Microscopic modeling of organic charge -transfer salts with electronic and structural transitions.

摘要

The instability of quasi one-dimensional metals towards dimerization at low temperatures has been a fascinating subject in solid-state theory for almost 50 years. A recent development is a related instability called a "quantum transition" in quasi one-dimensional organic charge-transfer (CT) salts. This structural instability, although similar to a Peierls transition, occurs in the electronic ground state (GS) of the system, and is complicated by the fact that it often coincides with an electronic neutral-ionic transition (NIT). As a result, CT salts exhibit a wide range of different behaviors and rich spectroscopy. Some salts dimerize without any NIT, while others undergo an NIT without dimerizing; when the structural instability does occur in connection with an NIT, the NIT can be either continuous, or discontinuous.;The unusual and widely varying structural and electronic properties of organic CT salts are modeled within the framework of a Peierls-Hubbard model. The goal is to obtain a comprehensive microscopic description of organic CT salts. The model includes strong electron correlations, electron-phonon coupling, polarization and disorder. It will be used to rationalize unexplained experimental measurements, and to characterize phase transitions. The model accounts naturally for thermal Peierls transitions in CT salts with low-energy magnetic excitations and for quantum transitions in CT salts with NITs. It can also be related to similar models, like the Heisenberg antiferromagnet (HAF) or the t-J model in order to develop an even broader description or organic molecular crystals.;Polarization in extended systems is an essential feature necessary to understand organic salts. Polarization is related to the dielectric response and vibrational spectra of crystals via the recent Berry-phase formulation, which is extended to correlated models. This allows for rationalization of the large dielectric peaks observed at Peierls transitions. In addition, polarization accounts for anomalous IR peaks that appear in the absence of dimerization and results in a general relation between IR intensity and magnetic susceptibility in the dimerized phase.