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Soutenance de thèse

Jeudi 29 novembre 2018 à 14h00,
Salle des Séminaires, Bâtiment A,
25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9

Orateur : Quentin Faure (UGA-INAC-Institut Néel)
"Transitions de phases quantiques dans le composé quasi-1D antiferromagnétique de type Ising BaCo2V2O8"


The aim of this thesis was to study various quantum phase transitions in BaCo2V2O8, a quasi-one dimensional antiferromagnet which presents an Ising-like magnetic anisotropy. This system consists of weakly coupled chains of magnetic ions Co2+ carrying an effective spin S = 1/2. Therefore BaCo2V2O8 can be mapped onto the XXZ model, largely studied theoretically with and without a magnetic field. Different quantum phase transitions are predicted depending on the orientations of the magnetic field, i.e. whether the field is applied perpendicularly (transverse) or parallel (longitudinal) to the Ising z-axis. The former case, namely the Ising-chain in a transverse field, is well known to be a paradigm of quantum phase transition while in the latter case, it has been theoretically predicted that the system enters in a gapless phase known as the Tomonoga Luttinger Liquid (TLL) phase. Both the static properties and the spin-dynamics of BaCo2V2O8 for these two configurations of the field have been investigated during this thesis to understand the quantum phase transitions occurring in this compound. We have shown that BaCo2V2O8 exhibits a very original quantum phase transition when subjected to a transverse field and we have explored the spin dynamics of the system under a longitudinal magnetic field characterized by dominant longitudinal fluctuations.

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