K. A. Regan

Strongly linked current flow in polycrystalline forms of the superconductor MgB2.

The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field–temperature (H–T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.

Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x|[thinsp]|AlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

Experimental determination of superconducting parameters for the intermetallic perovskite superconductor MgCNi3

We have measured upper-critical-field

Direct observation of nanometer-scale Mg- and B-oxide phases at grain boundaries in MgB2

{H}_{\mathrm{c}2},

Low temperature synthesis of MgB2

specific heat C, and tunneling spectra of the intermetallic perovskite superconductor

Low Temperature Fabrication of MgB2

{\mathrm{MgCNi}}_{3}

The suppression of superconductivity in MgCNi3 by Ni-site doping

with a superconducting transition temperature

Absence of a structural transition up to 40 GPa in MgB 2 and the relevance of magnesium nonstoichiometry

{T}_{\mathrm{c}}\ensuremath{\approx}7.6\mathrm{K}.

Anisotropic grain morphology, crystallographic texture and their implications for flux pinning mechanisms in MgB2 pellets, filaments and thin films

Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field