Magnetic analog of the isotope effect in cuprates

Abstract

We present extensive magnetic measurements of the (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy system with its four different families (x) having a T max c (x) variation of 28% and minimal structural changes. For each family, we measured the Néel temperature, the anisotropies of the magnetic interactions, and the spin glass temperature. Our results exhibit a universal relation Tc = cJns for all families, where c ∼ 1, J is the in plane Heisenberg exchange, and ns is the carrier density. This relates cuprate superconductivity to magnetism in the same sense that phonon mediated superconductivity is related to atomic mass.The critical temperature for superconductivity T c in metallic superconductors varies with isotope substitution [1]. This observation, known as the isotope effect, played a key role in exposing their mechanism for superconductivity. In contrast, the mechanism for superconductivity in the cuprate is still elusive, but is believed to be of magnetic origin [2]. Verifying this belief would require an experiment similar to the isotope effect, namely, a measurement of T c versus the magnetic interaction strength J, with no other structural changes in the compounds under investigation. Here we present such an experiment using the (Ca x La 1−x )(Ba 1.75−x La 0.25+x )Cu 3 O y (CLBLCO) system with its four different superconducting families, for which maximum T c (T max c ) varies by 28%. This is a large change compared to Sn, which has the strongest isotope effect in nature where T c varies only by 4%. For each family, we measured the Néel Temperature T N and the anisotropies of the magnetic interactions. This allows us to obtain the Heisenberg coupling J. In addition, we determine the spin glass temperature T g of underdoped samples. J, T g and T c allow us to generate a unified phase diagram for magnetism and superconductivity from no doping to over doping. We combine this result with a previous determination of the superconducting carrier density n s [3], and demonstrate experimentally a magnetic analog of the isotope effect.

Experimental investigation of the origin of the crossover temperature in cuprate superconductors via dc magnetic susceptibility

“…The density of states scenario is consistent with previous claims that the doping level in CLBLCO is x-independent, at least in the superconducting region [4,8]. The correlation length scenario is not cosistent with our previous claims that J(x) varies by 30% between families [6] since ξ has exponential J dependence [18].…”

“…This conclusion reinforces our previous claims that T c ≃ J(x)n s [6], and that in CLBLCO not all the holes condense to superfluid [8].…”

“…we are actually normalizing by the in-plane energy scale of each family J(x) [6]. If the cross-over was only a result of magnetic interaction between the spins in the planes (2D), T ⋆ should have scaled with T max c , demonstrated in Fig.…”

“…1(a). T c was measured by resistivity [4], and the spin glass temperature T g [5] and T N [6] by muon spin relaxation. Despite the rich phase diagram, the different CLBLCO families have negligible structural differences.…”

Soft x-ray absorption and high-resolution powder x-ray diffraction study of superconducting Ca La1−Ba1.75−La0.25+Cu3O system

“…The intimate mechanism which links the misfit strain between different layers to superconductivity is still unclear. An elaborate analysis of the whole phase diagram of CLBLCO performed by Keren et al [35] suggests that T c max scales with the antiferromagnetic exchange interaction, hence providing support that the misfit strain control both magnetic interaction and superconductivity. However more recently J.L.…”