Tunneling spectroscopy of deintercalated layered nitride superconductor ZrNCl0.7

Break junction tunneling spectroscopy measurements have been carried out on the novel layered nitride superconductor ZrNCl1-x (x congruent to 0.3) with T-c = 12 - 14 K. Two kinds of gap structures are reproducibly observed, both of which can be described by the BCS density of states. Values of the gap parameters at 4 K are obtained to be Delta = 1.6 - 2.3 meV and 4.0 - 5. 1 meV. Upon warming, these gap structures disappear at T-c thereby giving the ratio 2 Delta/k(B)T(c) = 2.7 - 3.8 and 6.6 - 8.4, respectively. The former is within the value of conventional superconductors, while the latter is extremely large in comparison with that of the conventional strong-coupling superconductors. The larger ratio is similar to the value of high-T-c cuprates. The origins of such two kinds of gaps are discussed.