References

*

Permanent address: Institute of Physics, Technical University, Budapest, Hungary

**

Permanent address: Research Institute for Solid State Physics, Budapest 1525, Pf. 49, Hungary

1

P. B. Allen and R.C. Dynes, Phys. Rev. 12, 905 (1975)

2

J.P. Carbotte, Rev. Mod. Phys. 62, 1027 (1990)

3

W.L. McMillan, Phys. Rev. B 167, 331 (1968)

4

O. Gunnarson et al., Phys. Rev. Letters 74, 1875 (1995); C.M. Varma, J. Zaanen, and K. Raghavachari, Science, 254, 989 (1991); M.Schluter et al., Phys. Rev. Letters, 68, 526 (1992)

5

L.D. Rotter, Z. Schlesinger, J.P. McCauley, N. Coustel, J.E. Fischer, A.B. Smith, Nature, 355, 532 (1992)

6

S.A. FitzGerlad, S.G. Kaplan, A. Rosenberg, A.J. Sievers, R.A.S. McMordie, Phys. Rev. B 45, 10165(1992)

7

L. Degiorgiet al., Phys. Rev. B 49, 7012 (1994)

8

L. Degiorgi, G. Briceno, M.S. Fuhrer, A. Zettl and P. Wachter, Nature 369, 541 (1994)

9

Z. Zhang et al., Science, 254, 1619 (1991) C.-C. Zhang et al. Nature, 353 333 (1991)

10

P. Jess, U. Hubler, S. Behler, V. Thommen-Geiser, H.P. Lang, H.J. Güntherodt, Synth. Met., 77, 201 (1996)

11

Chun Gu, et al., Phys. Rev. B 50, 16566 (1994)

12

Tycko et al., Phys. Rev. Letters, 68, 1912 (1992)

13

V.A. Stenger, C.H. Pennington, D.R. Buffinger, R.P. Ziebarth, Phys. Rev. Letters, 74, 1649 (1995)

14

R.F. Kiefl et al., Phys. Rev. Letters, 70, 3987 (1993)

15

W.A. Vareka, A. Zettl, Phys. Rev. Letters, 72, 4121 (1994)

16

For a summary of the intramolecular modes, see M.C. Martin et al., Phys. Rev. B 50, 173, (1994).

17

D. Koller, M.C. Martin, P.W. Stephens and L. Mihaly, Rev. Sci. Inst. 65, 760 (1994)

18

M.C. Martin, D. Koller, X.Q. Du, P.W. Stephens, and L. Mihaly, Phys. Rev. B 49, 10818 (1994)

19

T.T.M. Palstraa, R.C. Haddon, A.F. Hebard, J. Zaanen, Phys. Rev. Letters, 68, 1054 (1992)

20

R.E. Glover, M. Tinkham, Phys. Rev. 108 243 (1957)

21

D.C. Mattis and J. Bardeen, Phys. Rev. 111, 412 (1958)

22

D. Mandrus, L. Forro, D. Koller and L. Mihaly, Nature, 351, 460 (1991)

23

This formula accounts for the tunneling of single electrons (see, for example, M. Tinkham, Introduction to superconductivity McGraw-Hill, New York, 1975). The Josephson tunneling contributes to the DC current at zero bias. However, this contribution is typically small for large resistance junctions (V. Ambegaokar and A. Baratoff, Phys. Rev. Letters, 11, 104 (1963)).

24

It remains to be seen why did the optical reflectivity study of Degiorgi et al., Ref. [8], yield a smaller gap. Nevertheless, in that measurement a downturn was seen in the reflectivity curve at the energy close to the value found by us.

25

P.B. Allen and D. Rainer, Nature, 349, 396 (1991)

26

R.C. Dynes, V. Narayanamurti and J.P. Garno, Phys. Rev. Letters, 41, 1509 (1978)

27

E.L. Wolf, Principles of Tunneling Spectroscopy (Clarendon, Oxford, 1988)

28

R. Akis, C. Jiang and J.P. Carbotte, Physica C 176, 485 (1991)

29

P.B. Allen et al., Phys. Rev. B 34, 4331 (1986)