Enhancement of superconducting transition temperature in electrochemically etched FeSe/LaAlO3 films

Iron-based superconductors such as FeSe have been extensively investigated for various applications owing to their remarkably high superconducting transition temperature (T_c). Despite its simplest crystal structure, the superconducting transition temperature of FeSe can be increased to 30-50 K by electron doping. Among the available methods for doping FeSe, electrostatic doping of electrons into FeSe via electric-double layer transistor (EDLT) structure of ion gates is widely used due to its ability to increase the superconducting transition temperature for the single sample by changing the gate voltage (V_g). Nevertheless, despite the extensive research on electron-doped FeSe films, there are contradicting reports about the critical thickness and transition temperature of FeSe-doped films. For instance, the possible electrochemical reaction layer existing at the surface of the etched FeSe has been speculated to be the origin of enhanced superconducting properties. Unfortunately, limited research has been conducted to validate such assertions.

If the origin of the high T_c in the etched FeSe is the electrochemical reaction layer formed at the surface, etched FeSe will exhibit a different T_c –V_g relationship from that of electrostatic-doped FeSe because the amount of the doped carrier is determined by the gate voltage for the electrostatic-doping and the total charge of the gate current in the etched FeSe case. To this account, researchers at the University of Tokyo: Naoki Shikama (PhD graduate), Yuki Sakishita (MSc student), Dr. Fuyuki Nabeshima, Dr. Yumiko Katayama, Professor Kazunori Ueno, and led by Professor Atsutaka Maeda investigated the relationship between the gate voltage and superconducting properties of electrochemically etched FeSe film with electric-double layer transistor structure. In their approach, all the FeSe films were grown on the LaAlO₃(LAO) substrates via a pulsed laser deposition method (Fig. (a)), while the EDLT structure was fabricated on the synthesized FeSe films. A total of four gate voltages, 2.5, 3.3, 5.0, and 5.5 V were investigated. The main objective was to clarify the superconducting transition temperature enhancement mechanism in etched FeSe-EDLT. Their work is published in the journal, Applied Physics Express.

The research team observed enhancement of superconducting transition temperature with remarkable reproducibility at lower gate voltages. For instance, at lower gate voltages of V_g = 2.5 and 3.3 V, the superconducting transition zero-resistivity temperature reached 46 K (Fig. (b)), exceeding almost all the reported values from resistivity measurements of FeSe. Interestingly, the increased transition temperature remained unchanged even after the discharge process. The enhancement of T_c with a lower V_g is different from the results of the field-effect study for FeSe. These results suggest that the mechanism of T_c enhancement of etched FeSe is different from that of electrostatically doped FeSe. Discharge processes and X-ray diffraction measurement revealed that the electrochemical reaction at a gate voltage of 2.5 V was thin and stable while that at a gate voltage of 5.0 V was thick and unstable.

In a nutshell, the authors investigated the gate voltage dependence of the superconducting transition temperature of etched FeSe films. Results showed an increase in the transition temperature at lower gate voltages, attributed to the electrochemical reaction at the surface of the FeSe films. In a statement to Advances in Engineering, the authors stated that the results provide useful insights that would enable finding the best electrochemical reaction conditions for enhancing the superconducting transition temperature of iron-based superconductors.