THE EFFECT OF ALLOYING ON T-C IN SUPERCONDUCTING YNI(2-X)M(X)B(2)C (M=CO, CU)

In order to probe the influence of the density of states (DOS) at the Fermi level N(E(F)) on the superconducting transition temperature T-c, we investigated the effect of alloying on T-c in YNi(2-x)M(x)B(2)C (M = Co, Cu). The lattice contracts (expands) as a result of substitution of Ni with Co (Cu). From the observed drop in T-c for both substitutions, we infer that N(E(F)) may lie very close to a maximum in the DOS in the parent compound YNi2B2C, in agreement with published band-structure calculations. However, the much steeper drop in T-c for Co substitution (dT(c)/dx = -45 K compared to -19 K for Cu) indicates that either the peak in the DOS is asymmetric in energy, or alloying affects both N(E(F)) and the electron-phonon coupling strength.