Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB2. The materials included a hot-pressed sintered MgB2 pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB2 filament and a pulsed-laser-deposited thin film grown on a single crystalline [111] oriented SrTiO3 substrate. Thick plate...

The recently discovered intermetallic superconductor MgB2 appears to have a highly anisotopic upper critical field with H c2 /H min c2 ≡ γ > 5. In order to determine the temperature dependence of both H c2 and H min c2 we propose a method of extracting the superconducting anisotropy from the magnetization M(H,T ) of randomly oriented powder samples. The method is based on two features in (∂M/∂T...

MgB2 wires, tapes and bulk samples produced within the EU-funded HIPERMAG project have been studied by a combination of X-ray diffraction and electron microscopy. The reaction layers forming at the interface between the ceramic core and Fe or Ni sheaths can be studied with both methods. The complementary techniques enable to study both the microstructure and the formation kinetics of the interf...

We present a study of the effects of non-stoichiometry, boron purity, wire diameter and post-synthesis treatment (etching and Mg distilling) on the temperature dependent resistance and resistivity of sintered MgB2 pellets and wire segments. Whereas the residual resistivity ratio (RRR) varies between RRR ≈ 4 to RRR ≥ 20 for different boron purity, it is only moderately affected by non-stoichiome...

An overview of the theory of the upper critical field in dirty two-gap superconductors, with a particular emphasis on MgB2 is given. We focus here on the maximum Hc2 which may be achieved by increasing intraband scattering, and on the limitations imposed by weak interband scattering and paramagnetic effects. In particular, we discuss recent experiments which have demonstrated tenfold increase o...

Scanning Hall probe microscopy (SHPM) has been used to study vortex structures in thin epitaxial films of the superconductor MgB2. Unusual vortex patterns observed in MgB2 single crystals have previously been attributed to a competition between short-range repulsive and long-range attractive vortex-vortex interactions in this two band superconductor; the type 1.5 superconductivity scenario. Our...

We report a significant enhancement of the upper critical field Hc2 of different MgB2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ, we show a clear trend of an increase in Hc2 as ρ increases. One particular high resistivity film had a zero-temperature Hc2(0) well above the Hc2 values of competing non-cuprate superconductors ...

The microwave surface impedance = + 0 of MgB2 thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to by the standard BCS integral expression assuming the reduced energy gap (0) to be as low as 1.0–1.1 assuming (0) = 100–110 nm. These resul...