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Register a new userPossible Topological Phase Transition in Fe-Vacancy-Ordered $beta$-Fe$_{4+delta}$Se$_{5}$ Nanowires
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We studied the electrical transport on $beta$-Fe$_{4+delta}$Se$_{5}$ single-crystal nanowires, exhibiting $sqrt{5}timessqrt{5}$ Fe-vacancy order and mixed valence of Fe. We observed a first-order metal-insulator transition of the transition temperature at $sim$28~K at zero magnetic field. The dielectric relaxation reveals that the transition is related to an energy gap expansion of $sim$12~meV, involving the charge-orbital ordering. At nearly 28~K, colossal positive magnetoresistance emerges, resulting from the magnetic-field dependent shift of the transition temperature. Through the transition, the magnetotransport behavior transits from two-dimension-like to one-dimension-like conduction. The transition temperature demonstrates anisotropy with the $c$-axis as the preferred orientation in magnetic fields, suggesting the spin-orbital coupling. Our findings demonstrate the novel magnetoresistive transition intimating a topological transition in the Fe-vacancy-ordered $beta$-Fe$_{4+delta}$Se$_{5}$ nanowires. The results provide valuable information to better understand the orbital nature and the emergence of superconductivity in FeSe-based materials.
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The electronic structure of the vacancy-ordered K$_{0.5}$Fe$_{1.75}$Se$_2$ iron-selenide compound (278 phase) is studied using the first-principles density functional method. The ground state of the 278 phase is stripe-like antiferromagnetic, and its bare electron susceptibility shows a large peak around $(pi, pi)$ in the folded Brillouin zone. Near Fermi level, the density of states are dominated by the Fe-3d orbitals, and both electron-like and hole-like Fermi surfaces appear in the Brillouin zone. Unfolded band structure shows limited similarities to a hole doped 122 phase. With 0.1e electron doping, the susceptibility peak is quickly suppressed and broadened; while the two-dimensionality of the electron-like Fermi surfaces are greatly enhanced, resulting in a better nesting behavior. Our study should be relevant to the recently reported superconducting phase K$_{0.5+x}$Fe$_{1.75+y}$Se$_2$ with both $x$ and $y$ very tiny.
We have carried out a detailed study to investigate the existence of an insulating parent phase for FeSe superconductor. The insulating Fe4Se5 with specific Fe-vacancy order shows a 3D-Mott variable range hopping behavior with a Verwey-like electronic correlation at around 45 K. The application of the RTA process at 450 celcius degree results in the destruction of Fe-vacancy order and induces more electron carriers by increasing the Fe3+ valence state. Superconductivity emerges with Tc ~ 8K without changing the chemical stoichiometry of the sample after the RTA process by resulting in the addition of extra carriers in favor of superconductivity.
Vibrational properties of iron-chalcogenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_{2}$ with $T_{c}sim$ 30 K have been measured by Raman and optical spectroscopies over temperature range of 3-300 K. Sample undergoes textit{I4/m} $to $ textit{I4} structural phase transition accompanied by loss of inversion symmetry at $T_{1}$, below 250 K, observed as appearance of new fully-symmetric Raman mode at $sim$ 165 cm$^{-1}$. Small vibration mode anomalies are also observed at $T_{2}sim$ 160 K. From first-principles vibrational analysis of antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ utilizing pseudopotentials all observed Raman and infrared modes have been assigned and the displacement patterns of the new Raman mode identified as involving predominantly the Se atoms.
Various Fe-vacancy orders have been reported in tetragonal Fe1-xSe single crystals and nanowires/nanosheets, which are similar to those found in alkali metal intercalated A1-xFe2-ySe2 superconductors. Here we report the in-situ angle-resolved photoemission spectroscopy study of Fe-vacancy disordered and ordered phases in FeSe multi-layer thin films grown by molecular beam epitaxy. Low temperature annealed FeSe films are identified to be Fe-vacancy disordered phase and electron doped. Further long-time low temperature anneal can change the Fe-vacancy disordered phase to ordered phase, which is found to be semiconductor/insulator with (root 5) x (root 5) superstructure and can be reversely changed to disordered phase with high temperature anneal. Our results reveal that the disorder-order transition in FeSe thin films can be simply tuned by vacuum anneal and the (root 5) x (root 5) Fe-vacancy ordered phase is more likely the parent phase of FeSe.
Several superconducting transition temperatures in the range of 30-40 K were reported in the recently discovered intercalated FeSe sytem (A1-xFe2-ySe2, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal {beta}-Fe1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A1-xFe2-ySe2 are not exclusive in Fe-Se related superconductors. In addition, the magnetic {beta}-Fe1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of FeSe superconducting system, instead of the previously assigned {beta}-Fe1+{delta}Te.
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