Crystal structure, Fermi surface calculations and Shubnikov-de Haas oscillations spectrum of the organic metal $ heta$-(BETS)$_4$HgBr$_4$(C$_6$H$_5$Cl) at low temperature

摘要

The organic metal \theta$-(BETS)$_4$HgBr$_4$(C$_6$H$_5$Cl) is known toundergo a phase transition as the temperature is lowered down to about 240 K.X-ray data obtained at 200 K indicate a corresponding modification of thecrystal structure, the symmetry of which is lowered from quadratic tomonoclinic. In addition, two different types of cation layers are observed inthe unit cell. The Fermi surface (FS), which can be regarded as a network ofcompensated electron and hole orbits according to band structure calculationsat room temperature, turns to a set of two alternating linear chains of orbitsat low temperature. The field and temperature dependence of the Shubnikov-deHaas oscillations spectrum have been studied up to 54 T. Eight frequencies areobserved which, in any case, points to a FS much more complex than predicted byband structure calculations at room temperature, even though some of theobserved Fourier components might be ascribed to magnetic breakdown orfrequency mixing. The obtained spectrum could result from either an interactionbetween the FS's linked to each of the two cation layers or to an eventualadditional phase transition in the temperature range below 200 K.