Absence of Static Loop-Current Magnetism at the Apical Oxygen Site inHgBa2CuO4+δfrom NMR

Abstract

The simple structure of HgBa2CuO 4+δ (Hg1201) is ideal among cuprates for study of the pseudogap phase as a broken symmetry state. We have performed 17 O nuclear magnetic resonance (NMR) on an underdoped Hg1201 crystal with transition temperature of 74 K to look for circulating orbital currents proposed theoretically and inferred from neutron scattering. The narrow spectra preclude static local fields in the pseudogap phase at the apical site, suggesting that the moments observed with neutrons are fluctuating. The NMR frequency shifts are consistent with a dipolar field from the Cu +2 site. [14] show the appearance of broken time reversal symmetry that correlates with the onset of the pseudogap phase with a magnetic moment tilted away from the crystalline c-axis. A muon spin relaxation (µSR) experiment in zero field was unable to detect this magnetism [15] although screening effects may account for muon insensitivity. [16] To account for neutron data, extensions of 2D orbital currents to 3D models have been proposed which include the apical oxygen[17] as well as a quantum mechanical calculation [18] which has an out-of-plane component with a tilted local field. In this context, magnetic fields at the apical oxygen site, Fig.1(a) 89 Y is in a symmetry position which is insensitive to orbital current ordering and neutron data is not available in these materials for direct comparison. We report here our investigation of local fields in Hg1201 with 17 O NMR for which the apical oxygen is sensitive to orbital currents and polarized neutron scattering show evidence for magnetic ordering.Until recently, NMR results for Hg1201 have been constrained to c-axis magnetically aligned powdered samples, [22][23][24][25] which are more easily annealed and have a large NMR signal. However, these powder samples are disordered in the ab-plane and they can have a larger spectral linewidth due to imperfect alignment. In order to investigate possible effects of orbital currents, high arXiv:1304.6415v1 [cond-mat.supr-con]

Discovery of slow magnetic fluctuations and critical slowing down in the pseudogap phase of YBa 2 Cu 3 O y

“…IUC magnetic order would be retained, and the absent static field would be accounted for, if the ordered moments fluctuate among alternate orientations ( 15 , 18 , 19 ). This would average the local field B loc ( t ) at spin-probe sites to zero for time scales longer than a characteristic correlation time τ c .…”

Contrasting Phenomenology of NMR Shifts in Cuprate Superconductors

“…While all of these experiments made it clear that the approach used up to now that interprets the cuprate NMR shifts fails, and despite some support from the phenomenological theory [57] that used the earlier findings within a generalized two-fluid scenario, the single-fluid approach is still being adopted (e.g., [58,59]), perhaps since a unifying, new phenomenology for the NMR data analysis is missing. With the new understanding of the cuprate phase diagram in terms of NMR that has two charge components (at Cu and O) [34], we decided to take a closer look at all available Cu shift data, which led us eventually to introduce a new, contrasting NMR shift phenomenology that will be discussed below.…”

Numerical exploration of spontaneous broken symmetries in multiorbital Hubbard models

“…In addition, the Θ II phase (a q = 0 Fermi surface instability) faces the challenge of explaining the excitation gap in the pseudogap regime. Intense effort has been devoted to searching indirectly for orbital loop currents, using techniques such as NMR [47][48][49] and muon spin rotation, [50][51][52] but their existence in the cuprates remains inconclusive.…”