Influence of microstructure on superconductivity in KxFe2−ySe2 and evidence for a new parent phase K2Fe7Se8

Abstract

The search for new superconducting materials has been spurred on by the discovery of iron-based superconductors whose structure and composition is qualitatively different from the cuprates. The study of one such material, KxFe2−ySe2 with a critical temperature of 32 K, is made more difficult by the fact that it separates into two phases—a dominant antiferromagnetic insulating phase K2Fe4Se5, and a minority superconducting phase whose precise structure is as yet unclear. Here we perform electrical and magnetization measurements, scanning electron microscopy and microanalysis, X-ray diffraction and scanning tunnelling microscopy on KxFe2−ySe2 crystals prepared under different quenching processes to better understand the relationship between its microstructure and its superconducting phase. We identify a three-dimensional network of superconducting filaments within this material and present evidence to suggest that the superconducting phase consists of a single Fe vacancy for every eight Fe-sites arranged in a √8 x √10 parallelogram structure.

Structure and composition of the superconducting phase in alkali iron selenideKyFe1.6+xSe2

“…Alternatively, superconductivity in A y Fe 1.6+x Se 2 may arise from a chemically separated superconducting phase in the matrix of the insulating block AF phase. Although transmission electron microscopy (TEM) [18][19][20][21] , X-ray/neutron diffraction 13,[22][23][24] , scanning tunneling microscopy (STM) 26 , Mössbauer spectroscopy 27 , muon spin relaxation 28 , apertureless scattering-type scanning near-field optical microscopy 29 , and nuclear magnetic resonance 30 experiments have provided ample evidence for phase separation, where superconductivity comprises about 10-20% of the volume of A y Fe 1.6+x Se 2 , there is currently no consensus on the chemical composition or crystal structure for the superconducting A y Fe 1.6+x Se 2 . For example, while some TEM 21 , X-ray scattering 24 , and STM 26 measurements suggest that the superconducting phase of A y Fe 1.6+x Se 2 is A z Fe 2 Se 2 with the BaFe 2 As 2 iron pnictide crystal structure [1][2][3] , other TEM and STM measurements propose that the superconducting phase consists of a single Fe vacancy for every eight Fe-sites arranged in a √ 8× √ 10 parallelogram structure [ Fig.…”

“…For example, while some TEM 21 , X-ray scattering 24 , and STM 26 measurements suggest that the superconducting phase of A y Fe 1.6+x Se 2 is A z Fe 2 Se 2 with the BaFe 2 As 2 iron pnictide crystal structure [1][2][3] , other TEM and STM measurements propose that the superconducting phase consists of a single Fe vacancy for every eight Fe-sites arranged in a √ 8× √ 10 parallelogram structure [ Fig. 1(c)] 20 . In addition, single crystal neuron diffraction experiments indicate that superconductivity in A y Fe 1.6+x Se 2 may arise from a semiconducting AF phase with rhombus iron vacancy order [ Fig.…”

“…The superconducting phase arises in the region of the excess iron between the low temperature √ 5 × √ 5 iron vacancy ordered phase, and its volume fraction can be controlled through temperature cycling and quenching processes. Rietveld and pair distribution function (PDF) analysis of the data 20 . d) Iron-disordered, partially occupied orthorhombic phase with BaFe2As2 iron pnictide structure.…”

Impurity-induced subgap bound states in alkali-doped iron chalcogenide superconductors

“…4 It is now well documented that superconductivity phase separates from the BAFM and exists in filamentary regions parallel and perpendicular to the FeSe planes. [5][6][7][8][9][10][11][12][13][14][15] In addition to the presence of phase separation, any theoretical description of the superconducting state is necessarily altered compared to the usual iron pnictides by the absence of Fermi surface hole pockets.Experiments find only electron pockets around the M points of 1 Fe Brillouin zone at k z = 0 and the development of additional electron pockets around the Z point. [16][17][18] The description of a spin-fluctuation based pairing mechanism in iron based pnictide superconductors argues for a leading s± pairing instability based on (π, 0) nesting between the electron and hole Fermi surface sheets.…”

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

“…Different opinions, however, exist. For example, the SC phases are thought to originate from superstructures due to Fe vacancy, such as 2 × 436 or √8 × √10 37. Meanwhile, phases with disordered Fe vacancies were also reported to be SC 38, 39.…”