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有 機 導 体 -(BEDT-TTF) 2 I 3 における 非 線 形 伝 導 特 性Nonlinear Current-Voltage Characteristics in -(BEDT-TTF) 2 I 3筑 波 大 学1 , 物 材 機 構2 , 東 大 物 性 研3 4, 分 子 研小 玉 恒 太1,2 , 木 俣 基3 , 山 口 尚 秀2 , 寺 嶋 太 一2 , 栗 田 伸 之2 , 薩 川 秀 隆2 , 原 田 淳 之2 ,宇 治 進 也2,1 , 山 本 薫4 4, 薬 師 久 彌K. Kodama 1,2 , M. Kimata 3 , Y. Takahide 2 , T. Terashima 2 , N. Kurita 2 , H. Satsukawa 2 , A. Harada 2 ,S. Uji 2,1 , K. Yamamoto 4 , K. Yakushi 4Univ. of Tsukuba 1 , NIMS 2 , ISSP. Univ. of Tokyo 3 , IMS 4Abstract:Current-voltage (I-V) characteristics have been measured to clarify the charge transport mechanism in the charge orderstate of a two dimensional organic conductor -(BEDT-TTF) 2 I 3 . At low temperatures, we find strong non-linear I-Vcharacteristics that follow the power law approximately. The non-linear I-V characteristics are attributed to theelectric-filed-assisted unbinding of excited pairs of an electron and a hole that attract each other due to two-dimensionallong-range Coulomb interaction.Keywords: Strong electron correlation, charge ordering, organic conductorE-mail: KODAMA.Kouta@nims.go.jp1. IntroductionAn organic charge transfer salt, -(BEDT-TTF) 2 I 3(BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene) iswell known as a typical charge order (CO) insulator atambient pressure [1][2]. In spite of the 3/4 filled band ofBEDT-TTF molecules, the ground state is insulating due tostrong electron-electron Coulomb interaction. In this paper,we present I-V characteristics at various temperatures in awide electric field region for -(BEDT-TTF) 2 I 3 toinvestigate the charge transport mechanism of the CO state.2. ExperimentalAll I-V characteristics shown here were measured by atwo-terminal method. The DC voltage was applied alongthe ab conducting plane. The gold 10 m wires wereattached on the sample surface with a carbon paste and thetypical electrode spacing is on the order of 10 m. Thesamples were slowly cooled using a 3 He refrigerator.3. Results and discussions:The temperature dependence of the in-plane conductancefor -(BEDT-TTF) 2 I 3 is shown in Fig. 1. The conductancefollows the Arrhenius law V = 0 exp(-/2k B T) below90 K, giving the activation energy /k B =740 K.The in-plane I-V characteristics of -(BEDT-TTF) 2 I 3 atdifferent temperatures are shown in Fig. 2(a). Thenon-linear I-V characteristics appear below 35 K clearly. Inthe CO systems, the charge transport has been interpretedas thermally excited electron-hole pairs [3]. The nonlinearI-V characteristics is expressed in terms of electric fielddependent energy-gap (E):J / exp[-(E)/(2k B T)], (1)where J is the current density. Here, we assume that thetransfer integral is highly anisotropic (t // >>t⊥) and thedielectric constant of the BEDT-TTF layers is much largerthan that of the I 3 insulating layers: the excitedelectron-hole pairs remain at the same layer and the electricfluxes are well confined in each BEDT-TTF layer. In thiscase, the attractive Coulomb potential (r) between theelectron and hole can be assumed as logarithmic in distanceU 0 ln(r/a). In electric field, the energy gap is written as,( E)max[U0ln(( r / a) eEr ] U0ln[ U0/ eEa)](2)for E for simplicity. This value corresponds to the energygap at the zero-bias; = U 0 ln(/a) = 740 K for-(BEDT-TTF) 2 I 3 as shown in Fig. 1. Substituting (2) into(1), we obtain the power-law I-V characteristics,U 0 /( 2 k BT) U 0 /( 2 k BT) 1J ( ea / U ) E(3)00Figure 2 (b) shows the I-V characteristics calculated usingEq. (3) for U o /k B = 155 K, a = 0.5 nm, = 60 nm and thevoltage contact spacing L = 50 m. The experimentalresults are well reproduced except the data below T = 30 K.It should be noted that long-range 2D Coulomb potential,logarithmic potential, is essential for the power-law I-Vcharacteristics. Such long-range potential has not beentaken into account in conventional extended Hubbardmodels.- 15 -