Frontiers in Superconductivity Research by Barry P. Martins (Nova Science Publishers) Superconductivity is the ability of certain materials to conduct electrical current with no resistance and extremely low losses. High temperature superconductors, such as La2-xSrxCuOx (Tc=40K) and YBa2Cu3O7-x (Tc=90K), were discovered in 1987 and have been actively studied since. In spite of an intense, worldwide, research effort during this time, a complete understanding of the copper oxide (cuprate) materials is still lacking. Many fundamental questions are unanswered, particularly the mechanism by which high-Tc superconductivity occurs. More broadly, the cuprates are in a class of solids with strong electron-electron interactions. An understanding of such "strongly correlated" solids is perhaps the major unsolved problem of condensed matter physics with over ten thousand researchers working on this topic.
High-Tc superconductors also have significant potential for applications in technologies ranging from electric power generation and transmission to digital electron-ics. This ability to carry large amounts of current can be applied to electric power devices such as motors and generators, and to electricity transmission in power lines. For example, superconductors can carry as much as 100 times the amount of electricity of ordinary copper or aluminum wires of the same size. Many universities, research institutes and companies are working to develop high-Tc superconductivity applications and considerable progress has been made. This volume brings together leading research in this growth field.
Efforts to unify the Bardeen, Cooper & Schrieffer (BCS) and the Bose-Einstein condensation (BEC) formalisms in terms of a "complete boson-fermion (BF) model" (CBFM) are surveyed by M. de Llano. A vital distinction is that Cooper pairs (CPs) are indeed bosons that suffer BEC, in contrast with BCS pairs that are not bosons. Another crucial ingredient (particularly in 2D where ordinary BEC does not occur) is the linear dispersion relation of "ordinary" CPs, at least in leading order in the center-of-mass momentum (CMM) power-series expansion of the CP energy. This arises because CPs propagate not in vacuo but in the Fermi "sea." A many-body Bethe-Salpeter equation treatment of CPs based on the ideal Fermi gas (IFG) sea yields the familiar negative-energy, two-particle bound-state if 2h-CPs are ignored as in the ordinary CP problem. But it gives purely-imaginary energies, and is thus meaningless, if 2h-CPS are included as completness requires. However, when based on the BCS ground state instead of the IFG, in addition to the familiar trivial solution (or Anderson-Bogoliubov-Higgs) sound mode, legitimate two-particle moving "generalized CPs" emerge but as positive-energy, finite-lifetime, resonant nontrivial solutions for nonzero-CMM. This amounts to replacing the purely-kinetic-energy unperturbed Hamiltonian by the BCS one. The moving CPs again have a linear dispersion leading term. BEC of such pairs may thus occur in exactly 2D (as it can-not with quadratic dispersion) and in fact all the way down to (1 + e)D where e can be infinitesimally small, thus encompassing all empirically known superconductors. The unified theory reduces in limiting cases to all the main continuum (as opposed to "spin") statistical theories of superconductivity. These include both the BCS and BEC theories. The unified BF theory is "complete" in that not only two-electron (2e) but also two-hole (2h) CPs are allowed, and in arbitrary proportions. In contrast, BCS theory can be deduced from the CBFM but allows only equal (50%-50%) mixtures of them, a fact rarely if ever stressed. The CBFM shows that the BCS condensate is precisely a BE condensate of a mixture of kinematically independent electrons coexisting with weakly-coupled zero CMM 2e- and 2h-CPs in equal propor-tions. Without abandoning the electron-phonon mechanism, the CBFM has been applied in 2D and 3D. The BCS model interaction in moderately weak coupling is sufficient to reproduce the unusually high values of TT (in units of the Fermi temperature) of 0.01 - 0.1 empirically exhibited by the so-called "exotic" superconductors, including cuprates. This range is high relative to the low values of < 10-3 more or less correctly reproduced by BCS theory for conventional (mostly elemental) super-conductors. Also accounted for is the empirical fact that "hole superconductors" systematically have higher Tc's. Room temperature superconductors are predicted to be possible but only via 2h-CP BE condensates.
D. Dominguez, H. Kawamura and Mai Suan Li present a review of the results of their simulations on equilibrium and dynamic properties of the d-wave ceramic high-Ta superconductors. They represent disordered superconductors by a three-dimensional lattice model of randomly distributed 71- Josephson junctions with finite self-inductance. The paramagnetic Meissner effect observed experimentally in ceramic high-Ta superconductors is well reproduced by this model. In equilibrium, one can show that there is a low temperature chiral glass phase, which is characterized by chiralities frozen in time and in space. Experimental attempts on the search for the chiral glass phase are discussed. Dynamical phenomena such as anomalous microwave absorption, AC resistivity and enhancement of critical current by external electric fields are also considered by Langevin dynamics simulations. A satisfactory agreement of the simulations with several experiments in ceramic high TT superconductors was obtained.
In order to analyze pairing symmetry of an intrinsic superconducting true gap for inhomogeneous high-T, superconductors, Hyun-Tak Kim explains the intrinsic inhomogeneity of high-Ta superconductors on the basis of the metal-insulator instability and the extended Brinkman-Rice picture explaining the first-order metal-insulator transition, and justify a relation between a fractional effective charge and band filling (e' = pe; 0< p <1 is band filling) by means of measurement. An observed superconducting gap is given as a function of band-filling and intrinsic true gap (Robs= PZ )
and is the effect of measurement when 0< p <1. The true gap is observed only when p=1. The peak width of conductance curve as the density of states determines pairing symmetry. The identity of the observed superconducting gap anisotropy for Bi2Sr2CaCu2O8+X (Bi-2212)is also revealed that the gap anisotropy is attributed to inhomogeneity of a d-wave insulating phase and a metal phase in the measure-ment region and is not evidence of the d-wave symmetry. The Ambegaokar-Baratoff (AB) theory based on the BCS theory is extended. Small IIR,t values observed by experiments are explained by the extended AB theory. The magnitude of the true gap deduced from I~Ra values for Bi-2212 is 8.5< A <17 meV. In addition, the half-flux quantum (4)o/2) as evidence of the d-wave symmetry observed by the triple-7r-junction experiments, is caused by flux trap. Comments on a flux trap of the magnetic modulation experiment using Pb-YBCO dc SQUIDs are also given on the ground of experimental results for the observation of paramagnetic Meissner effect of Bao6Ko.4BiO3. Recent researches on tunnel effect as well as proximity effect are discussed by means of measurement. Finally, it is concluded that an analyzed pairing symmetry is s-wave.
Adriana C. Serquis and Xiaozhou Liao review their recent progress on the processing, structures and properties of MgB2. In particular,they have used a magnesium vapor infiltration process (MVIP) to make high quality MgB2 powder. Serquis and Liao present detailed studies of the field and temperature dependence of the magnetization as well as scanning and transmission electron microscopy characterizations of samples prepared under different conditions. They compare the environ-mental stability, the critical current densities (Je) and the creep relaxation rates of MgB2 samples with different microstructures. The MgB2 samples synthesized by MVIP were found to contain nanometer-sized Mg(B4O)2 precipitates. Magnetization data show that such precipitates act as effective flux pinning centers, without deteriorating other superconducting properties such as T. The sizes and distribution of these precipitates can be controlled through processing, providing an attractive way to introduce flux pinning centers. They also review their results on the effect of heat treatments on the microstructure, magnetization and transport properties of MgB2 wires prepared by the powder-in-tube method. They discuss the optimization of the annealing conditions that strongly improve the connectivity by eliminating most of the microcracks present in the un-annealed wires. On the other hand, hot isostatic pressed (HIPed) wires have a higher JJ than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning.
In the present work, R. Di Capua et al. report on the thin films fabrication and characterization of the recently discovered magnesium diboride (MgB2) inter-metallic superconducting compound. High-quality c-axis oriented thin films were grown by a d.c. planar magnetron sputtering technique in a UHV system (10-7 Pa) equipped with 3 focused 2" magnetron sources. A precursor deposition step was fol-
lowed by an in-situ annealing, in Mg atmosphere, at about 800C in a In sealed Nb box. Superconducting samples were obtained both on MgO and Al2O3 (sapphire) single crystal substrates, 10 mm x 10 mm wide. The process is highly reproducible and can be easily scaled to produce large area films. The best filers showed maximum Tc = 35 K (onset), a transition width of 0.5 K, a residual resisitivity ratio up to 1.6, a low temperature critical current density Jc > 106 A/cm2, with a thickness ranging between 0.5µm and 1pm. The films were characterized by a variety of structural and electronic measurements. Structural characterizations included profilometer, X-rays diffraction, SEM and AFM measurements. From them, a wide c-axis orientation was revealed, together with a granular structure of the surface. Resistivity measurements in external magnetic field up to 8 T have been performed both in parallel and perpendicular configuration. The upper critical magnetic field vs. temperature behavior has been determined from the experimental data. Measurement of upper critical field showed, for the best samples, an anisotropy ratio of about 2.5, very close to the values observed for single crystals. Di Capua et al. measured the temperature and magnetic field dependencies of the critical current density Jc(H,T). Three-dimensional point like normal core pinning was evidenced by measurements of the magnetic dependence of the pinning forces independently from the superconducting and structural quality of the investigated films. The analysis of the experimental data in terms of the collective pinning model has pointed out the presence of a crossover magnetic field from a single vortex to a small vortex bundle pinning regime. A OTc pinning mechanism, i.e., a pinning associated with spatial fluctuations of the transition temperature, has been evidenced by the temperature dependence of this crossover field, in agreement with previous observations performed on MgB2 bulk materials. In view of possible device applications, the microwave response has been studied in detail. They report and discuss the dependence of the surface impedance on temperature and r.f. field amplitude at 20 GHz and the temperature dependence of the penetration depth via a dielectric resonator technique.
An experimental review on the influence of nonlocal electrodynamics in the vor-tex pinning properties of non-magnetic borocarbide superconductors is presented by A. V. Silhanek, J. R. Thompson, L. Civale, S. L. Bud'ko, and P. Canfield. They show that the pinning force density FF exhibits a rich and complex anisotropic behavior that sharply contrast with the small mass anisotropy of these compounds. For magnetic fields H applied parallel to the crystallographic c-axis, the first order reorientation transition between two rhombic lattices manifests itself as a kink in FP(H). For H1c-axis, a much larger Fp(H) and a slower relaxation rate is ob-served. In this field configuration, nonlocality induces a fourfold periodicity in Fp when H is rotated within the square basal plane. Unlike the out-of-plane anisotropy, which persists for increasing impurity levels, the in-plane fourfold anisotropy can be strongly suppressed by reducing the electronic mean free path. This result unambiguously demonstrate that the in-plane anisotropy is a consequence of nonlocal effects. The unique combination of superconducting and material parameters of the cuprate high-temperature superconductors gives rise to tremendously rich vortex phase diagrams and raises many fundamentally and technologically important questions. Complex structured low-temperature superconductors may exhibit similarly intricate phase diagrams and can offer alternative and sometimes advantageous ways of addressing some of the unsolved problems.
Amorphous films of alternating TaNGe 1_x superconducting and Ge insulating layers provide such a system with arbitrary layer thicknesses and thus full control over anisotropy and coupling between adjacent superconducting layers. Extensive dc-conductance measurements over a large part of the H-T phase diagram were performed by Andreas Engel and Benjamin J. Ruck on a number of films, where in some cases the layering was augmented with a columnar microstructure. This columnar structure results in a network of extended strong pinning sites comparable to twin boundaries in twinned YBa2Cu3O7_6. For the films presented Tß(0) values were in the range from 1.7 to 2.8 K. They are strongly type-II superconductors with
90 or higher and the full range from weak to strong anisotropy. The current-voltage (I-V) curves measured from non-columnar samples can be collapsed according to the vortex glass scaling laws. However, a lack of universality in the critical exponents indicates that the scaling analysis may simply be highlighting a continuous crossover between strong pinning at low T and H and much weaker pinning at high T and H, with no actual phase transition separating the two regimes. For the most anisotropic samples, with essentially two-dimensional decoupled layers, the strongly pinned phase is absent, and the low T and H regime is characterized by thermally activated plastic vortex flow. The micro-structured columnar films show strongly enhanced vortex pinning for magnetic fields co-aligned with the extended pinning sites. The T and H range for which dissipation-free current transport was observed was considerably expanded relative to non-columnar films. For these films a critical scaling analysis clearly identified the pinned vortex phase as a Bose-glass, lending support to the notion that the Bose-glass is a universal phase exhibited by superconductors containing extended defects. It was noted, however, that for the most anisotropic of the micro-structured samples Bose-glass scaling was successful only at relatively low magnetic fields. The high-field vortex phase in this sample was instead best described as a very viscous vortex liquid, with no phase transition to a glass upon lowering the temperature. Further analysis of the I-V characteristics taken in the Bose-glass phase allowed for the extraction of the field, temperature and current dependent glass exponent µ. A markedly different T and H dependence of µ for films with different anisotropy suggests different mechanisms for vortex creep within the pinned vortex phases. Variable-range hopping of vortices and collective flux creep are considered as models to describe the experimental data.
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