Structural disorder and superconducting transition temperature of ion-irradiated NbC

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Abstract
Structural disorder and its influence on the Tc depression of He- and Ar-irradiated single-phase NbC thin films and single crystals have been investigated. X-ray diffraction experiments on thin films have revealed a lattice expansion ( Delta a0/a0=0.4% at irradiation state Phi 1) and a subsequent contraction ( Delta a0/a0=-0.5% at state Phi 2) with the preservation of the B1 structure for irradiation with both ion species. Static atomic displacements of Nb atoms show similar variations with a maximum value of 0.08 AA at state Phi 1 and with a subsequent decrease. Channelling measurements on a He-irradiated NbC0.98 single crystal have revealed small displacements in the Nb sublattice and a considerable disorder of the C sublattice at state Phi 1. Tc decrease continuously from above 11 K to around 4 K. It shows saturation for Ar-irradiated samples at higher fluences where a0 saturation is observed. From the annealing experiments of samples irradiated at Phi 1, stacking fault formation is inferred around 250 degrees C and recovery of Nb displacements has been observed around 700 degrees C. Lattice expansion is mainly attributed to interstitial Nb defects including antisite defects, and the subsequent contraction is attributed to their clustering or precipitation Tc depression is thought to be due mainly to the coexistence of Nb and C vacancies.