Abstract
Superconducting layered compound β-ZrNClx (x=0.7) with the critical temperature Tc=13–14K was investigated by means of scanning tunnelling microscopy/spectroscopy. The single-crystal domain facet of ∼100μm2 in the c-axis-oriented polycrystal was used as a probing surface. The STM image at 4.9K shows triangular atomic lattice spots with the period of ∼0.36nm, 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 ∼20mV is remarkably similar to the superconducting gap edge of the isostructural β-HfNClx with Tc=24K. Temperature, T, dependence of the dI/dV shows that the gap structure disappears above Tc∼13K. Fitting of the dI/dV curve by the broadened BCS density of states leads to the superconducting gap of 2Δ(4.9K)=11–13meV. This is in accordance with our former break-junction data confirming the intrinsic character of the previously obtained extremely large gap to Tc ratio 2Δ(0)/kBTc≈10 (kB is the Boltzmann constant), thereby pointing to the unusual superconducting properties of this compound.
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