Magnetic-Field Superconductivity in Layered Organic Molecular Crystals with Localized Magnetic Moments

The synthetic organic compound λ(BETS)₂FeCl₄ undergoes successive transitions from an antiferromagnetic insulator to a metal and then to a superconductor as a magnetic field is increased. We use a Hubbard-Kondo model to clarify the role of the Fe³⁺ magnetic ions in these phase transitions. In the high-field regime, the magnetic field acting on the electron spins is compensated by the exchange field H_e due to the magnetic ions. This suggests that the field-induced superconducting state is the same as the zero-field superconducting state which occurs under pressure or when the Fe³⁺ ions are replaced by non-magnetic Ga³⁺ ions. We show how H_c can be extracted from the observed splitting of the Shybnikov-de Haas frequencies. Furthermore, we use this method of extracting H_e to predict the field range for field-induced superconductivity in other materials. We also show that at high fields the spin fluctuations of the localized spins are not important.