We report on the preparation of dense monofilamentary MgB 2 /Ni and MgB 2 /Fe tapes with high critical current densities. In annealed MgB 2 /Ni tapes, we obtained transport critical current densities as high as 2.3 × 10 5 A/cm 2 at 4.2 K and 1.5 T, and for MgB 2 /Fe tapes 10 4 A/cm 2 at 4.2 K and 6.5 T. To the best of our knowledge, these are the highest transport j c values at 4.2 K reported for MgB 2 based tapes so far. An extrapolation to zero field of the MgB 2 /Fe data gives a critical current value of ~ 1 MA/cm 2 , corresponding to an critical current value well above 1000 A. The high j c values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB 2 and is thus an excellent sheath material candidate for tapes with self-field j c values at 4.2 K in excess of 1 MA/cm 2 .

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The range of critical thermal fluctuations in "classical" bulk superconductors is extremely small and especially in low fields hardly experimentally accessible. With a new type of calorimeter we have been able to resolve a small lambda anomaly within a narrow temperature range around the H c2 line. We show that the evolution of the anomaly as a function of magnetic field follows scaling laws expected in the presence of critical fluctuations. The lower onset of the fluctuation regime shows many characteristics of a continuous solid-to-liquid transition in the vortex matter. It can be driven into a first-order vortex melting transition by a small ac field which helps the vortex matter to reach equilibrium.

The critical current density of four MgB 2 samples was measured inductively and for one of them also by transport. Pure phased and dense bulk samples yield a critical current density which in value, as well as in its magnetic field and temperature dependence, is essentially the same as the intra-granular current density measured in a dispersed powder. Also the correspondence between the inductive and transport data indicates that the grain boundaries in the bulk samples are totally transparent for the current. The current-voltage relation becomes rapidly shallow in the vicinity of a depinning line, well below the second critical field. Between the depinning line and the second critical field the material is ohmic and shows a marked magnetoresisitivity, indicative of a flux-flow regime.

We report a pronounced peak effect in the magnetization and the magnetocaloric coefficient in a single crystal of the superconductor Nb 3 Sn. As the origin of the magnetization peak effect in classical type-II superconductors is still strongly debated, we performed an investigation of its underlying thermodynamics. Calorimetric experiments performed during field sweeps at constant temperatures reveal that the sharp increase in the current density occurs concurrently with additional degrees of freedom in the specific heat due to thermal fluctuations and a liquid vortex phase. No latent heat due to a direct first-order melting of a Bragg glass phase into the liquid phase is found, which we take as evidence for an intermediate glass phase with enhanced flux pinning. The Bragg glass phase can, however, be restored by a small ac field. In this case, a first-order vortex melting transition with a clear hysteresis is found. In the absence of an ac field, the intermediate glass phase is located within the field range of this hysteresis. This indicates that the peak effect is associated with the metastability of an underlying first-order vortex melting transition.

Mono- and multifilamentary Fe/MgB2 tapes and wires with high transport critical current densities have been prepared by the powder-in-tube technique using fine powders. The influence of the initial MgB2 grain size on critical current density, upper critical and irreversibility fields has been studied. After reducing the MgB2 grains to micrometer size by ball milling, the critical current density, Jc, was enhanced, while the upper critical field, μ0Hc2, remained unchanged. The anisotropy ratio between the upper critical fields parallel and perpendicular to the tape surface was determined to be 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive Jc values, measured at various temperatures between 4.2 and 25 K. On monofilamentary tapes, Jc values close to 105 A cm−2 were measured at 25 K/1 T, while Jc values ≈106 A cm−2 were extrapolated for 4.2 K/0 T. Fe/MgB2 tapes exhibit high exponential n factors for the resistive transition: n values of 60 and 30 were found at 4 T and 6 T, respectively. Multifilamentary wires (with seven filaments) show slightly lower Jc values, 1.1 × 105 A cm−2 at 4.2 K/2 T. The improvement of thermal and mechanical stability of MgB2/Fe tapes and wires appears clearly as a challenge for future developments.

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Single crystals of Bi 2 Sr 2 Ca 2 Cu 3 O 10 (Bi-2223) have been grown using the travelling solvent floating zone technique in an image furnace. Annealing the crystals under high pressures of O 2 increased their critical temperature to 109 K, and resulted in sharp superconducting transitions of T c = 1 K. The superconducting anisotropy of Bi-2223 was found to be ∼50, from measurements of the lower critical field with the magnetic field applied parallel and perpendicular to the c-axis. The anisotropy of Bi-2223 is significantly reduced compared to that of Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212), and this accounts for the enhanced irreversibility fields in Bi-2223. Furthermore, Bi-2223 has a higher critical current density, and a reduced magnetic relaxation rate compared to Bi-2212, which are both signatures of more effective pinning in Bi-2223 due to its reduced anisotropy.