Parent Compounds Of Iron-based Superconductors Research Articles

Tetragonal SnOFeSe: A possible parent compound of the FeSe-based superconductor

Recent experiments have reported that inserting metal atoms or small molecules in between the FeSe layers of $\beta$-FeSe can significantly enhance the superconducting transition temperature. Here, based on first-principles electronic structure calculations, we propose a stable compound SnOFeSe by alternatively stacking the SnO and $\beta$-FeSe layers. The predicted SnOFeSe has the same tetragonal structure as the well-known FeAs-based compound LaOFeAs, meanwhile their electronic structures in the nonmagnetic state are quite similar. The magnetic ground state of SnOFeSe is predicted to be the dimer antiferromagnetic (AFM) state, which is energetically only 2.77 (2.15) meV/Fe lower than the trimer (dimer-trimer-dimer-trimer) AFM state, indicating that strong magnetic fluctuations might be induced via slight modulation. Interestingly, SnOFeSe is at the verge of metal-insulator transition in these low-energy magnetic states, hence bridging the metallic parent compounds of iron-based superconductors and the insulating ones of cuprate superconductors. With the reduced dimensionality, monolayer SnOFeSe also shows great similarities in the electronic and magnetic properties to its bulk phase. Given that SnOFeSe is adjacent to magnetic frustration and resembles LaOFeAs in both crystal and electronic structures, we suggest that SnOFeSe is a possible superconductor parent compound, which may provide a promising platform to study the interplay between magnetism and unconventional superconductivity in FeSe-derived materials.

Local Atomic Configuration Control of Superconductivity in the Undoped Pnictide Parent Compound BaFe2As2

Emergent superconductivity is strongly correlated with the symmetry of local atomic configuration in the parent compounds of iron-based superconductors. While chemical doping or hydrostatic pressure can change the local geometry, these conventional approaches do not provide a clear pathway in tuning the detailed atomic arrangement predictably, due to the parent compounds complicated structural deformation in the presence of the tetragonal-to-orthorhombic phase transition. Here, we demonstrate a systematic approach to manipulate the local structural configurations in BaFe2As2 epitaxial thin films by controlling two independent structural factors orthorhombicity (in-plane anisotropy) and tetragonality (out-of-plane/in-plane balance) from lattice parameters. We tune superconductivity without chemical doping utilizing both structural factors separately, controlling local tetrahedral coordination in designed thin film heterostructures with substrate clamping and bi-axial strain. We further show that this allows quantitative control of both the structural phase transition, associated magnetism, and superconductivity in the parent material BaFe2As2. This approach will advance the development of tunable thin film superconductors in reduced dimension.

Spin dynamics in NaFeAs and NaFe0.53Cu0.47As probed by resonant inelastic x-ray scattering

The parent compounds of iron-based superconductors are magnetically-ordered bad metals, with superconductivity appearing near a putative magnetic quantum critical point. The presence of both Hubbard repulsion and Hund's coupling leads to rich physics in these multiorbital systems, and motivated descriptions of magnetism in terms of itinerant electrons or localized spins. The NaFe$_{1-x}$Cu$_x$As series consists of magnetically-ordered bad metal ($x=0$), superconducting ($x\approx0.02$) and magnetically-ordered semiconducing/insulating ($x\approx0.5$) phases, providing a platform to investigate the connection between superconductivity, magnetism and electronic correlations. Here we use X-ray absorption spectroscopy and resonant inelastic X-ray scattering to study the valence state of Fe and spin dynamics in two NaFe$_{1-x}$Cu$_x$As compounds ($x=0$ and 0.47). We find that magnetism in both compounds arises from Fe$^{2+}$ atoms, and exhibits underdamped dispersive spin waves in their respective ordered states. The dispersion of spin excitations in NaFe$_{0.53}$Cu$_{0.47}$As is consistent with being quasi-one-dimensional. Compared to NaFeAs, the band top of spin waves in NaFe$_{0.53}$Cu$_{0.47}$As is slightly softened with significantly more spectral weight of the spin excitations. Our results indicate the spin dynamics in NaFe$_{0.53}$Cu$_{0.47}$As arise from localized magnetic moments and suggest the iron-based superconductors are proximate to a correlated insulating state with localized iron moments.

Electronic and Magnetic Anisotropies in FeSe Family of Iron-Based Superconductors

Most parent compounds of iron-based superconductors (FeSCs) exhibit a tetragonal-to-orthorhombic lattice distortion below Ts associated with an electronic nematic phase that breaks the four-fold (C4) rotational symmetry of the underlying lattice, and then forms collinear antiferromagnetic (AF) below TN (TN ≤ Ts). Optimal superconductivity emerges upon suppression of the nematic and AF phases. FeSe, which also exhibits a nematic phase transition below Ts but becomes superconducting in the nematic phase without AF order, provides a unique platform to study the interplay amongst the nematic phase, AF order, and superconductivity. In this review, we focus on the experiments done on uniaxial pressure detwinned single crystals of FeSe and other FeSCs and highlight the importance of understanding the electronic and magnetic anisotropy in elucidating the nature of unconventional superconductivity.

Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+ Y Te Using a Spin-polarized Scanning Tunneling Microscope.

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental techniques and methodologies. Here we use a novel home-built uniaxial-strain device integrated into our variable temperature scanning tunneling microscope that enables us to controllably manipulate in-plane uniaxial strain in samples and probe their electronic response at the atomic scale. Using scanning tunneling microscopy (STM) with spin-polarization techniques, we visualize antiferromagnetic (AFM) domains and their atomic structure in Fe1+yTe samples, the parent compound of iron-based superconductors, and demonstrate how these domains respond to applied uniaxial strain. We observe the bidirectional AFM domains in the unstrained sample, with an average domain size of ~50-150 nm, to transition into a single unidirectional domain under applied uniaxial strain. The findings presented here open a new direction to utilize a valuable tuning parameter in STM, as well as other spectroscopic techniques, both for tuning the electronic properties as for inducing symmetry breaking in quantum material systems.

In-plane electrical impedance as a probe for the electron nematicity of BaFe2As2

In-plane electrical impedance has been examined in as-grown single crystals of BaFe2As2, one of the parent compounds of iron-based superconductors. From the results, it is found that the real part of the impeditivity, namely the AC resistivity, reveals the in-plane anisotropy of the material without any applied uniaxial strain. The imaginary part, i.e., the reactivity, also indicates strong in-plane anisotropy and is linearly dependent on the electrical frequency. Our study demonstrates that electrical impedance is a new and effective method of probing the electron nematicity of iron-based superconductors.

First-principles study of the magnetic and electronic properties of ACr2As2 (A = Sr, Ba)

Abstract We report a detailed density functional theory investigation for ACr2As2 (A = Sr, Ba), which are isostructural to their iron counterparts AFe2As2, parent compounds of iron-based superconductors. The calculated results for these two compounds are quite similar, indicating that the A site ions have only a relatively small impact on the magnetic and electronic structures. It is found that both ACr2As2 favor an itinerant antiferromagnetic ground state with G-type spin arrangement, and possess a sizable magnetocrystalline anisotropy energy with the easy axis directed along the c axis perpendicular to the CrAs plane. Furthermore, the optimized lattice constants and internal atomic parameter of ACr2As2 are found to be sensitive to magnetic ordering, which implies the presence of a magnetostructural coupling effect.

Anisotropy of the Seebeck and Nernst coefficients in parent compounds of the iron-based superconductors

In-plane longitudinal and transverse thermoelectric phenomena in two parent compounds of iron-based superconductors are studied. Namely, the Seebeck (S) and Nernst (n) coefficients were measured in the temperature range 10 - 300 K for BaFe2As2 and CaFe2As2 single crystals that were detwinned in-situ. The thermoelectric response shows sizeable anisotropy in the spin density wave state (SDW) for both compounds, while some dissimilarities in the vicinity of the SDW transition can be attributed to the different nature of the phase change in BaFe2As2 and CaFe2As2. Temperature dependences of S and n can be described within a two-band model that contains a contribution from highly mobile, probably Dirac, electrons. The Dirac band seems to be rather isotropic, whereas most of the anisotropy in the transport phenomena could be attributed to "regular" hole-like charge carriers. We also observe that the off-diagonal element of the Peltier tensor axy is not the same for the a and b orthorhombic axes, which indicates that the widely used Mott formula is not applicable to the SDW state of iron-based superconductors.

Antiphase Fermi-surface modulations accompanying displacement excitation in a parent compound of iron-based superconductors

We investigate the transient electronic structure of BaFe2As2, a parent compound of iron-based superconductors, by time- and angle-resolved photoemission spectroscopy. In order to probe the entire Brillouin zone, we utilize extreme ultraviolet photons and observe photoemission intensity oscillation with the frequency of the A1g phonon which is antiphase between the zone-centered hole Fermi surfaces (FSs) and zone-cornered electron FSs. We attribute the antiphase behavior to the warping in one of the zone-centered hole FSs accompanying the displacement of the pnictogen height, and find that this displacement is the same direction as that induced by substitution of P for As, where superconductivity is induced by a structural modification without carrier doping in this system.

Ultrafast melting of spin density wave order in BaFe2As2 observed by time- and angle-resolved photoemission spectroscopy with extreme-ultraviolet higher harmonic generation

Transient single-particle spectral function of BaFe$_{2}$As$_{2}$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin-density-wave (SDW) ordered state within $\sim$ 1 ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_{F}$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From the comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim$ 0.60 ps both around the zone center and the zone corner.

Common Electronic Features and Electronic Nematicity in Parent Compounds of Iron-Based Superconductors and FeSe/SrTiO3 Films Revealed by Angle-Resolved Photoemission Spectroscopy**Supported by the National Natural Science Foundation of China under Grant Nos 11190022, 11334010 and 11534007, the National Basic Research Program of China under Grant No 2015CB921000, and the Strategic Priority Research Program (B) of Chinese Academy of Sciences under Grant

We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and undoped FeSe/SrTiO3 films with 1, 2 and 20 layers. While the electronic structure near the Brillouin zone center Γ varies dramatically among different materials, the electronic structure near the Brillouin zone corners (M points), as well as their temperature dependence, are rather similar. The electronic structure near the zone corners is dominated by the electronic nematicity that gives rise to a band splitting of the dxz and dyz bands below the nematic transition temperature. A clear relation is observed between the band splitting magnitude and the onset temperature of nematicity. Our results may shed light on the origin of nematicity, its effect on the electronic structures, and its relation with superconductivity in the iron-based superconductors.

Suppression of a possible spin-density wave transition in Cr2GaN by Ge doping

We have successfully synthesized and investigated the compound with a space group. By chemical doping, we substituted gallium element with germanium element in the system. The solubility limit of germanium element is about = 0.25 in . The lattice parameter of -axis becomes larger, while -axis becomes smaller with doping germanium. In CrGaN, there is a phase transition at 170 K as revealed by both resistivity and magnetic susceptibility measurements at ambient pressure, this transition could correspond to a possible spin-density wave ordering transition. The electrical transport and magnetic behaviours show very similar properties to that of the parent compounds of iron-based superconductors. By doping germanium to the system, the possible SDW transition was successfully suppressed. While with the application of a hydrostatic pressure up to 1.1 GPa, the transition temperature does not show a clear change. No superconductivity was observed in either Ge-doped sample or under a hydrostatic pressure.

Resonant inelastic x-ray scattering study of electronic excitations in insulatingK0.83Fe1.53Se2

We report an Fe $K$-edge resonant inelastic x-ray scattering study of ${\mathrm{K}}_{0.83}{\mathrm{Fe}}_{1.53}{\mathrm{Se}}_{2}$. This material is an insulator, unlike many parent compounds of iron-based superconductors. We found a sharp excitation around 1 eV, which is resonantly enhanced when the incident photon energy is tuned near the pre-edge region of the absorption spectrum. The spectral weight and lineshape of this excitation exhibit clear momentum dependence. In addition, we observe momentum-independent broad interband transitions at higher excitation energies of 3--7 eV. Calculations based on a 70-band $dp$ orbital model, using a moderate ${U}_{\mathrm{eff}}\ensuremath{\approx}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, indicate that the $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ feature originates from the correlated Fe $3d$ electrons, with a dominant ${d}_{xz}$ and ${d}_{yz}$ orbital character. We find that a moderate ${U}_{\mathrm{eff}}$ yields a satisfying agreement with the experimental spectra, suggesting that the electron correlations in the insulating and metallic iron-based superconductors are comparable.

Electronic Structure Reconstruction across the Antiferromagnetic Transition in TaFe1.23Te3 Spin Ladder

Employing the angle-resolved photoemission spectroscopy we study the electronic structure of TaFe1.23Te3, a two-leg spin ladder compound with a novel antiferromagnetic ground state. Quasi-two-dimensional (2D) Fermi surface is observed, with sizable inter-ladder hopping. Moreover, instead of observing an energy gap at the Fermi surface in the antiferromagnetic state, we observe the shifts of various bands. Combining these observations with density-functional-theory calculations, we propose that the large scale reconstruction of the electronic structure, caused by the interactions between the coexisting itinerant electrons and local moments, is most likely the driving force of the magnetic transition. Thus TaFe1.23Te3 serves as a simpler platform that contains similar ingredients to the parent compounds of iron-based superconductors.

Electron correlations and orbital selectivities in multiorbital models for iron-based superconductors

We review the recent theoretical progress of the multiorbital effects on the electron correlations in iron-based superconductors. Studying the metal-to-insulator transitions of the multiorbital Hubbard models for parent compounds of iron-based superconductors, a Mott transition is generally realized. The natures of both the Mott insulating and the metallic phases are affected by the Hund's rule coupling. In alkaline iron selenides, Hund's rule coupling stabilizes a novel orbital-selective Mott phase, in which the iron 3d xy orbital is Mott localized, while other 3d orbitals are still itinerant. We discuss the effects of the orbital selectivity on normal state properties and the superconductivity of the iron-based systems.

Crossover from bad to good metal in BaFe2(As1−xPx)2induced by isovalent P substitution

Bad metallic behavior or incoherent charge dynamics is characteristic of the normal state of high-transition-temperature copper-oxide superconductors. A parent compound of iron-based superconductors, BaFe${}_{2}$As${}_{2}$, is also a bad metal in the high-temperature phase, whereas its phosphorous counterpart, BaFe${}_{2}$P${}_{2}$, is a good metal. Combined measurements of in-plane resistivity and optical conductivity for BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ make it possible to disentangle the coherent channel from the incoherent one in the charge transport. The isovalent P substitution promotes coherent motion of carriers without changing the balance of the electron and hole Fermi-surface volume and hence transforms a bad metal into a good metal.

Fermi-surface reconstruction involving two van Hove singularities across the antiferromagnetic transition in BaFe2As2

We report an angle-resolved photoemission study of BaFe2As2, a parent compound of iron-based superconductors. Low-energy tunable excitation photons have allowed the first observation of three-dimensional saddle-point dispersion at the Z point, as well as the Γ point. This resolves a discrepancy in Fermi-surface studies. We found that these van Hove singularities are not only involved in three-dimensional Fermi-surface reconstruction with antiferromagnetic ordering but also renormalized and predominant at low energies. Hence, the Fermi-surface instability due to these singularities would also underlie the associated superconductors.

Quasiparticle interference in antiferromagnetic parent compounds of iron-based superconductors

Recently reported quasiparticle interference imaging in underdoped Ca(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ shows pronounced $C$${}_{2}$ asymmetry, which is interpreted as an indication of an electronic nematic phase with a unidirectional electron band, dispersive predominantly along the $b$ axis of this orthorhombic material. On the other hand, even more recent transport measurements on untwinned samples show near isotropy of the resistivity in the $\mathit{ab}$ plane, with slightly larger conductivity along $a$ (and not $b$). We show that, in fact, both sets of data are consistent with the calculated ab initio Fermi surfaces, which have a decisively broken $C$${}_{4}$ and yet similar Fermi velocity in both directions. This reconciles completely the apparent contradiction between the conclusions of the scanning tunneling microscopy and the transport experiments.

Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe2As2, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe2As2 in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe2As2 illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.