Nanoscale phase separation of antiferromagnetic order and superconductivity in K0.75Fe1.75Se2

Abstract

We report an in-plane optical spectroscopy study on the iron-selenide superconductor K0.75Fe1.75Se2. The measurement revealed the development of a sharp reflectance edge below Tc at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Furthermore, the feature could be noticeably suppressed and shifted to lower frequency by a moderate magnetic field. Our analysis indicates that this edge structure arises from the development of a Josephson-coupling plasmon in the superconducting condensate. Together with the transmission electron microscopy analysis, our study yields compelling evidence for the presence of nanoscale phase separation between superconductivity and magnetism. The results also enable us to understand various seemingly controversial experimental data probed from different techniques.

Evolution of precipitate morphology during heat treatment and its implications for the superconductivity in KxFe1.6+ySeLiu1, Xing2, Dennis3 et al. 2012Phys. Rev. B

“…In contrast to formerly discovered "1111", "122", and "11" compounds etc., the coexistence of two spatially-separated phases is observed in K x Fe 2 Se 2 superconductors by high-resolution transmission electron microscopy (HRTEM) [5][6], as well as scanning nanofocused x-ray diffraction [7][8]. The unusual features observed in optical spectroscopy [9] and angle-resolved photoemission spectroscopy (ARPES) measurements [10] were explained by the phase separation scenario. The muon spin rotation (μSR) [11], Mössbauer [12], and nuclear magnetic resonance (NMR) [13] experiments have further revealed nearly 90% of the sample volumes exhibit large-moment antiferromagnetic (AFM) order, while 10% of the sample volumes remain paramagnetic (PM) and attributed to a metallic/superconducting phase in A x Fe 2-y Se 2 single crystals.…”

Direct observation of nanoscale interface phase in the superconducting chalcogenideKxFe2−ySe2with intrinsic phase separation

“…Among these materials, the intercalated layered iron-chalcogenide system with chemical formula of A x Fe 2−y Se 2 (A = K, Rb, Cs) [6][7][8][9] is a good example in which a large magnetic moment is associated with the iron vacancy order [10]. The A x Fe 2−y Se 2 system also shows an intrinsic phase separation [11,12] and a delicate balance between an insulating magnetic phase associated with the iron vacancy order and a metallic phase considered to be superconducting below a transition temperature T c of ∼32 K. Indeed, A x Fe 2−y Se 2 manifests peculiar microstructure, including iron vacancy order in the ab-plane, an antiferromagnetic order in the c-direction [10,14] along with an intrinsic phase separation [11][12][13][14][15][16][17][18][19] in which the majority phase with block antiferromagnetism has a stoichiometry of A 0.8 Fe 1.6 Se 2 (245) while the minority metallic phase is A x Fe 2 Se 2 (122). Incidentally, suppression of iron-vacancy order by high pressure produces a new phase with a T c of ∼56 K [20].…”

“…Incidentally, suppression of iron-vacancy order by high pressure produces a new phase with a T c of ∼56 K [20]. A variety of experimental techniques have been used to study the intrinsic phase separation [6][7][8][9][10][11][12][13][14][15][16][17][18][19][21][22][23][24], revealing a wealth of information on the peculiar microstructure of these materials.…”

Understanding Doping, Vacancy, Lattice Stability, and Superconductivity in KxFe2−ySe2

“…The results in ref 31. revealed that the obtained SC phases differ only in K contents, K 0.3 Fe 2 Se 2 with a T c = 44 K and K 0.6 Fe 2 Se 2 with a T c = 30 K. Superconductivity with T c of 30 K and 43 K was also observed in K x Fe 2− y Se 2 samples with √2 × √2 × 1 superstructure due to K vacancy ordering, but the phases for different T c have not been specified 25, 28, 42, 43. In addition, K‐intercalated iron selenide with excess Fe was also proposed to have T c of 44 K 40…”

“…The origin of this issue is largely due to the phase separation that inevitably occurs in these systems at 500–578 K,17 leading to the coexistence of the insulating phase (A 2 Fe 4 Se 5 ) and the SC phase 18, 19, 20, 21, 22, 23. The thus‐obtained SC phase is not standing freely; instead, it intergrows with the insulating phase in the form of nanostrip and its volume fraction is quite low, 10%–20%, as estimated by various measurements 24, 25, 26, 27, 28, 29. This is the main obstacle that prohibits the precise determination of the structure and the composition of the SC phase.…”