  One of central issues in iron-based superconductors is the role of structural
change to the superconducting transition temperature (T_c). It was found in
FeSe that the lattice strain leads to a drastic increase in T_c, 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
(DeltaE_{h-e}) between the hole and electron pockets in the normal state. The
change in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change
in T_c. Furthermore, the strength of nematicity is also found to be
characterized by DeltaE_{h-e}. These results suggest that the key to
understanding the phase diagram is the fermiology and interactions linked to
the semimetallic band overlap.
