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Possible superconductivity in Au(dmit)2 Langmuir–Blodgett films
Synthetic Metals 133±134 (2003) 663±664
Possible superconductivity in Au(dmit)2 Langmuir±Blodgett ®lms$ Y.F. Miura*, M. Horikiri, S. Tajima, T. Wakaita, S.-H. Saito, M. Sugi Department of Materials Science, Toin University of Yokohama, Yokohama, Japan
Abstract The complex magnetic susceptibility was measured for the Langmuir±Blodgett (LB) ®lms of ditetradecyldimethylammonium-Au(dmit)2 (2C14-Au(dmit)2) using an ac inductive method. A diamagnetic transition of the real part (w0 ) was observed at around 4 K under a weak ac ®eld of 0.1 mT (80 Hz) and it was shifted toward the lower side for about 0.6 K with superposing a dc ®eld of 60 mT. These results suggest existence of a superconducting phase in the 2C14-Au(dmit)2 LB system. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Langmuir±Blodgett (LB) ®lms; Au(dmit)2 anion; Electrochemical oxidation; Complex magnetic susceptibility; Superconductivity
1. Introduction Realization of superconductivity in Langmuir±Blodgett (LB) ®lms is one of the long-standing goals among various trials for fabricating LB ®lms with electronic and optoelectronic functions [1]. Since the LB technique allows us to assemble organic molecules into tailored two-dimensional (2D) molecular sheets, the realization of superconducting LB ®lms will open up the possibility for a variety of applications in the future. We have already reported that the complex magnetic susceptibility of the LB ®lms based on ditetradecyldimethylammonium-Au(dmit)2 (2C14-Au(dmit)2, Fig. 1) salt in which the real part w0 has a diamagnetic transition together with a single loss peak of w00 at around 3.9 K [2]. We have further reported that a blunt decrease of resistivity is seen below 3.9 K for some sample batches [3]. In this paper, we report preliminary results on the temperature dependence of the real part w0 measured with an ac magnetic ®eld of 0.1 mT (80 Hz) under the superposition of a dc magnetic ®eld of 60 mT. 2. Experiment 2.1. Sample preparation The ditetradecyldimethylammonium-Au(dmit)2 (2C14Au(dmit)2, Fig. 1) salt was synthesized following the $
Yamaha Conference LVI, the Fourth International Symposium on Crystalline Organic Metals Superconductors and Ferromagnets, ISCOM 2001, Abstract Number J1Mon. * Corresponding author. Tel.: 81-45-974-5290; fax: 81-45-974-5290. E-mail address: [email protected] (Y.F. Miura).
procedure of Steimecke et al. [4] and spread at the air/ water interface using a 1:1 mixture of acetonitrile and benzene as the solvent. After keeping the salt at the air/ water interface for 5 min, the ¯oating salt was compressed to 25 mN/m and then the ®lm at the air/water interface was transferred onto 0.1 mm thick PET (poly(ethylene) terephthalate) substrates by the horizontal lifting method. The substrates were pre-coated with ®ve layers of the LB ®lms of cadmium arachidate and then electrode strips of gold were vacuum-deposited onto the hydrophobized surface. The as-deposited ®lm was immersed in an aqueous solution of LiClO4 (0.1 M) and electrochemically oxidized using the gold electrode strips deposited underneath the LB layer. The details of the sample preparation are in our previous papers [2,3,5]. 2.2. Measurement of complex magnetic susceptibility The complex magnetic susceptibility was measured by an ac inductive method using a standard-type Hartshorn bridge. A pick-up coil system, which consists of coaxially-wound primary and secondary coils, was immersed in liquid 4He together with the LB samples. A Tinsley Type 4229 variable mutual inductor, an NF Type 5610B lock-in ampli®er and a phase-shift potentiometer were used for the bridge circuit. A weak ac magnetic ®eld (0.1 mT) of 80 Hz and a dc ®eld of 60 mT were both applied perpendicular to the ®lm plane. Eighteen pieces of 20 layered LB ®lms with the sample dimension of 6 mm 6 mm were stacked and loaded in the detection system. Temperature of the sample was monitored by a GaAlAs diode which was calibrated with 4 He vapor pressure for every run.
0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 4 4 2 - 3
664
Y.F. Miura et al. / Synthetic Metals 133±134 (2003) 663±664
4. Discussion and conclusion
Fig. 1. Molecular structure of 2C14-Au(dmit)2 salt.
As we previously reported, the typical shift of w0 at 3.9 K is the order of 10 6 CGS emu, which is remarkably large considering the small sample volume of 10 5 cm3 loaded in the detection system [2]. Furthermore, we have also reported that the 2C14-Au(dmit)2 LB system exhibits a blunt decrease of the resistivity below 3.9 K or an appreciable slope change in the Arrhenius plot [3]. In this paper, we have demonstrated that the diamagnetic transition of w0 at around 4 K is shifted toward the lower side for about 0.6 K with the application of the dc magnetic ®eld of 60 mT. The effect of the dc magnetic ®eld on w0 presented here further supports possible existence of a superconducting phase in the inhomogeneous LB system together with the previously-reported results. Further measurements under various magnetic ®elds, which are either parallel or perpendicular to the ®lm plane, are now underway. Acknowledgements
Fig. 2. The temperature dependence of the real part of the complex magnetic susceptibility (w0 ) measured on cooling with superposing a dc magnetic field of 60 mT (~) and without it ((*): first run before the dc field application; (*): second run after the dc-field cooling).
3. Results Fig. 2 shows the temperature dependence of the real part, w0 , measured with and without the superposition of the dc magnetic ®eld. As shown in Fig. 2, a diamagnetic transition of w0 is shifted toward the lower side for about 0.6 K with the application of the dc magnetic ®eld of 60 mT. After applying the dc ®eld, the second measurement of w0 was performed and reproducibility of the ac-®eld-only data was con®rmed, as shown in Fig. 2.
This work was supported in part by Grant-in-Aid for Scienti®c Research of Ministry of Education, Culture, Sports, Science and Technology (Japan) under Grant no. 12750019, and Kanagawa Academy of Science and Technology (Japan) under Grant no. 0012011. References [1] A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir± Blodgett Films to Self-Assembly, Academic Press, San Diego, 1991. [2] Y.F. Miura, M. Horikiri, S.-H. Saito, M. Sugi, Solid State Comm. 113 (2000) 603. [3] Y.F. Miura, M. Horikiri, S. Tajima, T. Wakaita, S.-H. Saito, M. Sugi, Synth. Met. 120 (2001) 727. [4] G. Steimecke, H.J. Sieler, P. Kirmse, E. Hoyer, Phosphorus sulfur 7 (1979) 49. [5] Y.F. Miura, Y. Okuma, H. Ohnishi, T. Kawasaki, M. Sugi, Jpn. J. Appl. Phys. 37 (1998) L1481.
Possible superconductivity in Au(dmit)2 Langmuir±Blodgett ®lms$ Y.F. Miura*, M. Horikiri, S. Tajima, T. Wakaita, S.-H. Saito, M. Sugi Department of Materials Science, Toin University of Yokohama, Yokohama, Japan
Abstract The complex magnetic susceptibility was measured for the Langmuir±Blodgett (LB) ®lms of ditetradecyldimethylammonium-Au(dmit)2 (2C14-Au(dmit)2) using an ac inductive method. A diamagnetic transition of the real part (w0 ) was observed at around 4 K under a weak ac ®eld of 0.1 mT (80 Hz) and it was shifted toward the lower side for about 0.6 K with superposing a dc ®eld of 60 mT. These results suggest existence of a superconducting phase in the 2C14-Au(dmit)2 LB system. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Langmuir±Blodgett (LB) ®lms; Au(dmit)2 anion; Electrochemical oxidation; Complex magnetic susceptibility; Superconductivity
1. Introduction Realization of superconductivity in Langmuir±Blodgett (LB) ®lms is one of the long-standing goals among various trials for fabricating LB ®lms with electronic and optoelectronic functions [1]. Since the LB technique allows us to assemble organic molecules into tailored two-dimensional (2D) molecular sheets, the realization of superconducting LB ®lms will open up the possibility for a variety of applications in the future. We have already reported that the complex magnetic susceptibility of the LB ®lms based on ditetradecyldimethylammonium-Au(dmit)2 (2C14-Au(dmit)2, Fig. 1) salt in which the real part w0 has a diamagnetic transition together with a single loss peak of w00 at around 3.9 K [2]. We have further reported that a blunt decrease of resistivity is seen below 3.9 K for some sample batches [3]. In this paper, we report preliminary results on the temperature dependence of the real part w0 measured with an ac magnetic ®eld of 0.1 mT (80 Hz) under the superposition of a dc magnetic ®eld of 60 mT. 2. Experiment 2.1. Sample preparation The ditetradecyldimethylammonium-Au(dmit)2 (2C14Au(dmit)2, Fig. 1) salt was synthesized following the $
Yamaha Conference LVI, the Fourth International Symposium on Crystalline Organic Metals Superconductors and Ferromagnets, ISCOM 2001, Abstract Number J1Mon. * Corresponding author. Tel.: 81-45-974-5290; fax: 81-45-974-5290. E-mail address: [email protected] (Y.F. Miura).
procedure of Steimecke et al. [4] and spread at the air/ water interface using a 1:1 mixture of acetonitrile and benzene as the solvent. After keeping the salt at the air/ water interface for 5 min, the ¯oating salt was compressed to 25 mN/m and then the ®lm at the air/water interface was transferred onto 0.1 mm thick PET (poly(ethylene) terephthalate) substrates by the horizontal lifting method. The substrates were pre-coated with ®ve layers of the LB ®lms of cadmium arachidate and then electrode strips of gold were vacuum-deposited onto the hydrophobized surface. The as-deposited ®lm was immersed in an aqueous solution of LiClO4 (0.1 M) and electrochemically oxidized using the gold electrode strips deposited underneath the LB layer. The details of the sample preparation are in our previous papers [2,3,5]. 2.2. Measurement of complex magnetic susceptibility The complex magnetic susceptibility was measured by an ac inductive method using a standard-type Hartshorn bridge. A pick-up coil system, which consists of coaxially-wound primary and secondary coils, was immersed in liquid 4He together with the LB samples. A Tinsley Type 4229 variable mutual inductor, an NF Type 5610B lock-in ampli®er and a phase-shift potentiometer were used for the bridge circuit. A weak ac magnetic ®eld (0.1 mT) of 80 Hz and a dc ®eld of 60 mT were both applied perpendicular to the ®lm plane. Eighteen pieces of 20 layered LB ®lms with the sample dimension of 6 mm 6 mm were stacked and loaded in the detection system. Temperature of the sample was monitored by a GaAlAs diode which was calibrated with 4 He vapor pressure for every run.
0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 4 4 2 - 3
664
Y.F. Miura et al. / Synthetic Metals 133±134 (2003) 663±664
4. Discussion and conclusion
Fig. 1. Molecular structure of 2C14-Au(dmit)2 salt.
As we previously reported, the typical shift of w0 at 3.9 K is the order of 10 6 CGS emu, which is remarkably large considering the small sample volume of 10 5 cm3 loaded in the detection system [2]. Furthermore, we have also reported that the 2C14-Au(dmit)2 LB system exhibits a blunt decrease of the resistivity below 3.9 K or an appreciable slope change in the Arrhenius plot [3]. In this paper, we have demonstrated that the diamagnetic transition of w0 at around 4 K is shifted toward the lower side for about 0.6 K with the application of the dc magnetic ®eld of 60 mT. The effect of the dc magnetic ®eld on w0 presented here further supports possible existence of a superconducting phase in the inhomogeneous LB system together with the previously-reported results. Further measurements under various magnetic ®elds, which are either parallel or perpendicular to the ®lm plane, are now underway. Acknowledgements
Fig. 2. The temperature dependence of the real part of the complex magnetic susceptibility (w0 ) measured on cooling with superposing a dc magnetic field of 60 mT (~) and without it ((*): first run before the dc field application; (*): second run after the dc-field cooling).
3. Results Fig. 2 shows the temperature dependence of the real part, w0 , measured with and without the superposition of the dc magnetic ®eld. As shown in Fig. 2, a diamagnetic transition of w0 is shifted toward the lower side for about 0.6 K with the application of the dc magnetic ®eld of 60 mT. After applying the dc ®eld, the second measurement of w0 was performed and reproducibility of the ac-®eld-only data was con®rmed, as shown in Fig. 2.
This work was supported in part by Grant-in-Aid for Scienti®c Research of Ministry of Education, Culture, Sports, Science and Technology (Japan) under Grant no. 12750019, and Kanagawa Academy of Science and Technology (Japan) under Grant no. 0012011. References [1] A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir± Blodgett Films to Self-Assembly, Academic Press, San Diego, 1991. [2] Y.F. Miura, M. Horikiri, S.-H. Saito, M. Sugi, Solid State Comm. 113 (2000) 603. [3] Y.F. Miura, M. Horikiri, S. Tajima, T. Wakaita, S.-H. Saito, M. Sugi, Synth. Met. 120 (2001) 727. [4] G. Steimecke, H.J. Sieler, P. Kirmse, E. Hoyer, Phosphorus sulfur 7 (1979) 49. [5] Y.F. Miura, Y. Okuma, H. Ohnishi, T. Kawasaki, M. Sugi, Jpn. J. Appl. Phys. 37 (1998) L1481.