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Superconducting transitions in β-(bedt-TTF)2I3
Synthetic Metals, 11 (1985) 207 - 212
207
SUPERCONDUCTING TRANSITIONS IN ~-(BEDT-TTF)2I 3 L. I. BURAVOV, M. V. KARTSOVNIK, V. F. KAMINSKII, P. A. KONONOVICH, E. E. KOSTUCHENKO, V. N. LAUKHIN, M. K. MAKOVA, S. I. PESOTSKII, I. F. SCHEGOLEV, V. N. TOPNIKOV and E. B. YAGUBSKII
Institute of Chemical Physics, U.S.S.R. Academy of Science, Chernogolovka (U.S.S.R.) (Received February 15, 1985; accepted April 14, 1985)
Abstract The investigation of superconducting transitions in some crystals of ~-(BEDT-TTF)213 points to the existence in the ( B E D T - T T F ) - I system of other superconducting phases, with Tcs o f ~ 4 K and ~ 7 K, respectively.
1. Introduction Triclinic fi-modification o f the ( B E D T - T T F ) - I system, ~-(BEDTTTF)213, has been initially reported [1] to be the normal pressure superc o n d u c t o r with Tc = 1.5 + 0.1 K. The ensuing investigation of many crystals o f this phase has revealed, however, some peculiarities in their low temperature behaviour, which have been interpreted [2] as an indication of the fact that admixtures of other superconducting phases with higher Tc s often exist within the same crystals. Here, we present evidence that Tcs of ~ 4 K and 7 K should be ascribed to these phases and discuss brefly their possible nature.
2. Experimental The ab-plane resistivity o f the fi-phase crystals, of the order of 3 × 10 -2 ~2 cm at room temperature, is rather high in comparison with that of other organic metals, b u t it falls considerably with cooling. In sufficiently perfect crystals the resistivity ratio, R3oo/R4.2, may a m o u n t to 5 - 7 × 10 2. It is a characteristic feature of the behaviour o f many crystals that their resistivity does n o t tend to a constant residual value at low temperatures b u t continues to fall d o w n to the 1.5 K superconducting transition. Moreover, it often happens that the falling rate of the resistivity is even accelerated beginning from some temperature a few Kelvins before the transition point. 0379-6779/85/$3.30
Elsevier Sequoia/Printed in The Netherlands
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Fig. 1. Typical examples of resistivity behaviour in crystals of ~-(BEDT-TTF)2I 3 in the pre-transitional temperature range. Fig. 2. Temperature dependence of resistivity of a crystal with a broad superconducting transition in different magnetic fields H]]c. 1, 0 kOe; 2, 6 kOe; 3, 34 kOe; 4, 50 kOe.
Such an acceleration may be realized in a number of ways. First, the resistivity may begin to fall from ~ 4 K, after some tendency to reach the residual value (Fig. 1, a). The second type of behaviour is a step at 7 - 8 K, more or less pronounced, again followed by the 1.5 K transition (Fig. 1, b). In addition, the 7 K step may be followed by a superconducting transformation beginning at ~ 4 K rather than by the 1.5 K transition (Fig. 1, c). Finally, we may have a nearly continuous broad transition that starts from 7 - 8 K and finishes below 1.3 K (the lowest temperature reached in our experiments). The last t y p e of behaviour is presented in Fig. 2, i n which the influence o f the magnetic field Hlic* on the resistivity is also shown. It is seen that in high fields the resistivity fall is suppressed from ~ 7 K. It should be stressed that the room temperature structure of each of the single crystals with different types of behaviour has been proved by X-ray analysis to be that of the ~-phase. In the case shown in Fig. 2, the superconducting transformation is spread over a wide temperature interval. Nevertheless, the analysis of R(H) curves measured at different temperatures allows us to indicate two temperature points characteristic of this transformation. The corresponding data for HHc* are presented in Fig. 3. It is seen that several regions of the R(H) curves may be separated, each characterized by a different rate of resistivity variation. At temperatures below ~ 4 K there are three such regions: the initial fast growth of the resistivity is first changed by a nearly linear intermediate increase and then slows down considerably. The initial fast-growing portions of the R(H) curves
209
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H,~0, Fig. 3. Field dependences of resistivityfor Hllc at different temperatures for the crystal of Fig. 2. 1, 1.5 K; 2, 3.0 K; 3, 4.2 K; 4, 5.3 K; 5, 6.25 K; 6, 7.3 K; 7, 9.3 K. Fig. 4. H - T diagram for two superconducting transformations in ~-(BEDT-TTF)213 with T c - 4 K and - 7 K. 1, 2, Hllc, crystal of Fig. 2; 3, Hllb, crystal of Fig. 5. o, firstcooling cycle; e, second cooling cycle.
disappear above ~ 4 K, and the intermediate regions above ~ 8 K. Noting that these temperatures are just those characteristic of the above described pre-transitional effects shown in Fig. 1, we may tentatively consider the field values dividing the different regions on the R(H) curves as some critical fields in the c*~tirection for the t w o superconducting transitions. These fields are plotted on the H - T diagram o f Fig. 4. The third set o f points in Fig. 4, for Hilb, is referred to another crystal whose resistivity is shown in Fig. 5 as a function of temperature for different magnetic fields. The 7 K step is quite large here and its shift under the action o f the magnetic field is easily observable. It should be stated that the data of Fig. 5 were obtained during the third cycle o f cooling on the same single crystal whose first passage data were shown in Fig. 1, c. The temperature cycling is seen to sometimes give rise to marked differences in the low temperature resistivity behaviour o f the crystals. All five cycles of cooling performed on the crystal of Fig. 1, c and Fig. 5 are shown in Fig. 6. The first three passages, within the measurement accuracy, did n o t change the absolute value o f the resistivity above ~ 8 K. During the fourth and fifth cooling cycles, small jumps in the resistivity were observed, and the 11 K resistivity has increased, in comparison with its value for the first three cyles, by 8.5 and 8.7 times, respectively. In Fig. 6 these curves have been matched to the first three at 11 K. The crystal broke during
210
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5-
~
T.K
Fig. 5. Shift of 7 K step under the action of magnetic field, e, 0 kOe;
(~, 14 kOe; x,
50 kOe.
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Fig. 6. Changes in R(T) curves caused b y five c o n s e c u t i v e cycles o f cooling.
the sixth cooling. Beginning from liquid nitrogen temperature, the cooling rate was a b o u t 30 K/h. It is seen from Fig. 6 that the temperature cycling gives rise to the disappearance of the ~ 4 K superconducting transition as if the transition temperature o f the corresponding part of the sample is increased. An analogous increase of the transition temperature with cycling has been observed on crystals of ~-(BEDT-TTF)213 [3]. Note, however, that such a behaviour is
211 n o t c o m m o n for all crystals o f this phase. The resistivity of many of them does n o t change greatly with temperature cycling.
3. Discussion The results presented in the preceding Section may be regarded as an indication of the fact that a number of superconducting transformations seem to occur in some crystals of ~-(BEDT-TTF)213. Besides the 1.5 K transition that is observed by itself, there also seem to exist transitions at 4 K and ~ 7 K, although, they have so far been observed only in the combinations 1.5 K + 4 K, 1.5 K + 7 K and 4 K + 7 K. The last fact suggests that we are dealing with a mixture of phases that exists, under some conditions, within an individual crystal. In this connection a question arises as to how it may be explained that we observe a number of superconducting transformations on the R(T) curves; in other words, why does the resistivity of a crystal not become zero after the first superconducting transition? The simplest explanation may be that different phases alternate along the needle-shaped samples and are connected in series rather than in parallel. Such layered crystals could grow because of instabilities in the synthesis conditions, for example. In this way we might also understand why even the 1.5 K transition is sometimes found to be incomplete. On the other hand, it may be imagined, although it seems less probable, t h a t we have so far observed the presence of phases with higher Tc in conditions when their concentration is close to the percolation edge but insufficient for complete shunting. In any case, the cycling phenomena described above make it doubtful t h a t variations in critical temperatures of different phases are associated with their different chemical compositions. It is more likely t h a t some structural or electronic transformations may take place in ~-phase crystals at low temperatures, which give rise to an increase in the superconducting transition temperature. It may also be that the high temperature superconducting phases are in fact the high pressure ones, which are stabilized in a portion of the crystal by strains arising during synthesis or cycling [4]. In conclusion, we report that, besides the 1.5 K superconducting transition, two more superconducting transformations occur in ~-(BEDT-TTF)213 crystals, with Tcs of the order of 4 and 7 K.
Acknowledgements We are indebted to R. P. Shibaeva for numberous discussions of crystal structure and to L. P. Gor'kov for very useful conversations.
212 References 1 2 3 4
E. B. Yagubskii, I. F. Schegolev, V. N. Laukhin et al. JETPPis'ma, 39 (1984) 12. E. B. Yagubskii, I. F. Schegolevetal. JETP, 68 (1985) 244. V. B. Ginodman, A. V. Rudenko et al. JETPPis'ma, 41 ( 1 9 8 5 ) 4 1 . We have found that at pressures of the order of 2 kbar the transition temperature in ~-(BEDT-TTF)2I 3 increases to 6 - 7 K. Details will be published elsewhere.
207
SUPERCONDUCTING TRANSITIONS IN ~-(BEDT-TTF)2I 3 L. I. BURAVOV, M. V. KARTSOVNIK, V. F. KAMINSKII, P. A. KONONOVICH, E. E. KOSTUCHENKO, V. N. LAUKHIN, M. K. MAKOVA, S. I. PESOTSKII, I. F. SCHEGOLEV, V. N. TOPNIKOV and E. B. YAGUBSKII
Institute of Chemical Physics, U.S.S.R. Academy of Science, Chernogolovka (U.S.S.R.) (Received February 15, 1985; accepted April 14, 1985)
Abstract The investigation of superconducting transitions in some crystals of ~-(BEDT-TTF)213 points to the existence in the ( B E D T - T T F ) - I system of other superconducting phases, with Tcs o f ~ 4 K and ~ 7 K, respectively.
1. Introduction Triclinic fi-modification o f the ( B E D T - T T F ) - I system, ~-(BEDTTTF)213, has been initially reported [1] to be the normal pressure superc o n d u c t o r with Tc = 1.5 + 0.1 K. The ensuing investigation of many crystals o f this phase has revealed, however, some peculiarities in their low temperature behaviour, which have been interpreted [2] as an indication of the fact that admixtures of other superconducting phases with higher Tc s often exist within the same crystals. Here, we present evidence that Tcs of ~ 4 K and 7 K should be ascribed to these phases and discuss brefly their possible nature.
2. Experimental The ab-plane resistivity o f the fi-phase crystals, of the order of 3 × 10 -2 ~2 cm at room temperature, is rather high in comparison with that of other organic metals, b u t it falls considerably with cooling. In sufficiently perfect crystals the resistivity ratio, R3oo/R4.2, may a m o u n t to 5 - 7 × 10 2. It is a characteristic feature of the behaviour o f many crystals that their resistivity does n o t tend to a constant residual value at low temperatures b u t continues to fall d o w n to the 1.5 K superconducting transition. Moreover, it often happens that the falling rate of the resistivity is even accelerated beginning from some temperature a few Kelvins before the transition point. 0379-6779/85/$3.30
Elsevier Sequoia/Printed in The Netherlands
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Fig. 1. Typical examples of resistivity behaviour in crystals of ~-(BEDT-TTF)2I 3 in the pre-transitional temperature range. Fig. 2. Temperature dependence of resistivity of a crystal with a broad superconducting transition in different magnetic fields H]]c. 1, 0 kOe; 2, 6 kOe; 3, 34 kOe; 4, 50 kOe.
Such an acceleration may be realized in a number of ways. First, the resistivity may begin to fall from ~ 4 K, after some tendency to reach the residual value (Fig. 1, a). The second type of behaviour is a step at 7 - 8 K, more or less pronounced, again followed by the 1.5 K transition (Fig. 1, b). In addition, the 7 K step may be followed by a superconducting transformation beginning at ~ 4 K rather than by the 1.5 K transition (Fig. 1, c). Finally, we may have a nearly continuous broad transition that starts from 7 - 8 K and finishes below 1.3 K (the lowest temperature reached in our experiments). The last t y p e of behaviour is presented in Fig. 2, i n which the influence o f the magnetic field Hlic* on the resistivity is also shown. It is seen that in high fields the resistivity fall is suppressed from ~ 7 K. It should be stressed that the room temperature structure of each of the single crystals with different types of behaviour has been proved by X-ray analysis to be that of the ~-phase. In the case shown in Fig. 2, the superconducting transformation is spread over a wide temperature interval. Nevertheless, the analysis of R(H) curves measured at different temperatures allows us to indicate two temperature points characteristic of this transformation. The corresponding data for HHc* are presented in Fig. 3. It is seen that several regions of the R(H) curves may be separated, each characterized by a different rate of resistivity variation. At temperatures below ~ 4 K there are three such regions: the initial fast growth of the resistivity is first changed by a nearly linear intermediate increase and then slows down considerably. The initial fast-growing portions of the R(H) curves
209
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H,~0, Fig. 3. Field dependences of resistivityfor Hllc at different temperatures for the crystal of Fig. 2. 1, 1.5 K; 2, 3.0 K; 3, 4.2 K; 4, 5.3 K; 5, 6.25 K; 6, 7.3 K; 7, 9.3 K. Fig. 4. H - T diagram for two superconducting transformations in ~-(BEDT-TTF)213 with T c - 4 K and - 7 K. 1, 2, Hllc, crystal of Fig. 2; 3, Hllb, crystal of Fig. 5. o, firstcooling cycle; e, second cooling cycle.
disappear above ~ 4 K, and the intermediate regions above ~ 8 K. Noting that these temperatures are just those characteristic of the above described pre-transitional effects shown in Fig. 1, we may tentatively consider the field values dividing the different regions on the R(H) curves as some critical fields in the c*~tirection for the t w o superconducting transitions. These fields are plotted on the H - T diagram o f Fig. 4. The third set o f points in Fig. 4, for Hilb, is referred to another crystal whose resistivity is shown in Fig. 5 as a function of temperature for different magnetic fields. The 7 K step is quite large here and its shift under the action o f the magnetic field is easily observable. It should be stated that the data of Fig. 5 were obtained during the third cycle o f cooling on the same single crystal whose first passage data were shown in Fig. 1, c. The temperature cycling is seen to sometimes give rise to marked differences in the low temperature resistivity behaviour o f the crystals. All five cycles of cooling performed on the crystal of Fig. 1, c and Fig. 5 are shown in Fig. 6. The first three passages, within the measurement accuracy, did n o t change the absolute value o f the resistivity above ~ 8 K. During the fourth and fifth cooling cycles, small jumps in the resistivity were observed, and the 11 K resistivity has increased, in comparison with its value for the first three cyles, by 8.5 and 8.7 times, respectively. In Fig. 6 these curves have been matched to the first three at 11 K. The crystal broke during
210
P, /0.
5-
~
T.K
Fig. 5. Shift of 7 K step under the action of magnetic field, e, 0 kOe;
(~, 14 kOe; x,
50 kOe.
1{)
~9 ~7 6 5
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~
5
4
5
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7
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9
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Fig. 6. Changes in R(T) curves caused b y five c o n s e c u t i v e cycles o f cooling.
the sixth cooling. Beginning from liquid nitrogen temperature, the cooling rate was a b o u t 30 K/h. It is seen from Fig. 6 that the temperature cycling gives rise to the disappearance of the ~ 4 K superconducting transition as if the transition temperature o f the corresponding part of the sample is increased. An analogous increase of the transition temperature with cycling has been observed on crystals of ~-(BEDT-TTF)213 [3]. Note, however, that such a behaviour is
211 n o t c o m m o n for all crystals o f this phase. The resistivity of many of them does n o t change greatly with temperature cycling.
3. Discussion The results presented in the preceding Section may be regarded as an indication of the fact that a number of superconducting transformations seem to occur in some crystals of ~-(BEDT-TTF)213. Besides the 1.5 K transition that is observed by itself, there also seem to exist transitions at 4 K and ~ 7 K, although, they have so far been observed only in the combinations 1.5 K + 4 K, 1.5 K + 7 K and 4 K + 7 K. The last fact suggests that we are dealing with a mixture of phases that exists, under some conditions, within an individual crystal. In this connection a question arises as to how it may be explained that we observe a number of superconducting transformations on the R(T) curves; in other words, why does the resistivity of a crystal not become zero after the first superconducting transition? The simplest explanation may be that different phases alternate along the needle-shaped samples and are connected in series rather than in parallel. Such layered crystals could grow because of instabilities in the synthesis conditions, for example. In this way we might also understand why even the 1.5 K transition is sometimes found to be incomplete. On the other hand, it may be imagined, although it seems less probable, t h a t we have so far observed the presence of phases with higher Tc in conditions when their concentration is close to the percolation edge but insufficient for complete shunting. In any case, the cycling phenomena described above make it doubtful t h a t variations in critical temperatures of different phases are associated with their different chemical compositions. It is more likely t h a t some structural or electronic transformations may take place in ~-phase crystals at low temperatures, which give rise to an increase in the superconducting transition temperature. It may also be that the high temperature superconducting phases are in fact the high pressure ones, which are stabilized in a portion of the crystal by strains arising during synthesis or cycling [4]. In conclusion, we report that, besides the 1.5 K superconducting transition, two more superconducting transformations occur in ~-(BEDT-TTF)213 crystals, with Tcs of the order of 4 and 7 K.
Acknowledgements We are indebted to R. P. Shibaeva for numberous discussions of crystal structure and to L. P. Gor'kov for very useful conversations.
212 References 1 2 3 4
E. B. Yagubskii, I. F. Schegolev, V. N. Laukhin et al. JETPPis'ma, 39 (1984) 12. E. B. Yagubskii, I. F. Schegolevetal. JETP, 68 (1985) 244. V. B. Ginodman, A. V. Rudenko et al. JETPPis'ma, 41 ( 1 9 8 5 ) 4 1 . We have found that at pressures of the order of 2 kbar the transition temperature in ~-(BEDT-TTF)2I 3 increases to 6 - 7 K. Details will be published elsewhere.