Synchrotron X-ray Scattering Studies of Complex Oxides

Abstract

Complex oxides, such as the copper-based high-temperature superconductors and the perovskite cobaltites, exhibit a vast range of properties and phases and are prominent research topics in condensed matter physics and materials science. Synchrotron X-ray techniques are powerful tools to study the electronic and magnetic properties of materials. In this Thesis, I present synchrotron X-ray scattering studies of four complex oxides: the cuprates HgBa2CuO4+d (Hg1201), La2-xSrxCuO4 (LSCO), and Nd2-xCexCuO4 (NCCO), and the cobaltite La0.5Sr0.5CoO3-d (LSCoO). I first describe a study of the dynamic charge correlations in the moderately-doped model cuprate Hg1201 (Tc = 70 K) using resonant inelastic X-ray scattering (RIXS) at the Cu L-edge. The cuprates exhibit a charge-density-wave (CDW) instability in the underdoped, pseudogap part of the phase diagram that competes with superconductivity. This study aims to discern the connection between CDW and pseudogap phenomena, and to understand the extent to which CDW correlations shape the phase diagram. With a new analysis method, I demonstrate that the charge correlations at the two-dimensional wave vector qCDW~(0.28, 0) feature three characteristic energy scales: (1) quasi-elastic; (2) 40 meV, in the optic-phonon range; (3) 150-200 meV, well beyond the phonon range. Intriguingly, the two dynamic energy scales identified here are comparable to previously measured pseudogap scales and to energy scales associated with the glue function deduced from optical and Raman spectroscopy. The paramagnon dispersion along [1,0] is also measured and found to be insensitive to the CDW correlations. The dynamic charge correlations may manifest themselves as anomalous softening or broadening of phonon modes. In order to investigate this possibility, I carried out non-resonant inelastic X-ray scattering (IXS) measurements of Cu-O bond-stretching phonons in hole-doped LSCO and electron-doped NCCO. In LSCO (x = 0.125), anomalous phonon broadening is observed at low temperature (T = 20 K) around the wavevector qph~(0.25, 0), to the CDW wave vector qCDW. The anomalous broadening is significantly weakened at higher temperatures (T = 150 K, 300 K) and higher doping (x = 0.20). Measurements across the structural transition temperature Ts suggest that the effect is not due to unit cell doubling below Ts. Rather, the dynamic charge correlations appear to be the cause of the observed phonon broadening. The anomalous softening and broadening behavior is also observed in NCCO, but found to be independent of temperature (up to T = 500 K) and doping (x = 0.078, 0.145), and therefore is likely the result of an anti-crossing between two phonon modes rather than due to dynamic charge correlations. Motivated in part by a recent phenomenological charge (de)localization model, which successfully captures the temperature and doping dependence of the pseudogap and strange-metal phenomena, I also studied the doping and temperature dependence of charge-transfer excitations in LSCO and NCCO via Cu K-edge RIXS. The measurements, which range from low temperature (15 K) to very high temperature (1200 K), reveal a distinct spectral weight decrease upon heating in both compounds that remains to be understood. Finally, I present a synchrotron X-ray study of ion-gel-gated LSCoO thin films. Electrolyte-based transistors utilizing ionic liquids/gels have been highly successful in the study of charge-density-controlled phenomena in diverse materials, particularly oxides. Experimental probes beyond electronic transport have played a significant role, despite challenges to their application in the electric double-layer transistor geometry. I demonstrate the application of synchrotron soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) as operando probes of the charge state and magnetism in ion-gel-gated ferromagnetic perovskite thin film LSCoO/LaAlO3(001). Application of gate voltages up to + 4 V is shown to dramatically suppress the O K-edge XAS pre-peak intensity and XMCD signal at the Co L-edges, and thus enables the Co valence and ferromagnetism to be tracked upon gate-induced reduction. This work lays the foundation for operando soft XAS/XMCD studies of other electrolyte-gated oxides, which could be especially illuminating in the case of battery, ionic conductor and supercapacitor materials.