Magnetic stress-driven metal-insulator phase transition and transport properties of CrN

Abstract

Metal-insulator transition is one of the most fascinating phenomena observed in condensed matter physics, wherein a material undergoes a transition from a metallic to an insulating state or vice versa. Understanding the origin of the metal-insulator transition has far-reaching implications in both fundamental science and technological applications. It sheds light on the complex interplay between the quantum phenomena and macroscopic behavior in condensed matter systems. Materials exhibiting metal-insulator transition have the potential for novel device applications in the field of electronics, optoelectronics, and energy sciences. Literature survey reveals that many of the materials exhibiting metal-insulator transition belong to mostly oxides, sulfides, and selenides. Chromium nitride (CrN) is the only material from the nitride family that exhibits metal-insulator transition. Nitride materials are widely known for their excellent electronic, optoelectronic, plasmonic, and mechanical properties that are harnessed for practical device applications. Therefore, a detailed understanding of the transport properties in CrN is necessary in terms of both fundamental science and practical device application. In this thesis, we have explored the spin-phonon coupled electronic and thermal transport properties in CrN. Though spin and phonon, the two fundamental quantum excitations are often analyzed independently, in the case of magnetic materials they could be coupled altering the materials’ properties significantly. Here we have chosen a prototype material CrN with strong spin-phonon coupling to investigate the magneto-structural metal-insulator phase transition and the transport properties. This thesis has been divided into seven chapters. In chapter 1, we briefly introduce the electronic and thermal transport properties of spin-phonon coupled transport properties in a material. In continuation to the electronic transport properties, we discussed the fundamental science and importance of the metal-insulator transition phenomenon. Besides, we discussed the basic physics and importance of thermal transport properties, which would be useful to analyze the fundamental physics and motivation of the subsequent working chapters. In chapter 2 we have demonstrated how the presence of secondary Cr2N grains hinders the observation of the metal-insulator transition in CrN, probed through the temperature-dependent resistivity measurements. Though the first-order phase transition in bulk CrN has been observed on more than one occasion from the early 1960s, the observation of metal-insulator of thin film CrN has been scarce. There are numerous reports on the diverse electronic properties of CrN thin film ranging from metal-to-metal, insulator-to-insulator, fully insulating, and fully metallic state. Using a combination of microscopic and structural analysis we have shown that, during the deposition of thin film CrN through magnetron sputtering, the presence of Cr atoms in high concentration leads to the formation of metallic Cr2N grains that hinder the observation of metal-insulator transition. Chapter 3 deals the origin of the metal-insulator transition in CrN. Traditionally, the metal-insulator transition in most materials is driven by strong Coulombic-repulsion (Mott transition) or the presence of disorder (Anderson transition) in the lattice. Besides, Peierls structural instability and Slater’s distortion are also known to cause metal-insulator transition. However, the origin of the metal-insulator transition in CrN is distinctively different. Using a combination of electrical, magnetic, structural characterization, and theoretical analysis we have There are numerous reports on the diverse electronic properties of CrN thin film ranging from metal-to-metal, insulator-to-insulator, fully insulating, and fully metallic state. Using a combination of microscopic and structural analysis we have shown that, during the deposition of thin film CrN through magnetron sputtering, the presence of Cr atoms in high concentration leads to the formation of metallic Cr2N grains that hinder the observation of metal-insulator transition. Chapter 3 deals the origin of the metal-insulator transition in CrN. Traditionally, the metal-insulator transition in most materials is driven by strong Coulombic-repulsion (Mott transition) or the presence of disorder (Anderson transition) in the lattice. Besides, Peierls structural instability and Slater’s distortion are also known to cause metal-insulator transition. However, the origin of the metal-insulator transition in CrN is distinctively different. Using a combination of electrical, magnetic, structural characterization, and theoretical analysis we haveThere are numerous reports on the diverse electronic properties of CrN thin film ranging from metal-to-metal, insulator-to-insulator, fully insulating, and fully metallic state. Using a combination of microscopic and structural analysis we have shown that, during the deposition of thin film CrN through magnetron sputtering, the presence of Cr atoms in high concentration leads to the formation of metallic Cr2N grains that hinder the observation of metal-insulator transition. Chapter 3 deals the origin of the metal-insulator transition in CrN. Traditionally, the metal-insulator transition in most materials is driven by strong Coulombic-repulsion (Mott transition) or the presence of disorder (Anderson transition) in the lattice. Besides, Peierls structural instability and Slater’s distortion are also known to cause metal-insulator transition. However, the origin of the metal-insulator transition in CrN is distinctively different. Using a combination of electrical, magnetic, structural characterization, and theoretical analysis we have shown that the metal-insulator transition in CrN stems from the unconventional magnetic stress. Chapter 4 focuses on the optical manipulation of weak electronic localization in CrN. Along with the metal-insulator transition at TN ∼ 277 K, CrN also exhibits weak electronic localization behavior at low temperatures due to the presence of defects at large concentrations. Until now, temperature-induced inelastic scattering and magnetic field-induced time-reversal symmetry breaking were the only known methods to manipulate weak electronic localization. Here we show that optical illumination can also serves as an external stimulus to manipulate the weak electron localization behavior. In chapter 5, we have demonstrated the Cr2N-CrN lateral metal-semiconductor heterostructure with improved thermoelectric performance. Artificially designed Metal-semiconductor heterostructure is known to improve the thermoelectric figure of merit (zT) by selectivity filtering out low energy electrons that improves the thermoelectric power factor and reduces the thermal conductivity by scattering the phonons at each metal-semiconductor interface. By utilizing the spontaneous phase segregation properties of CrxNy compounds at higher Cr-flux, we have shown Cr2N-CrN lateral metal-semiconductor heterostructure that exhibits 35% better thermoelectric properties compared to phase pure CrN. Chapter 6 explores the origin of anomalous phonon lifetime shortening in CrN. In most of the materials, the thermal conductivity at elevated temperatures decreases with the rise in temperature. However, as for CrN, the thermal conductivity increases or remains constant with the rise in temperature. Theoretical analysis predicts that such anomalous behavior results from the spin-phonon coupling. Utilizing inelastic x-ray scattering technique, we have measured the phonon lifetime at 3 different temperatures and demonstrated that the phonon lifetime in CrN indeed increases with the rise in temperature, thus unveiling the origin of anomalous phonon lifetime shortening in CrN. In chapter 7, we conclude our work, discuss the implications of the research outputs, and provide the future research direction on CrN.