Phase transitions in magnetic, thermoelectric and topological quantum materials : Raman and X-ray diffraction investigations at variable temperatures and high pressures

Abstract

Phase transitions of matter under external perturbations like pressure, temperature, and mag netic fields have captivated researchers for over a century. Recent technological advancements and the pressing need for energy-efficient and sustainable solutions have fueled the quest for next generation quantum materials with potential applications in energy storage, quantum comput ing, and spintronics. Unraveling phase transitions in complex quantum materials is challenging due to the interplay of spin, charge, orbital, and lattice degrees of freedom and their competing interactions. Experiments controlled by temperature and pressure are effective in understand ing these complex phase transitions if appropriate experimental techniques are employed to capture the various interactions occurring within a system. An in-depth understanding of phase transitions will aid in synthesizing new quantum materials with tailored properties for modern technological applications. This thesis demonstrates the versatility of Raman spectroscopy, a simple yet powerful tech nique, in probing different types of phase transitions in magnetic, thermoelectric, and topological quantum materials under varying temperatures and high pressures. The thesis highlights the advantage of combining Raman spectroscopy and X-ray diffraction in studying phase transitions in strongly correlated systems. It is divided into nine chapters, including an introduction, ex perimental methods, and six research chapters. The first two chapters encompass research on pressure-induced topological and electronic phase transitions in thermoelectric and topological materials. The next two chapters focus on temperature-induced magnetic phase transitions in two spintronic materials. The final two chapters includes studies on temperature-induced phase transitions in a Mott insulator and a perovskite oxide. Chapter 1 introduces fundamental concepts of quantum materials and phase transitions in condensed matter physics, along with the basics of Raman scattering, X-ray diffraction, temperature- and pressure-dependent Raman spectroscopy, pressure-dependent XRD, and the various excitations and interactions that influence phase transitions. Chapter 2 discusses the experimental techniques used in the subsequent research. Chapter 3 explores the hydrostatic pressure-induced topological quantum phase transitions and structural phase transitions in 3D topological insulator TlBiTe2 using Raman spectroscopy and synchrotron XRD. High-pressure Raman and XRD measurements confirm that the ambient structure of TlBiTe2 is stable up to ∼ 7.5 GPa, but it undergoes two structural phase transitions at higher pressures. Detailed structural analysis and Rietveld refinement have been done, and the high-pressure structures have been identified. The anomalous changes in the linewidth of Eg and A1g modes at 3 GPa confirm the presence of strong electron-phonon coupling and the associated isostructural phase transition in TlBiTe2. First-principles calculations have been carried out to understand the changes in electronic band structure under pressure and two consecutive band inversions at the Γ-point and F-point of the Brillouin zone and the changes in Z2 topological invariant and mirror Chern number establish that the phonon anomalies and the isostructural transition around 3 GPa is attributed to the pressure-induced transition to a topological crystalline phase. A detailed phase diagram of TlBiTe2 till 13 GPa is added in this chapter. Chapter 4 investigates the pressure-induced phase transitions in two chalcogenides. Chap ter 4A focuses on the electronic topological transition in the dual topological insulator and thermoelectric BiTe, which is a superlattice of Bismuth bi-layer and Bi2Te3 quintuple layers. ix High-pressure Raman and XRD studies confirm that BiTe undergoes a structural transition at ∼ 5 GPa. The anomalies in the peak position and linewidth of the vibrations of the Bismuth bi-layer and Bi2Te3 quintuple layers and the lattice parameter ratios confirm an isostructural phase transition at ∼ 1 GPa. First-principles calculations confirm that the anomalies are due to pressure-induced changes in the fermi surface, and the changes at 1 GPa are attributed to the electronic topological transition (Lifshitz transition). Another semiconductor-to-metal tran sition occurs in BiTe at higher pressures, as evident from the calculated bandgap reduction and anomalies in the linewidth of Raman modes. Chapter 4B discusses the pressure-induced electronic transition in the magnetic chalcogenide MnSb2Se4 around 2 GPa explored using high pressure Raman and synchrotron XRD. Unusual negative compression of phonon modes confirms the strong anisotropy present in the system. Chapter 5 discusses the spin-reorientation transition (SRT) in single crystals of the rare earth orthoferrite Sm1−xYxFeO3 (x= 0.7 and 0.3). The magnetization study reveals the temperature induced SRT owing to the changes in spin configuration near room temperature, around 380 K for Sm0.7Y0.3FeO3 and around 170 K for Sm0.3Y0.7FeO3. The temperature-dependent Raman study confirms the SRT from the anomalies in B2g and B3g phonon modes and the polarization dependent magnon studies. The role of strong anisotropy and spin-phonon coupling in deter mining the SRT and the spin configurations is discussed and also compared to existing research on Sm0.5Y0.5FeO3. In Chapter 6, the temperature-induced magnetic transitions in the skyrmion host Cu2OSeO3 and 10% Ni-doped Cu2OSeO3 are discussed using Raman and magnetization studies from 3- 300 K. Magnetic skyrmion hosting materials have potential applications in future information storage and magnetic devices. The appearance of strong magnon modes of the spin clusters below the magnetic ordering temperature confirms the magnetic phase transition and the anomalous changes in the phonon modes in 10% Ni-doped Cu2OSeO3 evidence strong spin-lattice coupling in the system. Chapter 7 examines the temperature-induced phase transitions in 4% Ga-doped V2O3. The phase transitions of archetypal Mott insulator V2O3 have been long-debated due to the occur rence of simultaneous magneto-structural and metal-insulator transitions (MIT) at ∼ 150 K. The effects of Ga-doping on decoupling the structural transition from the magnetic and elec tronic transitions are evidenced through the splitting of the A1g mode due to the degeneracy lifting, identified using temperature-dependent Raman and XRD. Chapter 8 discusses the temperature-induced phase transition in the Q phase of the complex perovskite NaNbO3. The phase transition sequence of the seven phases of NaNbO3 has been long-debated, and this material still intrigues researchers. A change in the epitaxial orientation of the NaNbO3 thin film grown on SrRuO3/MgO is identified using temperature-dependent Raman, dielectric measurement, and X-ray reciprocal space imaging techniques. The thesis aims to investigate the interactions between light and matter, as well as phase transitions in emerging quantum materials, utilizing two photon scattering techniques: Raman spectroscopy and X-ray diffraction. Summary and Outlook section comprehensively summarizes all the research conducted throughout the thesis and provides insights into future directions.