Abstract
The penetration depth \ensuremath{\lambda}(T) in ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${)}_{3}$${\mathrm{O}}_{7}$ films, with M=Ni or Zn and nominal concentrations, 0.02\ensuremath{\le}x\ensuremath{\le}0.06, is obtained from the mutual inductance of coaxial coils on opposite sides of the films. Both Ni and Zn increase \ensuremath{\lambda}(0) very rapidly, such that the superfluid density, ${\mathit{n}}_{\mathit{s}}$(0)\ensuremath{\propto}${\ensuremath{\lambda}}^{\mathrm{\ensuremath{-}}2}$(0), decreases by a factor of 2 for each percent of dopant. The rapid increase in \ensuremath{\lambda}(0) implies that disorder fills in the superconducting density of states at low energy, so that ${\mathit{N}}_{\mathit{S}}$(0) is roughly 80--95 % of the normal-state density of states. An analytic d-wave theory, valid at T=0, finds that \ensuremath{\lambda}(0) increases rapidly with disorder, but not as rapidly as observed. It is striking that the dependence of \ensuremath{\lambda}(T/${\mathit{T}}_{\mathit{c}}$) on T/${\mathit{T}}_{\mathit{c}}$ does not change significantly as x increases from 2% to 6%, although it is different from undoped ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films. An ad hoc phenomenological model finds that one should expect this result. Finally, the values of ${\mathit{N}}_{\mathit{S}}$(0) deduced from \ensuremath{\lambda} are somewhat larger than values deduced from specific-heat measurements on Zn-doped ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, which also indicate increasing gaplessness with doping.
Published Version
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