Muon Spin Relaxation Studies of Cuprates in the Normal State

Synopsis

Despite immense effort dedicated to understanding the physics of high-Tc superconducting cuprates, many questions regarding the origin of superconductivity in these materials remain unanswered. This thesis employs the muon spin relaxation/rotation (μSR) technique as a sensitive local probe of internal magnetic fields to investigate the magnetic properties of cuprates in their normal state.

Much of the normal state of cuprates is occupied by a “pseudogap” phase, characterized by the depletion of the electronic density of states near the Fermi energy below a characteristic temperature, T∗ [1]. The origin of the pseudogap phase and its relationship with superconductivity is unclear. Polarized neutron diffraction measurements have detected intra-unit-cell magnetic order in the pseudogap phase of several cuprates [2–11]. Investigations by local probe techniques nuclear magnetic resonance (NMR), nuclear quadrupolar resonance (NQR) and zerofield (ZF) μSR, however, have not found evidence of such magnetic order in YBa2Cu3O6+x (Y123) and La2−xSrxCuO4 (La214) [12–18]. Here, ZF-μSR is utilized for a search of intraunit cell magnetic order in Bi2Sr2CaCu2O8+δ (Bi2212). Earlier measurements detected weak, temperature-dependent quasistatic internal magnetic fields that appeared to be electronic in origin in the pseudogap and superconducting phases [19,20]. By extending the study to include a wider range of hole-doping concentrations (p = 0.16 − 0.23) and using a specialized ultralow background μSR apparatus, the internal magnetic fields are determined to be nuclear in origin and independent of hole-doping [21]. The weak temperature dependence of the ZF-μSR relaxation rate in Bi2212 is attributed to a slight modification of the nuclear field distribution at the muon site due to changes in the crystallographic lattice structure. The findings reaffirm the results of previous searches for of magnetic order in the pseudogap phase by local probe techniques.

The normal state of cuprate superconductors is also believed to harbour precursor superconducting pairing correlations [22,23]. Among mounting experimental evidence for phase fluctuating Cooper pairing above Tc [24–29] are high transverse-field (TF) μSR measurements that have recorded a universal inhomogeneous magnetic field response above Tc in Y123, La214 and Bi2212 [30, 31]. While the results from these studies are consistent with the presence of spatially inhomogeneous diamagnetic (i.e., superconducting) regions that proliferate with reduced temperature, it could not be determined if the source of the magnetic field broadening is diamagnetic in origin. To address this issue, muon Knight shift measurements exploiting a significant improvement in high TF-μSR instrumentation were carried out on Bi2212 single crystals over a wide hole-doping range. Aside from a hole-doping independent temperature dependence at high temperature attributed to muon diffusion, the muon Knight shift is temperature independent above Tc and consequently insensitive to the normal state pseudogap or normal state superconducting fluctuations. Potential reasons and the limits of the experiments are discussed.