Fluctuation Driven Collective Phenomena in Electron Doped Quantum Paraelectric: KTaO3

Abstract

The study of quantum phase transitions, in which a system undergoes a change in its ground state and physical properties due to quantum fluctuations at absolute zero temperature, stands as one of the most celebrated themes in modern condensed matter physics. In this context, quantum paraelectrics such as SrTiO3 and KTaO3 have garnered significant attention due to their proximity to a ferroelectric quantum critical point, making them easily tailorable into ferroelectric phase with external perturbations such as pressure, doping, strain, etc. While the microscopic intricacies of these transitions are still under investigation, the past two decades have witnessed a remarkable surge in research efforts focused on understanding the behavior of doped electrons in these systems. This has led to the observation of a myriad of coexisting novel quantum phases, including polar metal, unconventional Rashba spin-orbit coupling, and superconductivity. In this thesis, we aim to acquire a comprehensive understanding of the role of fluctuations in elucidating various collective phenomena or discovering new phases in electron doped KTaO3. One of the central challenges in condensed matter physics is to comprehend systems that have strong disorders and strong interactions. In the strongly localized regime, their subtle competition leads to emergence of glassy electron dynamics which ceases to exist well before the insulator-to-metal transition is approached as a function of doping. In the first part of the thesis, we present a discovery of glassy dynamics of the conduction electrons in the oxygen-deficient KTaO3, in the good metal regime. Even more astonishing is the observation that glassiness emerges in a regime where quantum fluctuations (which are generally considered a bottleneck for glass formation) are inherently present in the system. Using a combination of diverse experimental and theoretical techniques, we provide compelling evidence for quantum fluctuation-stabilized soft-polar modes in the creation of polar nano regions (PNRs) around the defect dipoles, which serve as the driving force behind the observed glassy behavior. Our finding of glassy dynamics in the good metal regime raises the question about the envisaged role of glassy freezing of electrons as a precursor to insulator-to-metal transition apart from the Anderson and Mott localization. In the second part of the thesis, we will present our experimental findings of real-space non-collinear spin texture, which emerges from the delicate interplay between quantum fluctuation-stabilized PNRs and localized magnetic moments around oxygen vacancies. When an electron moves in the presence of such non-trivial spin texture, it acquires a real-space Berry phase, leading to the observation of the topological Hall effect. Notably, this is a rare occurrence where real-space topology has been observed in a paramagnetic metal phase, offering an exceptional opportunity to engineer non-trivial spin textures in non-magnetic materials. One perplexing aspect of KTaO3 that has puzzled scientists for years is the lack of any evidence of superconductivity in the bulk of doped KTaO3, unlike SrTiO3. However, the recent discovery of 2D superconductivity at the [111] oriented interface of KTaO3 with TC one order of magnitude higher than the SrTiO3 has gained massive attention. In the final segment of this thesis, we delve into the nature of interfacial superconductivity by fabricating a new AlOX/KTaO3 heterostructure using pulsed laser deposition system. Apart from a few indirect implications of quantum fluctuations in the metallic phase, we observe certain interesting observations close to Berezinskii Kosterlitz Thouless phase transition temperature (TBKT) where the phase of the superconducting order parameter is disturbed due to vortex-antivortex unbinding transition. When subjected to an electric current, these vortices start moving leading to dissipation within the system. While the behavior of vortices under minimal driving currents is well understood, the dynamics under large current drives, especially near the critical current, remain unexplored. Our extensive transport measurements and in-depth analysis furnish several indications that near TBKT, vortices tend to collapse in a way predicted long back by Larkin and Ovchinnikov, leading to large dissipation in the system. Our findings not only furnish crucial insights into the microscopic structure of 2D superconductivity but also underscore the potential of the KTaO3 (111) based interfaces as an optimal platform for understanding dissipation mechanisms in 2D superconductors.