FeAs Layers Research Articles

Magnetism and electronic structures of novel layered CaFeAs2 and Ca0.75(Pr/La)0.25FeAs2

The magnetic and electronic properties of the parent material CaFeAs2 of new superconductors are investigated using first-principles calculations. We predict that the ground state of CaFeAs2 is a spin-density-wave (SDW)-type striped antiferromagnet driven by Fermi surface nesting. The magnetic moment around each Fe atom is about 2.1 μB. We also present electronic and magnetic structures of electron-doped phase Ca0.75(Pr/La)0.25FeAs2, the SDW order was suppressed by La/Pr substitution. The As in arsenic layers is negative monovalent and acts as blocking layers enhancing two-dimensional character by increasing the spacing distance between the FeAs layers. This favors strong antiferromagnetic fluctuations mediated pairing, implying higher Tc in Ca0.75(Pr/La)0.25FeAs2 than Ca0.75(Pr/La)0.25Fe2As2.

CaFeAs2: A staggered intercalation of quantum spin Hall and high-temperature superconductivity

We predict that CaFeAs$_2$, a newly discovered iron-based high temperature (T$_c$) superconductor, is a staggered intercalation compound that integrates topological quantum spin hall (QSH) and superconductivity (SC). CaFeAs$_2$ has a structure with staggered CaAs and FeAs layers. While the FeAs layers are known to be responsible for high T$_c$ superconductivity, we show that with spin orbital coupling each CaAs layer is a $Z_{2}$ topologically nontrivial two-dimensional QSH insulator and the bulk is a 3-dimensional weak topological insulator. In the superconducting state, the edge states in the CaAs layer are natural 1D topological superconductors. The staggered intercalation of QSH and SC provides us an unique opportunity to realize and explore novel physics, such as Majorana modes and Majorana Fermions chains.

Significant contribution of As4porbitals to the low-lying electronic structure of the 112-type iron-based superconductorCa0.9La0.1FeAs2

We report a systematic polarization-dependent angle-resolved photoemission spectroscopy study of the three-dimensional electronic structure of the recently discovered 112-type iron-based superconductor Ca1-xLaxFeAs2 (x = 0.1). Besides the commonly reported three hole-like and two electron-like bands in iron-based superconductors, we resolve one additional hole-like band around the zone center and one more fast-dispersing band near the X point in the vicinity of Fermi level. By tuning the polarization and the energy of incident photons,we are able to identify the specific orbital characters and the kz dependence of all bands. Combining with band calculations, we find As 4pz and 4px (4py) orbitals contribute significantly to the additional three-dimensional hole-like band and the narrow band, respectively. Also, there are considerable hybridization between the As 4p zand Fe 3d orbitals in the additional hole-like band, which suggests the strong coupling between the unique arsenic zigzag bond layers and the FeAs layers therein. Our findings provide a comprehensive picture of the orbital characters of the low-lying band structure of 112-type iron-based superconductors, which can be a starting point for the further understanding of their unconventional superconductivity.

Correlation-induced self-doping in the iron-pnictide superconductor Ba2Ti2Fe2As4O.

The electronic structure of the iron-based superconductor Ba2Ti2Fe2As4O (Tc(onset)=23.5 K) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the spacer layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect; i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the 3d shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.

Evidence for magnetic (AF SDW) 2D-phase transition in doped ferropnictides and ferroselenides

The evidence for magnetic (AF SDW) 2D phase transition preceding a superconducting one (Tc < Tm) is presented for the plane of the FeAs(Se) layers of doped ferropnictides (-selenides). In contrast to the parent undoped compounds, the incommensurability of SDW and the lattice period leads to the formation of CDW with one-half wavelength. The magnetic phase (H-T) diagram of doped ferropnictides (-selenides) is characteristic of systems in which dielectric (e-h) and superconducting (e-e) pairings coexist.

Synchrotron X-ray Diffraction Study of Structural Phase Transition in Ca10(Ir4As8)(Fe2−xIrxAs2)5

We report on the structural phase transition found in Ca10(Ir4As8)(Fe2−xIrxAs2)5, which exhibits superconductivity at 16 K. The c-axis parameter is doubled below a structural transition temperature of approximately 100 K, while the tetragonal symmetry with the space group P4/n (No. 85) is unchanged at all temperatures measured. Our synchrotron X-ray diffraction study clearly shows iridium ions at a non-coplanar position shift along the z-direction at the structural phase transition. We discuss the fact that iridium displacements affect superconductivity in Fe2As2 layers.

Critical current oscillations in the intrinsic hybrid vortex state of SmFeAs(O,F).

In layered superconductors the order parameter may be modulated within the unit cell, leading to nontrivial modifications of the vortex core if the interlayer coherence length ξ(c)(T) is comparable to the interlayer spacing. In the iron pnictide SmFeAs(O,F) (T(c)≈50 K) this occurs below a crossover temperature T(⋆)≈41 K, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around T(⋆), hybrid vortices between these two characteristics appear. Only in this region around T(⋆) and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the c-axis critical current j(c)(H) periodic in 1/sqrt[H] due to a delicate balance of intervortex forces and interaction with the layered potential. j(c)(H) shows pronounced maxima when a hexagonal vortex lattice is commensurate with the underlying crystal structure. The narrow temperature window in which oscillations are observed suggests a significant suppression of the order parameter between the superconducting layers in SmFeAs(O,F), despite its low coherence length anisotropy (γ(ξ)≈3-5).

Superconductivity by transition metal doping in Ca10(Fe1−xMxAs)10(Pt3As8) (M = Co, Ni, Cu)

We report the successful substitution of cobalt, nickel and copper for iron in the 1038-phase parent compound yielding , and ), respectively. Superconductivity is induced in Co and Ni doped compounds reaching critical temperatures up to 15 K, similar to known Pt substituted ), whereas no superconductivity was detected in . The obtained phase diagrams are very similar to those of other iron arsenide superconductors indicating rather universal behaviour despite the more complex structures of the 1038-type compounds, where the physics is primarily determined by the FeAs layer.

Heterogeneous Structural Distortions Induced by In-Plane and Out-of-Plane Doping in Iron-Based Superconductors

For iron-based superconductors, both the in-plane and out-of-plane doping can cause the superconductivity. In this contribution, we investigated both the electronic structure and local lattice structure induced by in-plane and out-of-plane doping in SmFeAsO-based superconductors probed by X-ray absorption spectroscopy. Data pointed out that upon out-of-plane doping the charge reservoir layers and superconductive layers become closer, enhancing the electron transfer from SmO to FeAs layers, and in the end, almost all the electron carriers were gathered around Fe atoms. On the other hand, for in-plane doping, we observed the elongation of Sm–As bonds but the shrinkage of FeAs layers that promoted the transfer of electron carriers on superconductive layers. Accordingly, it is because of the heterogeneous structural distortions that lead to the different superconducting mechanism of in-plane and out-of-plane doping in iron-based superconductors.

Lattice distortion and stripelike antiferromagnetic order inCa10(Pt3As8)(Fe2As2)5

Ca10(Pt3As8)(Fe2As2)5 is the parent compound for a class of Fe-based high-temperature superconductors where superconductivity with transition temperatures up to 30 K can be introduced by partial element substitution. We present a combined high-resolution high-energy x-ray diffraction and elastic neutron scattering study on a Ca10(Pt3As8)(Fe2As2)5 single crystal. This study reveals the microscopic nature of two distinct and continuous phase transitions to be very similar to other Fe-based high-temperature superconductors: an orthorhombic distortion of the high-temperature tetragonal Fe-As lattice below T_S = 110(2) K followed by stripe-like antiferromagnetic ordering of the Fe moments below T_N = 96(2) K. These findings demonstrate that major features of the Fe-based high-temperature superconductors are very robust against variations in chemical constitution as well as structural imperfection of the layers separating the Fe-As layers from each other and confirms that the Fe-As layers primarily determine the physics in this class of material.

Tuning the magnetic and structural phase transitions of PrFeAsO via Fe/Ru spin dilution

Neutron diffraction and muon spin relaxation measurements are used to obtain a detailed phase diagram of Pr(Fe,Ru)AsO. The isoelectronic substitution of Ru for Fe acts effectively as spin dilution, suppressing both the structural and magnetic phase transitions. The temperature of the tetragonal-orthorhombic structural phase transition decreases gradually as a function of x. Slightly below the transition temperature coherent precessions of the muon spin are observed corresponding to static magnetism, possibly reflecting a significant magneto-elastic coupling in the FeAs layers. Short range order in both the Fe and Pr moments persists for higher levels of x. The static magnetic moments disappear at a concentration coincident with that expected for percolation of the J1-J2 square lattice model.

First-Principles Investigation of Electronic Structure Features of TbOFeAs Compound

We report a theoretical investigation on the electronic and magnetic properties of rare-earth pnictide parent compound, such as TbOFeAs. Employing first-principles method supplemented by the local spin density approximation (LSDA), we discuss the electronic structure with the incorporation of the role of Coulomb on-site repulsion (U) of Tb 4f states as well as the spin-orbit (SO) coupling on the magnetic and nonmagnetic phases. For ferromagnetic (FM) and antiferromagnetic (AFM) phases, we have determined the spin and orbital magnetic moments of Tb ions and confer the significance of the spin-orbit interaction of Tb 4f states in this parent compound. In the FM state, the reduction of Fe moment is about a factor of 3.5 with respect to AFM configuration. The most energetically favorable state is AFM configuration. Our theoretical findings surmise that the magnetic moments on Fe sites carry an AFM order. Based on LSDA + U + SO approximation, we infer that the Tb magnetic moments also carry an AFM order, albeit the spin Tb sites in TbO layer possess the same orientation as the Fe spins in FeAs layer. With the incorporation of on-site Coulomb repulsion and spin-orbit interaction in AFM state, the Fe 3d states are large near the Fermi level and this phase is illustrating a metallic behavior. Moreover, the Fermi surface topology and nesting features are presented.

Coexistence of Bloch electrons and glassy electrons inCa10(Ir4As8)(Fe2−xIrxAs2)5revealed by angle-resolved photoemission spectroscopy

Angle-resolved photoemission spectroscopy of ${\text{Ca}}_{10}({\text{Ir}}_{4}{\text{As}}_{8})({\text{Fe}}_{2\ensuremath{-}x}{\text{Ir}}_{x}{\text{As}}_{2}{)}_{5}$ shows that the Fe $3d$ electrons in the FeAs layer form the holelike Fermi pocket at the zone center and the electronlike Fermi pockets at the zone corners as commonly seen in various Fe-based superconductors. The FeAs layer is heavily electron doped and has relatively good two dimensionality. On the other hand, the Ir $5d$ electrons are metallic and glassy probably due to atomic disorder related to the Ir $5d$ orbital instability. ${\text{Ca}}_{10}({\text{Ir}}_{4}{\text{As}}_{8})({\text{Fe}}_{2\ensuremath{-}x}{\text{Ir}}_{x}{\text{As}}_{2}{)}_{5}$ exhibits a unique electronic state where the Bloch electrons in the FeAs layer coexist with the glassy electrons in the ${\text{Ir}}_{4}{\text{As}}_{8}$ layer.

Influence on the Tc of Co and F Co-doping into SmFeAsO at Fe and O Site

Series of superconductive samples in which Co and F co-doping into SmFeAsO were prepared by mechanical alloying (MA) and sequent sintering method to investigate the influence on Tc of the charge carrier introduced into SmO layers and FeAs layers simultaneously. For the SmFe1 −xCoxAsO with optimum Co doping content of x= 0.1, F doping can still enhance the Tc until up to 23 K when doping content is 15 % (from our previous works, the highest Tc of SmFe1 −xCoxAsO is 12.5 K). The charge carrier density of SmFe0.9Co0.1AsO0.85F0.15 is larger than that of SmFe0.9Co0.1AsO, characterized by Hall coefficient measurement. On the other hand, for optimum F-doping SmFeAsO0.8F0.2, simultaneous Co doping decreases the Tc dramatically. For the under-doping SmFeAsO0.9F0.1, the Tc rises with the increasing Co doping content to 10 % and then falls down with more Co doping into the compounds. The results can offer some experimental evidence of the competition and restriction among the factors influencing the Tc of the SmFeAsO by doping Co and F simultaneously.

Framework structures of interconnected layers in calcium iron arsenides.

The new calcium iron arsenide compounds Ca(n(n+1)/2)(Fe(1-x)M(x))(2+3n)M'(n(n-1)/2)As((n+1)(n+2)/2) (n = 1-3; M = Nb, Pd, Pt; M' = □, Pd, Pt) were synthesized and their crystal structures determined by single-crystal X-ray diffraction. The series demonstrates the structural flexibility of iron arsenide materials, which otherwise prefer layered structures, as is known from the family of iron-based superconductors. In the new compounds, iron arsenide tetrahedral layers are bridged by iron-centered pyramids, giving rise to so far unknown frameworks of interconnected FeAs layers. Channels within the structures are occupied with calcium and palladium or platinum, respectively. Common basic building blocks are identified that lead to a better understanding of the building principles of these structures and their relation to CaFe4As3.

Strongly three-dimensional electronic structure and Fermi surfaces ofSrFe2(As0.65P0.35)2: Comparison withBaFe2(As1−xPx)2

The isovalent-substituted iron-pnictide superconductor ${\mathrm{SrFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$ ($x=0.35$) has a slightly higher optimum critical temperature than the similar system ${\mathrm{BaFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$, and its parent compound ${\mathrm{SrFe}}_{2}$${\mathrm{As}}_{2}$ has a much higher N\'eel temperature than ${\mathrm{BaFe}}_{2}$${\mathrm{As}}_{2}$. We have studied the band structure and the Fermi surfaces of optimally doped ${\mathrm{SrFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$ by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around $(0,0)$ and two electronlike FSs around $(\ensuremath{\pi},\ensuremath{\pi})$ have been observed as in the case of ${\mathrm{BaFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$. Measurements with different photon energies have revealed that the outermost hole FS is more strongly warped along the ${k}_{z}$ direction than the corresponding one in BaFe(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$, and that the innermost one is an ellipsoidal pocket. The electron FSs are almost cylindrical, unlike the corrugated ones in BaFe(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$. A comparison of the ARPES data with a first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character, but also between the hole and electron FSs of the same orbital character. By examining the nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the ${d}_{xy}$ and/or ${d}_{xz/yz}$ orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

Effect of As-chain layers inCaFeAs2

The new discovered iron-based superconductors have chain-like As layers. These layers generate an additional 3-dimensional hole pocket and cone-like electron pockets. The former is attributed to the Ca $d$ and As1 $p_z$ orbitals and the latter are attributed to the anisotropic Dirac cone, contributed by As1 $p_x$ and $p_y$ orbitals. We find that large gaps on these pockets open in the collinear antiferromagnetic ground state of CaFeAs$_2$, suggesting that the chain-like As layers are strongly coupled to FeAs layers. Moreover due to the low symmetry crystal induced by the As layers, the bands attributed to FeAs layers in $k_y=\pi$ plane are two-fold degenerate but in $k_x=\pi$ plane are lifted. This degeneracy is protected by a hidden symmetry $\hat{\Upsilon}=\hat{T}\hat{R}_y$. Ignoring the electron cones, the materials can be well described by a six-band model, including five Fe $d$ and As1 $p_z$ orbitals. We suggest that these new features may help us to identify the sign change and pairing symmetry in iron based superconductors.

Effects of phosphorous doping on the superconducting properties of SmFeAs(O,F)

The systematic doping effect induced by the isovalent substitution of P for As on the superconducting properties of F-doped SmFeAsO0.88F0.12 (Sm1111) has been studied by physical and magnetic measurements. The cell volume (V) decreases with P doping and the anisotropic chemical pressure might be induced. However, the superconducting transition temperature (Tc) and the upper critical field (Hc2) are suppressed. Thermoelectric power (S) indicates the majority of electron type charge carriers in support of Hall measurements and its magnitude does not change very much for different P concentrations. The present investigation depicts that isovalent substitutions in the FeAs layer strongly deteriorate the superconducting properties of Sm1111 as a result of increase in chemical pressure. These isovalent substitution effects are comparatively discussed with hole (Mn) and electron (Ni) type substitutions in the superconducting layer of Sm1111.

Depairing current density of Ba0.5K0.5Fe1.95Co0.05As2 microbridges with nanoscale thickness

We investigated the depairing current density of Ba0.5K0.5Fe1.95Co0.05As2 microbridges with width of 2μm and thickness of 150nm. The current vs. voltage characteristics of the microbridges show a Josephson-like behavior with the obvious hysteresis. The critical current density was observed as Jc=7.9MA/cm2 at temperature 21K, which is consistent with the Ginzburg–Landau depairing limit. However, the depairing current density is less than that of impurity-free crystal, although the Co ions provide additional pinning centers within the superconducting Fe2As2 layers. The Co impurity also enhances the anisotropic factor of the critical current density by 1.3 (1T) or 1.7 (3T).

Control of the superconducting properties of Sr2−xCaxVO3FeAs through isovalent substitution

The effect of the isovalent substitution of Sr2+ by Ca2+ on the structure and superconducting properties of Sr2−xCaxVO3FeAs is described in the compositional range 0≤x≤0.5. SQUID magnetometry measurements reveal that after an initial increase in Tc, which is maximised at 29.5K in Sr1.95Ca0.05VO3FeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr2−xCaxVO3FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in Tc with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with Tc maximised in samples with regular FeAs4-tetrahedra.