There are numerous potential applications for superconducting tapes based on YBa(2)Cu(3)O(7-x) (YBCO) films coated onto metallic substrates. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5-5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Twenty years ago in a series of amazing discoveries it was found that a large family of ceramic cuprate materials exhibited superconductivity at temperatures above, and in some cases well above, that of liquid nitrogen. Imaginations were energized by the thought of applications for zero-resistance conductors cooled with an inexpensive and readily available cryogen. Early optimism, however, was soon tempered by the hard realities of these new materials: brittle ceramics are not easily formed into long flexible conductors; high current levels require near-perfect crystallinity; and--the downside of high transition temperature--performance drops rapidly in a magnetic field. Despite these formidable obstacles, thousands of kilometres of high-temperature superconducting wire have now been manufactured for demonstrations of transmission cables, motors and other electrical power components. The question is whether the advantages of superconducting wire, such as efficiency and compactness, can outweigh the disadvantage: cost. The remaining task for materials scientists is to return to the fundamentals and squeeze as much performance as possible from these wonderful and difficult materials.

We present a comparative study of the angular dependent critical current density in YBa 2 Cu 3 O 7 films deposited on IBAD MgO and on single crystal MgO and SrTiO 3 substrates. We identify three angular regimes where pinning is dominated by different types of correlated and uncorrelated defects. We show that those regimes are present in all cases, indicating that the pinning mechanisms are the same, but their extension and characteristics are sample dependent, reflecting the quantitative differences in texture and defect density. In particular, the more defective nature of the films on IBAD turns into an advantage as it results in stronger vortex pinning, demonstrating that the critical current density of the films on single crystals is not an upper limit for the performance of the IBAD coated conductors.

We argue that superconductor/ferromagnet multilayers of nanoscale period should exhibit strong pinning of vortices by the magnetic domain structure in magnetic fields below the coercive field when ferromagnetic layers exhibit strong perpendicular magnetic anisotropy. The estimated maximum magnetic pinning energy for single vortex in such a system is about 100 times larger than the pinning energy by columnar defects. This pinning energy may provide critical currents as high as 106−107 A/cm2 at high temperatures (but not very close to Tc) at least in magnetic fields below 0.1 T.

Interlayer tunneling resistivity is used to probe the low-energy density-of-states (DOS) depletion due to the pseudogap in the normal state of Bi2Sr2CaCu2O8+y. Measurements up to 60 T reveal that a field that restores DOS to its ungapped state shows strikingly different temperature and doping dependencies from the characteristic fields of the superconducting state. The pseudogap closing field and the pseudogap temperature T ⋆ evaluated independently are related through a simple Zeeman energy scaling. These findings indicate a predominant role of spins over the orbital effects in the formation of the pseudogap.PACS numbers: 74.25. Dw, 74.25.Fy, 74.50.+r, 74.72.Hs A central unresolved issue of high temperature superconductivity is the connection of normal state correlations, referred to as the pseudogap [1][2][3][4][5], to the origins of high-T c . At the heart of the debate [6-11] is whether the pseudogap, which manifests itself as a depletion of the quasiparticle density of states (DOS) below a characteristic temperature T ⋆ , originates from spin or charge degrees of freedom and, in particular, whether it derives from some precursor of Cooper pairing [12] that acquires the superconducting coherence at T c . Energies of the order of the pseudogap have been accessed with elevated temperatures, with applied voltage in tunneling measurements, and with infrared frequencies in optical spectra [1]. But little is known about the effect of magnetic field. The magnetic field response may be unique: e.g., in the case of the superconducting state the upper critical field H c2 is determined by the superconducting coherence length, and not directly by the superconducting gap, since magnetic field strongly couples to the orbital motion of Cooper pairs.Current knowledge about the field dependence of the pseudogap derived from spectroscopic measurements is partly limited by the available dc field range [7][8][9][10][11]. More importantly, there is no systematic doping dependence in a single family of cuprates. Even in optimally doped YBa 2 Cu 3 O 7−δ alone, based on NMR relaxation rate measurements below 27.3 T, the pseudogap was claimed to decrease [7] or be independent of magnetic field [8]. In the underdoped YBa 2 Cu 4 O 8 no field effect on T ⋆ was reported up to 23.2 T [9], while a recent NMR study indicated a measurable field dependence in slightly overdoped TlSr 2 CaCu 2 O 6.8 [10]. In this paper, we report the interlayer (c-axis) resistivity ρ c measurements in fields up to 60 T in Bi 2 Sr 2 CaCu 2 O 8+y (BSCCO) crystals in a wide range of doping, from which we make a first systematic evaluation of the pseudogap closing field H pg that restores DOS to its ungapped state. Our results indicate a pronounced difference between field-temperature (H-T ) diagrams of the pseudogap and the superconducting states and a simple Zeeman scaling between H pg (0) and T ⋆ . Among various techniques that quantify DOS, the ρ c measurements are uniquely suited for exploring the highest magnetic field range available only in a pulsed mod...

Remarkable progress has been made in the development of YBa2Cu3O7−δ (YBCO)-based coated conductors, and the problems of continuous processing of commercially viable tape lengths are being rapidly solved by companies around the world. However, the current carried by these tapes is presently limited to about 100A for a 1-cm-wide tape, and this is due to a rapid decrease of critical current density (Jc) as the coating thickness is increased. We have now overcome this problem by separating relatively thin YBCO layers with very thin layers of CeO2. Using this multilayer technology, we have achieved Jc values on metal substrates of up to 4.0MA∕cm2 (75K, self-field) in films as thick as 3.5μm, for an extrapolated current of 1400A∕cm width.

We study the c-axis transport of stacked, intrinsic junctions in Bi 2 Sr 2 CaCu 2 O 81d single crystals, fabricated by the double-sided ion beam processing technique from single crystal whiskers. Measurements of the I-V characteristics of these samples allow us to obtain the temperature and voltage dependence of the quasiparticle c-axis conductivity in the superconducting state, the Josephson critical current, and the superconducting gap. We show that the BCS d-wave model in the clean limit for resonant impurity scattering, with a significant contribution from coherent interlayer tunneling, describes satisfactorily the low temperature and low energy c-axis transport of both quasiparticles and Cooper pairs. [S0031-9007 (99)09468-5] PACS numbers: 74.25.Fy, 74.50. + r, 74.72.HsThe observation of the pseudogap in the underdoped cuprate superconductors YBa 2 Cu 3 O 72d , La 22x Sr x CuO 41d , and Bi 2 Sr 2 CaCu 2 O 81d (Bi-2212) is indicative of the breakdown of the Fermi-liquid theory above T c in these systems [1]. On the other hand, the superconducting state is usually discussed in the BCS d-wave pairing model, which is based on the Fermi-liquid picture. Such an approach may be limited because (i) the properties of the normal state determine the mechanism of superconductivity, and (ii) the ratio 2D 0 ͞T c is well above the BCS ratio for d-wave pairing and is strongly doping dependent. Specifically, the BCS approach may fail in describing the properties of the superconducting state that are directly related to the quasiparticles, while the electrodynamics, based on supercurrents (macroscopic quantum phenomena), is insensitive to the pairing mechanism.The interlayer currents of both quasiparticles and Cooper pairs may be studied in highly anisotropic Bi-2212 crystals with Josephson interlayer coupling by measuring the I-V characteristic of the c-axis current. Such measurements provide information on the voltage and temperature dependence of the quasiparticle c-axis current, the energy gap, and the Josephson interlayer current. These data allow us to check the validity of the BCS d-wave model and determine the degree of the coherence of the interlayer tunneling. The question of coherence in both the normal and superconducting state is the focus of numerous studies (see, for example, [2-5]). Recently, Tanabe et al. [3] and Schlenga et al. [4] measured the quasiparticle c-axis transport in the superconducting state of Bi-2212 crystals and concluded that their data support the d-wave pairing scenario. However, their results for the quasiparticle current are insufficient to determine the nature of the interlayer transport and the effect of intralayer scattering on this transport.Our measurements of I-V characteristics have been performed on stacked, intrinsic mesa junctions, fabricated from high quality single crystal Bi-2212 whiskers by double-sided focused ion beam (FIB) processing [6]. For the fabrication, we used the conventional FIB machine of Seiko Instruments Corp., SMI 9800 (SP) with Ga 1 -ion beam. The details of...

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