The research described in this thesis is focused on utilizing physical pressure as a tuning parameter to modify and study various phase transitions and their associated ground states, including superconductivity, spin/charge density waves, structural transitions etc. Pressure provides a strong and unique way to tune these ground states as well as investigate the interplay between them. The thesis contains 9 chapters and 4 appendices outlined as follows.

Chapter 1 introduces the concept of physical pressure as well as some theoretical background to ideas that are encountered in this thesis.

Chapter 2 provides a detailed explanation and description of the measurement techniques used in this work. This includes resistivity measurements as well as various pressure cells that are used to perform electrical transport measurements under pressure.

Chapters 3-8 are research papers that have been published (Chaps. 3-6, Chap. 8) or are being drafted (Chap. 7). Chapters 3-7 are devoted to pressure tuning of a varity of correlated electron systems, which is the main topic of the thesis. In Chaps. 3, 4 and 5, studies of Fe-based superconductors including FeSe1-xSx, CaK(Fe1-xNixAs)4 and EuRbFe4As4 are presented. In Fe-based superconductors, different ground states related to electronic, magnetic, and structural degrees of freedom emerge in close proximity. Pressure tunability of these ground states provides great opportunity to investigate the interplay between them.

Chapter 6 presents the effects of pressure on a Kondo heavy fermion system, CeBi2. It is demonstrated that the antiferromagnetic transition TN of CeBi2 is moderately modified, whereas the pressure induced superconductivity is suggested to be extrinsic and due to Bi flux in the specimen.

Chapter 7 focuses on the effect of pressure on a newly discovered metallic ferromagnetic material, La5Co2Ge3. It is demonstrated that the ferromagnetic quantum critical point is avoided by the emergence of a new phase under pressure. The analysis and interpretation of the data is on-going.

Chapter 8 focuses on an important technical aspect of measuring pressure as a function of temperature in piston-cylinder pressure cells and the characterization of the pressure coefficient for manganin as well as the temperature evolution of pressure in a piston-cylinder cell. This work provides two main findings that are important for the pressure community in general. First, it is demonstrated that the temperature and pressure dependence of the pressure coefficient for manganin has to be taken into account for an accurate determination of pressure values at any given tempearture. Second, a detailed analysis of the temperature dependent pressure in a piston-cylinder cell is done to estimate the pressure value at any given temperature. Measurement results and analysis of other manometers such as InSb and Zeranin are further discussed in this work.

Chapter 9 summarizes the thesis as well as suggests some possible further research aspects that could address open questions or improve understandings of the work presented.

Appendix A describes the detailed process of assembling a piston-cylinder cell which enables electrical transport measurements of specimen under hydrostatic pressure up to ~ 2.5 GPa.

Appendix B describes the detailed process of assembling a modifield Bridgman Anvil Cell which enables electrical transport measurements of specimen under hydrostatic pressure up to ~ 6 -7 GPa.

Appendix C describes the detailed process of assembling a miniature Diamond Anvil Cell which enables electrical transport measurements of specimen under pressure up to ~ 20 - 30 GPa.

Appendix D briefly summarizes other projects that I led or was involved in and provides a full publication list during my Ph.D.