The Grüneisen parameter applied to critical phenomena and experimental investigations of correlated phenomena in molecular conductors

• Autor(a) principal: Squillante, Lucas Cesar Gomes [UNESP]
• Data de Publicação: 2023
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Repositório Institucional da UNESP
• Texto Completo: http://hdl.handle.net/11449/244127
• topic: Fenômenos críticos; Parâmetro de Grüneisen; Sistemas eletrônicos fortemente correlacionados; Critical phenomena; Grüneisen parameter; Strongly correlated electron systems

Critical phenomena are of great interest to the scientific community and can be widely extended to various fields of research, such as classical and quantum phase transitions, caloric effects, and even Biology in brain functionality, for instance. As broadly discussed in the literature, the smoking-gun physical quantity to experimentally access both finite-temperature and quantum critical points is the so-called Grüineisen ratio. In this Ph.D. Thesis, a systematic review is performed on the derivation and generalization of the Grüneisen parameter followed by its unprecedented applications to several distinct scenarios, such as magnetic model systems, zero-field quantum phase transitions, the maximization of caloric effects close to any critical-end point based on entropy arguments, the here-proposed adiabatic magnetization of a paramagnetic salt, as well as for Cosmology in the frame of the universe expansion. Since this Ph.D. Thesis is a symbiosis between theoretical and experimental results, an experimental investigation of correlated phenomena was carried out for molecular conductors of the (TMTTF)2X family, where TMTTF is the base molecule tetramethyltetrathiafulvalene and X a monovalent counter- anion such as PF6, SbF6, or AsF6. Such strongly correlated electron systems are considered suitable ones for the exploration of Mott insulating phase, charge-ordering, spin-Peierls, and superconductivity. In particular, the investigation of a possible multiferroic character in these salts was performed via quasi-static (low-frequency) dielectric constant ε′ measurements as a function of temperature where a maximum in ε′ as a function of temperature was observed at the corresponding charge-ordering temperature for both hydrogenated and 97.5% deuterated (TMTTF)2SbF6 salts. Furthermore, Raman measurements were performed on the 97.5% deuterated (TMTTF)2PF6, showing a possible magneto-optical effect on the ν4(ag) vibrational mode of the TMTTF molecule. Yet, fluorescence measurements demonstrated that the fully-hydrogenated (TMTTF)2AsF6 presents an expressive fluorescence background, which is roughly five orders of magnitude lower than that for the 97.5% deuterated variant of (TMTTF)2PF6.