Abstract
The magnetic field and temperature dependence of the electronic specific heat have been systematically investigated in (LSCNO) in order to study Ni-impurity effects on the superconducting (SC) gap. In LSCNO with = 0.15 and = 0.015, the value of () at = 0 K, , is enhanced under the magnetic field applied along the axis. The increment of , , follows the Volovik relation = , characteristic of the SC gap with line nodes, with prefactor similar to that of a pure sample. The versus curve under = 0 shows a -wave-like SC anomaly with an abrupt increase at and -linear dependence at , although the -value in the versus curve increases with increasing Ni concentrations. Interestingly, as the SC part of , , decreases in LSCNO, is reduced in proportion to the decrease of . These findings can be explained phenomenologically by a simple model in which Ni impurities bring about strong pair breaking at the edges of the coherent nodal part of the Fermi surface but in the vicinity of the nodes of the SC gap. The reduction of the SC condensation energy in LSCNO, evaluated from at , is also understood by the same model.
- Received 26 December 2011
DOI:https://doi.org/10.1103/PhysRevB.85.134522
©2012 American Physical Society