Pressure-induced phase transitions in strongly-correlated Van der Waals materials

Abstract

In strong correlated systems, there exists a dynamic interplay between various fundamental properties such as orbital motion, spin orientation, lattice structure, and charge distribution. These degrees of freedom engage in competitive interactions, where a delicate and precise equilibrium assumes a crucial significance in dictating the ground state of the system. A comprehensive comprehension of these degrees of freedom and their intricate interplay is paramount in elucidating the rich novel electronic phases in strongly correlated van der Waals systems. To gain insight into the behavior of strongly correlated materials, it is crucial to employ experimental techniques to investigate and analyze the dynamic physical phenomena that occur when external stimuli, such as magnetic fields, doping, strain, and pressure, are applied to the system. Specifically, pressure has the remarkable capability to manipulate the inherent lattice structure of a material, consequently inducing alterations in its electronic structure. As a result, pressure plays a pivotal and indispensable role in the investigation and analysis of strongly correlated materials. In this thesis, we conducted an analysis of the physical properties exhibited by strongly correlated materials, namely Ta2NiSe5, Ta2NiS5, and Mn3Si2Te6, utilizing three distinct pressure cells. Our investigation focused on studying the alterations in these materials' physical characteristics under varying pressure conditions. The results obtained provide valuable insights into the effects of pressure on the behavior of strongly correlated systems, shedding light on their unique properties and potential applications. The first part of this study will be dedicated to the investigation of the excitonic insulator Ta2NiSe5, which represents an intriguing phase characterized by exotic interactions between electrons and holes. Our study experimentally confirms the presence of an excitonic insulating phase in Ta2NiSe5 and demonstrates its suppression under pressure. The small but clear kink of temperature dependent resistivity and the low energy broad peak of Raman spectra, related with electron, show the existence of excitonic insulating phase on Ta2NiSe5. Our experimental findings are well explained by modifying semimetal band structure of Ta2NiSe5 under pressure. And the excitonic insulating phase suppressed with sliding-layer structure transition at about 3 GPa. The results indicate a possible connection between the excitonic insulating phase and structural transitions in Ta2NiSe5. In the second part of the thesis, we report another excitonic insulator candidate Ta2NiS5, the sister compound of Ta2NiSe5. Ta2NiS5 has isostructure of the high-pressure phase of Ta2NiSe5 but differs in its electronic structure. Ta2NiS5 exhibits a semiconductor electronic structure, unlike the semimetallic Ta2NiSe5. Ta2NiS5 is an excellent system to examine the influence of band structure on the excitonic insulating phase. In the pressure-dependent experiments conducted on Ta2NiS5, no excitonic insulating phase was observed. However, the experiments did confirm the occurrence of a sliding-layer structure transition, same as in Ta2NiSe5, at 5 GPa and a subsequent metal-insulator transition at 14 GPa. Furthermore, the observed structure transition in Ta2NiS5 was found to be a phase transition that increased the band gap energy. This finding contrasts with the behavior observed in Ta2NiSe5, where the gap closes during the excitonic insulating phase transition that coincides with a structural transition. This observation provides circumstantial evidence indicating the occurrence of an excitonic insulating phase transition within the Ta2NiSe5 system. Furthermore, within the Ta2NiS5 system, the excitonic insulating phase transition is found to be independent of structural transitions and solely influenced by the electronic structure. In the case of Mn3Si2Te6, a ferrimagnetic material characterized by self-intercalated nodal lines, Raman spectroscopy analysis conducted at 78 K has confirmed the existence of spin-orbit coupling within its ferrimagnetic phase. Furthermore, it has been confirmed that the ferrimagnetic phase transition in Mn3Si2Te6, exists a pronounced correlation between its electronic, magnetic, and structural properties, exhibits a dome-shaped dependence on pressure, with the transition temperature reaching room temperature at approximately 14 GPa. It has been experimentally verified that near 14 GPa, Mn3Si2Te6 undergoes simultaneous transitions involving a metal-insulator transition, spin-reorientation transition, and structural transition. These transitions, which align with the dome-shaped transition temperature (Tc) at 14 GPa, can be comprehended by considering the pressure-dependent changes in the band structure. Our study provides a comprehensive understanding of the interplay between crystal structure, electronic structure, and magnetic properties in strongly correlated materials under the influence of pressure. By examining these interactions, we gain valuable insights into the ground state and the degrees of freedom within each system, thus enhancing our understanding of these materials.