Superconductivity at 44.4 K achieved by intercalating EMIM⁺ into FeSe*

Abstract

Superconductivity with transition temperature T c above 40 K was observed in protonated FeSe (H y -FeSe) previously with the ionic liquid EMIM-BF 4 used in the electrochemical process. However, the real superconducting phase is not clear until now. And detailed structural, magnetization, and electrical transport measurements are lacking. By using similar protonating technique on FeSe single crystals, we obtain superconducting samples with T c above 40 K. We show that the obtained superconducting phase is not H y -FeSe but actually an organic-ion (C 6 H 11 N + 2 referred to as EMIM + )-intercalated phase (EMIM) x FeSe. By using x-ray diffraction technique, two sets of index peaks corresponding to different c-axis lattice constants are detected in the obtained samples, which belong to the newly formed phase of intercalated (EMIM) x FeSe and the residual FeSe, respectively. The superconductivity of (EMIM) x FeSe with T c of 44.4 K is confirmed by resistivity and magnetic susceptibility measurements. Temperature dependence of resistivity with different applied magnetic fields reveals that the upper critical field H c2 is quite high, while the irreversibility field H irr is suppressed quickly with increasing temperature till about 20 K. This indicates that the resultant compound has a high anisotropy with a large spacing between the FeSe layers.

Band Structure of Organic-Ion-Intercalated (EMIM)xFeSe Superconductor

“…The optimized lattice parameters are a = b = 11.321 Å, c = 10.450 Å. The length of c parameter is in excellent agreement with the known experimental data [11]. EMIM cations are localized in the space between the nearest Se atoms of the FeSe layers.…”

“…Therefore, change of the Fermi surface topology upon EMIM intercalation may be the most important ingredient in the increase of reported in ref. [ 11 ].…”

“…It was initially suggested that the protonation via the ionic-liquid-gating method [ 9 ] makes it possible to increase in iron selenide [ 10 ] due to the H -FeSe S formation. The later study [ 11 ] uncovered the formation of an organic ion-intercalated phase (EMIM) FeSe, where EMIM stands for C H N (see its structure in Figure 1 ). The EMIM cations were inserted into FeSe during the electrochemical process in the electrolytic cell with two platinum electrodes and EMIM-BF as an ionic liquid.…”

Coexisting superconductivity and charge-density wave in hydrogen-doped titanium diselenide via ionic liquid gating-induced protonation

“…Nuclear magnetic resonance. 1 H-NMR measurements at µ 0 H 3.5 T (magnetic field parallel to the crystallographic ab plane) were performed using a TecMag Apollo spectrometer coupled to a resonant circuit made of an in-series combination of a 470 pF capacitor and of a seven-loop solenoidal coil. The coil was deformed in order to mimic the flake-like shape of the crystals and to maximize the geometrical filling factor, in turn.…”

“…The recent observation of near-to-room-temperature superconductivity in hydrides under high pressure [1,2] has demonstrated that a superconducting (SC) state with remarkably high critical temperature not necessarily requires an unconventional electron-electron coupling mechanism. The key requirements here are high phonon frequencies, such as those associated to the hydrogen (H) vibration modes, a strong coupling of these modes to the electronic states at the Fermi level, and a non-conventional structural environment (in the specific case, stabilized by pressure) that favours such a coupling.…”