Control and investigation of metal-insulator transitions in novel oxides by light and electric field

• Main Author: Lourembam, James
• Other Authors: Wu Tao
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Materials::Functional materials; DRNTU::Science::Physics::Optics and light
• Online Access: https://hdl.handle.net/10356/65670
• Institution: Nanyang Technological University

Systems with strong interactions such as complex oxides show many emergent properties that are, still to this day, not fully understood. Of particular interest is the metal-insulator transition in complex oxides which arises from the competition between the tendencies of carrier localization and delocalization. To understand and ultimately control metal-insulator transitions in complex oxides, we need to explore the rules governing the interactions in charge, spin and lattice in these systems. To this end, the use of novel techniques in material fabrication, spectroscopy and lithography are invaluable for investigating the metal-insulator transition (MIT). Studies of electronic phases with advanced characterization techniques have revealed new insights into the correlated properties of these oxides. The focus of this dissertation is on understanding and tuning electronic phase transitions in complex oxide thin films and can be broadly divided into two parts. The first part of this thesis highlights the use of electrostatic modulation using electrolyte gating as a method to tune the electronic functionalities in a strongly phase-separated manganite Pr0.65(Ca0.75Sr0.25)0.35MnO3 and potentially control the emergence of colossal magneto-resistance. The device configuration of this powerful technique, known as the electric double layer technique, provides clean, quasi-continuous and high-density charge accumulation of carrier modulation in the manganite channel. A diverse behavior of transport modulation was observed and the modulation was remarkably enhanced by the application of a magnetic field. The experiments are also supported by the small polaron model, effective medium model and finite element simulation of inhomogeneous local electric field. The first part of the dissertation also discusses high quality thin film synthesis of manganites using Pulse Laser Deposition. Pr0.65(Ca0.75Sr0.25)0.35MnO3 manganite thin films under biaxial strain displayed presence of magnetic glassy state at low temperatures is accompanied by reemergence of insulating behavior. We discovered that the transport modulation in the channel increased multiple folds in the magnetic glassy state. The second part of this dissertation probes optical responses in the thin films of vanadium dioxide polymorphs using the techniques of terahertz and time-resolved femtosecond spectroscopy. Vanadium dioxide (VO2), just like manganites, is a prototypical MIT system. Although, VO2 exhibits a number of polymorphic forms, a major portion of research on this material has been devoted towards VO2(M1) polymorph because of its speedy and abrupt MIT (5−10 K) just above room temperature. VO2(B) polymorph, on the other hand, undergoes a broad MIT as wide as 150 K and remained mostly unexplored due to challenges in thin film fabrication. We studied for the first time optical conductivities of VO2(B) and construct the electronic phase diagram by indicating the critical temperatures deciding the width and the onset of the MIT in VO2(B). We also demonstrate ultrafast optical switching to a metallic state in this system using optical pump-probe technique. Compared to VO2(M1) the threshold value of laser fluence is about four times lower, and showed strong temperature dependence. The phase diagram of photoinduced transition in VO2(B), remarkably consists of two regions of photoinduced metallicity— one induced by weak pump excitation, and another by strong pump excitation.