Giant tunneling electroresistance effect driven by an electrically controlled spin valve at a complex oxide interface

A giant tunneling electroresistance effect may be achieved in a ferroelectric tunnel junction by exploiting the magnetoelectric effect at the interface between the ferroelectric barrier and a magnetic La(1-x)Sr(x)MnO3 electrode. Using first-principles density-functional theory we demonstrate that a few magnetic monolayers of La(1-x)Sr(x)MnO3 near the interface act, in response to ferroelectric polarization reversal, as an atomic-scale spin valve by filtering spin-dependent current. This produces more than an order of magnitude change in conductance, and thus constitutes a giant resistive switching effect.     

A novel proximity effect between superconductor and magnetic manganese oxide

A novel proximity effect between superconductor and magnetic manganese oxide has been found. To investigate interactions between superconductor and magnetic materials, we have fabricated both trilayered-type and coplanar-type junctions and investigated their current-voltage characteristics. The magnetic material was La_1-x Ca _x MnO _z (LCMO) or La_1-x Sr _x MnO _z (LSMO) manganese oxide. In YBCO/LCMO/YBCO junctions, supercurrents were observed through a magnetic barrier as thick as 500 nm. This phenomenon was also confirmed by coplanar junctions. The effect of magnetism of the barrier material was investigated. It was found that this proximity effect occurred only in a limited region where spin fluctuations are considered to exist.     

Experimental study of the feasibility of a spin valve based on superconductor/ferromagnet proximity effect

The feasibility of a superconducting spin valve based on superconductor/ferromagnet proximity effect is discussed. Experimental results obtained by the authors to date in studies of this problem are presented.     

Magnetic proximity effect in EuO-Cs interface

In the surface of the ferromagnetic insulator EuO is covered with a small amount of cesium, the photoelectric magnetization curves P(H) show magnetic saturation in contrast to those of the pure surface. The proximity of the Cs film causes the paramagnetism of the clean EuO surface to disappear.     

Commensurate spin dynamics in the superconducting state of an electron-doped cuprate superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped (n-type) superconducting (SC) cuprate Nd2-xCexCuO4 (x=0.15) with T(c)=18 and 25 K. Unlike the hole-doped (p-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the SC and in the normal state. A spin gap opens up when the n-type cuprate becomes SC, as in the optimally doped p-type La2-xSrxCuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from T(c) to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the SC state of p-type cuprates.     

Anomalous proximity effect in an inhomogeneous disordered superconductor

By combining very low temperature scanning tunneling microscopy and spectroscopy on a TiN film we have observed a nonuniform state comprising of superconducting (S) and normal (N) areas. The local density of states displays a spatial dependence between S and N different from the usual proximity effect. We conclude that mesoscopic fluctuations might play a major role in accordance with recent theories describing superconductor-normal-metal quantum transition.     

Giant proximity effect in a phase-fluctuating superconductor

When a tunneling barrier between two superconductors is formed by a normal material that would be a superconductor in the absence of phase fluctuations, the resulting Josephson effect can undergo an enormous enhancement. We establish this novel proximity effect by a general argument as well as a numerical simulation and argue that it may underlie recent experimental observations of the giant proximity effect between two cuprate superconductors separated by a barrier made of the same material rendered normal by severe underdoping.     

Surface polaritons at the interface between anisotropic superconductor and insulator

The dispersion, polarization, and energy characteristics of surface polaritons propagating along the interface between an isotropic insulator and an anisotropic superconductor are studied. The permittivity is calculated in the two-fluid model of superconducting and normal electrons. It is demonstrated that the properties of the surface wave are strongly dependent on the physical characteristics of the media, frequency, and temperature.     

Universal gap fluctuations in the superconductor proximity effect

Random-matrix theory is used to study the mesoscopic fluctuations of the excitation gap in a metal grain or quantum dot induced by the proximity to a superconductor. We propose that the probability distribution of the gap is a universal function in rescaled units. Our analytical prediction for the gap distribution agrees well with exact diagonalization of a model Hamiltonian.     

Null orbital frustration at the pseudogap boundary in a layered cuprate superconductor

We assess the relative importance of orbital frustration at the pseudogap closing field H(pg). Using interlayer tunneling transport in pulsed magnetic fields nearly up to 60 T, we track the field-temperature (H-T) phase diagram for fields parallel ( parallel ab) and normal ( parallel c) to the layered structure of Bi(2)Sr(2)CaCu(2)O(8+y). In contrast to large orientational anisotropy of the superconducting state related to the orbital motion of Cooper pairs, we find anisotropy of H(pg) temperature independent and small, due solely to the g factor. The obtained Zeeman relation with the pseudogap temperature T small star, filled, g( parallel c)micro(B)H( parallel c)(pg)=g( parallel ab)micro(B)H( parallel ab)(pg) approximately k(B)T small star, filled, is fully consistent with the correlations only in the spin channel.     

有机自旋阀的磁电阻性质研究

考虑到有机半导体中极化子和双极化子特殊的电荷-自旋关系,从自旋扩散方程和欧姆定律出发,理论研究了"铁磁/有机半导体/铁磁"有机自旋阀结构中的磁电阻性质.计算发现,磁电阻在数值上随有机半导体层中极化子比率的增加而增大,随有机半导体层厚度的增加而迅速减小.同时发现自旋相关界面电阻能在很大程度上提高系统的磁电阻.讨论了铁磁层和有机半导体电导率比率、铁磁层极化率等对系统磁电阻性质的影响. 关键词： 磁电阻 有机自旋电子学 极化子     

Nuclear spin effect in a metallic spin valve

We study electronic transport through a ferromagnet normal-metal ferromagnet system and we investigate the effect of hyperfine interaction between electrons and nuclei in the normal-metal part. A switching of the magnetization directions of the ferromagnets causes nuclear spins to precess. We show that the effect of this precession on the current through the system is large enough to be observed in experiment.     

Magnetic resonance in the spin excitation spectrum of electron-doped cuprate superconductors

We study the emergence of a magnetic resonance in the superconducting state of the electron-doped cuprate superconductors. We show that the recently observed resonance peak in the electron-doped superconductor Pr0.88LaCe0.12CuO4-delta is consistent with an overdamped spin exciton located near the particle-hole continuum. We present predictions for the magnetic-field dependence of the resonance mode as well as its temperature evolution in those parts of the phase diagram where dx2-y2-wave superconductivity may coexist with an antiferromagnetic spin-density wave.     

Recent experimental results on the superconductor/ferromagnet proximity effect

The recent experimental results on the proximity effect in heterostructures composed of superconducting and ferromagnetic thin films are reviewed. First, the experimental observation and investigation of the spin screening effect, i.e., a spin polarization in the V layer developing in the superconducting state under the influence of a spin polarization of conduction electrons in the ferromagnetic layer are discussed. This effect was predicted theoretically by Bergeret et al. [F. S. Bergeret, A. F. Volkov, and K. B. Efetov, EPL 66, 111 (2004); Phys. Rev. B 69, 174504 (2004)]. Then, the progress concerning the experimental realization of the superconducting spin switch device based on the superconductor/ferromagnet proximity effect is presented.     

Abrupt transition in quasiparticle dynamics at optimal doping in a cuprate superconductor system

We report time-resolved measurements of the photoinduced change in reflectivity, DeltaR, in the Bi2Sr2Ca(1-y)Dy(y)Cu2O8+delta (BSCCO) system of cuprate superconductors as a function of hole concentration. We find that the kinetics of quasiparticle decay and the sign of DeltaR both change abruptly where the superconducting transition temperature T(c) is maximal. These coincident changes suggest that a sharp transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system.     

Peeling stress analysis for an inhomogeneous high-Tc superconductor with a discontinuous interface at the substrate

This paper presents an analysis of a superconductor–substrate system to calculate the peeling stress of a high temperature superconductor (HTS) when the temperature decreases from ambient to operating conditions (cryogenic temperatures). Firstly, the values for the material properties of the inhomogeneous high temperature superconductor (HTS) were obtained by fitting a second order polynomial to the experimental data. It is assumed that the material properties of the inhomogeneous HTS vary with varying height coordinate and temperature. Then, through the proposed graded finite element method, the coupled thermo-mechanical equations were solved numerically. The numerical results show that the thermal stress generated in the inhomogeneous HTS is larger on a SiTiO₃ substrate than on a MgO substrate. The maximum thermal stresses, i.e., the peeling stresses, occur near the bottom corner of the inhomogeneous HTS and may induce fracture behavior at the bi-material interface. The inhomogeneous HTS cools at a slower pace than the homogeneous HTS from the room temperature to the operating temperature. It is also shown that the magnitude of the peeling stress for a homogeneous HTS is larger than that for an inhomogeneous HTS. It is intended that the model presented here be useful to researchers who are interested in the mechanical properties of an inhomogeneous HTS.     

Josephson effect in SIFS with an inhomogeneous superconductor/ferromagnet interface

The Josephson effect in a superconductor-insulator-ferromagnet-superconductor structure with a stepwise change in the transparency of the superconductor/ferromagnet interface has been investigated within the formalism of the Usadel equations. It is shown that, at a certain thickness of the ferromagnet layer, this feature leads to the formation of a point contact with an anomalous dependence of the critical current on the external magnetic field.     

Force between a magnetic tip and an inhomogeneous superconductor at zero field

The force exerted by a semi-infinite inhomogeneous superconductor with a planar interface to vacuum on a magnetic tip is studied theoretically in the absence of external magnetic fields. It is shown that the force has a contribution from inhomogeneities due to material defects with unique characteristics. Defects are taken into account in the London limit by allowing the mass parameter to vary spatially. The contribution from defects to the force is calculated analytically to first order in the deviation of the mass parameter from its constant value for the homogeneous superconductor, assuming that the tip is a point dipole perpendicular to the interface, and that it does not spontaneously create vortex matter. Random point defects and linear localized defects are considered phenomenologically. For each defect type the force dependence on the dipole position coordinates is obtained, and the force magnitudes are estimated numerically. The predictions for the dependence of the linear defect force on the dipole lateral position are found to agree qualitatively with experiment.