Fermiological interpretation of FeTe_1-xSex thin crystal by quantum conductance oscillation

We have successfully observed quantum oscillations in the magnetic-field dependence of resistivity in FeTe_1-xSex. Quantum oscillation measurements were performed using non-superconducting and superconducting thin crystals of FeTe_0.65Se0.35 cleaved from a bulk single crystal by mechanical exfoliation. We show that the Fermi surface (FS) of the non-superconducting crystals is in good agreement with the rigid band shift model based on electron doping by excess Fe, while that of the superconducting crystal is in good agreement with the calculated FS with no shift. From the comparison of the FS for both crystals, we determined the change of the cross-sectional area of the FS, suggesting that the suppression of the FS nesting with the vector due to excess Fe leads to the disappearance of superconductivity.