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Register a new userDirect observation of nanoscale interface phase in the superconducting chalcogenide K$_{x}$Fe$_{2-y}$Se$_2$ with intrinsic phase separation
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We have used scanning micro x-ray diffraction to characterize different phases in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (T$_csim$32 K), phase separation temperature (T$_{ps}sim$520 K) and iron-vacancy order temperature (T$_{vo}sim$580 K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found a clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below $sim$300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides a clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the T$_{ps}$ and T$_c$ is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in K$_{x}$Fe$_{2-y}$Se$_2$.
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Structural phase separation in A$_x$Fe$_{2-y}$Se$_2$ system has been studied by different experimental techniques, however, it should be important to know how the electronic uniformity is influenced, on which length scale the electronic phases coexist, and what is their spatial distribution. Here, we have used novel scanning photoelectron microscopy (SPEM) to study the electronic phase separation in K$_x$Fe$_{2-y}$Se$_2$, providing a direct measurement of the topological spatial distribution of the different electronic phases. The SPEM results reveal a peculiar interconnected conducting filamentary phase that is embedded in the insulating texture. The filamentary structure with a particular topological geometry could be important for the high T$_c$ superconductivity in the presence of a phase with a large magnetic moment in A$_x$Fe$_{2-y}$Se$_2$ materials.
K$_x$Fe$_{2-y}$Se$_2$ exhibits an iron-vacancy ordering at $T_{rm s} {sim}270{deg}$C and separates into two phases: a minor superconducting (iron-vacancy-disordered) phase and a major non-superconducting (iron-vacancy-ordered) phase. The microstructural and superconducting properties of this intermixture can be tuned by an appropriate control of the quenching process through $T_{rm s}$. A faster quenching rate leads to a finer microstructure and a suppression of formation of the non-superconducting phase by up to 50%. Nevertheless, such a faster cooling rate does induce a monotonic reduction in the superconducting transition temperature (from 30.7 K down to 26.0 K) and, simultaneously, a decrease in the iron content within the superconducting phase such that the compositional ratio changed from K$_{0.35}$Fe$_{1.83}$Se$_2$ to K$_{0.58}$Fe$_{1.71}$Se$_2$.
We report an in-plane optical spectroscopy study on the iron-selenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the development of a sharp reflectance edge below T$_c$ 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.
Pressure dependence of the electronic and crystal structures of K$_{x}$Fe$_{2-y}$Se$_{2}$, which has pressure-induced two superconducting domes of SC I and SC II, was investigated by x-ray emission spectroscopy and diffraction. X-ray diffraction data show that compressibility along the c-axis changes around 12 GPa, where a new superconducting phase of SC II appears. This suggests a possible tetragonal to collapsed tetragonal phase transition. X-ray emission spectroscopy data also shows the change in the electronic structure around 12 GPa. These results can be explained by the scenario that the two SC domes under pressure originate from the change of Fermi surface topology. Present results here show that the nesting condition plays a key role in stabilizing the superconducting state helping to address outstanding fundamental question as to why the SC II appears under pressure.
In this work, we study the A$_{x}$Fe$_{2-y}$Se$_2$ (A=K, Rb) superconductors using angle-resolved photoemission spectroscopy. In the low temperature state, we observe an orbital-dependent renormalization for the bands near the Fermi level in which the dxy bands are heavily renormliazed compared to the dxz/dyz bands. Upon increasing temperature to above 150K, the system evolves into a state in which the dxy bands have diminished spectral weight while the dxz/dyz bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature induced crossover from a metallic state at low temperature to an orbital-selective Mott phase (OSMP) at high temperatures. Furthermore, the fact that the superconducting state of A$_{x}$Fe$_{2-y}$Se$_2$ is near the boundary of such an OSMP constraints the system to have sufficiently strong on-site Coulomb interactions and Hunds coupling, and hence highlight the non-trivial role of electron correlation in this family of iron superconductors.
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