77Se NMR Evidence for the Jaccarino–Peter Mechanism in the Field Induced Superconductor, λ-(BETS)2FeCl4

We have performed ⁷⁷Se NMR on a single crystal sample of the field induced superconductor λ-(BETS)₂FeCl₄. Our results 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 in good agreement with that deduced from transport measurements (L. Balicas et al. Phys. Rev. Lett. 87, 067002 (2001)). This provides a direct microscopic evidence that the field induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter (Phys. Rev. Lett. 9, 290 (1962)). Furthermore, an anomalous broadening of the NMR line has been observed at low temperature, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.