Influence of Ni and Zn substitution on anisotropy of the penetration depth inLa1.85Sr0.15CuO4

Abstract

In this paper we present measurements of the penetration depths ${\ensuremath{\lambda}}_{\mathrm{ab}}(T)$ $(H\ensuremath{\parallel}c)$ and ${\ensuremath{\lambda}}_{\ensuremath{\perp}}(T)$ $(H\ensuremath{\perp}c)$ in ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{Cu}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{O}}_{4}$ for $x=0,$ 0.005, 0.01, 0.015, 0.025, and 0.035 for $M=\mathrm{Ni}$ and $x=0,$ 0.005, 0.01, and 0.02 for $M=\mathrm{Zn}.$ The penetration depth was obtained from ac susceptibility measurements of powdered samples, immersed in wax, and magnetically oriented in a static magnetic field of 10 T. The temperature dependencies of the penetration depths can be described by power laws, but with exponents n varying linearly with substituent content. The exponent n increases at a rate of about 1 per at. % for nickel substitution and 2.5 per at. % for zinc substitution. We also have found that the penetration-depth anisotropy is dependent on substituent content, decreasing to a minimum at $x\ensuremath{\simeq}0.015$ and increasing for higher substitutions. The penetration-depth anisotropy vs substituent content can be described by similar quadratic functions for both substituents. Our results strongly suggest that both the effective mass and the density of charge carriers must be taken into account in theories describing high-temperature superconductivity.