Superconductivity at 44 K in K intercalated FeSe system with excess Fe

Abstract

We report here that a new superconducting phase with much higher Tc has been found in K intercalated FeSe compound with excess Fe. We successfully grew crystals by precisely controlling the starting amount of Fe. Besides the superconducting (SC) transition at ~30 K, we observed a sharp drop in resistivity and a kink in susceptibility at 44 K. By combining thermodynamic measurements with electron spin resonance (ESR), we demonstrate that this is a new SC transition. Structural analysis unambiguously reveals two phases coexisting in the crystals, which are responsible respectively for the SC transitions at 30 and 44 K. The structural experiments and first-principles calculations consistently indicate that the 44 K SC phase is close to a 122 structure, but with an unexpectedly large c-axis of 18.10 Å. We further find a novel monotonic dependence of the maximum Tc on the separation of neighbouring FeSe layers.

New Superconducting Phase of Lix(C6H16N2)yFe2−zSe2 with Tc = 41 K Obtained through the Post-Annealing

“…T c 's of the FeSe-based superconductors obtained so far at ambient pressure are summarized in Fig. 6 as a function of d. 1,7,8,10,11,[13][14][15]17,19,20,[23][24][25][26][27] As for the origin of the decrease in T c at d > 9 Å, it may be due to the decrease of the interlayer coupling of cooper pairs. Considering the superconductivity above 100 K in single-layer FeSe films as mentioned above, 6) however, this may be incorrect.…”

Structure, magnetic order and excitations in the 245 family of Fe-based superconductors

“…Thus, an important question is whether the 245 superconductor is a doped semiconductor or only a minority phase in the phase-separated sample is superconducting [53]. A closely related question is whether inhomogeneity is intrinsic to a off-stoichiometric superconductor or a pure superconducting phase can be identified and hopefully isolated [44,[54][55][56][57][58][59][60]. The majority view at the moment is that the I4/m phase with the √ 5× √ 5 Fe superlattice and the large-moment antiferromagnetic or- der is irrelevant to the superconductivity.…”

Suppression of the antiferromagnetic order when approaching the superconducting state in a phase-separated crystal of KxFe2−ySe2

“…In our sample, it is possible that superconducting fluctuations that set in at higher temperatures impact local regions of the dominant √ 5× √ 5 phase by proximity effect, while coherent Josephson coupling among these domains only occurs at T c . Furthermore, although from our own susceptibility and resistivity measurements we do not observe any signatures of superconductivity above T c , there are some reports that this system may have a T c higher than 40 K. [33][34][35] If such a superconducting phase is present in our large sample, this suppression is expected. However, any such phase must be present as only a miniscule fraction.…”