Collective Electronic Phenomena in Layered Metal Oxides: From Static to Dynamic

Abstract

This thesis presents the theoretical, computational and/or experimental studies of the collective electronic phenomena in layered sodium transition-metal oxide (NaTMO2), with the TM being all seven types of 3d-transition metal Ti, V, Cr, Mn, Fe, Co, Ni, and in cuprate superconductors as well.

My research project in the Li Group started with the representative cases of Fe, Mn, which are Jahn-Teller active at 4+ and 3+ valence states, respectively. Since the structure of NaFeO2 is unstable upon battery charging, the element Fe is usually synthesized into NaTMO2 by mixing with other kinds of ions, to form Na(FeyTM1-y)O2. Using in-situ X-ray diffraction, electron microscopy and DFT simulation, we identified a phase transition from the low voltage phase (i.e., when Na composition x is high) with flat TMO2 layer to a novel high voltage phase (i.e., when Na composition x is low) in all kinds of Fe-containing Nax(FeyTM1-y)O2 with at least 25% of Fe (y > 0.25). The flat transition-metal-oxide layer in the low voltage phase spontaneously ripples when the battery is charged to high voltage, forming a rippled phase with inhomogeneous interlayer distance. Since Na ions diffuse faster in the region with wider interlayer distance, the formation of the rippled phase in the battery charge makes the discharge take a different structural evolution pathway, as reflected in the charge-discharge asymmetry in the battery voltage profiles. When further studying the charge-discharge evolution of NaxMnO2, a unique phenomenon that we call “super charge separation” was identified, where Mn3+ and Mn4+ ions tend to fully separate into alternating sets of hkl planes, called charge superplanes. The Mn3+ superplanes attract Na ions electronically and maintain the ordered planes with dense Na and Mn3+, surprisingly, until a very low Na composition of 1/18. From neutron diffraction measurements for different NaxMnO2 orderings at x = 1, 5/8, 1/2, 1/3, we further found a much stronger spin fluctuation in Na1/2MnO2 than NaxMnO2 at other Na compositions. An abnormal and strong Raman peak shift in Na1/2MnO2 further suggests the connection of the spin fluctuation with lattice dynamics. Detailed structural analysis combining X-ray diffraction, neutron diffraction, pair distribution function, and density functional theory (DFT) simulation suggests that Na1/2MnO2 exhibits an abnormal Jahn-Teller distortion that is dynamically very active. The structure unit can generate strong directional charge fluxes when the ions are perturbed in DFT. DFT calculation further shows a strong anharmonic phonon coupling in Na1/2MnO2, which can be the origin of the abnormal Raman shift. The strongly coupled phonon pairs can also induce strong changes in the magnetic coupling constants, which is likely related to the experimentally observed spin fluctuations, mediated by directional charge fluxes. Next, we consider the three kinds of early-3d transition metal, Ti, V, and Cr for NaTMO2. From the reported in-situ XRD patterns of NaTiO2 and NaVO2 in previous publications and NaCrO2 by us, we observed a general pattern of incommensurate Na ordering evolutions upon battery charge and discharge, and further solved a unified Na ordering evolution pattern that we call the Na density waves. Using insights gained from ab initio simulations, we proposed a sodium-modulated Peierls-like transition mechanism for the bonding formation of metal ion dimers, which achieves quantitative agreement with the DFT simulation result. We further show that the unique vanadium trimer phase in NaxVO2 is on a delicate balance between the orbital bonding effect and the on-site Coulomb repulsion, which can be broken and lead to the observed metal-insulator transition with experimental perturbations. Finally, in parallel with the study of NaTMO2, my research in the Li Group also builds a connection between NaxMnO2 and the attractive topic of unconventional superconductivity in cuprates. Inspired by the superplanes in NaxMnO2 and the associated anharmonic phonon coupling and dynamic charge fluxes, we identified a new emergent configuration in cuprates, called the charge pseudoplane, which involves strong anharmonic phonon coupling and confines the dynamic charge fluxes for high superconducting transition temperatures. We show that the charge pseudoplanes can likely regulate the flux behavior of the doped holes through a two-band Hamiltonian with the momentum-dependent and ultrafast localization-delocalization duality. Our model provides good agreements with previous experiments, including angle resolved photoemission spectroscopy and scanning tunneling microscopy, and predicts that the maximum strength of the duality interaction is strongly correlated with the superconducting temperature. Our result suggests that both pseudogap and superconducting phases can be born from and intertwined through the charge flux confinement property of the charge pseudoplane region.