FeSe Materials Research Articles

Janus monolayers Fe2SSeX2(X = Ga, In, and Tl): Robust nontrivial topology with high Chern-number

Abstract High-performance quantum anomalous Hall (QAH) systems are the crucial materials to explore emerging quantum physics and magnetic topological phenomena. Inspired by layered FeSe materials with excellent superconducting properties, the Janus monolayers Fe2SSeX2 (X = Ga, In, and Tl) are built by the decoration of Ga, In, and Tl atoms in monolayer Fe2SSe. Using the first-principles calculations, the Fe2SSeX2 have stable structures and prefer ferromagnetic (FM) orders, which belong to the Weyl semimetal without spin-orbit coupling (SOC). For the out-of-plane (OOP) magnetic anisotropy, the large nontrivial gaps are opened, and the Fe2SSeX2 are predicted to be large-gap QAH insulators with high Chern-number C = 2, proved by two chiral edge states and Berry curvatures. When the magnetization is flipped, two chiral edge states can be simultaneously changed and C = -2 can be obtained, revealing one fascinating behavior of chiral spin-edge-state locking. It is found that the QAH properties of the Fe2SSeX2 are robust against the strain. Especially, the nontrivial topological quantum states can spontaneously appear for the Fe2SSeGa2 and Fe2SSeIn2, because the easy magnetic axis orientations are adjusted from in-plane to OOP by the biaxial strain. Our studies provide excellent candidate systems to realize the QAH properties with high Chern-number, and advocate more experimental explorations in the combination of superconductivity and topology.

On the Remarkable Superconductivity of FeSe and Its Close Cousins

Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.

Crystal structures and sign reversal Hall resistivities in iron-based superconductors Lix(C3H10N2)0.32FeSe )**Project supported by the National Natural Science Foundation of China (Grant Nos. 51472266, 51532010, 91422303, and 51772323), the National Key Research and Development Program of China (Grant No. 2016YFA0300301), and the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDJ-SSW-SLH013).

Heavy electron-doped FeSe-derived materials have attracted attention due to their uncommon electronic structures with only ‘electron pockets’, and they are different from other iron-based superconductors. Here, we report the crystal structures, superconductivities and normal state properties of two new Li-doped FeSe-based materials, i.e., Li0.15(C3H10N2)0.32FeSe (P-4) and Lix(C3H10N2)0.32FeSe (P4/nmm, ) with superconducting transition temperatures ranging from 40 K to 46 K. The determined crystal structures reveal a coupling between Li concentration and the orientation of 1,3-diaminopropane molecules within the largely expanded FeSe layers. Superconducting fluctuations appear in the resistivity of the two superconductors and are fitted in terms of the quasi two-dimensional (2D) Lawrence–Doniach model. The existence of a crossing point and scaling behavior in the T-dependence of diamagnetic response also suggests that the two superconductors belong to the quasi-2D system. Interestingly, with the increase of temperature, a sign of Hall coefficient (RH) reversing from negative to positive is observed at ∼185 K in both phases, suggesting that ‘hole pockets’ emerge in these electron-doped FeSe materials. First principle calculations indicate that the increase in FeSe layer distance will lift up a ‘hole band’ associated with character and increase the hole carriers. Our findings suggest that the increase in two dimensionalities may lead to the sign-reversal Hall resistivity in Lix(C3H10N2)0.32FeSe at high temperature.

Strain effects on electronic structures of monolayer iron sulphide and selenide

Structural and electronic properties of single atomic layers of FeS and FeSe superconductors have been investigated from first principles within the density functional theory (DFT). For both systems the mixed single-stripe and block-checkerboard type antiferromagnetic ground states are predicted. Namely, the single-stripe and block-checkerboard orders are almost degenerate. The metal-insulator transition in a hypothetical block-checkerboard phase of FeS occurs only under significant tensile strain, i.e. for in-plane lattice parameters larger than 3.8 Å. Compressive strain leads to almost a complete disappearance of magnetism in FeS system, whereas tensile strain strengthens it. The electronic structure of FeS monolayer in nonmagnetic phase is similar to that of FeSe, exhibiting the (π,π) nesting between hole- and electron-like Fermi surface sheets. The findings presented here explain different characters of superconductivity in FeS and FeSe materials and encourage further experimental studies on FeS-based thin films and heterostructures.

Pair-checkerboard antiferromagnetic order in β−Fe4Se5 with 5×5 -ordered Fe vacancies

The electronic structure of recently discovered $\beta$-Fe$_4$Se$_5$ with $\sqrt{5} \times \sqrt{5}$ ordered Fe vacancies is calculated using first-principles density functional theory. We find that the ground state is an antiferromagnetic (AFM) insulator in agreement with the experimental observation. In K$_2$Fe$_4$Se$_5$, it is known that the ground state is $\sqrt{5} \times \sqrt{5}$-blocked-checkerboard AFM ordered. But for this material, we find that the ground state is $\sqrt{5} \times \sqrt{5}$-pair-checkerboard AFM ordered, in which the intrablock four Fe spins exhibit the collinear AFM order and the interblock spins on any two nearest-neighboring sites are antiparallel aligned. This state is about 130 meV/Fe lower in energy than the $\sqrt{5} \times \sqrt{5}$-blocked-checkerboard AFM one. Electron doping can lower the energy of the $\sqrt{5} \times \sqrt{5}$-blocked-checkerboard AFM state and introduce a transition between these two states, suggesting that there is strong AFM fluctuation in FeSe-based materials upon doping. This provides a unified picture to understand the AFM orders in $\beta$-Fe$_4$Se$_5$ and in alkali-metal intercalated FeSe materials.

Strongly enhanced temperature dependence of the chemical potential in FeSe

Employing a 10-orbital tight binding model, we present a new set of hopping parameters fitted directly to our latest high resolution angle-resolved photoemission spectroscopy (ARPES) data for the high temperature tetragonal phase of FeSe. Using these parameters we predict a large 10 meV shift of the chemical potential as a function of temperature. In order to confirm this large temperature dependence, we performed ARPES experiments on FeSe and observed a $\sim$25 meV rigid shift to the chemical potential between 100 K and 300 K. This unexpectedly strong shift has important implications for theoretical models of superconductivity and of nematic order in FeSe materials.

Effect of Ag Addition on Microstructure and Raman Vibrational Modes of Bulk FeSe

We have synthesized polycrystalline bulk FeSe materials with various Ag concentrations through double-sintering solid-state reaction. All the samples exhibited similar onset critical temperature of around 8 K. Atomic force microscopic observation revealed that the grain connectivity was improved through Ag addition. Raman spectroscopic studies confirmed that pristine and Ag-doped FeSe compounds have superconducting tetragonal structure with high crystalline quality. There was a clear red shift and peak broadening in Fe-related Raman modes when the Ag concentration is increased, which might be ascribed to the incorporation of Ag+ ions into the FeSe lattice. The critical current density decreased with Ag addition, although onset critical temperature remained constant.

Superconductivity in (NH3)yCs0.4FeSe

Alkali-metal-intercalated FeSe materials, (NH${}_{3}$)${}_{\mathrm{y}}$${M}_{0.4}$FeSe ($M$: K, Rb, and Cs), have been synthesized using the liquid NH${}_{3}$ technique. (NH${}_{3}$)${}_{\mathrm{y}}$Cs${}_{0.4}$FeSe shows a superconducting transition temperature (${T}_{\mathrm{c}}$) as high as 31.2 K, which is higher by 3.8 K than the ${T}_{\mathrm{c}}$ of nonammoniated Cs${}_{0.4}$FeSe. The ${T}_{\mathrm{c}}$s of (NH${}_{3}$)${}_{\mathrm{y}}$K${}_{0.4}$FeSe and (NH${}_{3}$)${}_{\mathrm{y}}$Rb${}_{0.4}$FeSe are almost the same as those of nonammoniated K${}_{0.4}$FeSe and Rb${}_{0.4}$FeSe. The ${T}_{\mathrm{c}}$ of (NH${}_{3}$)${}_{\mathrm{y}}$Cs${}_{0.4}$FeSe shows a negative pressure dependence. A clear correlation between ${T}_{\mathrm{c}}$ and lattice constant $c$ is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity.