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Low-temperature properties of organic conductor (DTEDT)3SbF6
ELSEVIER
SyntheticMetals86(1997)2121-2122
Low-temperature E. Ohmichi”,
properties of organic conductor (DTEDT)sSbFe
H. Ito”, T. Ishiguro”, Y. Misaki*, N. Higuchi*, T. Ohtab, and T. Yamabe* a Department of Physics, Kyoto University, Kyoto 606-01, Japan
b Division
of Molecular
Engineering,
Kyoto
University,
Kyoto
606-01,
Japan
~~The electrical resistivity of (DTEDT)sSbF s shows a metallic temperature dependence down to 10 K. After showing a resistance minimum, the resistivity increases with 1ogT dependence, which is followed by a decrease below -0.2 K. The 1ogT dependence is ascribed to weak localization in two-dimensional system. The decrement was suppressed by a magnetic field less than -1 T, suggesting that it is due to a superconducting transition. Keywords: Organic conductors
based on radical cation and/or anion salts, Superconducting
The TTF-fused donor molecule, DTEDT (Fig. l(a), 2-(1,3-dithiol-2-ylidene)-5(2-ethanediylidene-l,3dithiole)-1,3,4,6_tetrathiapentalene) is of unsymmetrical structure and its cation radical salt is lacking the inversion symmetry because the unsymmetrical molecules are stacked unidirectionally as shown in Fig. l(b). This indicates that not only the cation layers themselves are of bilayer structure of TTF and 2,2’-ethanediylidenebis(1,3_dithiole) but also the crystal is polarizable. It is interesting to know how the bilayer structure and the polarizability work in the electronic properties. Incidentally, it has been shown that this family yields a number of two-dimensional conductors exhibiting metallic temperature dependence down to liquid helium region and one of the salts, (DTEDT)sAu(CN) 2, undergoes a superconducting transition at 4 K at ambient pressure [l] . To investigate one of the metallic salts, (DTEDT)sSbFs, we measured the low temperature electrical conductance down to 50 mK. The crystals of (DTEDT)sSbFs were synthesized by electrochemical oxidation [l]. The samples were of needle-like or thin plate-like shapes and their typical dimensions are 1.5x0.1x0.02 mm3 or 1x0.5x0.02 mm3, respectively. The electrical resistivity was measured by a standard ac technique with/without a magnetic field directed perpendicularly to the conducting plane up to 8 T. Samples are cooled by a dilution refrigerator with a cell containing liquid 3He to ensure good thermal contact. The in-plane resistivity (~11) and interplane resistivity (~1) at room temperature are pll 2: lo-’ Qcm and pi = 10’ ncm, respectively, and the temperature dependences are similar with each other, implying that this salt belongs to two-dimensional conductors. In Fig. 2 we show the temperature dependences of the resistivity for two samples #l (plate-like) and 112 (needle-like) 0379-6779f97lS17.00Q 1997 ElsevierScienceS.A. AUrights resened PII SO379-6779(96)04757-1
phase transitions
Fig. 1. (a) DTEDT molecule ture of (DTEDT)sSbFs.
and (b)the
crystal
struc-
in the temperature range down to 50 mK. The resistivity decreases monotonically down to 6 N 8 K and the temperature (T) dependence is represented by Tc,5w1.5 above 10 K, implying that electron-electron interaction is not dominant in the scattering processes. After exhibiting a resistance minimum, the resistivity turns to increase in the low temperature side. We found that the increment is represented approximately by 1ogT dependence. Considering that the two-dimensionality and the effect of disorder, which is noteworthy from the small ratio of the resistivity at 273 K to that at resistance minimum, P2rsK/Pmin. (-5), the 1ogT dependence is
2122
E. Ohmichi et al. /Synthetic
Metals 86 (1997) 2121.2122
l!oi------“---j 2
loo Temperature
I
I
lo2
10-l Temperature
[K ]
loo [K ]
Fig. 2. Temperature dependence of in-plane resistivity for the samples 1 1 and fl 2. (Broken lines are guide to eyes.)
Fig. 3. Temperature dependence of in-plane resistivity for fl 1 below 1 K as a function of a magnetic field perpendicular to the conducting plane.
ascribable to weak localization. Incidentally, the c* efficients of the 1ogT term are -1 x 10e3 Rem (121) and -0.5x 10s3 Rem (#2), respectively. Then the values per one sheet are become - 6 kR and - 3 kR, which are smaller than h/e2 (426 kn), but close to the case of weak localization in two-dimensional conductors [3]. In the lower temperature side, the electrical resistivity turns to decrease again at 0.1 w 0.2 K as shown in Fig. 2. To clarify the reason for the resistivity decrease, we applied a magnetic field of up to 8 T perpendicularly to the conducting plane. It turned out that the resistivity decrease was suppressed at 1 T as shown in Fig. 3. It is noteworthy that the resistivity decrease is suppressed rapidly with lower field and the resistivity turns to exhibit gentle increase, implying that the rapid change is due to a superconducting transition and the following increase is due to the normal metal magnetoresistance. With respect to the superconducting state, we have to remark that the transition is broad and zero resistance is not achieved. Such behavior is reminiscent of a-(BEDT-TTF)zMHg(SCN)d (M = TI, Rb, K) where the superconductivity cannot be completed so as to provide the zero resistance [2]. Taking into account the fact that the salt is two-dimensional and the evaluated sheet resistance is in the order of h/4e2 (-6.5 kn), this has been explained by the Cooper pair localization [4]. Similar localization effect has been found also in K-(BEDT-TTF)2Cu[N(CN)2]C1 [5]. (DTEDT)sSbFs is also two-dimensional conductor and the estimated sheet resistance is -10 kfi at 4 K, which is close to the h/4e2
value. This implies that this salt is also considered to locate in the vicinity of crossover region of superconductor-insulator transition, resulting in the incompleteness of superconductivity. In summary in (DTEDT)sSbFs the resistivity decrease is found in the temperature range below 0.1 0.2 K. This is ascribed to superconductivity because the decrement is fully suppressed by a magnetic field less than -1 T. Taking into account the two-dimensionality and the sheet resistance in the order of h/4e2, the incompleteness of the superconducting transition is ascribed to the effect of Cooper pair localization. The authors thank Prof. Y. Nagaoka for useful discussion in weak localization effect. This work was supported by Proposal-Based Advanced Industrial Technology R&D Program by NEDO. REFERENCES 1.
2.
3. 4. 5.
Y. Misaki, N. Higuchi, H. Fujiwara, T. Yamabe, T. Mori, H. Mori, S. Tanaka, Angew. Chem. 34 (1995) 1222. H. Ito, M. V. Kartsovnik, H. Ishimoto, K. Kono, H. Mori, N. D. Kushch, G. Saito, T. Ishiguro, S. Tanaka, Synth. Mel. 70 (1995) 899, L. P. Gorkov, A. I. Larkin, D. E. Khmelnitzkii, Pis’ma Zh. Eksp. Teor. Fit. 30 (1979) 248, JETP Leit. 30 (1979) 228 M. P. A. Fisher, Phys. Rev. Lett. 65 (1990) 923 H. Ito, T. Ishiguro, M. Kubota, G. saito, J. Phys. Sot. Jpn. 65 (1996) No.9
SyntheticMetals86(1997)2121-2122
Low-temperature E. Ohmichi”,
properties of organic conductor (DTEDT)sSbFe
H. Ito”, T. Ishiguro”, Y. Misaki*, N. Higuchi*, T. Ohtab, and T. Yamabe* a Department of Physics, Kyoto University, Kyoto 606-01, Japan
b Division
of Molecular
Engineering,
Kyoto
University,
Kyoto
606-01,
Japan
~~The electrical resistivity of (DTEDT)sSbF s shows a metallic temperature dependence down to 10 K. After showing a resistance minimum, the resistivity increases with 1ogT dependence, which is followed by a decrease below -0.2 K. The 1ogT dependence is ascribed to weak localization in two-dimensional system. The decrement was suppressed by a magnetic field less than -1 T, suggesting that it is due to a superconducting transition. Keywords: Organic conductors
based on radical cation and/or anion salts, Superconducting
The TTF-fused donor molecule, DTEDT (Fig. l(a), 2-(1,3-dithiol-2-ylidene)-5(2-ethanediylidene-l,3dithiole)-1,3,4,6_tetrathiapentalene) is of unsymmetrical structure and its cation radical salt is lacking the inversion symmetry because the unsymmetrical molecules are stacked unidirectionally as shown in Fig. l(b). This indicates that not only the cation layers themselves are of bilayer structure of TTF and 2,2’-ethanediylidenebis(1,3_dithiole) but also the crystal is polarizable. It is interesting to know how the bilayer structure and the polarizability work in the electronic properties. Incidentally, it has been shown that this family yields a number of two-dimensional conductors exhibiting metallic temperature dependence down to liquid helium region and one of the salts, (DTEDT)sAu(CN) 2, undergoes a superconducting transition at 4 K at ambient pressure [l] . To investigate one of the metallic salts, (DTEDT)sSbFs, we measured the low temperature electrical conductance down to 50 mK. The crystals of (DTEDT)sSbFs were synthesized by electrochemical oxidation [l]. The samples were of needle-like or thin plate-like shapes and their typical dimensions are 1.5x0.1x0.02 mm3 or 1x0.5x0.02 mm3, respectively. The electrical resistivity was measured by a standard ac technique with/without a magnetic field directed perpendicularly to the conducting plane up to 8 T. Samples are cooled by a dilution refrigerator with a cell containing liquid 3He to ensure good thermal contact. The in-plane resistivity (~11) and interplane resistivity (~1) at room temperature are pll 2: lo-’ Qcm and pi = 10’ ncm, respectively, and the temperature dependences are similar with each other, implying that this salt belongs to two-dimensional conductors. In Fig. 2 we show the temperature dependences of the resistivity for two samples #l (plate-like) and 112 (needle-like) 0379-6779f97lS17.00Q 1997 ElsevierScienceS.A. AUrights resened PII SO379-6779(96)04757-1
phase transitions
Fig. 1. (a) DTEDT molecule ture of (DTEDT)sSbFs.
and (b)the
crystal
struc-
in the temperature range down to 50 mK. The resistivity decreases monotonically down to 6 N 8 K and the temperature (T) dependence is represented by Tc,5w1.5 above 10 K, implying that electron-electron interaction is not dominant in the scattering processes. After exhibiting a resistance minimum, the resistivity turns to increase in the low temperature side. We found that the increment is represented approximately by 1ogT dependence. Considering that the two-dimensionality and the effect of disorder, which is noteworthy from the small ratio of the resistivity at 273 K to that at resistance minimum, P2rsK/Pmin. (-5), the 1ogT dependence is
2122
E. Ohmichi et al. /Synthetic
Metals 86 (1997) 2121.2122
l!oi------“---j 2
loo Temperature
I
I
lo2
10-l Temperature
[K ]
loo [K ]
Fig. 2. Temperature dependence of in-plane resistivity for the samples 1 1 and fl 2. (Broken lines are guide to eyes.)
Fig. 3. Temperature dependence of in-plane resistivity for fl 1 below 1 K as a function of a magnetic field perpendicular to the conducting plane.
ascribable to weak localization. Incidentally, the c* efficients of the 1ogT term are -1 x 10e3 Rem (121) and -0.5x 10s3 Rem (#2), respectively. Then the values per one sheet are become - 6 kR and - 3 kR, which are smaller than h/e2 (426 kn), but close to the case of weak localization in two-dimensional conductors [3]. In the lower temperature side, the electrical resistivity turns to decrease again at 0.1 w 0.2 K as shown in Fig. 2. To clarify the reason for the resistivity decrease, we applied a magnetic field of up to 8 T perpendicularly to the conducting plane. It turned out that the resistivity decrease was suppressed at 1 T as shown in Fig. 3. It is noteworthy that the resistivity decrease is suppressed rapidly with lower field and the resistivity turns to exhibit gentle increase, implying that the rapid change is due to a superconducting transition and the following increase is due to the normal metal magnetoresistance. With respect to the superconducting state, we have to remark that the transition is broad and zero resistance is not achieved. Such behavior is reminiscent of a-(BEDT-TTF)zMHg(SCN)d (M = TI, Rb, K) where the superconductivity cannot be completed so as to provide the zero resistance [2]. Taking into account the fact that the salt is two-dimensional and the evaluated sheet resistance is in the order of h/4e2 (-6.5 kn), this has been explained by the Cooper pair localization [4]. Similar localization effect has been found also in K-(BEDT-TTF)2Cu[N(CN)2]C1 [5]. (DTEDT)sSbFs is also two-dimensional conductor and the estimated sheet resistance is -10 kfi at 4 K, which is close to the h/4e2
value. This implies that this salt is also considered to locate in the vicinity of crossover region of superconductor-insulator transition, resulting in the incompleteness of superconductivity. In summary in (DTEDT)sSbFs the resistivity decrease is found in the temperature range below 0.1 0.2 K. This is ascribed to superconductivity because the decrement is fully suppressed by a magnetic field less than -1 T. Taking into account the two-dimensionality and the sheet resistance in the order of h/4e2, the incompleteness of the superconducting transition is ascribed to the effect of Cooper pair localization. The authors thank Prof. Y. Nagaoka for useful discussion in weak localization effect. This work was supported by Proposal-Based Advanced Industrial Technology R&D Program by NEDO. REFERENCES 1.
2.
3. 4. 5.
Y. Misaki, N. Higuchi, H. Fujiwara, T. Yamabe, T. Mori, H. Mori, S. Tanaka, Angew. Chem. 34 (1995) 1222. H. Ito, M. V. Kartsovnik, H. Ishimoto, K. Kono, H. Mori, N. D. Kushch, G. Saito, T. Ishiguro, S. Tanaka, Synth. Mel. 70 (1995) 899, L. P. Gorkov, A. I. Larkin, D. E. Khmelnitzkii, Pis’ma Zh. Eksp. Teor. Fit. 30 (1979) 248, JETP Leit. 30 (1979) 228 M. P. A. Fisher, Phys. Rev. Lett. 65 (1990) 923 H. Ito, T. Ishiguro, M. Kubota, G. saito, J. Phys. Sot. Jpn. 65 (1996) No.9