High-resolution specific-heat measurements of the organic superconductor kappa-(BEDT-TTF)(2)-Cu[N(CN)(2)]Br in the superconducting ( B = 0) and normal ( B = 14 T) states show a clearly resolvable anomaly at T(c) = 11.5 K and an electronic contribution, C(es), which can be reasonably well described by strong-coupling BCS theory. Most importantly, C(es) vanishes exponentially in the superconducting state which gives evidence for a fully gapped order parameter.

We have studied the occurrence of superconductivity under pressure in the CaSi 2 family of compounds, namely the semimetallic trigonal phase and the ␣-ThSi 2 -type tetragonal one. Although only the latter is superconducting (T c ϭ1.56 K) at ambient pressure, starting from the trigonal phase and applying high pressure ͑Ͼ12 GPa͒ we have found a new superconducting phase with T c onset ϳ14 K, among the highest ever found in silicon based materials.Silicon and Si-based compounds are in general extensively studied as they are at the base of electronics and microdevices. Although intriguing and of practical interest, the appearance of superconductivity in these materials has not been so exhaustively explored. Yet the different bonding possibilities of silicon give rise to slightly different structural phases that can differ very much in their physical behavior. In 1985, the sp 2 simple hexagonal phase of pure Si was predicted and then found to be superconducting with transition temperature T c ϭ8.2 K under high pressure ͑15 GPa͒ by Chang and co-workers. 1 Similarly to C 60 fullerenes, the Si 20 fullerenes can be doped by alkali-metal atoms, when linked together through their dangling sp 3 bonds to form the structure of the clathrate (Na, Ba) x Si 46 , a superconductor with T c ϭ4 K. 2 Some silicides (NbSi 2 , TaSi 2 ) are also known to be superconductors 3 with typical T c 's of the order of 1 K, with the exception of the A-15 compounds, such as V 3 Si (T c ϭ17 K), where the linear chain bands of vanadium are at the origin of superconductivity. 4 More recently, the highpressure EuGe 2 -type phase ͓Fig. 1͑c͒, named as h1 in the following͔ of BaSi 2 , built by planes of boat-formed sp 3 silicon hexagons held together by hexagonal planes of calcium atoms, was found to be superconducting with T c onset ϭ6 K. 5 Pressure can indeed be a very useful variable allowing a gradual change of the lattice parameter of a particular silicide and forcing it to change to new configurations more favorable for superconductivity. As shown by the experiments on elementary silicon, in situ measurements using pressures in the range of 10 GPa may be adequate to scan a succession of interesting phases. Plausible candidates are large volume per unit formula silicides such as CaSi 2 . At ambient pressure CaSi 2 has a trigonal structure ͓also labeled TR6 in the following͔ 6 ͓Fig. 1͑a͔͒. In this form it is a semimetal 7 with an electrical resistivity that follows the conventional BlochGrüneisen temperature dependence and it is not superconducting down to 30 mK. 8 A high-pressure phase 9 with ␣-ThSi 2 type tetragonal structure ͓Fig. 1͑c͔͒ formed by interconnected sp 2 silicon atoms that rotate alternatively in the c-axis direction has been found to be superconducting with T c ϭ1.58 K. 10 In this report we describe the effects of high pressures ͑р22 GPa͒ on the behavior of the resistivity of these two CaSi 2 phases. We show that under pressure this system undergoes a series of phase transitions which finally develop superconductivity with one of the ...

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