Mössbauer study of the Bi-system superconductor at low temperature

Mossbauer measurements were performed in the temperature range from 77 to 300K. Temperature-dependent⁵⁷Fe Mossbauer recoil-free fraction in high-T_c superconducting phase of Bi-system display cvidence of a softening in lattice vibrations, which appears at T<150K as a precursor effect to the onset of superconductivity. The mean-square displacement (Inf) and the mean-square velocity (IS) of Fe³⁺ ion are both characterized by Debyc temperature O_D=320K within higher temperature range.     

First-principles study on superconductivity of solid oxygen

The superconductivity of solid oxygen in ζ phase was investigated by first-principles calculations based on the density functional theory. Using a monoclinic C2/m structure, we calculated the superconducting transition temperature by the Allen–Dynes formula and obtained 2.4 K at 100 GPa for the effective screened Coulomb repulsion constant μ* of 0.13. The transition temperature slowly decreases with increasing pressure and becomes 1.3 K at 200 GPa. The phonon analysis shows that the electron–phonon coupling is dominantly enhanced by the intermolecular vibrations of O₂ rather than the intramolecular ones. The phonon modes showing the strong electron–phonon coupling were found to be concentrated in the phonon frequency range of 100–150 cm^?1 at around the M-point in the Brillouin zone.     

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     

Influence of magnetic scattering on superconductivity of ferromagnet/superconductor/ferromagnet spin-valve

Ferromagnet/superconductor/ferromagnet (F/S/F) spin-valve systems in the dirty limit, described by Usadel equations, was theoretically investigated with respect to superconducting transition temperature. Their superconducting characteristics strongly depend not only on the mutual orientation and thickness of the ferromagnetic layers, but also on the interface transparency as well as magnetic scattering. Especially, the introduction of magnetic scattering drastically reduces the spin-valve effect in our work. The obtained results could be used to understand experimental values of and to provide guidelines for optimizing the experimental systems.     

Odd triplet superconductivity in superconductor ferromagnet structures: a survey

We review the main features of odd triplet superconductivity in superconductor-ferromagnet (S/F) structures. We discuss the different types of superconducting condensate that can be experimentally observed and pay special attention to the triplet component induced in a ferromagnet which is in contact with a superconductor. The triplet component is an even function of the momentum and an odd function of the frequency and leads to novel phenomena. PACS 74.78.Fk; 73.23.-b; 74.50.+r; 75.70.-i     

Effect of low-energy ion bombardment on the crystal structure and superconductivity of niobium films

The effect of ion bombardment on the growth of Nb films and their crystal structure is investigated. Epitaxial niobium films with the (001) orientation are grown on (01i2) Al₂O₃ substrates heated to 600°C and biased at ?20 V. Niobium films with pronounced axial texture in the [110] direction are grown on water-cooled Sitall (devitrified glass such as Pyroceram) substrates. In Nb films biased at ?50 V, which are in the superconducting state, the motion of individual magnetic vortices is observed with a magnetooptic indicator.     

Effect of electron irradiation on the structure and properties of the MgB2 superconductor

This paper reports on the results of an investigation into the effect of irradiation of the Bardeen-Cooper-Schriefer superconductor MgB₂ by electrons with a mean energy ē ～ 10 MeV at low doses (0 ≤ Φt ≤ ～5 × 10¹⁶ cm^?2) on the lattice parameters, the intensity and width of diffraction lines, the superconducting transition temperature T _c , and the temperature dependence of the resistivity ρ(T) in the normal state. The results of structural investigations have revealed regularities in the defect formation in the magnesium and boron sublattices of the MgB₂ compound as a function of the electron fluence. At the initial stage, irradiation leads to the formation of vacancies, originally in the magnesium sublattice and then in the boron sublattice. For fluences Φt ≥ ～1 × 10¹⁶ cm^?2, vacancies are formed in both sublattices. The evolution of the electrical and physical properties [T _c , ρ_{273 K}, residual resistivity ratio RRR = ρ_{273 K}/ρ_{50 K}, parameters of the dependence ρ(T)] under electron irradiation is in agreement with the regularities revealed in the formation of radiation-induced defects in the crystal lattice of the MgB₂ compound.     

Critical states of superconductivity and their crossover in multilayer superconductor/ferromagnet structures

In order to calculate the critical temperature of multilayer S/F structures (where S is a superconductor and F is a ferromagnet), a matrix method for solving linearized Usadel equations has been proposed. The spectrum of critical temperatures T ^(k) for the F/N _bl(S/F) structure has been obtained in the single-mode approximation. Eigenfunctions describing the spatial distribution of superconducting correlations in the direction perpendicular to the S-F interfaces have been calculated for each T ^(k) value. It has been found that dependences of T ^(k) on the thickness of F layers have a jump near the transition from 0 to π-state; any of the calculated T ^(k) values can be implemented in the region of jumps. It has been shown that the crossover of eigenstates is characterized by the suppression of superconductivity in outer S layers and by induced countercurrents in F layers. The possibility of the experimental implementation of a state corresponding to a given value from the spectrum of T ^(k) has been discussed.     

Electronic structure and superconductivity of europium

We have calculated the electronic structure of Eu for the bcc, hcp, and fcc crystal structures for volumes near equilibrium up to a calculated 90 GPa pressure using the augmented-plane-wave method in the local-density approximation. The frozen-core approximation was used with a semi-empirical shift of the f-states energies in the radial Schrödinger equation to move the occupied 4f valence states below the Γ₁ energy and into the core. This shift of the highly localized f-states yields the correct europium phase ordering with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the bcc and hcp structures are also found to agree with and follow a T_c trend similar to recent measurement by Debessai et al. [1].     

On the emergence of superconductivity and hysteresis in a cylindrical type I superconductor

One-dimensional vortex-free solutions of the system of Ginzburg-Landau equations (the so-called precursor states) are studied. These states describe the emergence of superconductivity in a long cylindrical type I superconductor, which was initially in the supercooled normal state in a magnetic field, and are formed upon subsequent reduction of the external field. The precursor states are responsible for the magnetic hysteresis in type I superconductors (for which κ < κ_c, where κ_c (R) is the critical value of the parameter κ in the Ginzburg-Landau theory, which is a function of radius). The range of fields is determined in which precursor states exist along with the Meissner state (and a hysteresis is possible) in the dependence of the cylinder radius R and parameter κ.     

Coexistence of ferromagnetism and superconductivity in normal mental/superconductor/ferromagnet structures

<正>We extend the Blonder,Tinkham and Klapwijk theory to the study of the inverse proximity effects in the normal mental/superconductor/ferromagnet structures.In the superconducting film,there are the gapless superconductivity and the spin-dependent density of states both within and without the energy gap.It indicates an appearance of the inverse-proximity-effect-induced ferromagnetism and a coexistence of ferromagnetism and superconductivity near the interface.The influence of exchange energy in the ferromagnet and barrier strength at the superconductor/ferromagnet interface on the inverse proximity effects is discussed.     

Separate re-entrant superconductivity in asymmetric ferromagnet/superconductor/ferromagnet trilayers

The superconducting and magnetic states of asymmetric ferromagnet/superconductor/ferromagnet (F/S/F′) nanostructures have been investigated using the boundary value problem for the Eilenberger function. It has been shown that 0- and π-phase superconducting states of pure thin F/S/F′ trilayers are controlled by the magnitude and sign of electron correlations in the F and F′ layers, as well as by the competition between homogeneous Bardeen-Cooper-Schrieffer (BCS) pairing and inhomogeneous Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) pairing. The LOFF-BCS-LOFF separate re-entrant superconductivity has been predicted for F/S/F′ trilayers. A continuous control of the pair-breaking factor in the Eilenberger function and transition to the state with re-entrant superconductivity is achieved by varying the thickness of the F′ layer. Sine-modulated 2D LOFF states in asymmetric F/S/F′ trilayers are possible not only for parallel, but also for antiparallel orientations of the magnetizations of the F and F′ layers; this fact significantly facilitates the experimental implementation of the predicted phenomena.     

锶掺杂钡位对YBCO晶体结构和超导电性的影响

利用XRD对系列样品YBa2-xSrxCu3O7-δ的(x=0~0.2)晶体结构进行研究,给出了体系相结构的掺杂效应,结果发现,随着掺杂量的增加,样品的晶体结构由正交相向四方相转变.同时采用四引线法对掺杂样品YBa2-xSrxCu3O7-δ的超导电性能进行了系统测量,发现随着锶掺杂量的增加,样品的超导转变温度总体呈明显降低趋势,并初步分析了钡位锶掺杂所引起的晶体结构变化对超导电性的影响.     

新型超导材料MgCNi3的电子结构与超导电性研究

用MS-Xα方法研究了非氧化物超导材料MgCNi3的电子结构.研究结果显示,态密度分布曲线的主峰靠近Fermi面,主要来自于Ni的d电子的贡献.用T(T=Co,Mn,Cu)替代MgCNi3中的部分Ni形成化合物MgCNi2T,替代使Ni的价电子数减小,价态发生变化,Fermi面处态密度N(EF)减小.计算结果表明:无论是电子掺杂(Cu)还是空穴掺杂(Co,Mn),MgCNi3的超导电性都被抑制;Mn掺杂比Co掺杂更快地抑制超导电性,Co是作为空穴掺杂而不是作为磁性杂质掺杂去抑制超导.     

Vortex patterns and nucleation of superconductivity in mesoscopic rectangles and in hybrid superconductor/ferromagnet structures

Nucleation of superconductivity and vortex patterns have been studied in mesoscopic samples providing the crossover between square and rectangular geometries. The measured nucleation line, T _c (H), has been analyzed in the framework of the linearized Ginzburg-Landau theory. A very good agreement has been found between the theoretical T _c (H) boundary and the experimental data for rectangles with different aspect ratios. The superconductor/ferromagnet hybrids, such as magnetic Co/Pd dot in a superconducting loop and the dot on top of a superconducting disk have also been investigated. Pronounced effects of the dot on the T _c (H) boundary have been found, including strong asymmetry with respect to the field polarity.Received: 26 February 2004, Published online: 3 August 2004PACS: 74.25.Dw Superconductivity phase diagrams - 74.78.Na Mesoscopic and nanoscale systems