Pressure effects on magnetism and superconductivity

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2018-01-01
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Kaluarachchi, Udhara
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Paul C. Canfield
Sergey L. Bud'ko
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Physics and Astronomy
Physics and astronomy are basic natural sciences which attempt to describe and provide an understanding of both our world and our universe. Physics serves as the underpinning of many different disciplines including the other natural sciences and technological areas.
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This thesis summarizes experimental work using high pressure as a tuning parameter in the study of selected correlated electron systems at low temperatures. This thesis contains 8 chapters and 2 appendices. After giving the introduction in Chapter 1, experimental techniques are discussed in Chapter 2. Chapters 3 and 5 are devoted to pressure studies of ferromagnetic materials. In ferromagnetic systems, the appearance of quantum criticality is avoided either by a change of the second order transition to one of the first-order at a tricritical point or by the appearance of modulated, antiferromagnetic, phase. Chapter 3 summarizes the pressure study of the itinerant ferromagnet LaCrGe3 which reveals a new possibility for the phase diagram of metallic quantum ferromagnets. Chapter 4 presents a pressure study of a Ce-based ferromagnet, Whereas many Ce-based compounds manifest an antiferromagnetic ground state, only very few systems are known with ferromagnetic order and pronounced Kondo effects. CeTiGe3 is a rare example of a Ce-based ferromagnet. Chapter 4 presents our observation of a quantum tricritical point in the temperature-pressure-magnetic field phase diagram of CeTiGe3.

Chapters 5, 6 and 7 are devoted to pressure studies of superconducting materials. The results of PbTaSe2 are summarized in Chapter 5. PbTaSe2 is an interesting non-centrosymmetric superconductor which shows two superconducting regions separated by an exceptionally pressure sensitive structural phase transition. Chapter 6 discusses the non-monotonic pressure evolution of the superconducting upper critical field of FeSe. Our analysis of the evolution of the upper critical field under pressure provided evidence of a change of the Fermi surface under pressure in FeSe. In Chapter 7, the newly discovered CaKFe4As4 superconductor shows a collapsed tetragonal phase transition under pressure. In CaKFe4As4, we find that this is a very specific type of collapse, so-called half collapsed, with As-As bonding take place across the Ca-layer but not across the K-layer. This is the first observation of the half-collapsed-tetragonal phase transition in a Fe-based superconductor.

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Mon Jan 01 00:00:00 UTC 2018