Strongly correlated f-electron systems are a rich reservoir 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 complex magnetism, unconventional superconductivity, and other intricate matter of states. The complex phase diagrams of temperature T versus a control parameter δ, such as chemical composition x, applied pressure P, and magnetic field H, have been observed, where they exhibit new and unexpected physics, including "non-Fermi-liquid" (NFL) behavior, "rattling" motion of atoms in a cage structure, local structure distortion, crossover of superconducting order parameters, and the coexistence of superconductivity and magnetism. In order to explore such new phase diagrams, therefore, the work in this dissertation focuses on several chemical substitution studies on f-electron systems: the layered superconductor La1-xYxO0.5F0.5BiS2, and the filled-skutterudite PrPt4Ge12-xSbx and Pr1-xEuxPt4Ge12 systems, whose parent compound PrPt4Ge12 is an unconventional superconductor. Polycrystalline samples were synthesized during the course of the studies and were characterized by means of x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements.

The research presented in this dissertation is a result of several chemical substitution studies on correlated electron systems. Through careful synthesis and measurement at low temperatures, we create lanthanide- and actinide-based compounds and environments in which various exotic phenomena can be studied in detail. Such phenomena include heavy fermion behavior (HF), superconductivity (SC), exotic magnetic ordering, the Kondo effect, valence fluctuations, non-Fermi liquid behavior (NFL), quantum criticality, among many others. Not only are these phenomena probed through measurements at extreme conditions like low temperatures down to 30 mK and high magnetic fields up to 65 T, but they are also studied as the electronic structure is modified through the substitution of atoms. By providing a detailed survey of certain correlated electron systems through multiple measurement techniques, we hope to better understand the underlying physical phenomena which drive the macroscopic behavior in these compounds which have been the subject of great interest for decades.

This dissertation includes a discussion of the results of measurements of electrical resistivity

for materials under applied pressure extending over a range in pressures from 0 to 27

GPa and temperatures from 1 to 300 K. The primary effect of applying pressure to a solid

is to reduce the interatomic distance and to increase the overlap of the electronic orbitals.

The secondary effects of applying pressure to a solid include the delocalization of electrons

and a broadening of the energy bands. In addition, the application of pressure can induce

a variety of transitions, both electronic and structural. Some of the phenomena observed

during the pressure experiments reported in this dissertation include the pressure-induced

enhancement of the superconducting transition temperature in a recently discovered class

of bismuth-sulfide layered superconductors, the anomalous “dome-like” behavior in the

pressure dependence of the Néel temperature in the first-known synthesis of an itinerant

antiferromagnetic metal with non-magnetic constituents TiAu, and the evolution of the

pressure-induced first-order transition to antiferromagnetism in the Fe-substituted heavy

fermion compound URu2Si2. These and other results including a semiconductor-metal

transition in the normal state of one of the bismuth-sulfide layered superconducting

compounds are a consequence of the electronic and structural transitions that can occur as a

result of the application of pressure. The theoretical context for the experimental results

includes a discussion of the effect of pressure on the relevant parameters in condensed

matter theory including the density of states, the exchange interaction in both the localized

and itinerant models of magnetism, and the electron-phonon coupling parameter, among

others. In particular, the pressure dependence of the magnetic ordering temperature and

the superconducting transition temperature in various materials can be explained in terms

of how the few parameters listed above respond to pressure.

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.

The goal of this dissertation is to study strongly correlated f–electron materials under extreme conditions using various novel imaging techniques. Strongly correlated electron materials exhibit complex physics, and when modified by a control parameter such as pressure, chemical composition, magnetic field, or temperature, can lead to novel quantum phases of matter. The underlying physics of these novel quantum phases of matter and phenomena are not completely understood and their properties, and potential applications in technology, are still being explored.In this dissertation I will review three selected works involving the application of a new optical magnetic resonance spectroscopy (ODMR) method on ferrimagnetic thin films and in-elastic x-ray spectroscopy at the Advanced Photon Source (APS) that was used to study the correlated f-electron superconductor UTe2 at large hydrostatic pressure up to 52 GPa. This dissertation will then describe the on-going projects to extend NV ODMR spectroscopy to high pressure experiments in search of emergent phases and phenomena such as superconductivity, magnetism, topological insulating states, valence fluctuations, heavy fermion phenomena, and quantum criticality in f-electron materials. First, I will describe research conducted using a new type of ODMR method using nitrogen vacancies (NV) defects in diamond to detect thermal magnons in ferrimagnetic thin films and thin film disks of yttrium iron garnet (YIG). This research demonstrated the detection sensitivity of NVs to a broad range of magnon wavevectors (k) in YIG, up to 5.1 × 107 m-1, as well as observation of parametrically excited magnons and quantized magnon wave modes in patterned disks with radius of 5 μm of 100 nm thin film YIG. A second follow up study used single NV centers to probe changes in the perpendicular magnetic anisotropy (PMA) in thin film YIG. By detecting variations in the single NV relaxation rate, Γ, we were able to extract the effective magnetization 4πM_eff in 8 nm and 12 nm YIG to be -442±7 Oe and -1470±12 Oe, which matches results gathered by traditional ferromagnetic resonance spectroscopy techniques. The spinwave stiffness constant D_s was found to be 8.458×10^(-40) J m^2. This study was followed by research into the structural and electronic properties of UTe2, a novel f- electron superconductor involving resonant x-ray emission spectroscopy (RXES), partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS), and x-ray diffraction experiments (XRD) on UTe2 at high pressure using diamond anvil cells (DACs), at the Argonne National Laboratory’s’ Advanced Photon Source, in search of quantum phase transitions in UTe2 under hydrostatic pressure. High pressure XRD measurements on single crystal UTe2 at room temperature resulted in the discovery of a crystal phase transition in UTe2 from an orthorhombic Immm ordered phase to a tetragonal I4/mmm ordered phase at ~ 7 GPa. Additionally, through a combination of RXES and PFY-XAS we were able to detect a pressure induced change in the U valence from U 3+ to U 4+ at low pressure which then stabilized towards a valence of U 3+ up to 52 GPa.

We report on a comprehensive optical, transport, and thermodynamic study of the Zintl compound Yb14MnSb11, demonstrating that it is the first ferromagnetic Kondo lattice compound in the underscreened limit. We propose a scenario whereby the combination of Kondo and Jahn-Teller effects provides a consistent explanation of both transport and optical data.