Protonation induced high- T c phases in iron-based superconductors evidenced by NMR and magnetization measurements

Abstract

Chemical substitution during growth is a well-established method to manipulate electronic states of quantum materials, and leads to rich spectra of phase diagrams in cuprate and iron-based superconductors. Here we report a novel and generic strategy to achieve nonvolatile electron doping in series of (i.e. 11 and 122 structures) Fe-based superconductors by ionic liquid gating induced protonation at room temperature. Accumulation of protons in bulk compounds induces superconductivity in the parent compounds, and enhances the T c largely in some superconducting ones. Furthermore, the existence of proton in the lattice enables the first proton nuclear magnetic resonance (NMR) study to probe directly superconductivity. Using FeS as a model system, our NMR study reveals an emergent high-T c phase with no coherence peak which is hard to measure by NMR with other isotopes. This novel electric-field-induced proton evolution opens up an avenue for manipulation of competing electronic states (e.g. Mott insulators), and may provide an innovative way for a broad perspective of NMR measurements with greatly enhanced detecting resolution.

FeSe-based superconductors with a superconducting transition temperature of 50 K

“…We attribute it to the easier formation of A 2 Se (A is the akaline metals) impurity for Na and K with the increase of reactivity. Recently, the hydrogen intercalated FeSe 0.93 S 0.07 single crystal is also reported to have an improved T c although the intercalation is not uniform [29]. The above results inspire us to intercalate new intercalator that is chemically inert in the sequence table of metal activity.…”

Enhancement of superconductivity in organic-inorganic hybrid topological materials

“…This is in sharp contrast to conventional electric double layer transistor (EDLT) using ionic liquids 16,17 , where charge carrier concentration is tuned through interfacial electrostatic effect while electrochemical reaction is intentionally avoided. It is interesting to note that recent studies reveal that the electrolysis of water residual within the ionic liquid could also lead to intercalation of oxygen and hydrogen ions 18 , which however has negligible effect on the interlayer spacing 19 . In strong contrast, here we achieve successful intercalation of large organic cation (for example, [C 2 MIm] + with chemical formula of [C 6 H 11 N] + 2 ) into these layered crystals with dramatically increased interlayer spacing.…”

Beyond electrostatic modification: design and discovery of functional oxide phases via ionic-electronic doping

“…Recently, Shi et al reported that under a large bias and using a KClO 4 /polyethylene glycol electrolyte, a electrochemical reaction between the electrolyte and channel material can occur, resulting in bulk ionic doping and an accumulation of carriers far beyond the level of electrostatic surface doping, leading to the observation of superconductivity in MoTe 2 [135]. Cui et al further utilized protonation process to induce the high T c phase in iron-based superconductors [26]. The proton doping process and setup are illustrated in Figure 16 (a) [26].…”

“…In the layered transition metal dichalcogenide TiS 2 , the thermoelectric figure of merit can be enhanced by at least a factor of four through ion intercalation [25]. Furthermore, recent studies have shown that the transition temperature of iron-based superconductors can be elevated by reversible ion doping, with results similar to those from substitutional alloy doping [26]. These experimental observations demonstrate that ion intercalation is capable of manipulating band structure and the properties of materials beyond traditionally explored paradigms such as strain engineering, alloying, electrostatic gating, and electromagnetic field excitation.…”

“…By driving ions in/ out or their spatial re-distribution, various physical and chemical properties of materials can be greatly modified. Representative examples of functionalities tuned by ion doping include but not limited to (i) electrical [4,20],(ii) optical [3,21], (iii) magnetic [16,104], (iv) catalytic [24], (v) electrochemical [5], (vi) thermoelectric [25], and (vii) superconducting properties [26]. Reproduced with permissions from Ref.…”