October 2, 1967

TO: Professor Bernd Matthias

5234 P-C Building

SUBJECT: Theory of Superconductivity

I think that it might be well worthwhile to carry out an attack on the problem of superconductivity by the use of the procedure described by me in my 1949 paper in the Proceedings of the Royal Society of London (196, 343).

In this paper I formulated a wave equation for metals and intermetallic compounds involving the motions of pairs of electrons from one site to another in the crystal. these pairs of electrons are, I think, what have recently been called Cooper pairs.

I believe that work along these lines could lead to a more detailed correlation of the superconductivity properties of substances with their other properties, such as crystal structure and bond numbers, than has been provided by the existing theories.

If someone were available who had a reasonably good grasp of quantum mechanics and an interest in work of this sort, I would be pleased to collaborate with him in this attack on the problem of superconductivity.

Linus Pauling

October 2, 1967

TO: Professor Bernd Matthias

5234 P-C Building

SUBJECT: Proposed Investigation of Tin-mercury Alloys

There are some simple crystal structures that I think are of special significance to the theory of superconductivity. I suggest the following investigation, in this connection.

Tin containing a small amount of mercury (between about 1% and 10%, atomic percent, mercury) has an unusual structure. The unit of structure is hexagonal, containing only one atom. Each atom is surrounded by six other atoms in the basal plane, at the same distance, and by two atoms on opposite sides along the directions of the hexagonal axis. The six bonds in the basal plane have lengths about 3.21 Å, corresponding to quarter bonds, and the other two have lengths about 3.00 Å, corresponding to half bonds.

I think that this structure is favorable to superconductivity, in the direction of the C axis.

Would it be possible for you to have alloys with a range of composition made and tested for superconductivity?

I note that white tin is a superconductor, with transition temperature 3.72º K. Rhombohedral mercury has transition temperature reported as 4.15º K, and tetragonal mercury 3.95º K.

There is the possibility that these alloys have already been studied as to their superconductivity. I note that Roberts in the American Institute of Physics Handbook lists HgSn as having critical 4.20º K. The structure is not given, and the reference is to Kamerlingh Onnes, 1913-1914

When I started this note to you I had not known that you had studied the indium-tin system, although I remembered that there was an indium-tin phase with simple hexagonal structure. I have now seen your 1961 paper in the Journal of Chemical Physics. I cannot be sure what the facts are, from reading your paper. The critical temperature 5º for 80 atomic percent tin may apply to the gamma phase. I do not know why there is a dashed line between the experimental points at 50 atomic percent tin and 80 atomic percent tin, and the solid line from 80 to 100 percent atomic percent tin.

I predict that below the critical temperature these simple hexagonal phases would show a superstructure line corresponding to the basal plane, with spacing about 12 Å, that is four times the C-axis length above the critical temperature.

Linus Pauling