Physics and Astronomy Colloquium
			   Tuesday, February 5, 2002
		4:10 p.m., Room 224 Physics-Astronomy Building

	    (Dr. Larbalestier will meet with students in Room 224 
		        from 3:15-3:45 p.m. on Tuesday)

	       Refreshment served at 3:45 p.m. outside Room 224

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	     MAGNESIUM DIBORIDE - A SURPRISING NEW SUPERCONDUCTOR

			     David C Larbalestier
		       University of Wisconsin-Madison

Twelve months ago a surprising announcement was made that the "simple"
compound MgB2 was superconducting at 39 K.  It was very quickly established
that a strong isotope effect occurred in the layered hexagonal compound and
that it was an electron-phonon superconductor.  However the material
consists of Mg and B layers with significant charge transfer from the Mg to
the B layer, leading to significant electronic anisotropy.  My colleagues
and I posed the question whether the material was weak linked and what the
anisotropy might be.  The answers that we developed were a.) that grain
boundaries are not obstacles to current flow, as they sadly are in high-Tc
materials (1), b.) that c-axis textured thin films have anisotropy of order
two (2), and c.) that thin films can be alloyed with unknown elements -
probably O - take MgB2 all the way to the dirty limit, maximizing Hc2 but
also somewhat depressing Tc (3).  These results, obtained by mid-2001 have
generally stood the test of time, but various newer measurements are not
always in agreement.  It is now clear that "well-made" MgB2 can exhibit a
variety of Tc values that range from below 20 K to 40 K.  Anisotropy values
range from 2 to more than 6 and it is now common to think that there are two
gaps in the compound.  So an apparently simple superconductor may not be so
simple after all!  A review of present knowledge of the material from my
viewpoint of its potential for high critical current density and high field
use will be made, situating my view in the context of the broader work being
done on this surprising new superconductor (4).

(1)  D. C. Larbalestier, L. D. Cooley, M. O. Rikel, A. A. Polyanskii, J.
Jiang, S. Patnaik, X. Y. Cai, D. M. Feldmann, A. Gurevich, A. A. Squitieri,
M. T. Naus, C. B. Eom, E. E. Hellstrom, R. J. Cava, and K. A. Regan,
"Strongly Linked Current Flow in Polycrystalline Forms of the Superconductor
MgB2," Nature, 410: 186-189, 2001.

(2) 1.  C.B. Eom, M.K. Lee, J.H. Choi, L. J. Belenky, X. Song, L.D. Cooley,
M.T. Naus, S. Patnaik, J. Jiang, M. Rikel, A. Polyanskii, A. Gurevich, X.Y.
Cai, S.D. Bu, S.E. Babcock, E.E. Hellstrom, and D.C. Larbalestier, "Very
High Critical Current Density and Enhanced Irreversibility Field in
Magnesium Diboride Films", Nature 411: 558-560, 2001

(3) 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).

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