Electronic spin susceptibilities and superconductivity inHgBa2CuO4+δfrom nuclear magnetic resonance

Abstract

Nuclear magnetic resonance (NMR) experiments on single crystals of HgBa2CuO 4+δ are presented that identify two distinct temperature-dependent spin susceptibilities: one is due to a spin component that is temperature-dependent above the critical temperature for superconductivity (Tc) and reflects pseudogap behavior; the other is Fermi-liquid-like in that it is temperature independent above Tc and vanishes rapidly below Tc. In addition, we demonstrate the existence of a third, hitherto undetected spin susceptibility: it is temperature independent at higher temperatures, vanishes at lower temperatures (below T0 ≠ Tc), and changes sign near optimal doping. This susceptibility either arises from the coupling between the two spin components, or it could be given by a distinct third spin component.

Magnetic susceptibility of optimally doped HgBa2CuO4+δ

“…χ levels off above about 250 K and decreases with cooling to T c = 98 K, which is a pseudogap behavior. Since the temperature dependence of χ resembles those of the plane-site 63 Cu and 17 O Knight shifts [7][8][9], one may regard the present bulk magnetic susceptibility χ as an intrinsic behavior.…”

Contrasting Phenomenology of NMR Shifts in Cuprate Superconductors

“…In NMR, one finds various degrees of line broadening in the various cuprates (see e.g., [24]) typically caused by concomitant charge and spin density variations [25]. The results cannot be dismissed as being due to sample quality, since even chemically highly ordered systems can show tremendous electronic inhomogeneity [26][27][28]. More recently, as will be addressed below, even the magnetic field was shown to influence the NMR splittings [29] and broadening [30].…”

“…On the other hand, Hg NMR showed that also the optimally doped system is not a single fluid [56]. In order to understand this somewhat mysterious behavior, more systems were studied [27]. With the new samples, it became apparent that there was a third shift component that had a different Cu shift anisotropy, easily recognizable for underdoped and overdoped samples.…”

“…Thus, it appeared that the single-fluid picture emerging through NMR was rather accidental. In a parallel set of experiments, the HgBa 2 CuO 4+δ family of materials was investigated [26,27,41]. These materials are tetragonal and have a single CuO 2 plane similar to the La 2−x Sr x CuO 4 family.…”

“…This can be rather difficult, but comparison with the NMR shift of other nuclei within the same sample can be helpful. This has been employed rarely [27,[39][40][41], rather, the diamagnetism in the mixed state was estimated based on phenomenological theory. This adds some uncertainty to the NMR shifts at low temperatures in the typical fields used (the distance between fluxoids is small compared to the penetration depth, which makes the overall field variation small in terms of shift and linewidth effects).…”