Spin-zero anomaly in the magnetic quantum oscillatins of a two-dimensional metal

We present de Haas-van Alplhen (dHvA) and Shubnikov-de Haas (SdH) measurements on the quasi-two-dimensional organic superconductor β"-(BEDT-TTF)₂SF₅CH₂CF₂SO₃. The measurements were carried out by the torque method and by four-point low-frequency ac-resistance measurements in magnetic fields up to 15 T. Unlike theoretical expectations for two-dimensional metals the dHvA signal shows an unconventional sawtooth wave-form, e. e., an 'inversed sawtooth' wave form is observed.In order to investigate the behaviour in more detail we performed angle-dependent dHvA and SdH measurements. The SdH effect displays the behaviour predicted by the grand-canonical Lifshitz-Kosevich theorie. In contrast, the dHvA signal can be explained by a theory, which includes a slightly oscillating chemical potential. Even for simultaneous measurements of both effects the behaviour does not change. This means that the dHvA signal is not affected by an external charge carrier reservoir.