One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (Tc). It was found in FeSe that the lattice strain leads to a drastic increase in Tc, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap (ΔEh-e) between the hole and electron pockets in the normal state. The change in ΔEh-e modifies the Fermi-surface volume, leading to a change in Tc. Furthermore, the strength of nematicity is also found to be characterized by ΔEh-e. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap.
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