High-Pressure Studies of Some Heavy Fermion Materials

Abstract

This thesis presents the results of measurements on three heavy fermion materials under pressure: CeSb2, YbPtBi and CeAgSb2. In Chapter 1, I give an introduction to the key concepts involved in this work, such as unconventional superconductivity, quantum critical points and quantum oscillations, and I explain why heavy fermion materials under pressure are good systems for studying these effects. Chapter 2 describes the experimental techniques used in my measurements. I give an overview of anvil pressure cell techniques, the tunnel diode oscillator circuit and the various cryostats and magnets which were used to generate the low temperatures and high magnetic fields needed to study heavy fermion materials. CeSb2 undergoes a transition under pressure into a structure hosting a heavy fermion state with a large effective mass and antiferromagnetic order. Antiferromagnetism can be suppressed to a quantum critical point around 30 kbar, where there exists a dome of superconductivity with Tc ≈ 0.2 K. Chapter 3 focuses on this superconducting phase, which has a large upper critical field, exceeding the conventional Pauli limit, with an unusual temperature dependence. Chapter 4 contains an analysis of the pressure, temperature and magnetic field dependence of the normal state resistivity of CeSb2, which provides insights into the material’s electronic and magnetic properties. YbPtBi has traditionally been interpreted as a heavy fermion material with an extremely large effective mass due to its very large low-temperature heat capacity. However, quantum oscillations detect light masses, and an alternative scenario is possible in which f electrons act as local moments. Chapter 5 presents quantum oscillation measurements on YbPtBi; at ambient pressure, I have investigated the temperature, magnetic field and angular dependence of quantum oscillations to investigate the role of f electrons in the electronic structure, and I have extended these measurements to high pressures. CeAgSb2 is a clean ferromagnet at ambient pressure, with a ferromagnetic quantum phase transition and antiferromagnetic phase under pressure. In Chapter 6, I discuss phase transitions seen under pressure and my attempts to observe quantum oscillations near pressure-induced quantum phase transitions.