Search for ferromagnetic order in overdoped copper-oxide superconductors

J Wu,H Ambaye,V Lauter,I Božović,X He

doi:10.1038/srep45896

Abstract

In copper-oxides that show high-temperature superconductivity (HTS), the critical temperature (Tc) has a dome-shaped doping dependence. The cause of demise of both Tc and superfluid density ns on the overdoped side is a major puzzle. A recent study of transport and diamagnetism in a large number of overdoped La2−xSrxCuO4 (LSCO) films shows that this cannot be accounted for by disorder within the conventional Bardeen-Cooper-Schrieffer theory. This brings to focus an alternative explanation — competition of HTS with ferromagnetic order, fluctuating in superconducting samples and static beyond the superconductor-to-metal transition. Here, we examine this proposal by growing single-crystal LSCO thin films with doping on both sides of the transition by molecular beam epitaxy, and using polarized neutron reflectometry to measure their magnetic moments. In a heavily overdoped, metallic but non-superconducting LSCO (x = 0.35) film, the spin asymmetry of reflectivity shows a very small static magnetic moment (~2 emu/cm3). Less-doped, superconducting LSCO films show no magnetic moment in neutron reflectivity, both above and below Tc. Therefore, the collapse of HTS with overdoping is not caused by competing ferromagnetic order.

Highlights

The superconducting temperature Tc in cuprates shows an unusual dome-shaped dependence on the doping level, which presumably originates from an unconventional pairing mechanism
reflection high-energy electron diffraction (RHEED) is used to monitor every step of the film growth in real time, and in particular to study the oscillations in the intensity of the specular spot (Fig. 1b) that originate from the contrast between completely-filled layers and half-filled (‘rough’, maximal diffuse scattering) layers
RHEED oscillations persist throughout the entire film growth, and even more remarkably, the amplitude of oscillations does not decay till the end, indicating essentially ideal perfect layer-by-layer growth

Summary

Introduction

The superconducting temperature Tc in cuprates shows an unusual dome-shaped dependence on the doping level, which presumably originates from an unconventional pairing mechanism. It has been speculated that the demise of Tc and the superfluid density ns with overdoping originates from pair breaking caused by impurities, disorder, and phase separation, which could be accounted for within the conventional dirty-BCS (Bardeen-Cooper-Schrieffer) picture[3] This hypothesis has been thoroughly scrutinized and refuted in the recently released detailed study of penetration depth and magnetoresistance in a huge number of overdoped LSCO films[4] grown by atomic-layer-by layer molecular beam epitaxy (ALL-MBE). No ferromagnetism has been detected so far in the vicinity of the quantum (T = 0) superconductor-to-metal transition (SMT) that occurs at the overdoped dome edge (xc2 ≈ 0.26) With this motivation, we used polarized neutron reflectometry (PNR) to search for magnetic order in single-crystal films of overdoped LSCO, covering both the superconducting and metallic states. This difference is attributed largely to the epitaxial strain, the high oxidation power of ozone plays a role[24]

Methods

Results

Conclusion