FeAs Layers Research Articles

Local magnetic anisotropy in BaFe2As2: A polarized inelastic neutron scattering study

The anisotropy of the magnetic excitations in BaFe$_2$As$_2$ was studied by polarized inelastic neutron scattering which allows one to separate the components of the magnetic response. Despite the in-plane orientation of the static ordered moment we find the in-plane polarized magnons to exhibit a larger gap than the out-of-plane polarized ones indicating very strong single-ion anisotropy within the layers. It costs more energy to rotate a spin within the orthorhombic {\it a-b} plane than rotating it perpendicular to the FeAs layers.

Ba2Ti2Fe2As4O: A New Superconductor Containing Fe2As2 Layers and Ti2O Sheets

We have synthesized a new oxypnictide, Ba2Ti2Fe2As4O, via a solid-state reaction under a vacuum. The compound crystallizes in a body-centered tetragonal lattice, which can be viewed as an intergrowth of BaFe2As2 and BaTi2As2O, thus containing Fe2As2 layers and Ti2O sheets. Bulk superconductivity at 21 K is observed after annealing the as-prepared sample at 773 K for 40 h. In addition, an anomaly in resistivity and magnetic susceptibility around 125 K is revealed, suggesting a charge- or spin-density wave transition in the Ti sublattice.

Electronic and Phonon Structures of BaFe2As2 Superconductor by Ab-initio Density Functional Theory

Electronic and phonon structures of the BaFe2As2 superconductor in the magnetic–orthorhombic phase have been investigated by the ab-initio density functional theory using the pseudopotential Quantum Espresso code. Density of state and electronic band structure for this phase has been studied, but phonon dispersion has been obtained only for the nonmagnetic–orthorhombic phase. Electronic band structure and density of states are in good agreement with other calculations in the literature. The electronic state near the Fermi energy are essentially made from Fe3d and As4p orbital that indicate in-plane conductivity in FeAs layers in this system. Comparing calculated phonon dispersions with experimental data justify magnetic dependence of some phonon modes.

Unsynchronized resonance of covalent bonds in the superconducting state

DFT calculations performed on different cluster models of cuprates (LaBa2Cu3O6.7, La1.85Sr0.15CuO4, YBa2Cu3O7, TlBa2Ca2Cu3O8.78, HgBa2Ca2Cu3O8.27), metallic systems (Nb3Ge, MgB2) and the pnictide LaO0.92F0.08FeAs made evident the occurrence of unsynchronized resonance of covalent bonds in the superconducting state, as predicted by Pauling´s resonating valence bond (RVB) theory. For cuprates, the unsynchronized resonance involves electron transfer between Cu atoms accompanied by a decrease in the charge of the La, Sr, Y and Ca atoms. For MgB2, electron transfer occurs in the Mg layer, while the B layer behaves as charge reservoir. For Nb3Ge, unsynchronized resonance occurs among the Ge atoms, which should be responsible for charge transfer. For LaO0.92F0.08FeAs, the results suggest that both La-O and Fe-As layers are involved in the mechanism of superconductivity. The identification of unsynchronized resonances in these systems provides evidence which supports RVB as a suitable theory for high-temperature superconductivity (high-T C).

Effect of Ru substitution on atomic displacements in the layered SmFe1−xRuxAsO0.85F0.15superconductor

The effect of Ru substitution on the local structure of layered SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ superconductor has been studied by As $K$- and Sm $L_3$ - edges x-ray-absorption spectroscopy. The extended x-ray-absorption fine-structure measurements reveal distinct Fe-As and Ru-As bondlengths in the Ru substituted samples with the latter being $\sim$0.03 \AA\ longer. Local disorder induced by the Ru substitution is mainly confined to the FeAs layer while the SmO spacer layer sustains a relative order, consistent with the x-ray-absorption near-edge structure spectra. The results suggest that, in addition to the order/disorder in the active active iron-arsenide layer, its coupling to the rare-earth\textminus oxygen spacer layer needs to be considered for describing the electronic properties of these layered superconductors.

Combinative energy between two structural blocks and superconductivity in LaFe1−xCoxAsO

The cohesive energy of the whole cell and the combinative energy between the FeAs block and the LaO block in LaFe1−xCoxAsO were calculated using a block model. A relationship between the combinative energy and Tc is found similar to the previous reports in SmOFeAs systems to some extent. Remarkably, the combinative energy in LaFe1−xCoxAsO appears nearly unchanged regardless of the positions we locate electron at the Fe, As site or at the centre of the FeAs layer, which further confirms an itinerant character in LaFe1−xCoxAsO. In addition, the relationship between the combinative energy and Tc is also extended to the whole LnOFeAsF(LnLnLa, Pr, Nd, Sm, Gd) systems and the relationship can be well explained with the charge-transfer model.

Field dependence of the Eu2+ spin relaxation in EuFe2−xCoxAs2

The layered compound EuFe2As2 is an interesting model system to investigate the effects of well-defined local Eu2+ 4f states on the itinerant electronic and magnetic properties of the FeAs layers. To address this subject, we investigated the series EuFe2−xCoxAs2 (0.1 ⩽ x ⩽ 0.75) by electron spin resonance (ESR) of Eu2+ to probe the spin dynamics of the itinerant subsystem. We relate the results to dc-susceptibility measurements and band structure calculations. As a consequence of the weak coupling between the local and itinerant subsystems, we found that the spin relaxation is well understood in terms of the exchange coupling among the local Eu2+ spins. A pronounced field dependence of the Eu2+ spin relaxation demonstrates the direct influence of magnetic fluctuations at the Fe2−xCoxAs2 layers.

Effect of rare earth substitution in the density of electronic states of LnOFeAs

Measurements of the Fe K-edge x-ray absorption near edge (XANES) spectra of LnOFeAs (Ln being a lanthanide) high Tc superconductors exhibit significant changes in the pre-edge peak region upon rare earth substitution. Ab initio XANES calculations, based on the local structure centered at the Fe site obtained by crystallographic investigations, reproduce the observed changes in the spectra, indicating variations of the Fe d-local unoccupied electronic states. The calculated Fe density of states (DOS) at the Fermi energy shows an increase of the Fe d-density of states with increasing height of the arsenic atomic position with respect to the iron plane, similar to that observed for the superconducting transition temperature, Tc. These calculations show that not only the atomic position variation of the Fe-As layers induced by the rare earth substitution is relevant in the increase of the DOS at the Fermi-level, but the actual change in electronic configuration of the rare earth also plays a role in the increase of Fe d-density of states at the Fermi energy.

Superconducting fluctuations and anomalous phonon renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature Tc = 28.3 K, reveal that the Raman mode at ∼230 cm−1 shows a sharp jump in frequency by ∼2% and linewidth increases by ∼175% at To ∼ 60 K. Below To, anomalous softening of the mode frequency and a large decrease by ∼10 cm−1 in the linewidth are observed. These precursor effects at T0 (∼2Tc) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weak coupling between the well separated (∼15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K and 60 K (∼7%) indicates strong spin-phonon coupling in this superconductor.

Interpretation of optical conductivity in normal state of Iron-Based Superconductors CeOFeAs

Quantitative analysis of the optical conductivity σ(ω) for the normal state of Iron-Based superconductors CeOFeAs have been made within the two-component scheme: one is the coherent Drude free carrier excitations and other is incoherent motion of carriers leading to a polaron formation, originated from inter and intra layer transitions of charge carriers. The model successfully accounts for the anomalies reported in the optical measurements for metallic state of the superconductors. The frequency dependent relaxation rates are expressed in terms of memory functions and the coherent Drude carriers from the effective interaction potential leads to a sharp peak at zero frequency which is an indication of metallic conduction and a long tail at higher frequencies, i.e. in the infrared region. While to that the hopping of carriers from Fe to Fe in the FeAs layer and from FeAs layer to CeO layer (incoherent motion of carriers) yields two-peak value around 100 cm−1 and 425 cm−1 respectively in the optical conductivity centred at mid-infrared region. Both the Drude and hopping carriers contribute to the optical process of conduction in the iron-based superconductors and shows similar results on optical conductivity in the mid-infrared as well as infrared frequency regions as those revealed from experiments.

Ab initioinvestigation of FeAs/GaAs heterostructures for potential spintronic and superconducting applications

Ultra-thin FeAs is of interest both as the active component in the newly identified pnictide superconductors, and in spintronic applications at the interface between ferromagnetic Fe and semiconducting GaAs. Here we use first-principles density functional theory to investigate the properties of FeAs/GaAs heterostructures. We find that the Fermi surface is modified from that characteristic of the pnictide superconductors by interactions between the FeAs layer and the As atoms in the GaAs layers. Regardless of the number of FeAs layers, the Fe to As ratio, or the strain state, the lowest energy magnetic ordering is always antiferromagnetic, suggesting that such heterostructures are not promising spintronic systems, and offering an explanation for the failure of spin injection across Fe/GaAs interfaces.

Gd3+ electron spin resonance spectroscopy on LaO1 − x F x FeAs superconductors

We report an electron spin resonance (ESR) spectroscopy study on polycrystalline samples of the LaO1 − xFxFeAs (x = 0 and 0.1) compound with small levels of Gd doping (2% and 5%). The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent LaO1 − xFxFeAs compounds at TSDW ∼ 130 K. Interestingly, the analysis of the low-temperature ESR spectra of the c-axis oriented Gd1 − yLayOFeAs samples gives evidence for the magnetically nonequivalent Gd sites and also for sites having a different local charge environment. The analysis of the temperature dependence of the ESR linewidths gives evidence for a coupling of the localized 4f electrons of Gd to the conduction electrons in the FeAs layers. The ESR data reveal that the fluorine substitution, which provides electron doping, suppresses the SDW order and enhances the density of states in the electronic bands stemming from the xz and yz orbital states of Fe to which the 4f electrons are most strongly coupled.

Fermi-surface topology and low-lying electronic structure of the iron-based superconductor Ca10(Pt3As8)(Fe2As2)5

We report a first study of low energy electronic structure and Fermi surface topology for the recently discovered iron-based superconductor Ca10(Pt3As8)(Fe2As2)5 (the 10-3-8 phase, with Tc = 8K), via angle-resolved photoemission spectroscopy (ARPES). Despite its triclinic crystal structure, ARPES results reveal a fourfold symmetric band structure with the absence of Dirac-cone-like Fermi dots (related to magnetism) found around the Brillouin zone corners in other iron-based superconductors. Considering that the triclinic lattice and structural supercell arising from the Pt3As8 intermediary layers, these results indicate that those layers couple only weakly to the FeAs layers in this new superconductor, which has implications for the determination of its potentially novel pairing mechanism.

Magnetic Interactions on Single-Crystal EuFe2As2Studied by57Fe Mössbauer Spectroscopy

Magnetic interactions on single-crystal AFe 2 As 2 (A: Eu and Sr) were studied by 57 Fe Mossbauer spectroscopy. An abrupt appearance of the effective magnetic hyperfine field ( B eff ) is observed at the spin density wave transition temperature (∼190 K for Eu and ∼200 K for Sr) in both samples. The value of B eff in EuFe 2 As 2 shows a small but significant increase around the Neel temperature T N Eu (=19 K) of the Eu 2+ sublattice, while B eff in SrFe 2 As 2 almost saturates at lower temperatures. Consistent with this, the isomer shift of EuFe 2 As 2 indicates an anomalous temperature dependence below approximately T N Eu . These results provide direct evidence of the coupling between the localized Eu 2+ moments and the conduction electrons from the FeAs layers and suggest that this coupling may affect the observed superconductivity in EuFe 2 As 2 under pressure.

Determination factors of magnetic anisotropy in a layered iron-based pnictide and its related compounds

From magnetic orientation under static and rotating magnetic fields for RE OFeAs, RE OCuS and Fe Ch , we discuss determination factors of magnetic anisotropies for the above compounds. Behaviours of the magnetic orientation were quite different between RE OFeAs and RE OCuS, and revealed that FeAs layers and RE3+ ions played main roles as determination factors of magnetic anisotropies in RE OFeAs and RE OCuS, respectively. Furthermore, FeSe and FeTe compounds with the same crystal structure possess different magnetic axes, and we clarified that FeTe was the one and only compound with the easy axis parallel to the c -axis among the Fe- based layered compounds.

Iron–platinum–arsenide superconductors Ca10(PtnAs8)(Fe2−xPtxAs2)5

An overview of the crystal structures and physical properties of the recently discovered iron–platinum–arsenide superconductors, Ca10(PtnAs8)(Fe2−xPtxAs2)5 (n=3 and 4), which have a superconducting transition temperature up to 38 K, is provided. The crystal structure consists of superconducting Fe2As2 layers alternating with platinum–arsenic layers, PtnAs8. The upper critical field Hc2, hydrostatic pressure dependence of superconducting transition temperature Tc, and normal-state magnetic susceptibility are reported.

Correlated Trends of Coexisting Magnetism and Superconductivity in Optimally Electron-Doped Oxypnictides

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe(1-x)Ru(x)AsO(0.85)F(0.15) for 0.1≤x≲0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold x(c)~0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold x(c).

High T c electron doped Ca 10 (Pt 3 As 8 )(Fe 2 As 2 ) 5 and Ca 10 (Pt 4 As 8 )(Fe 2 As 2 ) 5 superconductors with skutterudite intermediary layers

It has been argued that the very high transition temperatures of the highest T c cuprate superconductors are facilitated by enhanced CuO 2 plane coupling through heavy metal oxide intermediary layers. Whether enhanced coupling through intermediary layers can also influence T c in the new high T c iron arsenide superconductors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca 10 (Pt 3 As 8 )(Fe 2 As 2 ) 5 (the “10-3-8 phase”) and Ca 10 (Pt 4 As 8 )(Fe 2 As 2 ) 5 (the “10-4-8 phase”). Based on -Ca-(Pt n As 8 )-Ca-Fe 2 As 2 - layer stacking, these are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to T c of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The often-cited empirical rule in the arsenide superconductor literature relating T c to As-Fe-As bond angles does not explain the observed differences in T c of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase arising from the two-channel interlayer interactions and the metallic nature of its intermediary Pt 4 As 8 layer. The interlayer coupling is thus revealed as important in enhancing T c in the iron pnictide superconductors.

Superconductivity at 38 K in Iron-Based Compound with Platinum–Arsenide Layers Ca10(Pt4As8)(Fe2-xPtxAs2)5

We report superconductivity in novel iron-based compounds Ca10(PtnAs8)(Fe2-xPtxAs2)5 with n = 3 and 4. Both compounds crystallize in triclinic structures (space group P-1), in which Fe2As2 layers alternate with PtnAs8 spacer layers. Superconductivity with a transition temperature of 38 K is observed in the n = 4 compound with a Pt content of x ~ 0.36 in the Fe2As2 layers. The compound with n = 3 exhibits superconductivity at 13 K.

Hydrogen in layered iron arsenides: Indirect electron doping to induce superconductivity

Utilizing the high stability of calcium and rare-earth hydrides, CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--1.0) and SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--0.47) have been synthesized using high pressure to form hydrogen-substituted 1111-type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H${}^{\ensuremath{-}}$ ions occupying F${}^{\ensuremath{-}}$ sites in the blocking layer of CaFeAsF. The resulting CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ is nonsuperconducting, whereas, SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ is a superconductor, with an optimal ${T}_{\mathrm{c}}$ $=$ 55 K at $x$ \ensuremath{\sim} 0.2. It was found that up to 40% of the O${}^{2\ensuremath{-}}$ ions can be replaced by H${}^{\ensuremath{-}}$ ions, with electrons being supplied into the FeAs layer to maintain neutrality (O${}^{2\ensuremath{-}}$ $=$ H${}^{\ensuremath{-}}$ $+$ ${e}^{\ensuremath{-}}$). When $x$ exceeded 0.2, ${T}_{\mathrm{c}}$ was reduced corresponding to an electron overdoped region.