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.     

Superconducting fluctuations in a magnetic field

The rounding of the transition curve is measured for superconducting bismuth films in a perpendicular magnetic field. The contribution of the fluctuating superconducting wave function to the conductivity aboveT _c in an applied magnetic field is calculated with a simple model. The allowed states of the fluctuations are cylinders in momentum space. During their life time the fluctuating superconducting electrons can be accelerated by an electrical field and contribute to the conductivity. Experiment and theory are in fair agreement. We obtain some information about the Pauli spin paramagnetism of the electrons.     

Upper critical field of niobium nitride thin films

The temperature dependences of the superconducting transition of niobium nitride (NbN) thin films have been investigated via the first harmonic of the voltage in dc magnetic fields of up to 8 T. The temperature dependence of the second critical field of NbN has been determined. The parameter responsible for the effect of spin paramagnetism in this material and the temperature dependence of the upper critical field that describes well the experimental data have been found in terms of the Werthamer–Helfand–Hohenberg (WHH) theory. The key parameters of the superconductor have been estimated from the transport and optical measurements.     

Paraconductivity of superconducting tin films

Experimental results on paraconductivity of superconducting thin tin films are compared with theoretical predictions of the Maki-Thompson theory which includes a small correction term proposed by Imamura et al. It is found that the corrected Maki-Thompson expression for paraconductivity of thin superconducting films improves the fit between theory and experimental data at relatively higher temperatures. It also gives a smaller value of δ, a fitting variable in the M-T theory which is known as the pair- breaking interaction parameter, than the value obtained in the unmodified M-T expression for the paraconductivity.     

Magnetic surfaces

The magnetic ground state, magnetic anisotropies and spin excitations of surfaces, interfaces and ultra-thin films of ferromagnetic 3d-metals are discussed. Enhanced magnetic ground state moments and altered hyperfine fields as predicted by ab initio band calculations have not been conclusively verified by experiments up to now. Future calculations should take into account dipolar fields and the role of interface roughness. Very large magnetic anisotropies are observed at magnetic surfaces and interfaces. In Ni/Cu multilayered films, the superposition of surface and stress-induced anisotropies was used to switch the easy axis of magnetization from the film plane to a perpendicular orientation by a proper choice of the Ni layer thickness. This could be an attractive possibility to develop new magnetic materials for technical applications. The temperature dependence of the spontaneous magnetization at surfaces and in ultra-thin films deviates from the behaviour of bulk material. Size effects as well as surface effects of spin wave excitations are discussed, comparing theoretical and experimental results. The need for more complete theories including surface exchange, surface anisotropy and realistic surface structures is emphasized.     

Magnetic behavior of nanoscopic superconductors

The magnetic behavior of nanometer-scale superconducting grains at finite temperature is investigated using the correlated static path approximation to the partition function. In these systems, the field penetration depth is much larger than the spatial dimension and Pauli paramagnetism becomes dominant. It is shown that deviations from both the bulk behavior and the BCS results for fixed number parity become significant due to gap fluctuations, which lead to appreciable pairing effects in the spin magnetization and susceptibility beyond the BCS critical fields or sizes. Differences between even and odd systems are also discussed.     

Spin injection and relaxation in a mesoscopic superconductor

We study spin transport in a superconducting nanowire using a set of closely spaced magnetic tunnel contacts. We observe a giant enhancement of the spin accumulation of up to 5 orders of magnitude on transition into the superconducting state, consistent with the expected changes in the density of states. The spin relaxation length decreases by an order of magnitude from its value in the normal state. These measurements, combined with our theoretical model, allow us to distinguish the individual spin-flip mechanisms present in the transport channel. Our conclusion is that magnetic impurities rather than spin-orbit coupling dominate spin-flip scattering in the superconducting state.     

Josephson and Andreev transport through quantum dots

In this article, we review the state of the art on the transport properties of quantum dot systems connected to superconducting and normal electrodes. The review is mainly focused on the theoretical achievements, although a summary of the most relevant experimental results is also given. A large part of the discussion is devoted to the single-level Anderson-type models generalized to include superconductivity in the leads, which already contains most of the interesting physical phenomena. Particular attention is paid to the competition between pairing and Kondo correlations, the emergence of π-junction behavior, the interplay of Andreev and resonant tunneling, and the important role of Andreev bound states that characterized the spectral properties of most of these systems. We give technical details on the several different analytical and numerical methods which have been developed for describing these properties. We further discuss the recent theoretical efforts devoted to extend this analysis to more complex situations like multidot, multilevel or multiterminal configurations in which novel phenomena is expected to emerge. These include control of the localized spin states by a Josephson current and also the possibility of creating entangled electron pairs by means of non-local Andreev processes.     

First-principles study of FeSe epitaxial films on SrTiO_3

The discovery of high temperature superconductivity in Fe Se films on SrTiO_3 substrate has inspired great experimental and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of Fe Se epitaxial films, emphasizing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermore, the comparison between Fe Se monolayer and bilayer films on SrTiO_3 is discussed.     

Non-equilibrium superconductivity in homogeneous thin films

A review is given of non-equilibrium phenomena in thin homogeneous superconducting films. The basic theoretical concepts necessary for understanding the experiments are discussed. Experiments on non-equilibrium states created by microwave, laser or phonon irradiation, quasiparticle injection through tunnel barriers, time-dependent externally supplied currents and thermal gradients are described. The emphasis of the paper is on the resulting physical phenomena such as the enhancement of superconductivity, charge imbalance, order parameter relaxation and changes in the superconducting energy gap.     

Surface and interface magnons: Magnetic structures near the surface

After a short review of the recent experimental data in the study of static and dynamic properties of magnetism in finite samples with attention to the surface effect, a theoretical treatment of the surface effect is presented. Different experiments are concerned: magnetization measurements, Mössbauer analysis, spin polarized photoemission, low energy electron diffraction, spin wave resonance and Brillouin scattering of spin waves. The choice of a typical Hamiltonian consistent with the experimental results is carefully made, with either a quite local surface perturbation leading to a discrete treatment or a quasi-continuous surface perturbation leading to a continuum treatment. Then the ground state of this spin Hamiltonian is derived in both cases with evidence for a surface of interface magnetic rearrangement with standard conditions. The first excited states, i.e. the one-magnon eigenstates are then derived in a quite general way, which allows non-uniformities in the sample. Several simple cases are completely treated, with, for instance, the study of the magnetic decoupling of the different parts of coupled thin films. These results enable us to discuss the temperature effects, and by means of a Tyablikov-Bogoliubov renormalization technique, high temperatures are correctly studied, including the Curie transitions. Now a careful investigation of the experimental situation compared to these theoretical results is given, with attention to new effects such as spin wave polarization and to a surface classification with evidence for “hard” and “soft” surface different behavior.     

Local measurements of magnetic characteristics of high-temperature superconducting films

A new experimental technique for measuring magnetic characteristics of high-temperature superconducting (HTS) films is described. The measurement system includes a laser providing for local measurements. The measured magnetic characteristics of the HTS films are of scientific and practical interest.     

Superconducting proximity effect in the presence of strong spin scattering

We report measurements of the four terminal temperature dependent resistance of narrow Au wires implanted with 100 ppm Fe impurities in proximity to superconducting Al films. The wires show an initial decrease in resistance as the temperature is lowered through the superconducting transition of the Al films, but then show an increase in resistance as the temperature is lowered further. In contrast to the case of pure Au wires in contact with a superconducting film, the resistance at the lowest temperatures rises above the normal state resistance. Analysis of the data shows that, in addition to contributions from magnetic scattering and electron-electron interactions, the temperature dependent resistivity shows a substantial contribution from the superconducting proximity effect, which exists even in the presence of strong spin scattering.     

Curie law, entropy excess, and superconductivity in heavy fermion metals and other strongly interacting fermi liquids

Low-temperature thermodynamic properties of strongly interacting Fermi liquids with a fermion condensate are investigated. We demonstrate that the spin susceptibility of these systems exhibits the Curie-Weiss law, and the entropy contains a temperature-independent term. The excessive entropy is released at the superconducting transition, enhancing the specific heat jump deltaC and rendering it proportional to the effective Curie constant. The theoretical results are favorably compared with the experimental data on the heavy-fermion metal CeCoIn5, as well as 3He films.     

Low temperature ferromagnetic properties of CdS and CdTe thin films

The magnetic property in a material is induced by the unpaired electrons. This can occur due to defect states which can enhance the magnetic moment and the spin polarization. In this report, CdS and CdTe thin films are grown on FTO glass substrates by chemical bath deposition and close-spaced sublimation, respectively. The magnetic properties, which are introduced from oxygen states, are found in CdS and CdTe thin films. From the hysteresis loop of magnetic moment it is revealed that CdS and CdTe thin films have different kinds of magnetic moments at different temperatures. The M–H curves indicate that from 100 K to 350 K, CdS and CdTe thin films show paramagnetism and diamagnetism, respectively.A superparamagnetic or a weakly ferromagnetic response is found at 5 K. It is also observed from ZFC/FC curves that magnetic moments decrease with temperature increasing. Spin polarized density functional calculation for spin magnetic moment is also carried out.     

Interplay of localization and superconducting fluctuations above the critical point

A review of some developments in localization effects on conductivity and magnetoconductivity is given. The determination of inelastic scattering rates for electrons in thin disordered metallic films is emphasized. In two-dimensional disordered superconductors aboveT _c , the superconducting fluctuations play an essential role. Recent work on the interplay of localization and superconducting fluctuation effects in determining the magnetoconductivity and the inelastic rate is described.     

Electronic properties of metallic small particles

The electronic properties of very small metallic particles are reviewed. The emphasis is placed on the experimental work done by the author's group using nuclear magnetic resonance. The anomalies in the properties such as the spin susceptibility and the nuclear spin lattice relaxation arising from the discreteness of the orbital energies are discussed. For superconducting particles, the fluctuation of the order parameter and the critical magnetic field are also discussed.     

正常导体-超导体(n-s)多层薄膜系统的电子态

本文是文献[1—3]工作的继续,在前面工作中发展的方法被推广到讨论半无限n-s多层膜结构的电子态,构造这种系统的步骤如下:首先把一完整无限的晶体沿两个分开的原子平面切割开得一晶体薄膜,它可以是正常金属也可以是超导体;然后把薄膜A和B通过金属型接触形成一混合单元O;最后沿垂直于界面方向依次排上单元O构成一半无限多层膜结构,我们导出了上述每一步所对应的单位子格林函数,对两种不同正常金属组成的多层膜结构,给出了单粒子状态密度和能带的数值计算结果,当两种正常金属相同时,所得的公式结果与文献[7]等价,进而,对薄膜 关键词：     

Spin fluctuations of nonequilibrium electrons and excitons in semiconductors

Effects that are related to deviations from thermodynamic equilibrium have a special place in modern physics. Among these, nonequilibrium phenomena in quantum systems attract the highest interest. The experimental technique of spin-noise spectroscopy has became quite widespread, which makes it possible to observe spin fluctuations of charge carriers in semiconductors under both equilibrium and nonequilibrium conditions. This calls for the development of a theory of spin fluctuations of electrons and electron–hole complexes for nonequilibrium conditions. In this paper, we consider a range of physical situations where a deviation from equilibrium becomes pronounced in the spin noise. A general method for the calculation of electron and exciton spin fluctuations in a nonequilibrium state is proposed. A short review of the theoretical and experimental results in this area is given.     

Spin density of states of proximity effect superconducting sandwiches in high fields

We use a tunneling Hamiltonian model to study the behavior of a thin film superconducting-normal metal sandwich in a parallel magnetic field including the effects of Pauli spin paramagnetism. The field acts to spin-split the density of states of the superconductor and the normal metal. With increasing magnetic field the sandwich displays zero temperature electron spin paramagnetism and a first-order transition to the normal state. We present a calculation of the field dependence of the density of states and order parameter.