MAGNESIUM DIBORIDE – AN UNEXPECTED NEW SUPERCONDUCTOR

by   David Larbalestier
University of Wisconsin-Madison

History: January 2002 marks the first anniversary of a very unexpected discovery, that the simple binary compound MgB₂ became superconducting at 39 K. This discovery, made in the group of Professor Akimitsu at Aoyama-Gakuin University in Tokyo, was announced in mid-January 2001. Within less than 6 weeks more than 50 groups world-wide were working hard on the compound and some half dozen papers on it had already been published in leading journals such as Nature and Physical Review Letters . Within 3 months several groups had shown the feasibility of powder-in-tube wires and thin films. The potential for both large-scale and electronic applications of MgB₂ beckons attractively in the view of many. A brief survey of the key issues may help IEEE-CSC members follow the developments, steering between opposite poles of excessive optimism and excessive caution.

Crystal Structure: MgB₂ is a hexagonal layered compound in which the larger Mg atoms confine the B atoms. A first question was whether or not higher T_c analogs of MgB₂ existed. Sadly, so far¸ the answer is no. Alloying of MgB₂ has so far succeeded only in decreasing T_c, though there is at least one report of precursor transitions at higher temperatures. It appears that specific details of the charge transfer from the Mg to B are vital in making the compound superconducting. The layered nature of the compound suggests both mechanical and electronic anisotropy. This leads to an H_c2 anisotropy of at least 2-3 and some recent papers suggest even higher numbers. Lower values are seen for fields parallel to the Mg and B planes.

Grain Boundary Effects: A decisive issue for the copper oxide, high temperature superconductors is their tendency to become weak-linked at grain boundaries. It was soon established that MgB₂ does not suffer from this effect, greatly simplifying conductor design.

Magnetic Field-Temperature Phase Diagram: Working with textured thin films grown by Eom, Patnaik et al. presented a diagram contrasting the behavior of perhaps clean-limit bulk samples with T_c of 39K with dirty-limit thin films having reduced T_c of 30-32K. As expected from GLAG theory, decreasing the electron mean free path strongly enhances H_c2, approximately doubling it from ~16T to ~39T in the favorable direction in the zero temperature limit.

Figure 1: Irreversibility fields, H*(T), (dashed lines) and Upper Critical Fields, H_c2(T), (solid lines) of high resistivity thin films (red) and standard bulk samples (black) of MgB₂. H* and H_c2 are higher for fields parallel to the B planes, H*(T) being about 0.85 H_c2. Bulk samples are untextured and exhibit dissipation onset when H exceeds H*(T) for grains having B planes perpendicular to the field. Superconductivity is lost only when the parallel grains are driven normal.

Large Scale Applications Potential: The anisotropy of the compound, whether as low as 2 or higher as some recent reports are suggesting, imposes a significant limit on performance. Dissipation begins at the irreversibility field, which is less than half the parallel upper critical field. At 4.2K this is 7-9T, well below that of Nb-Ti, which is about 10.5T and even further below that of Nb₃Sn which is about 25T. Thus to compete with low T_c materials, MgB₂ must be used at temperatures of order 15-30K, in fields of ~5-2T. These limits may be raised if the alloying exhibited in the thin film can be developed in bulk wire forms too. For now most wires are being made as rectangular tape. Rolling to tape form makes the composites denser, raising their connectivity and their critical current density. Rolling may also lead to some texture and in favorable geometries raise H* and H_c2. However, one potential advantage of the compound may be its use as a round wire because there is no need to texture to avoid weak-link behavior at grain boundaries. Round wires are much easier to cable, making conductors of arbitrary amperage much easier to implement than for tape form high-T_c conductors. Many groups have already made wires and it should soon be possible to evaluate their long-length performance in realistic devices.

General References:

L. D. Cooley, C. B. Eom, E. E. Hellstrom, and D. C. Larbalestier, "Potential application of Magnesium Diboride for accelerator magnet applications", Proceedings of the 2001 Particle Accelerator Conference to appear.

P Canfield and S Bud’ko "MgB₂: one year on" Physics World January 2002, p 29-34

C. Buzea and T. Yamashita, "Review of the Superconducting Properties of MgB₂", Sup. Sci and Tech. 14, R115-R146 (2001).

S. Patnaik, L.D. Cooley, A. Gurevich, A.A. Polyanskii, J.Y. Jiang, X.Y. Cai, A.A. Squitieri, M.T. Naus,

M.K. Lee, J.H. Choi, L. Belenky, S.D. Bu, J. Letteri, X. Song, D.G. Schlom, S. E. Babcock, C. B. Eom E.E. Hellstrom and D. C. Larbalestier "Electronic Anisotropy, Magnetic Field-Temperature Phase Diagram and their Dependence on Resistivity in c-Axis Oriented MgB₂ Thin Films", Superconductor Science and Technology 14, 315-319, (2001).

David Larbalestier, Alex Gurevich, Matthew Feldmann and Anatoly Polyanskii, "High Transition Temperature Superconducting Materials For Electric Power Applications", Nature 414, 368-377, (2001).