UC San Diego

In condensed matter physics, the term "extreme condition" mostly refers to a environment of extreme temperatures, high pressures, and/or high magnetic fields. With the extension of these dimensions, we are facing a new world with many more exotic states of matter than what has been explored at ambient conditions. The mechanical, chemical, and physical properties of matter may dramatically changes in such conditions.

The primary subject of this thesis is the study of electron-electron and electron-phonon interplay of $d$- and $f$-electron materials at extreme conditions. It is well known that these materials are rich reservoirs for exotic and intriguing physical phenomena; their competition and interplay between localized magnetic moments in partially filled $d$- or $f$-electron shells, and conduction electron states lead to novel quantum criticality, magnetic ordered state, superconductivity, and other interesting states of matters. Part of this thesis covers the research on synthesis of BiS$_2$-based superconductors, which has a layered structure that is very similar to high-$T_c$ cuprate and Fe pnictide superconductors. The lattice parameters, crystal structure, and superconductivity of these materials turn out to be very sensitive to external environments. Evidence of pressure and magnetic field induced changes in superconductivity and structure of BiS$_2$ compounds are also presented. The latter part is focused on novel metallic ground state in FeSi. For over 50 years, this compound was believed to be a intrinsic narrow gap semiconductor. The study shows strong evidence of surface electron conducting of FeSi at low temperatures. Possibility of FeSi as a 3D topological insulator, surface magnetic ordered state, and novel Hall effects are also emphasized.