Muon spin relaxation studies of cuprates in the normal state

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 explores the use of 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 using local probe techniques such as nuclear magnetic resonance (NMR), nuclear quadrupolar resonance (NQR) and zero-field (ZF) μSR, however, have not found evidence for such magnetic order in YBa2Cu3O6+x (Y123) and La2−xSrxCuO4 (La214) [12–18]. Here, ZF-μSR is utilized to search for intra-unit-cell magnetic order in Bi2Sr2CaCu2O8+δ (Bi2212). Earlier studies of this material 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 that work to include a wider range of hole-doping concentrations and using a specialized ultra-low 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 that is observed in Bi2212 is attributed to a slight modification of the nuclear field distribution at the muon site caused by changes in the crystallographic lattice structure. These findings reaffirm the results of previous searches for 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 reveal a universal inhomogeneous magnetic field response above Tc in Y123, La214 and Bi2212 [30, 31]. While the data from these studies are consistent with the presence of spatially inhomogeneous diamagnetic (i.e., superconducting) regions that proliferate with reduced temperature, it has not been possible to determine 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 explanations for this, as well as limits to the experimental data are discussed.