Investigation of emergent phenomena in quantum materials induced via chemical substitution, plastic deformation and ionic-liquid gating

Abstract

The emergent properties of quantum materials are the subject of intense research interest. The identification of techniques to control these properties is extremely important in order to tailor materials toward potential technological applications. In this Thesis, three different manipulation techniques, namely chemical substitution, plastic deformation and ionic-liquid gating, are used to tweak the properties of three different classes of materials - the ferromagnetic Mott insulator YTiO3, the dilute superconductor SrTiO3, and the antiferromagnetic Mott insulator NiS2. The resultant structural, electronic and magnetic properties are studied with a battery of experimental techniques which, coupled with theoretical calculations, provide important insights into the physics of these materials. The first, and most extensively studied material in this Thesis is the Mott-insulating ferromagnet YTiO3. Isovalent La substitution at the Y site transforms the system into an antiferromagnetic Mott insulator, whereas Ca doping at the Y site initially converts it into a paramagnetic Mott insulator, and further doping subsequently leads to a paramagnetic metallic phase. In order to enable this work, single crystals were grown with the traveling-solvent floating-zone technique. Several challenges associated with inhomogeneous overoxidation of the Ti3+ ion across large single crystals were overcome through careful characterization to produce high-quality samples. The ferromagnetic-antiferromagnetic and ferromagnetic-paramagnetic transitions in La-substituted and Ca-doped YTiO3 are studied via magnetometry, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, neutron scattering and muon spin rotation. The thermal magnetic phase transition in YTiO3 is found to exhibit first-order rather than conventional second-order behavior, and the first-order nature is found to become increasingly prominent with increasing La substitution and Ca doping. A decrease in the magnitude of the bulk ordered magnetic moment is found with increasing La substitution and Ca doping, albeit stronger in the former case, as a result of both a decreasing local magnetic moment and a decreasing magnetic volume fraction. The evolution of the spin-wave spectra in the ferromagnetically-ordered state is mapped out with increasing La substitution and Ca doping. A heuristic description using an anisotropic Heisenberg model with nearest- and next-nearest-neighbor interactions is used to compare to prior theoretical work. Neither a significant increase in spatial spin-exchange anisotropy nor an overall decrease in the spin-exchange constants is observed with increasing La substitution, contrary to prior theoretical predictions. An increasing spin-exchange anisotropy is, however, observed with increasing Ca doping. In both cases, a strong broadening indicative of shortened spin-wave lifetimes is observed. The absence of significant changes in the spin-wave dispersion, the life-time effect, and the decreasing ordered magnetic moment in the La-substituted system are found to be qualitatively consistent with calculations for a disordered isotropic nearest-neighbor Heisenberg model. This result indicates that the physics of the La-substituted system is dominated by substitutional disorder. The insulator-metal transition induced via hole doping (Ca doping) in YTiO3 is studied with a combination of x-ray absorption spectroscopy and density functional theory calculations. A picture for the insulator-metal transition is developed, whereby hole doping induces electronic phase separation into hole-poor Mott-insulating and hole-rich metallic regions. The results indicate that the insulator-metal transition in Ca-doped YTiO3 is of inherent first-order character. The second material investigated in this Thesis is SrTiO3 which, upon doping, becomes a dilute superconductor. In particular, we study the effects of plastic deformation, which involves the creation and rearrangement of dislocations. The resultant structural, electronic and magnetic properties are studied via transport measurements, magnetometry, Raman scattering, and diffuse x-ray and neutron scattering. We uncover an enhancement of the bulk superconducting transition temperature (Tc) by up to a factor of two in plastically-deformed samples. In addition, we find possible evidence for superconducting correlations at 30 to 50 K, about two orders of magnitude above the bulk Tc. The Tc enhancement is shown to be associated with the formation of self-organized dislocation structures in plastically-deformed samples, which lead to locally-enhanced soft ferroelectric fluctuations. The latter have previously been argued to cause the superconducting pairing in SrTiO3. Quite generally, this work demonstrates the great potential of plastic deformation in quantum materials research. The third and final material studied in this Thesis is the antiferromagnetic Mott insulator NiS2. The well-established technique of ionic-liquid gating is utilized to explore the possibility of electric-field induced metallicity and superconductivity. A clear gating-induced insulator-metal transition is observed at positive gate-bias. However, no superconductivity is detected, down to the lowest measured temperature of 450 mK. The gating mechanism is found to be strongly electrochemical in nature. It involves sulfur reduction and is non-volatile and irreversible. The results are compared to the reversible electrostatic mechanism for the ionic-liquid gating induced insulator-metal transition in FeS2. This study highlights the important role played by the sulfur diffusion coefficient in determining electrostatic vs. electrochemical gating mechanisms in sulfides.