Electronic states and fermi surface in (BEDT-TTF)2X: Hall effect and magnetoresistance

Synthetic Metals, 41-43 (1991) 2163-2166

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ELE~rRQN~C STATES AND FERMI SI=}RFACEIN (BEDT-TFF)2~X: HALL EFFECTAND MAGNETORESISTANCE

K. Murata, M. Ishibashi,* N. A. Fortune, T. Komazaki,** Y. Honda,* M. Tokumoto, N. Kinoshita and H. Anzai*** Electrotechnical Laboratory, Tsukuba, Ibaraki 305, (Japan)

ABSTRACT The Hall effect in [~-(BEDT-TTF)2I 3 and ~ - ( B E D T - T T F ) 2 C u ( N C S ) 2 and the preliminary results on the pressure dependence of the Shubnikov de Haas effect in [ ~ - ( B E D T - T T F ) 2 I 3 and their implications are presented. Possible changes in the electronic states of both material are discussed in relation to superconductivity. 1. INTRODUCTION Our recent studies on the transport properties such as the Hall effect and the Shubnikov de Haas effect on typical ~ - a n d ~-type B E D T - T T F salt are presented. A detailed description of the Hall effect in [ 3 - ( B E D T - T T F ) 2 I 3 at ambient pressure is presented in Ref. [1] and under pressure up to 10 kb in Ref. [2], and for • - ( B E D T - T T F ) 2 C u ( N C S ) 2 in Ref. [3]. Preliminary results on the Shubnikov de Haas effect under pressure on [ L ( B E D T - T T F ) 2 I 3 are presented in Ref. [4]. The field direction for the Hall effect study is always along the least conducting axes, i.e. HI~c* for [ ~ - ( B E D T - T T F ) 2 I 3, and H//a* for ~( B E D T - T T F ) 2 C u ( N C S ) 2 . The field direction for the Shubnikov de Haas experiment under pressure for ~ - ( B E D T - T T F ) 2 I 3 is nearly parallel to the c* -axis. 2. ON [3-(BEDT-TTF)2I 3 Figure 1 shows the temperature dependence of the Hall coefficient RH(T) at P = 0 and P

= 3 kb.

In the high temperature region,

although

the

* Visiting student from Tokyo Metropolitan University. ** Visiting student from University of Tsukuba. *** Present Address: Himeji Institute of Technology, 2167 Shosha Himeji 67122 (Japan) 0379-6779/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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temperature d e p e n d e n c e of the resistivity appears metallic, since the apparent mean free path is smaller than the lattice constant, it is important to verify the metallic nature by measuring the Hall effect. Our experimental result of constant carrier concentration with changing temperature confirms the metallic nature of this material. Through 175 K, where incommensurate superstructure appears, the Hall coefficient start to decrease to lower temperature. With the appearance of the superstructure if the band is very simple two-dimensional, a charge transfer ratio may not change. It might suggest that the band is not so simple. At least electronic state is affected by the occurrence of the superstructure. Around 20 K, a sharp decrease in R H ( T ) is observed from high temperature value, which is considered to be that of stoichiometry. The value at 4.2 K is close to 60 % of the high temperature value. With increasing pressure anomalies are always observed as kinks in RH(T). At 3 kb, R H ( T ) exhibits a peak at 10 K as shown in Fig. 1. By increasing pressure, the temperature of this anomaly increases up to 18 - 20 K at 7 kb. Below 18 - 20 K, the decrease in RH(T) is not prominent. The behavior in RH(T) at 10 kb is the same as that of 7 kb. The decrease in R H ( T ) below around 20 K at P = 0 has been confirmed by another group [5], but with a hump in RH(T) which is similar to our data at 3 kb except for the peak temperature. If we define the anomalies as kinks in R H ( T ) , we can plot the temperatures where anomalies occur in the temperature-pressure diagram together with the superconducting T c, as shown in Fig. 2. Figure 2 we can read that the temperature of the anomaly is high when the superconducting T c is low, and also that the temperature of the anomaly is discontinuous between pressures where the low-T c and high-T c states are stable. The NMR at P = 0 obeys different Korringa relations above and below the temperature of the anomaly[6,7]. These results suggest a phase transition or a precursor to a new electronic state. A different possibility has been proposed for the e n h a n c e m e n t of RH(T)[5]. When the electron-electron scattering is dominant, the Umklapp process becomes nonneglible and the scattering become anisotropic. The anisotropy in the carrier lifetime x gives rise to the factor, / 2 > 1 in the formula R H = (1/nec)/<'c>2. At low temperature, where isotropic electron-impurity scattering becomes dominant, the enhancement is reduced, At high temperature, thermal effects make the scattering isotropic. Thus, the enhancement is only pronounced around 20 K( referred to as 40 K in Ref. [5]). If this mechanism is the primary factor that determines how R H

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changes with temperature, we expect a more gradual change in RH(T ) with temperature, instead of kinks, which we observe at all pressures. Also, the mechanism that enhances R H can not explain the reduction in RH(T) from its high temperature value that we observe at P = 0 below 20 K[1]. Figure 3 shows typical data from our measurement of the Shubnikov de Haas at 3.5 kb at 106 mK. From our preliminary results, the effective mass seems to increase slightly from 5.9 m 0 to 7.1 m 0 as pressure increases from 1.8 to 3,5 kb, respectively, with the analysis shown in Fig. 4. This variation of the effective mass may be related to changes in the electronic properties that govern the gradual increase of the temperature of the anomaly in the Hall effect with pressure. 3. ON ~-(BEDT-TTF)2Cu(NCS) 2 The temperature dependence of the resistivity is known to exhibit a hump around 100 K. The reason for the resistivity hump has been controversial[8]. Our experimental observation is that although a gradual increase is observed in RH(T) down to 60 K, no anomalous change is observed as shown in Fig. 5. Below 60 K, a sudden increase in RH(T ) is observed. At this temperature, the temperature dependence of the resistivity has an inflection point[3,9]. This temperature coincides with the anomaly in the thermoelectric power and with the temperature below which low frequency conductivity increases, as measured by infrared . These results suggests the 60 K feature is related to an electronic transition. REFERENCES 1. K. Murata, M. Ishibashi, Y. Honda, M.Tokumoto, N. Kinoshita and H. Anzai, J, Ph.y~. So~, Jon. 58 (1989) 3469. 2. K. Murata, M.Ishibashi, N. A. Fortune, Y. Honda, M. Tokumoto, N. Kinoshita and H. Anzai, preprint(1990, August). 3. K. Murata, M. Ishibashi, Y. Honda, N. A. Fortune, M. Tokumoto, N. Kinoshita, and H. Anzai, Solid State ¢ommun. 76 (1990) 377. 4. K. Murata, M. Ishibashi, Y. Honda, T. Komazaki, M. Tokumoto, N. Kinoshita and H. Anzai, Springer Pro~, in Physics ~1 (1990) 224. 5. B. Korin-Hamzic, L. Fort6 and J. R. Cooper, Phy$, Rev. B41 (1990) 11646. 6. Y. Maniwa, T. Takahashi, M. Takigawa, H. Yasuoka, G. Saito, K. Murata, M.Tokumoto and H. Anzai, Jon. J.Appl, Phys. Suppl. 26-3 (1987) 1361. 7. F. Creuzet, C. Bourbonnais, D. J6rome, D. Schweitzer, H. J. Keller, Eur0. Phys~ Lett, !. (1986) 467. 8. N.Toyota, T. Sasaki and H. Satoh,preprint, and Solid State ¢ommun. 74 (74 (1990) 361. Also see reference in Ref. [3]. 9. L.I. Buravov, A.V. Zvarykina, N.D. Kushch, V.N. Laukhin, V.A. Merzhanov, A.G. Khomenko and E.B. Yagubskii, Zh, Eksp, Teor. Fiz, 95 (1989) 322.

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