Guy Quirion


Activity Our society's current technological development is strongly based on the use of magnetic materials. The understanding of the magnetic properties of solids is therefore very important. However, our understanding is still far from complete and this is attributed to the theoretical difficulties involved in tackling real materials with realistic models. There is, therefore, a need of sensitive experiments to test which of the different theoretical models are the most relevant.

This program of research is aimed at the investigation of quasi-one-dimensional insulating magnetic systems through measurement of their phase diagrams as a function of temperature, magnetic field, and pressure. These systems are particularly interesting because the theoretical models used to describe their magnetic properties can be considerably simplified. In spite of that, low-dimensional systems yield very rich phase diagrams as a result of competing magnetic interactions and spin-phonon coupling.

The modification of the magnetic structure in these compounds is obtained by measuring changes in sound velocity. The sound velocity measurement is a very sensitive tool for locating transition points, determining phase diagrams, and in favorable cases, obtaining the order parameter. By combining the ultrasonic and high pressure techniques, we open a new way to study phase transitions induced by pressure in a broad range of materials as a function of temperature and magnetic field. The application of pressure has proven to be very useful in elucidating the fundamental properties of new materials. It is a powerful method that can be used to tune, in a controllable and reversible manner, the distance between the atoms of the crystal under investigation. Even if the induced atomic displacements are in general very small, they can be sufficient to produce structural, electronic, and magnetic phase transitions. As an example, pressure has played a major role in studies of organic low-dimensional conductors, including the discovery of the first organic superconductor, (TMTSF)2PF6 at 9kbar and the development of the generic phase diagram for the Bechgaard salts series (TMTSF)2X and their sulphur analog (TMTTF)2X with X=PF6, AsF6, ClO4.

The main activity of this group is to study the spin-Peierls phase transition (SP) and see how it evolves as the transverse magnetic coupling or second nearest neighbor spin coupling along the chain is increased by applying pressure. Other types of quasi-one-dimensional insulating magnetic systems with a strong magnetic anisotropy, such as the ABX3-type hexagonal antiferromagnets, are also investigated. Here the objective is to use high pressure to vary the interchain and intrachain exchange coupling ratio J'/J, and to study its influence. Moreover, since ultrasonic velocity measurements under pressure are not limited to insulating materials, the investigation of non-conventional superconductors such as the bi-dimensional organic superconductors is also possible.

G. Quirion
Department of Physics
Memorial University of Newfoundland
St. John's, Nfld.
A1B 3X7
Office: C3018
Lab: C1067
Phone: (709) 737-3006
Fax: (709) 737-8739

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