May 29 2019
High-temperature (Tc) superconductivity essentially originates from antiferromagnetic insulators, and ferromagnetism and superconductivity are always contradictory to each other.
This is the phase diagram of the gate-tuned (Li,Fe)OHFeSe thin flake. (Image credit: Science China Press)
Xianhui Chen’s team from the University of Science and Technology of China recently noticed an electric-field-controlled reversible transition from superconductor to ferromagnetic insulator in (Li,Fe)OHFeSe thin flake. This study provides an exclusive platform to analyze the relationship between ferromagnetism and superconductivity in Fe-based superconductors and may offer some clue about gaining insights into the electron pairing mechanism beyond traditional electron-phonon superconductivity.
The link between magnetism and superconductivity plays a vital role in gaining insights into the electron pairing mechanism beyond traditional electron-phonon superconductivity. The intertwined or competing electronic states in magnetic and superconducting phases could be explained by controlling the magnetism close to the superconducting region. One of the most effective methods to control the collectively ordered electronic states in condensed matter physics is to modulate the carrier density using field electric transistors (FET).
However, traditional gating method can be used to tune only the carrier concentration on the surface of materials and manipulation of the charge density in the bulk is hindered due to the Thomas-Fermi screening. In the recent past, scientists developed an innovative type of FET using solid ion conductor (SIC) as the gate dielectric. In such a SIC-FET, the electric field can not just tune the carrier density to cause electronic phase transitions but also drive ions into a crystal to convert it from one crystalline phase to another.
Using the newly developed gating method, Xianhui Chen’s team at the University of Science and Technology of China noticed an electric-field-controlled reversible transition from superconductor to ferromagnetic insulator in (Li,Fe)OHFeSe thin flake. With the help of SIC-FET, it is possible to drive Li ions into or extract them out from the (Li,Fe)OHFeSe thin flake using electric field. Upon driving the Li ions initially into the thin flake, they replace the Fe in the hydroxide layers. The Fe ions evicted by Li can travel away from the hydroxide layers to fill the vacancies found in the selenide layers. As soon as the vacancies are filled, the thin flake attains the optimal Tc of ~43 K.
Further Li injection makes the Fe ions extruded from the hydroxide layers to travel to the interstitial sites. Subsequently, the interstitial Fe ions turn ordered and ultimately result in a long-range ferromagnetic order. Therefore, a dome-shaped superconducting phase possessing optimal Tc (=43 K) is constantly tuned into a ferromagnetic insulating phase that displays an electric-field-controlled quantum critical behavior. The device is developed on a solid ion conductor with the potential to reversibly manipulate electronic states of the materials that are collectively ordered and to stabilize new metastable structures using electric field. This study opens the door to access metastable phases and to manipulate structural phase transformation and physical properties using the electric field.
These stunning outcomes provide an exclusive platform to analyze the relationship between ferromagnetism and superconductivity in Fe-based superconductors. This study also shows the excellent performance of SIC-FET in controlling the physical properties of layered crystals and its possible applications for multifunctional devices.
This study is supported by the National Key R & D Program of China (Grant Nos.2017YFA0303001 and 2016YFA0300201), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No.XDB25010100), the National Natural Science Foundation of China (Grant No. 11888101, 11534010), Science Challenge Project (Grants No. TZ2016004), and Hefei Science Center CAS (2016HSC-IU001).