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Transport properties of organic conductor (BEDT-TTF)2KHg(SCN)4: I. Resistance and magnetoresistance anomaly
Solid State Ccnxnunications, Vol. 75, No. 2, pp. 93-96, 1990. Printed in Great Britain.
TRANSPORT
0038-1098/90$3.00+.00 FergamonPress plc
PROPERTIES
OF ORGANIC CONDUCTOR
I. RESISTANCE
AND MAGNETORESISTANCE
(BEDT-‘I”I’F)2KHg(SCNl4: ANOMALY
T. Sasaki and N. Toyota Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980, Japan M. Tokumoto, N. Kinoshita and H. Anzait Electrotechnical Laboratory, Umezono l-l-4, Tsukuba, Ibaraki 305, Japan (Received 27 April 1990 by T. Tsuzuki)
The electrical resistance of the single crystalline organic salt (BEDT‘ITF)zKHg(SCN)4 has been measured at temperatures down to 0.5 K and under magnetic fields up to 13 T. The zero-field resistance decreases almost monotonously with decreasing temperature down to 9-10 K and then exhibits a ste -like anomal accompanying the 30-40 96 resistance drop around below Ha = 8 K. Un Ber fields perpendicular to the conducting a-c plane, the resistance below T, is so enhanced that an upturn in the temperature-dependent resistance appears and the resistance, for example, at 13 T and 0.5 K, becomes higher by a factor of 12 than the resistance at T, and H = 13 T. This anomalous magnetoresistance is never observed under fields recisely aligned parallel to the a-c lane. The ows a broad maximum at T,, whlc *R seems to at weak anti-ferromagnetic correlations like spindensity-wave ordering takes place among p(n) electrons itinerating in the metallic states.
$1. INTRODUCTION
The electrical resistance”, the electron spin resonance (ESR16and Shubnikov-de Haas (SdH) effects have been so far studied. The present paper (referred as [II) concerns the low temperature anomalies observed in both zero-field and highfield resistance and in the magnetic susceptibility. The subse uent paper (referred as [II]‘) will re ort on the Sd?I effect and the first observation oP the of SdH oscillations of (BEDTspin-splittin ?TF)zKHg(S EN)4.
Char e-transferred organic conductors (BEDT-T&X h ave attracted much attention because of their interesting physical properties. The superconducting transition temperature T, has been raised to lo-11 K in K-(BEDT‘ITF)$u(SCN12 with a single anion sheet consistof -SCN-Cu-NCS-.’ ing of ordered Recently, Oshima et nthesized a new conducwhich is one of the tor (BEDT-‘ITF)&II modifications of BE T-‘M’F salts with a pseudohalide metal anion of potassium- and mercurvthiocyanate. This material takes a layered structure: BEDT-‘ITF molecules are stacked in the a-c plane of the triclinic lattice in a similar way to a-3 or &‘phase, and each c nductin sheet is sandconsistine wiched bv the thick (6.8 9r) anion *f avers ., of tri 16 sheets with the s uare yramidal K(SC&4 and tetrahedral Hg(S~Nl4. Tie chemical valence of the anion is exoected to be -1 ([KHg(SCN)41-‘1 and this leads’to (BEDT-‘M’F1+0.5 as the other stoichiometric (BEDT-‘ITF)zX salts. Taking into account that the triclinic unit cell contains iwo chemical formulae, then two holes are shared on the average by the four BEDT-‘ITFs in a unit cell.
$11.EXPERIMENTAL The single of (BEDTcrystals ‘lTF)2KHg(SCN)4 were grown b the usual electrochemical oxidation method.2 T Ee typical sha e of the as-grown crystals is thick rectangular WI*tl! the size of 1.2 X 0.8 X 0.4 mm3. The longest direction is parallel to the c-axis. We define the direction normal to the crystal plane as the be-axis. Gold films were evaporated onto the crystal and gold wires (25 pm@) were attached with the gold paint. Resistance measurements have been carried out by the four terminal method using dc currents (lo-200 pA) along the c-axis in the conducting plane. The magnetic fields are generated bv the 13 T sunerconducting magnet. “The tern e;ature of the s&nle is determined and contra Pled in the adiabatic ! He cryostat by the germanium resistor in zero field and by the each-run calibrated capacitor in the fields. The dc magnetic susceptibility measure-
tPresent Address: Himeji Institute of Technology, Himeji 671-22, Japan 93
94
vol.
ORGANIC CONDUCTOR (&EDT-TTF)2K&(SCN)4
ments are done by use of the commercial SQUID from Quantum Design Co. Ltd.
15,
yo.
a 1 ’ 1 u , 0 ’ - ’ , ’ 6 r * I - r . m
150 -
(BEDT-TTF)2KHg(SCNt
#l-l0
5111.RESULTS Figure 1 shows the logarithmic plots of the resistance R(H=O) versus temperature for samples #l-10 and #l-11. With decreasing tern erature, the resistance of #l-11 monotonous12 Becreases with the power law dependence of T’. down to 30 K, while that of #l-10 exhibits a shoulder-like anomal around 130 K as already has been reported2, anB follows the Ti.O-dependence between 130 and 20 K. At low temperatures around 10 K, the resistance of both samples show the level-off and then the step-like anomaly accompanying the 3040 % resistance drop at temperatures between 8 and 3 K. Below 3 K, the resistance is almost independent on temperature. We tentatively define the onset temperature 8-9 K of the resistance drop as Ta. Figure 2(a) and (b) show the temperature dependence of the resistance R(T, Ho) at constant magnetic fields applied perpendicular to the a-c plane, for #l-10 and #l-11, respectively. The insets show the lo arithmic lots. With decreasin 4 fields (2 2-3 T), R( 8 temperature under the hlg Ho) is so enhanced that an upturn of R(T, Ho) apl ears below T,. As shown in(b), for example, R(0.5 E ,13 T) reaches by a factor of 12 higher value than R(T,, 13 T). Above T,, however, magnetic fields induce only small increase of R(T, Ho). In short, when the sample is cooled through Ta, a drastic change of R(T, Ho) appears below T,, at which temperature R(T, H=O) starts to drop. It should be noted that the magnetic fields precisely aligned parallel to the a-c plane do not induce any magnetoresistance effect within the ex erimental accuracy, as will be described in [III. T!o show more clearly the resistance enhancement below T,, the than es of the resistance with fields, AR = R(T, Ho) -R(f’, Ho=O) are plotted as a function of temperature in Fi ’ 3(a) and 3(b) for #l-10 and #l-11, respectively. 1 s shown by solid lines in the fi ure, the data except at low temperatures and high krelds well show the linear dependence of AR =A( T - T,), where A is some function of H, and T, is determined to be 8.5 K and 8.0 K for #l-10 and #l-11,
1
Temperature
(K 1
(K )
Fig. 2 Temperature dependence of the resistance under the constant magnetic fields ap lied perpendicular to the conducting a-c p Pane; (a) #l-10 and(b) #l-11. The insets show the logarithmic plots.
respectively. Therefore, as mentioned above, T, is the common onset temperature for both the zero-
I
EDT-TTF+KHqtSCtQ
Temperature
field resistance and magnetoresistance anomalies
1
100
Te’:peroture (K
300
)
Fig.1 Logarithmic plots of the resistance normalized at 273 K as a function of temperature for samples #l-10 and #l-11.
to appear. Fi ure 4(a) and 4(b) show the field dependence of the isothermal -magnetoresistance AR(To, HYR(To, H =O) measured by sweeping fields perpendicular to the u-c plane of #l-11. Figure 4(c) shows the logarithmic-plots, where the arr’;ws indicate the fields at which the single power-law dependence starts to break down. Above 10 K the magnetoresistances change with fields as H13-l.l over the entire range of the fields, while , below Ta, the magnetoresistance tends to saturate and then takes a broad maximum at high fields. In Fig. 4(b), the SdH oscillations are observed below 2 K which will be the sub’ect of the subsequent paper [II]. Finally ai 1 the data shown here are reversible with respect to up and down of temperatures or magnetic fields.
2
vol,
75,
No.
2
ORGANIC
CONDUCTOR
(BEDT-TTF)~Iw~CXN)~
95
g,O
&f a;20 0
-0
10 Magnkc
15
Field (T)
--‘*-
0 I
0
I
I
I
1 Tem;erature (K
Fig. 3 Temperature dependence of the resistance change under the fields, AR=R(H)-R(O); (a) #l-10 and(b) #l-11.
#IV. DISCUSSIONS First of all, we discuss the resistance drop from the view of the possibility of some contaminations from the su erconducting materials included in the crystals. $ here has been a well known superconducting mercury chain compound, Hgs_ recipitated mercury with sAsFa, due to T,=4.152K.s-I0 4th this in mind, Schirber et al” observed the pressure dependence of T, (4.0 K at ambient pressure) of (BEDT-‘ITF)4Hg2,sgBrs and found that the T, increases with ressure and then takes a maximum (6.7K) aroun a 4 kbar. This result is quite different from the case of above mercury chain corn ound’O and therefore the superconductivity in (BE DT-‘!XF)4Hg2,sgBrs was ascribed to be intrinsic. In the present salt, the resistance drop starts at Ta = 8K, much higher than Tc of metalhc mercury. Furthermore, as clearly shown in
Fig. 4 Isothermal magnetoresistance as a function of the fields applied perpendicular to the a-c lane of #l-11; (a) 0.6-6 K, (b) 6-15 K and (c) Pogarithmic plots of these data. The arrows in (c) indicate the fields at which the single power law dependence starts to break down. Fig. 2(a), the resistance drop is still observed at lT, much hi her than Hc ( =O.O412T, superconducting critical Bleld) of a metallic mercury and T, depends little on H. These facts exclude the presence of the precipitated mercury in the present case,
96
ORGANIC CONDUCTOR
Next we have to check whether or not the resistance drop is induced by some ‘inhomogeneous’ superconducting phase (Tc = 7-8K) as often seen in PI,-(BEDT-TTFl& at ambient pressure.‘2 Our measurements of dc magnetic susceptibility X/l between 240K under fields applied parallel to the u-c plane reveal that Xl/ showsa broad maximum at 8 9K iust eaual to T,. and this temoerature and XII depend little on H Gf l-5T. Therefore we cannot re: gard the resistance dro and, of course, the associated enhancement of tKe magnetoresistance as a superconductin transition. There has %een extensively studied the structural than e in a quasi-one-dimensional (TMTSF)&! 4.13J4 This material, when slowly cooled, undergoes a phase transition at 25K due to the ordering of the noncentrosymmetric Cl04 anions. However, the rapidly cooled sample shows the metal-insulator transition accompanying spindensity-wave ordering.16 For the present salt, we have examined the cocling speed d
(BEDT-TTF)~~(scN)~
Vol. 75, No.
found from the ESR experiments that the g-factor and the real part of the spin susceptibility starts to decrease from_ 10K. These results might support above suggestions. Obertelli et all7 have revealed that a’-(BEDT‘ITFlzX (X =AuBrz,CuCl7. and Ag(CN)2) are MottHubbard insulators due to the narrow bandwidth, resulting in antiferroma etic short range order with S = l/2 per BEDT- & dimer. It is noted that our salt is still metallic down to 0.5K but some antiferromagnetic correlations seem to coexist. It is likely that the itinerant o(n)-electrons in our svstern might have a small l&d spin even in a me&llit state. Therefore the present system (BEDT‘ITFIxKHg(SCN14 might be one of the key comounds to study from the view of extended Hubg ard model how the on-site and inter-site Coulomb repulsions play roles in two-dimensional metallic states in a variety of BEDT-TTF salts.ls,lo Theoretical studies for the nearly quater-filling case in the two-dimensional extended Hubbard model will be highly encouraged. Through these studies, the reason will come out why the present system does not show such as superconductivit or metalinsulator transition as other related cEarge transferred BEDT-!ITF salts. Acknowled ements- The authors appreciate Dr. K. Murata (E# L) for helpful discussions and showing them his data of pressure ex advice in SQUID ex-
from the Ministry of Education, ture of Japan. Profs. Y. Muto and T. Fukase (IMR) are acknowledged for their encouragements.
REFERENCES H. Ura ama, H. Yamochi, G. Saito, K. Nozawa, T. Bugano, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto and J. Tanaka, Chem. Lett. 55 (1988). 2. M. Oshima, H. Mori, G. Saito and K. Oshima, Chem. Lett. 1159 (1989). 3. K. Bender, I. Henning, D. Schweitzer, K. Dietz, H. Endres and H. J. Keller, Mol. Cryst. Li . Cryst. 108,359 (1984). 4. H.& o bayashi, R. Kato, A. Kobayashi, Y. Nishio, K. Kajita and W. Sasaki, Chem. Lett. 833 (1986). 5. M. Oshima, H. Mori, G. Saito and K. Oshima, The Physics and Chemistry of Or anic Superconductors, Springer-Verlag, a eiderberg, (19901. 6. T. Osada, R. Yagi, A. Kawasumi, S. Kagoshima, N. Miura, M. Oshima and G. Saito, Phys. Rev. B41,5428 (1990). 7. T. Sasaki, N. Toyota, M. Tokumotq, N. Kinoshita and H. Anzai, subsequent article in this volume of Solid State Commun. 8. R. J. Gillespie, D. Brown, W. R. Datars, K. R. Morgan, Z. Tun and P. K. Ummat, Phil. Trans. R. Sot. Lond. A 314 (1985) 105 9. W. R. Datars, F. S. Razavi, R. J. Gillespie and P. K. Ummat, Phil. Trans. R. Sot. Lond. A314, 115 (1985). 10. J. E. Schirber, A. J. Heeger and P. J. Nigery, Phys. Rev. B26,6291(1982). 1.
11. J. E. Schirber, D. L. Overmyer, E. L. Venturini, H. H. Wan K. D. Carlson, W. K. Kwok, S. Kleinjan and f: M. Williams, Physica C 161, 412 (1989). 12. H. Bando. M. Tokumoto. K. Murata. H. Anzai. G. Saito, K. Kajimura and T. Ishiguro, J. Phys: Sot. Jnn. 54.4265 (1985). 13. S. To&c, D: Jerome, P. Monod and K. Bechgaard, J. Phys. (Paris) Lett. 43, L839 (1982). 14. L. Ferro, K. Biljakovic, J. R. Cooper and K. Bechgaard, Ph s. Rev. B 29,2839 (1984). 1.5. T. Takahashi, 5 . Jerome and K. Bechgaard, J. Phys. (Paris) 43, L565 (1982). 16. N. Kinoshita, M. Tokumoto and H. Anzai, private communications and presentations at ;gi;g meeting of Japanese Physical Society, 17. S. D. Obertelli, R. H. Friend, D. R. Talham, M. Kurmoo and P. Day, Synthetic Metals 27, A375 (1988). 18. N. Toyota,. E.W. Fenton, T. Sasaki and M. Tachiki, Solid State Commun. 72, 859 (1989) and The Physics and Chemistry of Organic Superconductors, Springer-Verlag, Heiderberg, 119901. ~____,. 19. N. Toyota and T. Sasaki, Solid State Commun. 74,361(1990).
TRANSPORT
0038-1098/90$3.00+.00 FergamonPress plc
PROPERTIES
OF ORGANIC CONDUCTOR
I. RESISTANCE
AND MAGNETORESISTANCE
(BEDT-‘I”I’F)2KHg(SCNl4: ANOMALY
T. Sasaki and N. Toyota Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980, Japan M. Tokumoto, N. Kinoshita and H. Anzait Electrotechnical Laboratory, Umezono l-l-4, Tsukuba, Ibaraki 305, Japan (Received 27 April 1990 by T. Tsuzuki)
The electrical resistance of the single crystalline organic salt (BEDT‘ITF)zKHg(SCN)4 has been measured at temperatures down to 0.5 K and under magnetic fields up to 13 T. The zero-field resistance decreases almost monotonously with decreasing temperature down to 9-10 K and then exhibits a ste -like anomal accompanying the 30-40 96 resistance drop around below Ha = 8 K. Un Ber fields perpendicular to the conducting a-c plane, the resistance below T, is so enhanced that an upturn in the temperature-dependent resistance appears and the resistance, for example, at 13 T and 0.5 K, becomes higher by a factor of 12 than the resistance at T, and H = 13 T. This anomalous magnetoresistance is never observed under fields recisely aligned parallel to the a-c lane. The ows a broad maximum at T,, whlc *R seems to at weak anti-ferromagnetic correlations like spindensity-wave ordering takes place among p(n) electrons itinerating in the metallic states.
$1. INTRODUCTION
The electrical resistance”, the electron spin resonance (ESR16and Shubnikov-de Haas (SdH) effects have been so far studied. The present paper (referred as [II) concerns the low temperature anomalies observed in both zero-field and highfield resistance and in the magnetic susceptibility. The subse uent paper (referred as [II]‘) will re ort on the Sd?I effect and the first observation oP the of SdH oscillations of (BEDTspin-splittin ?TF)zKHg(S EN)4.
Char e-transferred organic conductors (BEDT-T&X h ave attracted much attention because of their interesting physical properties. The superconducting transition temperature T, has been raised to lo-11 K in K-(BEDT‘ITF)$u(SCN12 with a single anion sheet consistof -SCN-Cu-NCS-.’ ing of ordered Recently, Oshima et nthesized a new conducwhich is one of the tor (BEDT-‘ITF)&II modifications of BE T-‘M’F salts with a pseudohalide metal anion of potassium- and mercurvthiocyanate. This material takes a layered structure: BEDT-‘ITF molecules are stacked in the a-c plane of the triclinic lattice in a similar way to a-3 or &‘phase, and each c nductin sheet is sandconsistine wiched bv the thick (6.8 9r) anion *f avers ., of tri 16 sheets with the s uare yramidal K(SC&4 and tetrahedral Hg(S~Nl4. Tie chemical valence of the anion is exoected to be -1 ([KHg(SCN)41-‘1 and this leads’to (BEDT-‘M’F1+0.5 as the other stoichiometric (BEDT-‘ITF)zX salts. Taking into account that the triclinic unit cell contains iwo chemical formulae, then two holes are shared on the average by the four BEDT-‘ITFs in a unit cell.
$11.EXPERIMENTAL The single of (BEDTcrystals ‘lTF)2KHg(SCN)4 were grown b the usual electrochemical oxidation method.2 T Ee typical sha e of the as-grown crystals is thick rectangular WI*tl! the size of 1.2 X 0.8 X 0.4 mm3. The longest direction is parallel to the c-axis. We define the direction normal to the crystal plane as the be-axis. Gold films were evaporated onto the crystal and gold wires (25 pm@) were attached with the gold paint. Resistance measurements have been carried out by the four terminal method using dc currents (lo-200 pA) along the c-axis in the conducting plane. The magnetic fields are generated bv the 13 T sunerconducting magnet. “The tern e;ature of the s&nle is determined and contra Pled in the adiabatic ! He cryostat by the germanium resistor in zero field and by the each-run calibrated capacitor in the fields. The dc magnetic susceptibility measure-
tPresent Address: Himeji Institute of Technology, Himeji 671-22, Japan 93
94
vol.
ORGANIC CONDUCTOR (&EDT-TTF)2K&(SCN)4
ments are done by use of the commercial SQUID from Quantum Design Co. Ltd.
15,
yo.
a 1 ’ 1 u , 0 ’ - ’ , ’ 6 r * I - r . m
150 -
(BEDT-TTF)2KHg(SCNt
#l-l0
5111.RESULTS Figure 1 shows the logarithmic plots of the resistance R(H=O) versus temperature for samples #l-10 and #l-11. With decreasing tern erature, the resistance of #l-11 monotonous12 Becreases with the power law dependence of T’. down to 30 K, while that of #l-10 exhibits a shoulder-like anomal around 130 K as already has been reported2, anB follows the Ti.O-dependence between 130 and 20 K. At low temperatures around 10 K, the resistance of both samples show the level-off and then the step-like anomaly accompanying the 3040 % resistance drop at temperatures between 8 and 3 K. Below 3 K, the resistance is almost independent on temperature. We tentatively define the onset temperature 8-9 K of the resistance drop as Ta. Figure 2(a) and (b) show the temperature dependence of the resistance R(T, Ho) at constant magnetic fields applied perpendicular to the a-c plane, for #l-10 and #l-11, respectively. The insets show the lo arithmic lots. With decreasin 4 fields (2 2-3 T), R( 8 temperature under the hlg Ho) is so enhanced that an upturn of R(T, Ho) apl ears below T,. As shown in(b), for example, R(0.5 E ,13 T) reaches by a factor of 12 higher value than R(T,, 13 T). Above T,, however, magnetic fields induce only small increase of R(T, Ho). In short, when the sample is cooled through Ta, a drastic change of R(T, Ho) appears below T,, at which temperature R(T, H=O) starts to drop. It should be noted that the magnetic fields precisely aligned parallel to the a-c plane do not induce any magnetoresistance effect within the ex erimental accuracy, as will be described in [III. T!o show more clearly the resistance enhancement below T,, the than es of the resistance with fields, AR = R(T, Ho) -R(f’, Ho=O) are plotted as a function of temperature in Fi ’ 3(a) and 3(b) for #l-10 and #l-11, respectively. 1 s shown by solid lines in the fi ure, the data except at low temperatures and high krelds well show the linear dependence of AR =A( T - T,), where A is some function of H, and T, is determined to be 8.5 K and 8.0 K for #l-10 and #l-11,
1
Temperature
(K 1
(K )
Fig. 2 Temperature dependence of the resistance under the constant magnetic fields ap lied perpendicular to the conducting a-c p Pane; (a) #l-10 and(b) #l-11. The insets show the logarithmic plots.
respectively. Therefore, as mentioned above, T, is the common onset temperature for both the zero-
I
EDT-TTF+KHqtSCtQ
Temperature
field resistance and magnetoresistance anomalies
1
100
Te’:peroture (K
300
)
Fig.1 Logarithmic plots of the resistance normalized at 273 K as a function of temperature for samples #l-10 and #l-11.
to appear. Fi ure 4(a) and 4(b) show the field dependence of the isothermal -magnetoresistance AR(To, HYR(To, H =O) measured by sweeping fields perpendicular to the u-c plane of #l-11. Figure 4(c) shows the logarithmic-plots, where the arr’;ws indicate the fields at which the single power-law dependence starts to break down. Above 10 K the magnetoresistances change with fields as H13-l.l over the entire range of the fields, while , below Ta, the magnetoresistance tends to saturate and then takes a broad maximum at high fields. In Fig. 4(b), the SdH oscillations are observed below 2 K which will be the sub’ect of the subsequent paper [II]. Finally ai 1 the data shown here are reversible with respect to up and down of temperatures or magnetic fields.
2
vol,
75,
No.
2
ORGANIC
CONDUCTOR
(BEDT-TTF)~Iw~CXN)~
95
g,O
&f a;20 0
-0
10 Magnkc
15
Field (T)
--‘*-
0 I
0
I
I
I
1 Tem;erature (K
Fig. 3 Temperature dependence of the resistance change under the fields, AR=R(H)-R(O); (a) #l-10 and(b) #l-11.
#IV. DISCUSSIONS First of all, we discuss the resistance drop from the view of the possibility of some contaminations from the su erconducting materials included in the crystals. $ here has been a well known superconducting mercury chain compound, Hgs_ recipitated mercury with sAsFa, due to T,=4.152K.s-I0 4th this in mind, Schirber et al” observed the pressure dependence of T, (4.0 K at ambient pressure) of (BEDT-‘ITF)4Hg2,sgBrs and found that the T, increases with ressure and then takes a maximum (6.7K) aroun a 4 kbar. This result is quite different from the case of above mercury chain corn ound’O and therefore the superconductivity in (BE DT-‘!XF)4Hg2,sgBrs was ascribed to be intrinsic. In the present salt, the resistance drop starts at Ta = 8K, much higher than Tc of metalhc mercury. Furthermore, as clearly shown in
Fig. 4 Isothermal magnetoresistance as a function of the fields applied perpendicular to the a-c lane of #l-11; (a) 0.6-6 K, (b) 6-15 K and (c) Pogarithmic plots of these data. The arrows in (c) indicate the fields at which the single power law dependence starts to break down. Fig. 2(a), the resistance drop is still observed at lT, much hi her than Hc ( =O.O412T, superconducting critical Bleld) of a metallic mercury and T, depends little on H. These facts exclude the presence of the precipitated mercury in the present case,
96
ORGANIC CONDUCTOR
Next we have to check whether or not the resistance drop is induced by some ‘inhomogeneous’ superconducting phase (Tc = 7-8K) as often seen in PI,-(BEDT-TTFl& at ambient pressure.‘2 Our measurements of dc magnetic susceptibility X/l between 240K under fields applied parallel to the u-c plane reveal that Xl/ showsa broad maximum at 8 9K iust eaual to T,. and this temoerature and XII depend little on H Gf l-5T. Therefore we cannot re: gard the resistance dro and, of course, the associated enhancement of tKe magnetoresistance as a superconductin transition. There has %een extensively studied the structural than e in a quasi-one-dimensional (TMTSF)&! 4.13J4 This material, when slowly cooled, undergoes a phase transition at 25K due to the ordering of the noncentrosymmetric Cl04 anions. However, the rapidly cooled sample shows the metal-insulator transition accompanying spindensity-wave ordering.16 For the present salt, we have examined the cocling speed d
(BEDT-TTF)~~(scN)~
Vol. 75, No.
found from the ESR experiments that the g-factor and the real part of the spin susceptibility starts to decrease from_ 10K. These results might support above suggestions. Obertelli et all7 have revealed that a’-(BEDT‘ITFlzX (X =AuBrz,CuCl7. and Ag(CN)2) are MottHubbard insulators due to the narrow bandwidth, resulting in antiferroma etic short range order with S = l/2 per BEDT- & dimer. It is noted that our salt is still metallic down to 0.5K but some antiferromagnetic correlations seem to coexist. It is likely that the itinerant o(n)-electrons in our svstern might have a small l&d spin even in a me&llit state. Therefore the present system (BEDT‘ITFIxKHg(SCN14 might be one of the key comounds to study from the view of extended Hubg ard model how the on-site and inter-site Coulomb repulsions play roles in two-dimensional metallic states in a variety of BEDT-TTF salts.ls,lo Theoretical studies for the nearly quater-filling case in the two-dimensional extended Hubbard model will be highly encouraged. Through these studies, the reason will come out why the present system does not show such as superconductivit or metalinsulator transition as other related cEarge transferred BEDT-!ITF salts. Acknowled ements- The authors appreciate Dr. K. Murata (E# L) for helpful discussions and showing them his data of pressure ex advice in SQUID ex-
from the Ministry of Education, ture of Japan. Profs. Y. Muto and T. Fukase (IMR) are acknowledged for their encouragements.
REFERENCES H. Ura ama, H. Yamochi, G. Saito, K. Nozawa, T. Bugano, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto and J. Tanaka, Chem. Lett. 55 (1988). 2. M. Oshima, H. Mori, G. Saito and K. Oshima, Chem. Lett. 1159 (1989). 3. K. Bender, I. Henning, D. Schweitzer, K. Dietz, H. Endres and H. J. Keller, Mol. Cryst. Li . Cryst. 108,359 (1984). 4. H.& o bayashi, R. Kato, A. Kobayashi, Y. Nishio, K. Kajita and W. Sasaki, Chem. Lett. 833 (1986). 5. M. Oshima, H. Mori, G. Saito and K. Oshima, The Physics and Chemistry of Or anic Superconductors, Springer-Verlag, a eiderberg, (19901. 6. T. Osada, R. Yagi, A. Kawasumi, S. Kagoshima, N. Miura, M. Oshima and G. Saito, Phys. Rev. B41,5428 (1990). 7. T. Sasaki, N. Toyota, M. Tokumotq, N. Kinoshita and H. Anzai, subsequent article in this volume of Solid State Commun. 8. R. J. Gillespie, D. Brown, W. R. Datars, K. R. Morgan, Z. Tun and P. K. Ummat, Phil. Trans. R. Sot. Lond. A 314 (1985) 105 9. W. R. Datars, F. S. Razavi, R. J. Gillespie and P. K. Ummat, Phil. Trans. R. Sot. Lond. A314, 115 (1985). 10. J. E. Schirber, A. J. Heeger and P. J. Nigery, Phys. Rev. B26,6291(1982). 1.
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