1995 Fiscal Year Final Research Report Summary
Preparation of Ultrathin and Granular Superconducting Films, and Studies on Electronic Properties at Very Low Temperatures
| Project/Area Number |
06452058
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
固体物性Ⅱ(磁性・金属・低温)
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| Research Institution | TOKYO INSTITUTE OF TECHNOLOGY |
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Principal Investigator |
OKUMA Satoshi TOKYO INSTITUTE OF TECHNOLOGY,ASSOCIATE PROF., 極低温システム研究センター, 助教授 (50194105)
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| Project Period (FY) |
1994 – 1995
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| Keywords | 2D superconductors / Ultrathin films / Granular films / S-I transition / Hall effect / Bose-glass phase / Vortex-glass phase / Mixed state |
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| Research Abstract |
We have constructed a cryogenic evaporator which allows us to fabricate clean and ultrathin (or granular) films of indium, which could not be obtained by conventional deposition at room temperatures, and to measure them in situ below 0.5 K under magnetic fields up to 11 T.Using these superconducting films, we have studied effects of quantum fluctuations as well as thermal fluctuations on the phase transition.
1. We have measured the Hall resistance as well as the longitudinal resistance for disordered thin indium films and found the unusual insulating phase where R_<xx> diverges but R_<xy> is small or zero at T*0. From the result, we have proposed a possibility that the insulating phase corresponds to the Bose-glass insulator where the phase fluctuates quantum-mechanically and Cooper pairs are localized. In order to make a definitive statement on Bose-glass insulator, further experiments at even lower temperatures (-10mK) are necessary.
2. Nonlinear resistivities (I-V characteristics) for indium films have been studied in various magnetic fields B.(1) The Meissner phase (B=0) : The I-V characteristics for thin films (<14nm) are well described by the two-dimensional (2D) KT theory. As the films become thicker, I-V curves are altered and are explained in terms of the recent theory considering 3D vortex loops. Thus, the dimensionality crossover for vortices from 2D to 3D is clearly observed. (2) The mixed state (B*0) : The I-V curves for thick films are analyzed according to the 3D vortex-glass theory. We have found the field-independent critical exponents and the scaling functions, which are similar to those obtained in high T_C superconductors, indicating that the VG transition is universal in type II superconductors. Finite-size effects are visible for thinner films (2D dirty superconductor) as well as for thick films with larger grain sizes (3D Josephson network), which are also consistent with the VG picture.
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