Evolution of high-temperature superconductivity from low-Tc phase tuned by carrier concentration in FeSe thin flakes (1509.00620v1)

Abstract: In contrast to bulk FeSe superconductor, heavily electron-doped FeSe-derived superconductors show relatively high Tc without hole Fermi surfaces and nodal superconducting gap structure, which pose great challenges on pairing theories in the iron-based superconductors. In the heavily electron-doped FeSe-based superconductors, the dominant factors and the exact working mechanism that is responsible for the high Tc need to be clarified. In particular, a clean control of carrier concentration remains to be a challenge for revealing how superconductivity and Fermi surface topology evolves with carrier concentration in bulk FeSe. Here, we report the evolution of superconductivity in the FeSe thin flake with systematically regulated carrier concentrations by liquid-gating technique. High-temperature superconductivity at 48 K can be achieved only with electron doping tuned by gate voltage in FeSe thin flake with Tc less than 10 K. This is the first time to achieve such a high temperature superconductivity in FeSe without either epitaxial interface or external pressure. It definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with Tc as high as 48 K in bulk FeSe. Intriguingly, our data also indicates that the superconductivity is suddenly changed from low-Tc phase to high-Tc phase with a Lifshitz transition at certain carrier concentration. These results help us to build a unified picture to understand the high-temperature superconductivity among all FeSe-derived superconductors and shed light on further pursuit of higher Tc in these materials.

Examination of High-Temperature Superconductivity Transition in FeSe Thin Flakes

The paper titled "Evolution of high-temperature superconductivity from low-Tc phase tuned by carrier concentration in FeSe thin flakes" presents a meticulous exploration of superconductivity transitions in iron-based superconductors. Notably, it elucidates how electron doping via gate voltage in FeSe thin flakes induces a transition from low to high superconducting transition temperatures (Tc), achieving Tc as high as 48 K without an epitaxial interface or external pressure—an unprecedented feat in this parent compound.

The authors utilize a liquid-gating technique, employing an electric-double-layer transistor (EDLT) setup, to systematically regulate carrier concentrations in FeSe flakes. This method permits precise modulation of electron density, thereby allowing an insightful investigation of superconductivity's evolution in response to such changes. The paper identifies a Lifshitz transition at a specific carrier concentration, marking a distinct shift from low-Tc to high-Tc superconducting phases. This transition underscores the critical role of electron doping in the Tc enhancement of FeSe-derived materials. By successfully manipulating carrier concentration, this work provides crucial evidence supporting the hypothesis that electron-dominant Fermi surfaces in FeSe-derived superconductors significantly contribute to raising Tc.

The experimental apparatus involved liquid gating using ionic liquid DIE-ME-TFSI, which significantly enhances carrier concentration without altering the crystal structure. Key observations include anomalous behavior of electron-dominated Hall signal transitions and a dependency of superconductivity on gate voltage, exemplified by the shift from a low-Tc phase with an onset critical temperature (Tc_onset) of approximately 5.2 K to a high-Tc phase exceeding 40 K when the gate voltage surpasses 4.25 V.

The implications of these findings are multifold:

1. Theoretical Impact: The discovery of electron-dominated transport characteristics introduces a substantial challenge to existing theories, primarily those postulating the necessity of a mixed electron-hole Fermi surface topology for high-Tc in iron-based superconductors. The evidence suggests that electron pocket configurations alone predispose FeSe to higher critical temperatures, aligning with electronic structures similar to FeSe/SrTiO3 interfaces.
2. Technological Potential: The ability to achieve high-temperature superconductivity in bulk FeSe via a controllable, clean electron-doping process paves the way for utilizing EDLT methods in tuning electronic structures across various materials. The implications for creating superconductors operating at comparatively high temperatures are substantial, with potential applications in quantum computing and other cutting-edge technologies.
3. Future Research Directions: While this paper sets a precedent, the exploration of optimal doping levels is constrained by current device limitations. Further research could explore alternative methods to increase electron doping without risking electrochemical degradation, aiming for critical temperatures beyond the observed 48 K.

In conclusion, this investigation not only contributes to a comprehensive understanding of the mechanisms governing high-Tc phases in FeSe superconductors but also highlights new pathways for enhancing superconductivity through meticulous control of electron doping, offering a strategic approach to future research and technological advancement within the domain.