Electric field-induced superconducting transition of insulating FeSe thin film at 35 K

Abstract

<p id="d15025885e197">One of the key strategies for obtaining higher superconducting critical temperature ( <i>T</i> _c) is to dope carriers into an insulator parent material with strong electron correlation. Here, we examined electrostatic carrier doping to insulator-like thin (∼10-nm-thick) FeSe epitaxial films using an electric double-layer transistor (EDLT) structure. The maximum <i>T</i> _c obtained is 35 K, which is 4× higher than that of bulk FeSe. This result demonstrates that EDLTs are useful tools to explore the ultimate <i>T</i> _c for insulating parent materials, and opens a way to explore high- <i>T</i> _c superconductivity, where carrier doping is difficult by conventional chemical substitution. </p><p class="first" id="d15025885e225">It is thought that strong electron correlation in an insulating parent phase would enhance a critical temperature ( <i>T</i> _c) of superconductivity in a doped phase via enhancement of the binding energy of a Cooper pair as known in high- <i>T</i> _c 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 <i>T</i> _c 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 <i>T</i> _c of 35 K, which is 4× 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 1.4 × 10 ¹⁵ cm ^–2 (the average volume density of 1.7 × 10 ²¹ cm ^–3), is the origin of the high- <i>T</i> _c superconductivity. This result demonstrates that EDLTs are useful tools to explore the ultimate <i>T</i> _c for insulating parent materials. </p>