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

Significance 

Iron-based superconductors such as FeSe have been extensively investigated for various applications owing to their remarkably high superconducting transition temperature (Tc). 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 (Vg). 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 Tc in the etched FeSe is the electrochemical reaction layer formed at the surface, etched FeSe will exhibit a different Tc Vg 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 Vg = 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 Tc with a lower Vg is different from the results of the field-effect study for FeSe. These results suggest that the mechanism of Tc 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.

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

About the author

Naoki SHIKAMA is a Ph.D student of the University of Tokyo. He received his master degree in 2020 at the University of Tokyo. His current research interests are the enhancement of the superconducting properties of the epitaxial film of iron chalcogenide superconductors by using gating techniques.

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About the author

Yuki Sakishita is a master student in Graduate School of Arts and Sciences in the University of Tokyo since 2019.

His research interest is in condensed matter physics, especially superconductivity phenomena.

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About the author

Fuyuki Nabeshima received his PhD degree in 2015 on growth of thin films of iron chalcogenide superconductors and fabrication of superlattice films based on iron chalcogenides.

He is currently a research associate of Maeda Lab at the University of Tokyo. His research interests focus on the interface superconductivity, especially in FeSe on SrTiO3, and development of new methods to evaluate superconducting properties in thin film samples.

About the author

Yumiko Katayama is an assistant professor in Graduate School of Arts and Sciences in the University of Tokyo since 2016. She received her Ph.D. of Human and Environmental Studies from Kyoto University in 2014.

She was Research fellowship for Young Scientists of JSPS from 2012 to 2016 and a visiting scientist for national chemical engineering institute in Paris in PSL Research University in 2015.

Her research interest is 2D wan der walls materials and strongly correlated systems. Especially she studies transport properties of these materials using electric double layer transistors.

About the author

Kazunori Ueno is an Associate Professor in the Department of Basic Science at the University of Tokyo.

His main research interests are material design for superconductor, ferromagnets, and semiconductors, and device applications with these materials.

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About the author

Prof. Atsutaka MAEDA is a full professor of Department of Pure and Applied Sciences/ Department of Basic Science, The University of Tokyo, since 2008. He received the Ph. D. in Engineering from The University of Tokyo in 1989.

He is an experimental scientist of condensed matter physics, material science. He has been engaged in studies of broad aspects of quantum condensate, such as charge-density wave and superconductivity, probed by microwave techniques in many cases. His recent interests focus on the physical property investigation of the epitaxial film of Fe chalcogenides, and microwave flux flow Hall effect etc.

Reference

Shikama, N., Sakishita, Y., Nabeshima, F., Katayama, Y., Ueno, K., & Maeda, A. (2020). Enhancement of superconducting transition temperature in electrochemically etched FeSe/LaAlO3 filmsApplied Physics Express, 13(8), 083006.

Go To Applied Physics Express

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