MAGNETIC PENETRATION DEPTH IN NI-DOPED AND ZN-DOPED YBA2(CU(1-X)M(X))(3)O-7 FILMS

The penetration depth λ(T) in YBa2(Cu1-xMx)3O7 films, with M=Ni or Zn and nominal concentrations, 0.02≤x≤0.06, is obtained from the mutual inductance of coaxial coils on opposite sides of the films. Both Ni and Zn increase λ(0) very rapidly, such that the superfluid density, ns(0)λ-2(0), decreases by a factor of 2 for each percent of dopant. The rapid increase in λ(0) implies that disorder fills in the superconducting density of states at low energy, so that NS(0) is roughly 80-95 % of the normal-state density of states. An analytic d-wave theory, valid at T=0, finds that λ(0) increases rapidly with disorder, but not as rapidly as observed. It is striking that the dependence of λ(T/Tc) on T/Tc does not change significantly as x increases from 2% to 6%, although it is different from undoped YBa2Cu3O7-δ films. An ad hoc phenomenological model finds that one should expect this result. Finally, the values of NS(0) deduced from λ are somewhat larger than values deduced from specific-heat measurements on Zn-doped YBa2Cu3O7-δ, which also indicate increasing gaplessness with doping. © 1995 The American Physical Society.