Spin splitting at the high-magnetic-field phase transition of the organic conductor (formula presented)

Spin splitting of the de Haas–van Alphen (dHvA) effect is studied in the quasi-two-dimensional organic conductor (Formula presented) by means of the magnetic torque method. The magnetic field direction dependence of the dHvA oscillation amplitude shows that the spin-splitting zero angles change at 23 T, where the phase transition magnetic field between the low-temperature, low-magnetic-field phase [antiferromagnetic (AF) phase] and the high-magnetic-field phase (Formula presented) phase) is. This change implies the increase of the (Formula presented) value in the AF phase, where g is the g value, (Formula presented) and (Formula presented) the cyclotron effective and free electron masses, respectively. The enhancement of the electron-electron interaction may be the origin of the large (Formula presented) value in the AF phase. This result supports a model which requires a modification of the Lifshitz-Kosevich formulation in the AF phase in order to evaluate the effective mass and Dingle temperature correctly. In addition, anomalous structures on the magnetic torque curves are observed in both the AF and the (Formula presented) phases. Each structure may correspond to the subsequent phase transitions by changing the magnetic field direction.