Enhancement of electron correlations in ion-gated FeSe film by in situ Seebeck and Hall measurements

In situ Seebeck and Hall measurements on FeSe films are carried out to understand the intrinsic evolution of the band structure as superconducting critical temperature $({T}_c)$ is tuned by ionic liquid gating. At the initial state, the nonmonotonous behavior of Hall and Seebeck curves indicates the multiband feature of FeSe film. As ${T}_c$ increases via gating, the hole band gradually sinks below the Fermi level. Meanwhile, the Fermi energy of the electron band reduces while its carrier density increases, indicating that the electron band is flattening as ${T}_c$ enhances. When ${T}_c$ reaches the maximum at the final state, only the electron band exists and the effective electron mass is about $3.4{m}_e$.