Theoretical Studies on Phase Transitions in Quasi-One-Dimensional Molecular Conductors

Abstract

A review is given for recent theoretical studies on phase transitions in quasi-one-dimensional molecular conductors with a quarter-filled band. By lowering temperature, charge transfer salts exhibit a variety of transitions accompanying symmetry breaking, such as charge ordering, lattice dimerization, antiferromagnetic transition, spin-Peierls distortion, and so on. Analyses on microscopic quasi-one-dimensional models provide their systematic understandings, by the complementary use of different analytical and numerical techniques; they can reproduce finite-temperature phase transitions, whose results can be directly compared with experiments and give feedbacks to material design.

Bond patterns and charge-order amplitude in quarter-filled charge-transfer solids

“…CO amplitudes for this second class of materials are large, with high transition temperatures. One example is (EDO-TTF) 2 PF 6 , where the transition temperature is 280 K and the CO amplitude below the transition is approximately 0.9:0.1 [3][4][5]. It has been assumed that the simplest model incorporating electron-electron (e-e) and electron-phonon (e-p) interactions, the Peierls-extended Hubbard model (see Sec.…”

“…Prior theoretical work [2] has focused almost entirely on the competition between BCDW1 and the 4k F CO, and only large U and V near V c has been studied. In this region the amplitude of the 2k F CO is weak [12][13][14][15].…”

“…In Fig. 3 we also include the boundary for the 4k F CO phase [2,9] from Ref. [13], which is determined from the condition that the Luttinger liquid exponent K ρ > 1 3 , indicating dominant 4k F charge fluctuations [20].…”

“…Small structural changes can lead to very different electronic behaviors. These effects have been studied extensively in the quasi-one-dimensional (quasi-1D) CTS, in particular for the three-quarter-filled (with density ρ = 0.5 holes per molecule) materials (TMTSF) 2 X and (TMTTF) 2 X, which become superconducting under the application of pressure [1]. The ground state of a 1D system of electrons with coupled lattice degrees of freedom is an insulating Peierls state.…”

“…It is often assumed for strongly correlated ρ = 0.5 systems that this insulating ground state is reached through two thermodynamic transitions, with bond dimerization or period two charge order (CO) occurring first at the higher metal-insulator (MI) transition, followed by a small amplitude bond alternation between the localized charges or dimers (for a recent review, see Ref. [2]). Examples of materials with two transitions are MEM(TCNQ) 2 and (TMTTF) 2 …”