| dc.description.abstract | According to Landau’s theory, the organizing principle for understanding phase transitions is rooted in the change in a system’s underlying symmetry, which is quantitatively captured by the order parameter. In strongly correlated transition metal oxides (TMOs), the intricate interplay among various degrees of freedom (DOF), such as spin, charge, orbital, and lattice, gives rise to diverse symmetry-broken collective electronic and magnetic phenomena. Intriguingly, reconstructions of the DOF at the interface of TMO-based artificial heterostructures lead to emergent electronic phenomena, which are absent in their bulk counterparts. Disentangling the exact origin of such collective behavior in these quantum materials remains a long-standing challenge. The aim of this thesis is not only to elucidate the mechanisms driving various symmetry-broken phases but also to design and engineer novel emergent phenomena in a specific subclass of TMOs, namely perovskite titanates (ATiO3, A = alkaline earth, rare earth elements).
ATiO3 hosts a plethora of symmetry broken phases, such as ferroelectricity, superconductivity, magnetism etc., originating from broken inversion, gauge and time reversal symmetries, respectively. Moreover, seemingly antagonistic order parameters associated with various symmetries have also been shown to coexist in these systems, posing a challenge to current theoretical understanding. In this thesis, we have addressed some of these questions by primarily employing epitaxial thin films grown by pulsed laser deposition. We have employed a combination of transport measurement and state-of-the-art synchrotron-based measurement techniques to probe the underlying mechanisms.
In the first part of the thesis, we delve into the origin of orbital ordering phenomena in a prototypical antiferromagnetic 3d1 Mott insulator PrTiO3. The observation of antiferromagnetism in RETiO3 (RE = rare-earth) series has been puzzling since no Jahn-Teller distortion was observed and therefore, the celebrated Kugel-Khomskii model of spin-orbital superexchange predicts ferromagnetism in an orbitally degenerate d1 system. Further, the existence of the orbitally ordered vs. orbital liquid phase in both antiferromagnetic (AFM) and paramagnetic phases remains highly debated. We employ x-ray linear dichroism (XLD) technique to unravel the origin of orbital ordering, as its order parameter - the quadrupole moment of the anisotropic charge distribution of the occupied orbital – is directly connected to the XLD intensity. Our results, in conjunction with DFT calculations, conclusively demonstrate that a sizable crystal field having orthorhombic (D2h) symmetry lifts the orbital degeneracy and predominantly drives robust ferro orbital ordering which persists across both antiferromagnetic and paramagnetic phases, thereby resolving a long-standing debate.
In the second part of the thesis, we investigated the electronic structure and transport properties of two-dimensional electron gas formed at a Mott insulator/band insulator interface of NdTiO3/SrTiO3. The inversion symmetry breaking at the interface gives rise to Rashba spin orbit coupling (SOC). The interplay between Rashba SOC and local moments, originating from electrons localized at oxygen vacancy centers, leads to the observation of skew scattering driven anomalous Hall effect.
In the final segment of the thesis, we demonstrate the stabilization of a robust polar metal phase in an otherwise non-polar band insulator CaTiO3. Intrinsic polar metals are exceptionally rare, owing to the inherent incompatibility between metallicity and polar behavior as constrained by Gauss’s law. To overcome this limitation, we artificially induce a polar metal phase in the incipient ferroelectric CaTiO3 by employing epitaxial strain engineering combined with oxygen vacancies. We observe that the temperature evolution of the polar behavior in strained CaTiO3 exhibits a crossover (Tc ~ 125 K) between two distinct phases: i) an epitaxial strain induced long-range ordered polar phase (T < Tc), and, ii) a nano-polar phase (T > Tc), having short-range polar ordering. Remarkably, Tc remains unaffected up to a carrier density as high as 1.7×1020 cm−3, at which the Tc of a similar system, SrTiO3, is significantly suppressed. Our DFT results clarify that the robustness of the polar metal phase originates from Ca-site-driven polar behavior, which is decoupled from the electronic states at the Fermi energy primarily contributed by Ti 3d states.
In summary, this thesis underscores transition metal oxides (TMOs) as a highly versatile platform for investigating a wide range of symmetry-broken collective electronic phenomena. Intriguingly, novel emergent phenomena can be engineered by precisely tuning various degrees of freedom, offering significant potential for the development of next-generation electronic devices. | en_US |
