^<77>Se NMR Evidence for the Jaccarino-Peter Mechanism in the Field Induced Superconductor, λ-(BETS)_2FeCl_4(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We performed ^<77>Se NMR on a single crystal sample of the field-induced superconductor λ-(BETS)_2FeCl_4. Our result obtained in the paramagnetic state provide a microscopic insight on the exchange interaction J between the spins s of the BETS π conduction electrons and the Fe localized d spins S. The absolute value of the Knight shift K decreases when the polarization of the Fe spins increases. This reflects the "negative" spin polarization of the π electrons through the exchange interaction J. The value of J has been estimated from the temperature and the magnetic field dependence of K and found to be in good agreement with that deduced from transport measurements [Balicas et al.: Phys. Rev. Lett. 87 (2001) 067002]. This provides direct microscopic evidence that the field-induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter [Phys. Rev. Lett. 9 (1962) 290]. Furthermore, an anomalous broadening of the NMR line has been observed at low temperatures, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.
社団法人日本物理学会の論文
2007-12-15