Direct search for a ferromagnetic phase in a heavily overdoped nonsuperconducting copper oxide

Abstract

The doping of charge carriers into the CuO 2 planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case, a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La 2−x Sr x CuO 4 using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping.A ttempts within the framework of standard theories have failed to explain how high-temperature superconductivity emerges from charge carrier doping of an antiferromagnetic (AF) Mott insulator (1). The conventional Bardeen-CooperSchrieffer (BCS) theory of low-temperature superconductors (2) assumes that above the critical transition temperature (T c ) the electrons form a Landau Fermi liquid, and that superconductivity arises from pair condensation of the associated low-energy excitations (quasiparticles). What is clear in the case of copper oxides is that over the initial doping range where superconductivity first appears, ordinary Landau Fermi liquid theory does not apply. This realization has prompted theories in which the properties of the ordinary Fermi liquid are hidden (3), or alternatively only some properties of the Fermi liquid persist (4).A BCS-type theory may be applicable in the heavily overdoped region, where an ordinary Fermi liquid is observed (5-7). However, the recent proposal by Kopp et al. (8) that ferromagnetic (FM) fluctuations compete with d-wave superconductivity is a challenge to the notion that overdoped copper oxides strictly conform to such conventional wisdom. The primary motivation for their hypothesis is a strong upturn in the magnetic susceptibility immediately above T c for doping levels greater than p ∼ 0.19 (9-14). A tendency toward FM order for high charge doping (Fig. 1) is supported by electronic band calculations for super cells of La 2−x Ba x CuO 4 (15). However, these calculations favor the appearance of weak ferromagnetism about concentrated regions of ...

Dichotomy in the T -linear resistivity in hole-doped cuprates

“…For the highest x-values (p ≥ 0.24), dr ab /dT (T ) does exhibit downward curvature, but only at high temperatures. The latter can be interpreted either as a tendency towards resistivity saturation [7,15,16] or as the development of a non-integer power law in r ab (T ) at high temperatures, for example, owing to development of ferromagnetic critical fluctuations near the apex of the superconducting dome [17,18]. The form of r ab (T ) described in equation (2.1) is also consistent with the form of the in-plane transport scattering rate G extracted from angle-dependent magnetoresistance (ADMR) measurements in overdoped Tl2201 [19,20].…”

Search for ferromagnetic order in overdoped copper-oxide superconductors

“…The dramatic evolution of Fermi surface with overdoping in LSCO may provide a chance for magnetic moments to be interacting and correlated in a preferred direction, in some composition window. Several experimental observations provide some support to this scenario8910111213141516. The temperature dependence of magnetic susceptibility ( χ ) in overdoped LSCO crystals indicated the absence of local magnetic moments below the critical doping x c  ≈ 0.18, and dramatic increase in their density with further overdoping89101112.…”

“…The temperature dependence of magnetic susceptibility ( χ ) in overdoped LSCO crystals indicated the absence of local magnetic moments below the critical doping x c  ≈ 0.18, and dramatic increase in their density with further overdoping89101112. Muon spin rotation (μSR) experiments15 detected the onset of static magnetic moments at low temperature in heavily overdoped ( x  = 0.33) LSCO, metallic but non-superconducting (in what follows, we refer to such samples simply as ‘metallic’). However, 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 ( x c2  ≈ 0.26).…”

Hole-Pair Formation in Cuprate Superconductors despite Antiferromagnetic Fluctuations

“…He also suggested that at the end of superconductivity dome, there should be genuine ferromagnetism at zero temperature. Sonier et al [29] observed gradual disappearance of antiferromagnetism on doping of cuprates and onset of static magnetic order in the highly doped regimes. But this magnetism is not of long range order but magnetic moments appear in dilute form.…”

“…It may be possible that the transitory FM order in such a shot spell is not detected in neutron scattering experiments. Kopp et al [28] and Sonier et al [29] have maintained that in superconductivity dome in the phase diagram, FM order coexists with AFM order.…”