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

Abstract

Tunneling measurements have been carried out on layered superconductors of the β(SmSI)-type -L i0.48(THF)xHfNCl (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 2Δ(4. 2K ) =1 1 −12 meV for Li0.48(THF)xHfNCl with the highest Tc = 25.5 K. Some of our measurements revealed multiple gaps and dip-hump structures, the largest gap 2Δ (4.2 K) ≈ 17-20 meV closing at Tc. This was shown both by break-junction and scanning-tunneling spectroscopy. From these experiments it stems that the highest obtained gap ratio 2Δ/kBTc ∼ 8 substantially exceeds the BCS weak-coupling limiting values: ≈3.5 and ≈4.3 for s-wave and d-wave order parameter symmetry, respectively. Such large 2Δ/kBTc ratios are rather unusual for conventional superconductors but quite common to high-Tc 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.