Physica C 185-189 (1991) 2685-2686 North-Holland PHYSICA

HALL EFFECT OF THE ORGANIC SUPERCONDUCTORS OF (TM-I'SF)2X, (DMET)2X and (BEDT-TTF)2X

Keizo MURATA, a Masayoshi ISHIBASHI,a, b Nathanael A. FORTUNE, a Madoka TOKUMOTO, a Nobumori KINOSHITA, a Hiroyuki ANZAI, c Koichi KIKUCHI, b Kazuya SAITO, b Isao IKEMOTO, b Toshihiro TAKAHASHI, d

a: Electrotechnical Laboratory, Tsukuba, Ibaraki 305 Japan b: Tokyo Metropolitan University, Haohioji, Tokyo 192-03 Japan c: Himeji Institute of Technology, Himeji, Hyogo 671-22, Japan d: Gakushuin University, Mejiro, Toshima-ku, Tokyo 171, Japan

We found that the Hall coefficient,RH, varies srongly with temperature even in the metallic state of these materials especially at low temperature. Since the variation of RH with temperature, RH(T), is usually quite sharp, some new electronic phase or fluctuation is expected in the low temperature region. Further, such temperature dependence of RH(T) varies with pressure. However, the variation of RH(T) with pressure seems to be remarkable only in the pressure region where superconductivity is present, it is plausible that the low temperature metallic state is influenced by some fluctuation relating to superconductivity.

1. INTRODUCTION Hall coefficient studies are basically a method to

estimate the number of carriers, n using the tirst ap- proximation RH=llnec, with usual notations. Therefore, RH is expected to be a temperature in- dependent constant in the metallic state. However, our recent work on organic superconductors re- vealed that such expectations are not valid. This short paper summarizes our work of the last few years on typical organic superconductors.

2. EXPER!MENTAL RESULTS All of the crystal shapes are plate-like. The Hail

effect measurements are carried out with the do cur- rent along the most conducting axis. The Hall volt- age contacts are located on both sides of the fiat surface, which is usually along the second most conducting axis. The field is applied perpendicular to the plane. Experimental details are described elsewhere. 1

2-1. ~-(BEDT-TTF)213 This salt has a two-dimensional closed Fermi sur-

face. The superconductivity is well-known with both Iow-T o (T o~ 1 K) and high-T o (T c ~ 8 K) states. These two transition temperatures are associated with a superstructure that appears below 175 K for pressures below 0.4 kbar.

The Hall coefficient at ambient pressure is found to be almost independent of temperature. However in detail, below 175 K, RH(T) decreased by 8 %. Moreover, below 20 K, a sudden decrease of RH(T) is observed. 1 With increasing pressure, the RH(T) shows a cusp-shaped peak below around 20 K, which varies with pressure. These temperatures

seem to vary continuously with pressure in th8 pres- sure region where high-To state is realized, but dis- continuously with that of the low-To state. Above the pressure of the high-To state, "/c seems to saturate with pressure. Therefore, the temperatures corre- sponding to the peaks in RH(T) seem to correlate with that of the suparconducting Tc's. 2

2-2. n-(BEDT-TTF)2Cu(NCS)2 The temperature dependence of the resistivity ex-

hibits a hump around 100 K. However, RH(T) only increases slightly with decreasing temperature, with no significant anomaly around 100 K. By further cooling, a sudden increase in RH(T) is observed bemow 60 K and a decrease below 10 K as shown in Fig. 1.2,3 As is the case with ~-(BEDT-TrF)213, the pressure variation of these features in RH(7} seems to correlate with that of T c.

• ]ii ( B E D T - T T F } 2Cu {NCS) 2 # 1 3

<~ Ambient P r e s s u r e

[ . H

LLU- 5 " , 2 ~ . . . . _ " E

J 1 < I H t I I I ¢ n ~ l I I ~ " J I I ~ l l l ¢ 1 1 ~ I R I A

100 2 0 0 3 0 0 TEM~ERATUP, E {KI

FIGURE 1 Temperature dependence of RH(T] of ~c-(BEDT- TTF)2Cu(NCS)2 at P = 0.

0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers BV. All rights reserved.

2686 K Murata et aL / Hall effect of the organic superconductors of (TMTSF)J~, (DM£T)2J( and (BEDT-TTF)2X

2-3. (DMET)2Au(CN) 2 The superconductivity is observed around 1 K

between 0.5 - 6.5 kbar. A spin density wave(SDW) appears below around 25 K. At high temperature, the carriers are holes as expected from stoichiome- try. The interesting phenomena is that RH(T) starts to vary already from 50 - 60 K, which is much higher than the real TSD W . The variation of RH(T) at such a high temperature is associated with SDW fluctua- tion, because the pressure region where such large variation in RH(T) is observed coincides with the pressure region for the real SDW. 4

2-4. (DMET)213 From the temperature dependence of resistivity,

this material shows a simple metallic and supercon- ducting behavior, like 13-(BEDT-TTF)213 but more one-dimension. The variation in RH(T) is significant as shown in Fig. 2. 5

2-5. (TMTSF)2PF 6 We paid attention to the pressure region where

neither SDW nor Fl(field induced)SDW are ob- served in order to compare with (DMET)213, What we found is a striking variation of RH('D, change in sign as shown in Fig. 3. A single carrier model, which is naturally expected in the normal metallic state, meets serious difficulties in explaining the polarity change in RH(T).

3. SUMMARY All the organic conductor we examined show a

strong temperature dependence at low temperature. The polarity change in (TMTSF)2PF 6 is not under- stood. These results suggest a new electronic phase or fluctuation at low temperature. The way of variation of RH(~ with pressure implies a relation between this low temperature electronic state, as suggested by Hall effect, and superconductivity.

1. K. Murata et al., J. Phys. Soc. Jpn. 58 (1989) 3469.

2. K. Murata etai., Synth. Met. 41-43 (1990)2163. 3. K. Murata et aL, Solid State Commun. 76 (1990)

377. 4. K. Murata et at., Springer Proc. in Phys., 51

(1990) 234. 5. M. Ishibashi et al., Synth. Met. 41-43 (1990)

2167.

4,-I

C

. 0 L

W U Z

W E

J J < T

;. ,..~,, :: :.i:.:.:'.i'- :.G L,C.~.~,~.,,3!..:..:. .- .:: .. ~: :! ~ l:" : :" " ' " : " ': ~'"~::""'h::'c;v::::it~,-'::":':;:':!'.i..:';-: '

..:.;

(DMET) 213 #13 P = 2 . 0 kbar LT

100 200 TEHPERATURE (K)

FIGURE 2 Temperature dependence of RH(T) of (DMET)213 at P = 2 kbar.

~ 2 . 0 . . . . l ' ' ' ' I . 2 o . , .

• ~ | 1 51" I E x p e r l m g n l

/ " '- ' , l \ ,oF . . . ol y_ _t , /I

J ' • . . : . . i ) : ~ .. '. : . . : O S 10 1S 2 0

b ..i. ! ' " . . . ' . . L . , , , . : , . ,:; ": , .

E l '. . ' . . . - . , ~ " ..

~:. : J ~.

'*- ~JF:,' ; .: , W / I 1

• 0 5 0 1 0 0 1 5 0 2 0 0 TEMPERATURE (K}

FIGURE 3 Temperature dependence of RI-i(~ of (TMTSF)2PF 8 at P= 12 kbar. The line is the guide to the eye. The inset shows the pressure of this experiment in the schematic temperature-pressure phase diagram