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Title: Magnetism and superconductivity in heavy-fermion metals
Author: Agarwal, P.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2001
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One of the key challenges of contemporary condensed matter physics is to understand how the large variety of states of matter arises in electron systems. While the basic force is in all cases the electrostatic Coulomb interaction, the variety of states of matter spans an impressively broad spectrum, ranging from insulators to superconductors, and including a variety of magnetic phases. In recent years, a series of new, emergent phenomena have added more exotic states to the list of known behaviour in metallic systems, and some of them defy the standard descriptions of 'normal' metals, known as Fermi liquid theory. Examples of these new phenomena include the fractional quantum-Hall effect, high temperature superconductivity and magnetic quantum phase transitions, which are at the focus of this dissertation. Such phase transitions occur when magnetic order is suppressed at low temperatures by means of an external control parameter, such as pressure, doping or applied field. In the vicinity of such a transition, unusual properties are expected due to strong, long range magnetic interactions, which can lead to unconventional normal states and, quite generally, to exotic forms of superconductivity. More specifically, I have studied high purity single crystals of two f-electron metals, namely CeNi2Ge2 and UGe2, both close to their critical lattice densities, and examined the behaviour of their electrical resistivity as a function of temperature and pressure. CeNi2Ge2 at ambient pressure is naturally very close to antiferromagnetic ordering, and for the first time, CeNi2Ge2 is shown to be an unconventional superconductor at ambient pressure at about Tc ≃ 200mK. Furthermore, the normal state of CeNi2Ge2 displays an unconventional power-law of the form Δp ˜ T1.2 over 2 decades in temperature. As pressure is applied a slow return to Fermi liquid behaviour is observed at the lowest temperatures, before a second (and as yet unidentified) ordered phase sets in at high pressures. The second system studied in this dissertation is the ferromagnet UGe2, in which magnetism can be suppressed by pressures of about 1.4 GPa. In this material, too, superconductivity is observed at low temperatures over a narrow pressure range, but in the ferromagnetically ordered phase. I present preliminary measurements together with a phase diagram. It seems that UGe2 is the first example of an itinerant ferromagnetic superconductor. Experimental work was carried out using a new adiabatic demagnetization cryostat, specially designed and built as an important part of the research project. The thesis describes the design of the instrument, which allows high sensitivity measurements of the resistivity over a wide range in temperature with high reliability.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available