Superconducting β-ZrNClx probed by scanning-tunnelling and break-junction spectroscopy

Superconducting layered compound beta-ZrNClx (x = 0.7) with the critical temperature T-c = 13-14 K was investigated by means of scanning tunnelling microscopy/spectroscopy. The single-crystal domain facet of similar to 100 mu m(2) in the c-axis-oriented polycrystal was used as a probing surface. The STM image at 4.9 K shows triangular atomic lattice spots with the period of similar to 0.36 nm, which agrees with the X-ray diffraction measurements. The STS measurements of the local conductance, dI/dV, exhibit broadened gap structures with a substantial distribution of the gap-edge values. Most frequently observed peak-to-peak value of similar to 20 mV is remarkably similar to the superconducting gap edge of the isostructural beta-HfNClx with T-c = 24 K. Temperature, T, dependence of the dI/dV shows that the gap structure disappears above T-c similar to 13 K. Fitting of the dI/dV curve by the broadened BCS density of states leads to the superconducting gap of 2 Delta(4.9 K) = 11-13 meV. This is in accordance with our former break-junction data confirming the intrinsic character of the previously obtained extremely large gap to T-c ratio 2 Delta(0)/k(B)T(c) approximate to 10 (k(B) is the Boltzmann constant), thereby pointing to the unusual superconducting properties of this compound. (C) 2013 Elsevier B.V. All rights reserved.