  It is thought that strong electron correlation in an insulating parent phase
would enhance a critical temperature (Tc) of superconductivity in a doped phase
via enhancement of the binding energy of a Cooper pair as known in high-Tc
cuprates. To induce a superconductor transition in an insulating phase,
injection of a high density of carriers is needed (e.g., by impurity doping).
An electric double-layer transistor (EDLT) with an ionic liquid gate insulator
enables such a field-induced transition to be investigated and is expected to
result in a high Tc because it is free from deterioration in structure and
carrier transport that are in general caused by conventional carrier doping
(e.g., chemical substitution). Here, for insulating epitaxial thin films (~10
nm thick) of FeSe, we report a high Tc of 35 K, which is four times higher than
that of bulk FeSe, using an EDLT under application of a gate bias of +5.5 V.
Hall effect measurements under the gate bias suggest that highly accumulated
electron carrier in the channel, whose area density is estimated to be
1.4x10^15 cm-2 (the average volume density of 1.7x10^21 cm-3), is the origin of
the high-Tc superconductivity. This result demonstrates that EDLTs are useful
tools to explore the ultimate Tc for insulating parent materials.
