第36回 原研・兵県大合同コロキウム

日時: 平成20年2月7日(火) 15:00〜
Date and time: 7th Feb. (Tue.) 15:00〜
場所: SPring-8, 萌光館
Place: SPring-8, "HOUKOUKAN" seminar room
Title: Ab initio electronic structure study for molecular conductors: single-component molecular metals and TTF-TCNQ
講演者: 石橋章司 氏 (産総研・計算科学研究部門)
Speaker: Prof. Shoji Ishibashi (RICS-AIST)
Abstract:

In contrast with inorganic materials, it had been thought up to a few years ago that metallic organic solids should consist of more than two kinds of molecules. Their metallic nature is brought by (partial) charge transfer between the highest occupied molecular orbital (HOMO) of donors and the lowest unoccupied molecular orbital (LUMO) of anions. The discovery of the first single-component molecular metal Ni(tmdt)2 (tmdt = trimethylenetetrathiafulvalenedithiolate) [1] has opened a new paradigm. Subsequent to this discovery, various related compounds have been synthesized and they form a new class of materials. We have studied electronic structures of several materials such as Ni(tmdt)2[1], Au(tmdt)2[2], Au(tmstfdt)2, Cu(dmdt)2[3] and Zn(tmdt)2[4] by means of ab initio calculations [5-7]. First, we investigated the central metal dependence of the character of states around HOMO (or singly occupied molecular orbital (SOMO)) and LUMO for isolated molecules and show that the stable structure of each isolated molecule is properly predicted. For the solid state of Ni(tmdt)2, Au(tmdt)2 and Au(tmstfdt)2, their structures are similar to each other though there are systematic variations in the electronic structure. From the present calculations, Ni(tmdt)2 is a non-magnetic semimetal while Au(tmdt)2 is an antiferromagnetic metal. Antiferromagnetic spin arrangement is observed between two ligands and there is no spin on Au. As for Au(tmstfdt)2, although its electronic structure is similar to that of Au(tmdt)2, the Fermi surface of the former has less nesting feature and its magnetic solution was not obtained in the GGA level calculation. With non-magnetic calculations, Cu(dmdt)2 is a metal and Zn(tmdt)2 is a semimetal. As for Cu(dmdt)2, we could obtain a stable antiferromagnetic solution with a spin polarization also on Cu. The existence of stable magnetic moment is consistent with the susceptibility measurement but the long range antiferromagnetic order has not been confirmed experimentally.

TTF-TCNQ is a typical quasi-one-dimensional conductor showing a CDW transition around 60 K. Recently, Yasuzuka et al. almost succeeded in suppressing the CDW transition by applying hydrostatic pressure of 8 GPa [8]. Stimulated with this experimental result, we performed a systematic study of electronic structures of TTF-TCNQ under hydrostatic pressures. When applying hydrostatic pressure, the b axis shows a larger compression than the other axes. On contrary to a naive expectation that three dimensionality of the Fermi surface may increase with hydrostatic pressure, it has been revealed that quasi-one dimensionality increases with hydrostatic pressure. This is due to the non-uniform compression mentioned above. The variation of the electronic structure under hydrostatic pressure seems similar to that for uniaxial compression along the b axis [9].