Tunneling spectroscopy of layered superconductors: intercalated Li0.48(C4H8O)xHfNCl and De-intercalated HfNCl0.7

Tunneling measurements have been carried out on layered superconductors of the beta(SmSI)-type - Li-0.48(THF)(x)HfNCl (THF = C4H8O) and HfNCl0.7 - by means of break-junction and scanning tunneling spectroscopy. Break-junction technique reveals Bardeen-Cooper-Schrieffer (BCS) - like gap structures with typical gap values of 2 Delta (4.2 K) = 11-12 meV for Li-0.48(THF)(x)HfNCl with the highest T-c = 25.5 K. Some of our measurements revealed multiple gaps and dip-hump structures, the largest gap 2 Delta (4.2 K) a parts per thousand 17-20 meV closing at T-c. This was shown both by break-junction and scanning-tunneling spectroscopy. From these experiments it stems that the highest obtained gap ratio 2 Delta/k(B)T(c) similar to 8 substantially exceeds the BCS weak-coupling limiting values: a parts per thousand 3.5 and a parts per thousand 4.3 for s-wave and d-wave order parameter symmetry, respectively. Such large 2 Delta/k(B)T(c) ratios are rather unusual for conventional superconductors but quite common to high-T-c cuprates, as well as to organic superconductors. Our studies allowed to collect much more evidence concerning the huge pairing energy in those materials and to investigate in detail the complexity of their superconducting gap spectra. An origin of the observed phenomena still remains to be clarified.