin-Triplet Supercurrents

The mutual interplay between superconductivity and magnetism in superconductor/ferromagnet heterostructures may give rise to unusual proximity effects beyond current knowledge. Especially, spin-triplet Cooper pairs could be created at carefully engineered superconductor/ferromagnet interfaces. Here we report a giant proximity effect on spin dynamics in superconductor/ferromagnet/superconductor Josephson junctions. Below the superconducting transition temperature T_C, the ferromagnetic resonance field at X-band(~9.0 GHz) shifts rapidly to a lower field with decreasing temperature. In strong contrast, this phenomenon is absent in ferromagnet/superconductor bilayers and superconductor/insulator/ferromagnet/superconductor multilayers. Such an intriguing phenomenon can not be interpreted by the conventional Meissner effect. Instead, we propose that the strong influence on spin dynamics could be due to spin-transfer torque associated with spin-triplet supercurrents in ferromagnetic Josephson junctions with precessing magnetization.     

Proximity effects in ferromagnet/superconductor bilayers

The local density of states (LDOS) in a ferromagnet (FM)/superconductor (SC) bilayer is obtained by using the Nambu Green's function approach. Besides the transition from the “0” state to “π” state found in the spatial variation of the LDOS in the FM, the dependence of the LDOS spectrum on the thicknesses of the FM and SC is also investigated. It is shown that the LDOS in a thin FM film in contact to the SC shows a strong superconducting feature suggesting the possible coexistence of the ferromagnetism and s-wave superconductivity induced by the proximity effects.     

Spin-dependent proximity effects in ferromagnetic semiconductor/superconductor structures

Proximity effects in normal metal/insulator/ferromagnetic semiconductor/superconductor (NM/I/FS/SC) and NM/I/SC/FS junctions are studied based on an extended Blonder-Tinkham-Klapwijk (BTK) theory. It is found that the magnitude of the proximity effects depends to a great extent on the mismatches of the effective mass and band between the FS and SC. For NM/I/FS/SC junction, the transition of the tunneling conductance from “0” to “π” state is determined by the mass, magnetic exchange energy in FS and the thickness of FS. For NM/I/SC/FS junctions, the conductance spectrum is spin-dependent, indicating a local coexistence of weak ferromagnetism and s-wave superconductivity.     

EuS/Ta异质结的极大磁电阻效应

在铁磁/超导异质结中,铁磁体的交换场通过近邻效应将导致超导体准粒子态密度的塞曼劈裂.基于该效应,在外磁场不强的情况下,通过外加磁场可以有效地调节铁磁/超导界面处的交换作用,从而实现超导体在正常态和超导态之间转换,产生极大磁电阻.本文利用脉冲激光沉积方法制备了EuS/Ta异质结并研究了其电磁特性.Ta在3.6 K以下为超导态,EuS在20 K以下为铁磁态.在2 K时,EuS/Ta异质结中可观测蝴蝶型磁滞回线,证明在低磁场下(<±0.18 T)异质结中EuS铁磁态和Ta超导态共存.磁输运测试表明,通过施加外磁场可以有效调节EuS的交换场,随着交换场的增大,同时也加强了界面处的交换作用,从而抑制Ta的超导态,实现了Ta在超导态和正常态之间的转变,在EuS/Ta异质结中观测到了高达144000%的磁电阻.本文制备的EuS/Ta异质结具有极大磁电阻效应,在自旋电子学器件中有潜在的应用前景.     

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.     

A theoretical study of tunnelling conductance in ferromagnet/ PrOs4Sb12 junctions

The tunnelling conductance spectra of ferromagnet/PrOs4Sb12 junctions are theoretically investigated by using the Blonder-Tinkham-Klapwijk theory. Three pairs of possible candidate for the pairing symmetry of superconducting energy gap of the recently discovered heavy-fermion unconventional superconductor PrOs4Sb12 are chosen for calculation. We have studied the spin-polarization effect on the conductance spectra, with respect to different strength of ferromagnetism of the ferromagnet and different strength of the interface barrier. Moreover, we have discussed the influence of nodal structures of the superconducting energy gap on the conductance spectra. Different features of the tunnelling conductance spectra were got, which may serve as useful theoretical comparisons for future experiments.     

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     

Proximity effect in ferromagnet/triplet p-wave superconductor structures

The superconducting proximity effect in normal metal/insulator/ferromagnet/triplet p  -wave superconductor (N/I/FP) structures is studied based on an extended Blonder–Tinkham–Klapwijk (BTK) theory. Three kinds of pairings for the P side are chosen: px$p_x$, py$p_y$, px+ipy$p_x+i{p}_y$ waves. The transition from the “0” to “π” state is found in the conductance spectra with increasing the thickness of F or the ferromagnetic exchange energy. The large amplitude of the normalized conductance suggests the possible coexistence of the ferromagnetism and p-wave superconductivity in a small region near the F/P interface induced by the proximity effect.     

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.     

Influence of spin-polarized current on superconductivity and the realization of large magnetoresistance

The superconducting state can be influenced by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator). Because of the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T(C)) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. The T(C) shift is attributed to ferromagnet-induced nonequilibrium spin carriers in the superconductors.     

Long-range proximity effects in superconductor-ferromagnet structures

We analyze the proximity effect in a superconductor/ferromagnet (S/F) structure with a local inhomogeneity of the magnetization in the ferromagnet near the S/F interface. We demonstrate that not only the singlet but also the triplet component of the superconducting condensate is induced in the ferromagnet due to the proximity effect. The singlet component penetrates into the ferromagnet over a short length xi(h) = sqrt[D/h] ( h is the exchange field and D the diffusion coefficient), whereas the triplet component penetrates over a long length sqrt[D/epsilon] and leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature Tc.     

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.     

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．     

Experimental observation of the spin screening effect in superconductor/ferromagnet thin film heterostructures

When studying the nuclear magnetic resonance (NMR) of ⁵¹V nuclei in superconductor/ferromagnet thin film heterostructures, anomalous behavior of the NMR signal was observed. This effect was manifested as a systematic distortion of the high-field wing of the resonance line. We consider this to be the first experimental evidence for penetration of ferromagnetism into a superconductor.     

Conductance in normal metal/insulator/ferromagnetic superconductor tunnel junctions

In this paper, the superconducting order parameter and the energy spectrum of the Bogoliubov excitations are obtained from the Bogoliubov-de Gennes equation for a ferromagnetic superconductor (FS). In the framework of the Blonder-Tinkham-Klapwijk model, we present the differential conductance of the normal metal/insulator/FS junctions. It is shown that the exchange energy h in the FS can lead to the Zeeman splitting of the conductance peaks and the energy difference between the two splitting peaks is equal to 2h. The observation of such Zeeman splitting in the conductance spectrum can be taken as evidence for the coexistence between superconductivity and ferromagnetism.