Metallic Nonsuperconducting Phase andD-Wave Superconductivity in Zn-SubstitutedLa1.85Sr0.15

Abstract

Measurements of the resistivity, magnetoresistance and penetration depth were made on films of La1.85Sr0.15CuO4, with up to 12 at.% of Zn substituted for the Cu. The results show that the quadratic temperature dependence of the inverse square of the penetration depth, indicative of dwave superconductivity, is not affected by doping. The suppression of superconductivity leads to a metallic nonsuperconducting phase, as expected for a pairing mechanism related to spin fluctuations. The metal-insulator transition occurs in the vicinity of kF l ≈ 1, and appears to be disorder-driven, with the carrier concentration unaffected by doping. 74.72.Dn, 74.76.Bz, 74.25.Fy, 74.20.Mn Although there is strong evidence for d-wave symmetry of the order parameter in high-T c superconductors [1], earlier experiments do not distinguish between mechanisms that lead to pure d x 2 −y 2 symmetry [2], and others that allow an admixture of s-wave pairing [3].In this letter we describe the suppression of superconductivity by disorder, with the conclusion that pure d-wave symmetry continues until the superconductivity disappears. The experiment is based on the fact that disorder strongly suppresses d-wave pairing, and may therefore lead to a transition from the superconducting state to a normal-metal state. Any s-wave pairing would be less strongly affected, so that in its presence superconductivity would be expected to persist until, with greater disorder, it is destroyed at the metal-insulator transition.Studies of the T c -suppression in electron-irradiated YBa 2 Cu 3 O 7−δ (YBCO) [3], or in Zn-doped YBCO and La 2−x Sr x CuO 4 (LSCO) [4] did not address the question of the nature of the nonsuperconducting phase. Our previous study showed the existence of a metallic nonsuperconducting phase in LSCO with variouys impurities [5], but was subject to criticism because it was done on polycrystalline, ceramic specimens.We studied a series of single-crystalline La 1.85 Sr 0.15 Cu 1−y Zn y O 4 films, with zinc content, y, from 0 to 0.12, and complete suppression of superconductivity for y > 0.055. We find that with increasing y the transition from the superconducting state is to a metallic state, with the carrier concentration unaffected by the addition of the zinc. This is in contrast with the carrier-driven transition that is observed with a change in the strontium content. We also measured the superconducting penetration depth (λ), and find that it remains proportional to T 2 when y is increased, suggesting that there is no s-wave component. As y increases to 0.12, the metal-insulator transition is approched in the vicinity of k F ℓ = 1, where k F is the Fermi wave vector and ℓ is the electronic mean free path, suggesting that the transition is disorder-driven.The c-axis oriented films, with thicknesses between 5000 and 9000Å , were grown by pulsed laser deposition on LaSrAlO 4 substrates. The films were patterned by photolithography and wires were attached with indium to evaporated silver pads. Standard six-probe geometry was used to meas...

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Metal-Insulator Transition in Zinc-Doped LaSrCuO

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