Penetration Depth in Pure and Zinc-Substituted La2−x Sr x CuO4

Abstract

In the paper we present measurements of the in-plane λab(T) (H ‖ c) and out-of-plane λ⊥(T) (H⊥c) penetration depths in La2−x Sr x Cu1-yZnyO4 for x = 0.08, 0.1, 0.125, 0.15, and 0.2, and for y = 0, 0.005, 0.01, and 0.02. 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. For unsubstituted, underdoped samples (x 0.15) the measured points can be fitted by the exponential function of temperature. Our results support the view that for underdoped samples we are dealing with Bose-Einstein condensation while for overdoped ones the superconductivity is BCS-like. Extrapolated to T = 0, penetration depth values may be described by the quadratic function of strontium concentration similarly as the T c(x) dependence. For zinc-doped, underdoped La1.85Sr0.15Cu1−yZnyO4 the temperature dependences of penetration depths can be described by power laws, but with exponents n varying linearly with substituent content. These exponents n increase at a rate of about 2.5 per at% of zinc substitution. We found that the penetration- depth anisotropy is dependent on substituent content in La1.85Sr0.15Cu1-y,Zny04, decreasing to a minimum at x ≃ 0.015 and increasing for higher substitutions and can be described by a quadratic function. 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.