Fermiological Interpretation of Superconductivity/Non-superconductivity of FeTe_{1-x}Se_{x} Thin Crystal Determined by Quantum Oscillation Measurement

We have successfully observed quantum oscillation (QO) for FeTe 1- x Se x . QO measurements were performed using non-superconducting and superconducting thin crystals of FeTe 0.65 Se 0.35 fabricated by the scotch-tape method. We show that the Fermi surfaces (FS) of the non-superconducting crystal are 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 FS comparison of both crystals, we demonstrate the change of the cross-sectional area of the FS, suggesting that the suppression of the FS nesting with the vector Q s = (  ,  ) due to excess Fe results in the disappearance of the superconductivity. we expect that the non-superconductivity for Fe 1.07 Te 0.65 Se 0.35 crystal is probably attributed to the suppression of FS nesting due to the change of FS area and Fe 1.00 Te 0.65 Se 0.35 crystal shows the superconductivity with the nesting vector Q s . Our results suggest that the suppression of the FS nesting by the excess Fe suppresses the superconductivity.