HIGH-FIELD ESR MEASUREMENTS OF THE π-d MOLECULAR CONDUCTOR λ-( BETS ) 2 FeCl

The two-dimensional molecular conductor λ-(BETS)2FeCl4 is composed from the conducting π-electrons of BETS molecules and the magnetic d-electrons of the Fe ions. Thanks to the non-negligible interaction between πand d-electrons, λ-(BETS)2FeCl4 has a fascinating phase diagram as a function of temperature and magnetic field that includes a paramagnetic metal (PM) phase, an antiferromagnetic insulating (AFI) phase, and a fieldinduced superconducting (FISC) phase. Although the mechanism of the FISC phase is well understood by the Jaccarino-Peter compensation mechanism [1], the AFI phase of the λ-(BETS)2FeCl4 is still under a strong debate whether the d-electrons are long-range ordered or not [2, 3]. Recent specific heat measurement has observed a broad Schottky anomaly, and claim that is due to the paramagnetic Fe [2]. However, this contradicts with the observation of a conventional antiferromagnetic resonance (AFMR) mode [3]. To have more microscopic information about the AFI ground state, we have investigated the AFI and the PM phases up to 8 T using our high-field ESR system (40 400 GHz). At low field, we have observed single AFMR mode that is consistent with the previous data [3]. However, two ESR lines have been observed in the high field region (Fig. 1). From the temperature dependence of the g-value and the frequency-field plots of the resonance (Fig. 2), resonance A and B can be attributed to the AFMR mode and the EPR line, respectively. We presume that the additional observation of the EPR line is due to the delocalization of π-electrons at high magnetic field. The localization/delocalization of the π-electrons might play an important role to the AFI ground state.