2P049
Charge order and its fluctuation in !-type BEDT-TTF charge transfer salts
Y. Yue1.2; C. Nakano2; K. Yamamoto1.2; M. Uruichi2; K. Yakushi1.2; T. Hiejima3, and A. Kawamoto4
1

Graduate University for Advanced Studies, Okazaki, Japan
2

3

Institute for Molecular Science, Okazaki, Japan

Department of Nanochemistry, Tokyo Polytechnic University, Atsugi, Japan
4

Department of Physics, Hokkaido University, Sapporo, Japan
Email: yakushi@ims.ac.jp

Among a variety of BEDT-TTF charge-transfer salts, the !-type BEDT-TTF salts showsrich properties such as charge ordering, superconductivity, zero-gap state or Dirac fermion,persistent photoconduction, photoinduced phase transition, and non-linear optical response.
We investigate the charge order and fluctuation of chargeorderin!-type

BEDT-TTF

salts

including!"-(BEDT-TTF)2IBr2, !-(BEDT-TTF)2I3, and !-(BEDTTTF)2NH4Hg(SCN)4 by means of infrared, Raman, andfar-infrared spectroscopy. We estimated the kinetic energyof !’-(BEDT-TTF)2IBr2, !-(BEDT-TTF)2I3, and !-(BEDTTTF)2NH4Hg (SCN)4 from the integration of the opticalconductivity at room temperature. Figure 1 shows the dataof kinetic energy of !-type BEDT-TTF salts Fig.1 kinetic energy of !-type BEDT-TTF salts (fromeestimated from band calculation and opticalleft

conductivity. The kinetic energy of all three!-(BEDT-TTF)2I3,

compounds is much smaller than the calculated!-(BEDT-TTF)2NH4Hg(SCN)4) estimated from band

kinetic energy and they are in a good relationshipcalculation (") and optical conductivity (#)

sitetorightsiteare!’-(BEDT-TTF)2IBr2,!-(BEDT-TTF)2CsCd(SCN)4andwith the lattice parameter ratio c/a.
As shown in Fig. 1, the bandwidth of$$ !’-(BEDT-TTF)2IBr2 is narrowest among the!-type BEDT-TTF salts. This compound shows a semiconductor-to-insulator phase transitionat ~200 K in electrical resistivity. We investigated the infrared and Raman spectra of!’-(BEDT-TTF)2IBr2 using 13C-substituted BEDT-TTF salt, and showed clear evidence thatthe low-temperature high-resistivity phase is a charge-ordered state. The horizontal stripe ofcharge order is most stable, but dislocation can be easily generated because a diagonal stripe
is energetically very close to the horizontal stripe. The increase of inter-site hopping rate,
symmetry change from P1 to P 1 , and the appearance of density of state near Fermi energy inthe high-temperature phase indicate that this semiconductor-to-insulator transition is anorder-disorder phase transition of localized charge. This means that the conduction electrons

are localized (incoherent) in the whole temperature range; they are diffusively moving fromone site to the adjacent site above ~200 K, and they are crystallized with long-range orderforming a horizontal stripe below ~200 K. This localized electronic state in the wholetemperature range is ascribed to the very narrow bandwidth.
###!-(BEDT-TTF)2I3

bandwidth showsawith

intermediate

metal-insulator

phase

400

transition at ~135 K in electrical resistivity. We

298K
200K
150K
138K
136K
134K
130K
50K//ainvestigated the infrared and Raman spectra of
low-temperature phase using 13C- and d8substituted BEDT-TTF salts. The complete
assignment of %2, %3, and %27 is consistent withconductivity (Scm-1)

300

100

the charge-ordered state. The amplitude ofchargeorderissmaller

thanthat20000of1000

2000

3000

4000!’-(BEDT-TTF)2IBr2, which is consistent with298 K
200 K
150 K
138 K
136 K
134 K
130 K
50 K//b

the infrared-active %27 mode of !-(BEDTTTF)2I3 is qualitatively different from that of!’-(BEDT-TTF)2IBr2, which appears to beconsistent with the metal-insulator transition.
However, the linewidth in the metallic phase ofconductivity (Scm-1)

400

the bandwidth. The temperature dependence of5000-1

wavenumber (cm )
500

300

200

100001000

2000

3000

4000

5000

-1

wavenumber (cm )!-(BEDT-TTF)2I3 is much broader than that ofFig. 2 Optical conductivity measured on the (001)the Raman spectrum of wide-bandwidth metallic

plane with polarizations of E//a and E//b.

compound, !-(BEDT-TTF)2NH4 Hg(SCN)4. The
Drude response by conduction electrons does not
grow in the metallic phase of !-(BEDT-TTF)2I3 incontrast to the well developed Drude response atroom temperature in !-(BEDT-TTF)2NH4Hg(SCN)4. This characteristic behavior means that thescattering rate of conduction electrons is so largethat the wavevector (momentum) cannot be welldefined any more. This result suggests that theconduction electrons in the high-temperaturephase of !-(BEDT-TTF)2I3 have not metallic(coherent) character but diffusive (incoherent) Fig 3 Optical conductivity of !-(BEDT-TTF)2I3 measured
character. The broad linewidth of %2 and %27 of on [110] with the polarization of E //c.
!-(BEDT-TTF)2I3 suggests the fluctuation of charge order. This suggestion is consistent withthe localized nature of the electronic state in high-temperature phase.