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An anomalous magnetoresistance in α-(BEDT-TTF)2I3 under high pressure: a possible new phase
PHYSICA
Physica B 184 (1993) 494-497 North-Holland
An anomalous magnetoresistance in et-(BEDT-TTF)2I 3 under high pressure: a possible new phase T. O j i r o ~, K. K a j i t a a, Y. Nishio a, H . K o b a y a s h i b, A . K o b a y a s h i c, R . K a t o d a n d Y. Iye d ~Department of Physics, Toho University, Chiba, Japan bDepartment of Chemistry, Toho University, Chiba, Japan CDepartment of Chemistry, University of Tokyo, Japan alnstitute for Solid State Physics, University of Tokyo, Japan Transport properties of the organic conductor a-(BEDT-TTF)2I 3 are investigated at low temperatures below 60 K in magnetic fields up to 7 T. The metal-nonmetal (M-I) transition which occurs at 135 K under the ambient pressure is suppressed by applying high pressure of about 15 kbar. We have found that the M-I transition is suppressed by the pressure but revives with the aid of the magnetic field. The phase diagram of the M-I transition induced by the magnetic field is discussed.
1. Introduction The organic conductor, o~-type crystal of ( B E D T - T F F ) 2 I 3 (where B E D T - T F F means bis(ethyle~edithiolo)tetrathiafulvalene) is known to behave as a two-dimensional metal at room temperatures but is an insulator at low temperatures [1,2]. The transition takes place at 135 K, below which the resistivity rises by about five orders of m a g n i t u d e . Transport experiments [1,2], X-ray analysis [3,4], ESR measurements [5], DC-susceptibility measurements [6], optical investigation [7] and specific-heat measurements [8] have been done to clarify the nature of this transition. Nogami et al. [4] show that the transition is accompanied by a modification of the molecule arrangement. However, the origin of this change of the crystal structure and thus the true cause of the transition is not yet understood. In our laboratory, a series of experiments are in progress to make the mechanism of this transition clear. Both high-frequency (24 GHz) transport and D C transport are being studied. In the course of the DC-conductivity experiments, we have found that the M - I transition can be fully Correspondence to: K. Kajita, Department of Physics, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274, Japan.
suppressed by applying a high pressure of about 15 kbar. Moreover, for metallic samples under high pressures, we found that the magnetic field strongly affects the electrical conductivity below 60 K [9]. The temperature dependence of the resistivity suggests that the M - I transition, once suppressed by the pressure, revives again in the magnetic field. In this paper, we discuss the magnetotransport p h e n o m e n a of oL-(BEDT-TFF)2I 3 in the metallic state and show the phase diagram of the M - I transition caused by the magnetic field.
2. Experiments 2.1. Crystal structure and the fermiology This crystal has a layered structure, in which the sheets of B E D T - T F F molecules and the sheets of 13 molecules pile up alternately. Since the sheets of 13 are insulating, the electrical current flows only in the plane of the B E D T T F F sheet (hereafter, we call this plane as a conductive plane) and the conduction in the direction normal to the sheet is very low. At room temperatures, a - ( B E D T - T F F ) z I 3 is considered to be a two-dimensional metal.
0921-4526/93/$06.00 © 1993- Elsevier Science Publishers B.V. All rights reserved
T. Ojiro et al. / Anomalous magnetoresistance in a-( BEDT-TFF)213 b*
I Y
F
V
X
a*
Fig. 1. Fermi surface calculated by the tight-binding approximation [10]. Figure 1 is the Fermi surface of this crystal calculated by A. Kobayashi et al. [10] based on the tight-binding approximation. The Fermi surface consists of small pockets on the Brillouin zone boundary. These pockets with two-dimensional nature are created from the one-dimensional Fermi surface which crosses the Brillouin zone boundary. It implies that though this system looks two-dimensional, it is essentially onedimensional. Thus, this system may be unstable due to the Pierles instability typical for the onedimensional system. 2.2. Temperature dependence o f the resistivity A t the ambient pressure, a - ( B E D T - T F F ) 2 I 3 undergoes the metal-nonmetal transition at 135 K. This phase transition is very sensitive to the pressure (fig. 2). The pressure affects the
495
transition in two ways. First, it lowers the critical temperature, and second, it reduces the resistivity jump at the transition. Under high pressures, thus, the system is no more an insulator even at low temperatures but has a residual conductivity. The higher the pressure, the higher the residual conductivity. Finally, under the pressure of about 15 kbar, the M - I transition is fully suppressed. This metal, however, seems to be a bad metal with a high residual resistivity. The resistivity drop from room temperature to liquidhelium temperature is only by a factor 2.
2.3. Magnetoresistance at liquid-helium temperatures Figure 3 shows the magnetic field dependence of the resistivity of the sample under a pressure of about 14.7kbar. Under this pressure, the sample is metallic over the whole temperature region. The magnetic field is applied along the direction normal to the conducting plane and the other one in the plane. The magnetic field in the conducting plane gives a minor effect on the resistivity, while the field normal to the plane gives a very strong effect. A field of 0.2 T causes the rise of the resistivity by a factor of about 2. A n o t h e r feature of this p h e n o m e n o n to be noticed is the resistivity saturation. The resistivity first increases linearly with the magnetic field and then saturates.
Z~
A
,x
~
Z~
ix
Z~
ZX
.oolo,
A
1
c-
2/
r-I:H±ab-plane A:H//ab-plane 14.7kbar
O
J
.| l ZO.k.
A A &
-
o . .i
10o
1 ..... i
101 10 2 Temperature(K)
Fig. 2. Temperature dependence of the resistivity under several pressures [9].
o
~
: .
m .
.
a .
.
.
a .
.
o .
a
a
a
a
a
I
,
,
1
Magneticfield(T) Fig. 3. Magnetic field dependence of the resistance.
T. Ofiro et al. / Anomalous magnetoresistance in a-(BEDT-TFF)213
496 .
.
.
.
.
.
.
.
i
.
.
.
.
.
.
.
.
i
..,.~..\ 6T -~::./.--~... ~. . \ 3T ~....___..~ 2T ~..... .....".~-,\.~\,..,. 1T '.~',.. '-
t'=
"~¢'~~t~10(~ "° 0.4707. ""'"''...,......... ""\ """~ n-
~ 10 1-10 °
.
.
.
.
.
.
.
.
20kbar I ¢ ab-plane I
.
,
t
i i i i i i
101
idual resistivity increases with increasing field. These two states both apply to the resistivity in zero magnetic field, if we replace the word 'increasing magnetic field' by the word 'decreasing pressure'. Based on these observations, we interpret the kink of the resistivity in the curves in fig. 4 at low temperatures as the revival of the m e t a l - n o n m e t a l transition. In fig. 5 we have plotted the magnetic field dependence of the critical temperature of the field-induced M - I transition.
102
Temperature (K)
Fig. 4. Temperature dependence of the resistance in the magnetic field which is applied perpendicular to the conducting plane.
2.4. Temperature dependence of the resistivity in the magnetic field The most important aspect of this phenomenon is shown in the temperature dependence of the resistivity in the magnetic field. The curves in the magnetic field (fig. 4) look similar to those in zero magnetic field but under lower pressures (fig. 2). For example, compare the curve in the magnetic field H = 6 T in fig. 4 and the curve for P = 12 kbar in fig. 2. Two points should be mentioned about the curves in the magnetic fields. First, the critical temperature below which the resistivity rises increases with the magnetic field. Second, the res6C
g
== 4C
¢D
20kbar
O.
E
EO
2C 0
'
~
"'
~
'
Magnetic field (T)
Fig. 5. The critical temperature of the metal-nonmetal transition induced by the magnetic field.
3. Discussion
T o understand the M - I transition induced by the magnetic field, we proceed as follows. First, we assume that the M - I transition in this material is caused by the one-dimensional nature of the electron system. As we have mentioned before, the two-dimensional Fermi surface of this system is brought about by the crossing of the 'onedimensional' Fermi surface with the Brillouin zone boundary. Standing on this assumption, the suppression of the M - I transition under pressure should be interpreted that the pressure makes the twodimensionality of the sample stronger. This is probable because such a crossover from one dimension to two dimensions under high pressures is often observed in organic conductors. Finally, above a critical pressure of about 15 kbar, the system becomes metallic down to liquid-helium temperatures. Under the pressure around this critical value, the system is at a critical state in which the instability typical for the one-dimensional system is just suppressed. In our experiment, the magnetic field is applied on such an electron system. As has been discussed in connection with the magnetic field induced SDW transition of (TMTSF)2PF 6 [11,12], the magnetic field makes the one-dimensional nature of the electron system stronger. So in the magnetic field the m e t a l nonmetal transition could appear as we have observed. This is our tentative interpretation. Two problems remain to be answered. First, the scattering lifetime of carriers in this sample
T. Ojiro et al. / Anomalous magnetoresistance in ct-(BEDT-TFF)213
s e e m s n o t to be high e n o u g h for the magnetic field of the c o n v e n t i o n a l strength to give such a strong effect as we have observed. F o r the Lorentz effect to affect the system, 'to~-' should be larger t h a n one. Figure 3 indicates that in the p r e s e n t system, this is achieved at a very low field such as 1 T. This can never be consistent with the picture that the present system is a bad metal with a high residual resistivity (fig. 2). T h e s e c o n d p r o b l e m is the resistance saturation s h o w n in fig. 3. This saturation and the c o r r e s p o n d i n g saturation of the critical t e m p e r a ture s h o w n in fig. 5 seem to imply that the o n e - d i m e n s i o n a l nature of the system is not m o n o t o n i c a l l y increased by the magnetic field, but it saturates at very low fields such as 1 T, We c a n n o t u n d e r s t a n d w h y this is so.
References [1] K. Bender, I. Hennig, D. Schweitzer, K. Dietz, H.
Endres and H.J. Keller, Mol. Cryst. Liq. Cryst. 108 (1984) 359.
497
[2] H. Schwenk, F. Gross, C.P. Heidmann, K. Andres, D. Schweitzer and H. Keller, Mol. Cryst. Liq. Cryst. 119 (1985) 329. [3] T.J. Emge, P.C.W. Leung, M.A. Beno, H.H. Wang, J.M. Williams, M.H. Whango and M. Evain, Mol. Cryst. Liq. Cryst. 138 (1986) 393. [4] Y. Nogami, S. Kagoshima, T. Sugano and G. Saito, Synth. Met. 16 (1986) 367. [5] E.L. Venturini, L.J. Azevedo, J.E. Schirber, J.M. Williams and H.H. Wang, Phys. Rev. B 32 (1985) 2819. [6] B. Rothaemel, L. Forro, J.R. Cooper, J.S. Schilling, M. Weger, P. Bele, H. Brunner, D. Schweitzer and H.J. Keller, Phys. Rev. B 34 (1986) 704. [7] K. Yakushi, H. Kanbara, H. Tajima, H. Kuroda, G. Saito and T. Moil, Bull. Chem. Soc. Jpn. 60 (1987) 4251. [8] N.A. Fortune, K. Murata, M, Ishibashi, M. Tokumoto, N. Kinoshita and H. Anzai, Solid State Commun. 77 (1991) 265. [9] K. Kajita, T. Ojiro, H. Fujii, Y. Nishio, H. Kobayashi, A. Kobayashi and R. Kato, J, Phys. Soc. Jpn. 61 (1992) 23. [10] A. Kobayashi et al., to be published. [11] L.P. Gor'kov and A.G. Lebed', J. Phys. Lett. (Paris) 45 (1984) L-433. [12] K. Yamaji, Mol. Cryst. Liq. Cryst. 119 (1985) 105.
Physica B 184 (1993) 494-497 North-Holland
An anomalous magnetoresistance in et-(BEDT-TTF)2I 3 under high pressure: a possible new phase T. O j i r o ~, K. K a j i t a a, Y. Nishio a, H . K o b a y a s h i b, A . K o b a y a s h i c, R . K a t o d a n d Y. Iye d ~Department of Physics, Toho University, Chiba, Japan bDepartment of Chemistry, Toho University, Chiba, Japan CDepartment of Chemistry, University of Tokyo, Japan alnstitute for Solid State Physics, University of Tokyo, Japan Transport properties of the organic conductor a-(BEDT-TTF)2I 3 are investigated at low temperatures below 60 K in magnetic fields up to 7 T. The metal-nonmetal (M-I) transition which occurs at 135 K under the ambient pressure is suppressed by applying high pressure of about 15 kbar. We have found that the M-I transition is suppressed by the pressure but revives with the aid of the magnetic field. The phase diagram of the M-I transition induced by the magnetic field is discussed.
1. Introduction The organic conductor, o~-type crystal of ( B E D T - T F F ) 2 I 3 (where B E D T - T F F means bis(ethyle~edithiolo)tetrathiafulvalene) is known to behave as a two-dimensional metal at room temperatures but is an insulator at low temperatures [1,2]. The transition takes place at 135 K, below which the resistivity rises by about five orders of m a g n i t u d e . Transport experiments [1,2], X-ray analysis [3,4], ESR measurements [5], DC-susceptibility measurements [6], optical investigation [7] and specific-heat measurements [8] have been done to clarify the nature of this transition. Nogami et al. [4] show that the transition is accompanied by a modification of the molecule arrangement. However, the origin of this change of the crystal structure and thus the true cause of the transition is not yet understood. In our laboratory, a series of experiments are in progress to make the mechanism of this transition clear. Both high-frequency (24 GHz) transport and D C transport are being studied. In the course of the DC-conductivity experiments, we have found that the M - I transition can be fully Correspondence to: K. Kajita, Department of Physics, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274, Japan.
suppressed by applying a high pressure of about 15 kbar. Moreover, for metallic samples under high pressures, we found that the magnetic field strongly affects the electrical conductivity below 60 K [9]. The temperature dependence of the resistivity suggests that the M - I transition, once suppressed by the pressure, revives again in the magnetic field. In this paper, we discuss the magnetotransport p h e n o m e n a of oL-(BEDT-TFF)2I 3 in the metallic state and show the phase diagram of the M - I transition caused by the magnetic field.
2. Experiments 2.1. Crystal structure and the fermiology This crystal has a layered structure, in which the sheets of B E D T - T F F molecules and the sheets of 13 molecules pile up alternately. Since the sheets of 13 are insulating, the electrical current flows only in the plane of the B E D T T F F sheet (hereafter, we call this plane as a conductive plane) and the conduction in the direction normal to the sheet is very low. At room temperatures, a - ( B E D T - T F F ) z I 3 is considered to be a two-dimensional metal.
0921-4526/93/$06.00 © 1993- Elsevier Science Publishers B.V. All rights reserved
T. Ojiro et al. / Anomalous magnetoresistance in a-( BEDT-TFF)213 b*
I Y
F
V
X
a*
Fig. 1. Fermi surface calculated by the tight-binding approximation [10]. Figure 1 is the Fermi surface of this crystal calculated by A. Kobayashi et al. [10] based on the tight-binding approximation. The Fermi surface consists of small pockets on the Brillouin zone boundary. These pockets with two-dimensional nature are created from the one-dimensional Fermi surface which crosses the Brillouin zone boundary. It implies that though this system looks two-dimensional, it is essentially onedimensional. Thus, this system may be unstable due to the Pierles instability typical for the onedimensional system. 2.2. Temperature dependence o f the resistivity A t the ambient pressure, a - ( B E D T - T F F ) 2 I 3 undergoes the metal-nonmetal transition at 135 K. This phase transition is very sensitive to the pressure (fig. 2). The pressure affects the
495
transition in two ways. First, it lowers the critical temperature, and second, it reduces the resistivity jump at the transition. Under high pressures, thus, the system is no more an insulator even at low temperatures but has a residual conductivity. The higher the pressure, the higher the residual conductivity. Finally, under the pressure of about 15 kbar, the M - I transition is fully suppressed. This metal, however, seems to be a bad metal with a high residual resistivity. The resistivity drop from room temperature to liquidhelium temperature is only by a factor 2.
2.3. Magnetoresistance at liquid-helium temperatures Figure 3 shows the magnetic field dependence of the resistivity of the sample under a pressure of about 14.7kbar. Under this pressure, the sample is metallic over the whole temperature region. The magnetic field is applied along the direction normal to the conducting plane and the other one in the plane. The magnetic field in the conducting plane gives a minor effect on the resistivity, while the field normal to the plane gives a very strong effect. A field of 0.2 T causes the rise of the resistivity by a factor of about 2. A n o t h e r feature of this p h e n o m e n o n to be noticed is the resistivity saturation. The resistivity first increases linearly with the magnetic field and then saturates.
Z~
A
,x
~
Z~
ix
Z~
ZX
.oolo,
A
1
c-
2/
r-I:H±ab-plane A:H//ab-plane 14.7kbar
O
J
.| l ZO.k.
A A &
-
o . .i
10o
1 ..... i
101 10 2 Temperature(K)
Fig. 2. Temperature dependence of the resistivity under several pressures [9].
o
~
: .
m .
.
a .
.
.
a .
.
o .
a
a
a
a
a
I
,
,
1
Magneticfield(T) Fig. 3. Magnetic field dependence of the resistance.
T. Ofiro et al. / Anomalous magnetoresistance in a-(BEDT-TFF)213
496 .
.
.
.
.
.
.
.
i
.
.
.
.
.
.
.
.
i
..,.~..\ 6T -~::./.--~... ~. . \ 3T ~....___..~ 2T ~..... .....".~-,\.~\,..,. 1T '.~',.. '-
t'=
"~¢'~~t~10(~ "° 0.4707. ""'"''...,......... ""\ """~ n-
~ 10 1-10 °
.
.
.
.
.
.
.
.
20kbar I ¢ ab-plane I
.
,
t
i i i i i i
101
idual resistivity increases with increasing field. These two states both apply to the resistivity in zero magnetic field, if we replace the word 'increasing magnetic field' by the word 'decreasing pressure'. Based on these observations, we interpret the kink of the resistivity in the curves in fig. 4 at low temperatures as the revival of the m e t a l - n o n m e t a l transition. In fig. 5 we have plotted the magnetic field dependence of the critical temperature of the field-induced M - I transition.
102
Temperature (K)
Fig. 4. Temperature dependence of the resistance in the magnetic field which is applied perpendicular to the conducting plane.
2.4. Temperature dependence of the resistivity in the magnetic field The most important aspect of this phenomenon is shown in the temperature dependence of the resistivity in the magnetic field. The curves in the magnetic field (fig. 4) look similar to those in zero magnetic field but under lower pressures (fig. 2). For example, compare the curve in the magnetic field H = 6 T in fig. 4 and the curve for P = 12 kbar in fig. 2. Two points should be mentioned about the curves in the magnetic fields. First, the critical temperature below which the resistivity rises increases with the magnetic field. Second, the res6C
g
== 4C
¢D
20kbar
O.
E
EO
2C 0
'
~
"'
~
'
Magnetic field (T)
Fig. 5. The critical temperature of the metal-nonmetal transition induced by the magnetic field.
3. Discussion
T o understand the M - I transition induced by the magnetic field, we proceed as follows. First, we assume that the M - I transition in this material is caused by the one-dimensional nature of the electron system. As we have mentioned before, the two-dimensional Fermi surface of this system is brought about by the crossing of the 'onedimensional' Fermi surface with the Brillouin zone boundary. Standing on this assumption, the suppression of the M - I transition under pressure should be interpreted that the pressure makes the twodimensionality of the sample stronger. This is probable because such a crossover from one dimension to two dimensions under high pressures is often observed in organic conductors. Finally, above a critical pressure of about 15 kbar, the system becomes metallic down to liquid-helium temperatures. Under the pressure around this critical value, the system is at a critical state in which the instability typical for the one-dimensional system is just suppressed. In our experiment, the magnetic field is applied on such an electron system. As has been discussed in connection with the magnetic field induced SDW transition of (TMTSF)2PF 6 [11,12], the magnetic field makes the one-dimensional nature of the electron system stronger. So in the magnetic field the m e t a l nonmetal transition could appear as we have observed. This is our tentative interpretation. Two problems remain to be answered. First, the scattering lifetime of carriers in this sample
T. Ojiro et al. / Anomalous magnetoresistance in ct-(BEDT-TFF)213
s e e m s n o t to be high e n o u g h for the magnetic field of the c o n v e n t i o n a l strength to give such a strong effect as we have observed. F o r the Lorentz effect to affect the system, 'to~-' should be larger t h a n one. Figure 3 indicates that in the p r e s e n t system, this is achieved at a very low field such as 1 T. This can never be consistent with the picture that the present system is a bad metal with a high residual resistivity (fig. 2). T h e s e c o n d p r o b l e m is the resistance saturation s h o w n in fig. 3. This saturation and the c o r r e s p o n d i n g saturation of the critical t e m p e r a ture s h o w n in fig. 5 seem to imply that the o n e - d i m e n s i o n a l nature of the system is not m o n o t o n i c a l l y increased by the magnetic field, but it saturates at very low fields such as 1 T, We c a n n o t u n d e r s t a n d w h y this is so.
References [1] K. Bender, I. Hennig, D. Schweitzer, K. Dietz, H.
Endres and H.J. Keller, Mol. Cryst. Liq. Cryst. 108 (1984) 359.
497
[2] H. Schwenk, F. Gross, C.P. Heidmann, K. Andres, D. Schweitzer and H. Keller, Mol. Cryst. Liq. Cryst. 119 (1985) 329. [3] T.J. Emge, P.C.W. Leung, M.A. Beno, H.H. Wang, J.M. Williams, M.H. Whango and M. Evain, Mol. Cryst. Liq. Cryst. 138 (1986) 393. [4] Y. Nogami, S. Kagoshima, T. Sugano and G. Saito, Synth. Met. 16 (1986) 367. [5] E.L. Venturini, L.J. Azevedo, J.E. Schirber, J.M. Williams and H.H. Wang, Phys. Rev. B 32 (1985) 2819. [6] B. Rothaemel, L. Forro, J.R. Cooper, J.S. Schilling, M. Weger, P. Bele, H. Brunner, D. Schweitzer and H.J. Keller, Phys. Rev. B 34 (1986) 704. [7] K. Yakushi, H. Kanbara, H. Tajima, H. Kuroda, G. Saito and T. Moil, Bull. Chem. Soc. Jpn. 60 (1987) 4251. [8] N.A. Fortune, K. Murata, M, Ishibashi, M. Tokumoto, N. Kinoshita and H. Anzai, Solid State Commun. 77 (1991) 265. [9] K. Kajita, T. Ojiro, H. Fujii, Y. Nishio, H. Kobayashi, A. Kobayashi and R. Kato, J, Phys. Soc. Jpn. 61 (1992) 23. [10] A. Kobayashi et al., to be published. [11] L.P. Gor'kov and A.G. Lebed', J. Phys. Lett. (Paris) 45 (1984) L-433. [12] K. Yamaji, Mol. Cryst. Liq. Cryst. 119 (1985) 105.