Charge Ordering and Π–d Interaction in Electron-Doped 3/4-Filling Molecular System Α′′-(Bedt-ttf)2rb2xco(scn)4 (x = 0.6)

Satoshi Iguchi,Syuhei Yamada,Ryota Kobayashi,Koutaro Morita,Yuki Kudo,Megumi Kurosu,Naoki Yoneyama,Yuka Ikemoto,Taro Moriwaki,Takahiko Sasaki

Journal of the Physical Society of Japan（2021）SCI 4区

Tohoku Univ

Cited 0|Views27

Abstract

We have investigated a newly found alpha ''-phase bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecular arrangement system, namely, alpha ''-(BEDT-TTF)(2)Rb2xCo(SCN)(4) (alpha ''-Rb2xCo), with localized S = 3/2 Co2+ spins. From X-ray structural analyses, we found that owing to the nonstoichiometric ratio of Rb ions (x = 0.6), this compound takes an intermediate value of pi-electron band filling between those of alpha ''-(BEDT-TTF)(2)CsHg(SCN)(4) and alpha ''-(BEDT-TTF)(2)K1.4Co(SCN)(4). alpha ''-Rb2xCo (x = 0.6) is a paramagnetic metal at room temperature and exhibits a first-order transition to an insulating state at 100 K, which is the lowest among the three compounds. The results of the two-dimensional optical conductivity spectra and peak splitting of charge-sensitive nu(27) molecular vibrational modes indicate the phase transition to a state with a gap of 600 cm(-1), which is attributed to charge ordering consisting of at least four differently charged BEDT-TTF molecules. The pi-spin susceptibility suddenly decreases at the transition temperature, and another anomaly can be seen at about 40 K. Given that pi-spin disappears at 100K (i.e., formation of a spin-singlet state), it is difficult to systematically explain other physical properties as a whole at low temperatures. However, by simply assuming the presence of pi spins, we can account for all the observed results without contradiction. We also discuss anomalies in the dielectricity of pi electrons under magnetic fields mediated by pi-d interactions and the spin-charge coupling.

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