Charge and spin dynamics in antiferromagnetic metallic phase of iron-based superconductors

The electronic states, charge dynamics, and spin dynamics in the antiferromagnetic metallic phase of iron-arsenide superconductors are investigated by mean-field calculations for a five-band Hubbard model. Taking into account the difference of observed magnetic moments between LaFeAsO (1111 system) and BaFe₂As₂ (122 system), we investigate the effect of the magnitude of the moments on band dispersion, optical conductivity, and dynamical spin susceptibility. We clarify how the magnitude affects on these quantities and predict different behaviors between the 1111 and 122 systems in the antiferromagnetic metallic phase.     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

Dependence of magnetic ordering temperature of doped and undoped EuFe2As2 on hydrostatic pressure to 0.8 GPa

The ac susceptibility of single crystalline tetragonal EuFe₂As₂, EuFe₂As_1.4P_0.6, and EuFe_1.715Co_0.285As₂ has been measured over the temperature and hydrostatic (He-gas) pressure ranges 10–60 K and 0–0.8 GPa, respectively. For all three samples the magnetic ordering temperature (17–19 K) from the Eu sublattice increases linearly with pressure, presumably due to the enhanced exchange coupling between Eu-layers. No evidence for a superconducting transition was observed in the susceptibility for any sample over the measured temperature/pressure range.     

Photoexcited Eu2+ spin dynamics in EuFe2As2

Employing temperature dependent time-resolved optical femtosecond spectroscopy, we investigated the quasiparticle and Eu²⁺ spin relaxation dynamics in EuFe₂As₂ (EFA). As previously reported in other undoped iron-based pnictides, we observe the quasiparticle relaxation bottleneck due to the charge gap opening in the spin density wave (SDW) state below T _SDW = 189 K. Below the Eu²⁺ antiferromagnetic (AFM) spin ordering temperature, T _AFM = 19 K, we observe another slower relaxation component, which we attribute to the Eu²⁺ AFM order dynamics. The slow dynamics of this component suggests a weak coupling between the Eu²⁺ spins and the carriers in the Fe-d derived bands.     

Effect of the distortion of FeX4 (X = P,As) tetrahedron for the electronic structure of iron-pnictide system

We have performed an ab initio band structure calculation for the new high-T_c related iron-pnictide compounds LaFeXO (X = P, As), BaFe₂As₂, CaFe₂As₂ and LiFeAs (X = P, As). We found that LaFeXO and CaFe₂As₂ have many similarities in their band structures, which is expected by an ionic model. We found that the degree of distortion of FeAs₄ tetrahedra in LaFeAsO considerably changes the slope of the density of states near the Fermi level, and this result may explain why REFeAsO (RE = Nd, Sm, …) have higher T_c than LaFeAsO when electrons are doped. For all the above compounds, the density of states at the Fermi level decreases when X atoms approaches to the Fe–Fe plane, which means that the hybridization between Fe and X orbitals considerably expands the Fe d-bands.     

Heteroepitaxial film growth of layered compounds with the ZrCuSiAs-type and ThCr2Si2-type structures: From Cu-based semiconductors to Fe-based superconductors

FeAs-based layered superconductors such as F-doped LaFeAsO have recently been investigated intensively because of their high superconducting transition temperatures. Epitaxial films of these compounds are important to examine their intrinsic materials properties as well as to transfer them to device applications. In this review, we first present our research route from transparent p-type oxides semiconductors to the Fe-based superconductors. Then we review growth of epitaxial thin films for the layered oxychalcogenides and oxypnictides. Reactive solid-phase epitaxy technique was inevitable to prepare epitaxial thin films of the oxychalcogenides and Zn-based oxypnictides. On the other hand, epitaxial thin films of Mn-based oxypnictides were grown by standard pulsed laser deposition. These techniques, however, did not grow epitaxial thin films for LaFeAsO. Thus, we developed a modified pulsed laser deposition process and succeeded in obtaining epitaxial thin films of FeAs-based superconductors, LaFeAsO and cobalt-doped SrFe₂As₂.     

Ni2X2 (X = pnictide,chalcogenide, or B) based superconductors

We review the properties of Ni-based superconductors which contain Ni₂X₂ (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi₂As₂. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in T_c in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.     

Inelastic neutron scattering studies of the spin and lattice dynamics in iron arsenide compounds

Although neutrons do not couple directly to the superconducting order parameter, they have nevertheless played an important role in advancing our understanding of the pairing mechanism and the symmetry of the superconducting energy gap in the iron arsenide compounds. Measurements of the spin and lattice dynamics have been performed on non-superconducting ‘parent’ compounds based on the LaFeAsO (‘1111’) and BaFe₂As₂ (‘122’) crystal structures, and on electron and hole-doped superconducting compounds, using both polycrystalline and single crystal samples. Neutron measurements of the phonon density-of-state, subsequently supported by single crystal inelastic X-ray scattering, are in good agreement with ab initio calculations, provided the magnetism of the iron atoms is taken into account. However, when combined with estimates of the electron–phonon coupling, the predicted superconducting transition temperatures are less than 1 K, making a conventional phononic mechanism for superconductivity highly unlikely. Measurements of the spin dynamics within the spin density wave phase of the parent compounds show evidence of strongly dispersive spin waves with exchange interactions consistent with the observed magnetic order and a large anisotropy gap. Antiferromagnetic fluctuations persist in the normal phase of the superconducting compounds, but they are more diffuse. Below T_c, there is evidence in three ‘122’ compounds that these fluctuations condense into a resonant spin excitation at the antiferromagnetic wavevector with an energy that scales with T_c. Such resonances have been observed in the high-T_c copper oxides and a number of heavy fermion superconductors, where they are considered to be evidence of d-wave symmetry. In the iron arsenides, they also provide evidence of unconventional superconductivity, but a comparison with ARPES and other measurements, which indicate that the gaps are isotropic, suggests that the symmetry is more likely to be extended-s_± wave in character.     

Electronic structure of novel multiple-band superconductor SrPt<Subscript>2</Subscript>As<Subscript>2</Subscript>

The electronic structure of the recently discovered superconductor SrPt₂As₂ with T _c = 5.2 K has been calculated in the local-density approximation. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt₂As₂ is very much different. The crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs₄ and AsPt₄ tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt₂As₂ is essentially three-dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt₂As₂ with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni₂As₂ superconductors. Brief discussion of superconductivity in SrPt₂As₂ is also presented.     

Elastic properties and chemical bonding in ternary arsenide SrFe2As2 and quaternary oxyarsenide LaFeAsO – Basic phases for new 38–55 K superconductors from first principles

Here we report the first-principle FLAPW-GGA calculations of the elastic properties of two related layered phases, namely, the ternary arsenide SrFe₂As₂ and the quaternary oxyarsenide LaFeAsO – basic phases for the newly discovered “1 2 2” and “1 1 1 1” 38–55 K superconductors. The independent elastic constants (C_ij), bulk moduli, compressibility, and shear moduli are evaluated and discussed. The numerical estimates of the elastic parameters of the polycrystalline SrFe₂As₂ and LaFeAsO ceramics are performed for the first time. Additionally, the peculiarities of chemical bonding in these phases are discussed.     

The physical properties of ThCr2Si2- type nickel-based superconductors BaNi2T2 (T = P,As): An ab-initio study

Employing first-principles computations depending on the density functional theory (DFT), we have investigated the structural, mechanical, electronic, bonding, optical and thermodynamics properties of the newly discovered bulk superconductors BaNi₂P₂ (T_c ∼ 3 K) and BaNi₂As₂ (T_c ∼ 0.7 K). Our optimized lattice parameters are in good agreement with the experimental results. The positive elastic constants reveal that both the superconductors are stable in nature. The analysis of the mechanical properties insures that both the phases are ductile in nature and show anisotropic behavior. The analysis of the electronic band structures and density of states (TDOS and PDOS) reveals the metallic nature for both the compounds, and the major contribution comes from the Ni-3d states for both the phases. The calculations of the chemical bonding specify that a mixture of covalent, ionic and metallic bonds exist in bothsuperconductors. The negative value of the real part of the dielectric constant of BaNi₂T₂ (T = P, As) also ensures that these compounds exhibit metallic behavior. The large reflectivity in the low energy region indicates that both the compounds might be useful as coating materialsfor reducing solar heating. By using the elastic constants information, the calculated Debye temperatures of BaNi₂P₂ and BaNi₂As₂ are 323.70 K and 272.94 K, respectively, which are in good accord with the experimental values. Finally, we have calculated the thermal conductivity of these compounds, which is 0.56 for BaNi₂P₂ and 0.46 for BaNi₂As₂.     

Electronic structure and magnetic coupling properties of EuFe2P2: First-principles calculations

We have investigated the electronic structure and magnetic properties of EuFe₂P₂ using first-principles density functional theory within the generalized gradient approximation (GGA)+U schemes. Our calculated ground state magnetic configurations of EuFe₂P₂ is ferromagnetic which Eu²⁺ spins order along c axis. We argue that this kind of magnetic structure of Eu is determined by the indirect RKKY interactions between Eu and direct coupling interaction between Eu 4f with Fe 3d state by our spin-polarized density of states calculations. From the charge density and the Laplace charge density of EuFe₂P₂, we believe that the magnetic moment of Fe is determined by not only Fe-P coupling interactions but also Fe-Fe directly exchange interactions.     

First-principle calculation for the phonon structure on iron-based superconductors

We perform first-principle phonon calculations for three typical iron-based superconductors, i.e., LaFeAsO,BaFe₂As₂, and FeSe. Though those crystals have different structures, we find that the optical modes associated with Fe vibration have almost similar characters. Moreover, we examine the pressure effect on phonons in FeSe. By increasing the external pressure, the phonon mode frequency related to Fe vibration effectively rises up and the electronic density of states at Fermi level also increases. These results may correlate to the critical temperature enhancement under high pressure.     

ESR in X-irradiated As2O3 glass

X-irradiation of glassy As₂O₃ at 77K or 300K produces an unusually large density (～5×10¹⁸ cm^-3) of paramagnetic centers which are stable at 300K. The average spin-Hamiltonian parameters (g = 1.998, g_⊥ = 1.984, A₆ = 243G, A_⊥ = 114G) indicate that these centers are analogous to those previously observed in As₂Se₃ and As₂S₃ glasses and that they consist of unpaired electrons localized on a non-bonding 4p orbital of an As atom. Unlike the results obtained for As₂Se₃ and As₂S₃, the concommitant holes in As₂O₃ are trapped on Fe²⁺ impurity sites which become Fe³⁺ and not on non-bonding oxygen p orbitals. The radiation induced ESR is also accompanied by a stable optical absorption tail which lies within the band gap and increases exponentially with energy. This absorption can be partially bleached with the application of sub-band-gap light.     

Mössbauer studies of the peculiar magnetism in parent compounds of the iron-based superconductors

A review of the magnetism in the parent compounds of the iron-based superconductors is given based on the transmission Mössbauer spectroscopy of ⁵⁷Fe and ¹⁵¹Eu. It was found that the 3d magnetism is of the itinerant character with varying admixture of the spin-polarized covalent bonds. For the ‘122’ compounds, a longitudinal spin density wave (SDW) develops. In the case of the EuFe₂As₂, a divalent europium arranges in an anti-ferromagnetical order at a much lower temperature as compared with the onset of SDW. These two magnetic systems remain almost uncoupled one to another. For the non-stoichiometric Fe_1+xTe parent of the ‘11’ family, one has a transversal SDW and magnetic order of the interstitial iron with relatively high and localized magnetic moments. These two systems are strongly coupled one to another. For the ‘grand parent’ of the iron-based superconductors FeAs, one observes two mutually orthogonal phase-related transversal SDW on the iron sites. There are two sets of such spin arrangements due to two crystallographic iron sites. The FeAs exhibits the highest covalency among the compounds studied, but it has still a metallic character.     

Visualization of the magnetic flux structure in phosphorus-doped EuFe<Subscript>2</Subscript>As<Subscript>2</Subscript> single crystals

Magnetic flux structure on the surface of EuFe₂(As_1-x P _x )₂ single crystals with nearly optimal phosphorus doping levels x = 0.20 and x = 0.21 is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with x = 0.21 in the temperature range between the critical temperatures T _SC= 22 K and T _C = (18 ± 0.3) K of the superconducting and ferromagnetic phase transitions, respectively, has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below T _C. The nature of this structure is discussed.     

Temperature dependence of the lower critical field Hc1 in SmFeASO0.9F0.1 and Ba0.6K0.4Fe2As2 iron-arsenide superconductors

We report detailed measurements of the temperature dependence of the lower critical field H_c1 of the FeAs-based superconductor SmFeAsO_0.9F_0.1 (Sm-1111) and Ba_0.6K_{0. 4}Fe₂As₂ (BaK-122) by global and local magnetization measurements. It is found that the obtained H_c1 for both kinds of samples show a weak temperature dependence in low temperature region. We argue that this weak T-dependence of H_c1 of Sm-1111 does not indicate a conventional s-wave state, instead it satisfies a T² dependence up to a temperature of (0.5–0.6) T_c. In contrast, the data of H_c1(T) of BaK-122 superconductor clearly show a multigap behavior. Excellent fitting to the data can be reached with two s-wave superconducting gaps. Comparison of the absolute values of H_c1(0) between Sm-1111 and BaK-122 shows a relatively large superfluid density for the latter. This may indicate the distinction between the electron doped and hole-doped FeAs-based superconductors.     

Comparative study of electronic structure of new superconductors (Sr, Ca)Pd2As2 and related compound BaPd2As2

This paper presents the comparative study of LDA calculated electronic structure of new isostructural to iron based systems superconductors (Sr, Ca)Pd₂As₂ with T _c about 1 K and similar but structurally different system BaPd₂As₂. Despite chemical formula looks similar to iron superconductors and even main structural motif is the same—layers of Fe square lattices, electronic structure of (Sr, Ca)Pd₂As₂ and BaPd₂As₂ differs from Fe (As, Se)-HTSC completely. All these systems have essentially three dimensional Fermi surfaces in contrast to Fe (As, Se) materials. The Fermi level is crossed by low intensive tails of Pd-4d and As-4p states. However, (Sr, Ca)Pd₂As₂ and BaPd₂As₂ materials have rather well developed peaks of Pd-4d (x ² ? y ²) band. Thus, by doping of about 2 holes per unit cell one can increase density of states at the Fermi level by a factor about 2.5. Since experimentally these compounds were found to be simple BCS superconductors the hole doping may considerably increase T _c . LDA calculated total densities of states at the Fermi level for stoichiometric systems perfectly agree with experimental estimates signifying rather small role of electronic correlations.     

Magnetic uni- and tri-axial grain-orientation in superconductors with layered structures

We report the grain-orientation effects under a modulated rotation magnetic field for Y-based cuprate superconductors and LaFeAsO (La1111). Tri-axial orientation has been successfully achieved only for orthorhombic Y₂Ba₄Cu₇O_y and YBa₂Cu₄O₈ powders without a twin microstructure, while separation of three crystallographic axes could not be observed in twinned YBa₂Cu₃O_y (Y123) and tetragonal La1111 powders. The morphology of grains, in addition to the symmetry of crystal structures, seriously affects the degrees of tri-axial orientation, which means that the control of twin microstructures is required for the tri-axial magnetic orientation in Y123.     

Study of the structure of a superconducting state of Co-doped BaFe2As2 multiband compounds

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting Ba(Fe_{1 ? x} Co _x )₂As₂ compounds, have been analyzed within a Bardeen-Cooper-Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: Δ₁ ≈ 15 cm^?1, Δ₂ ≈ 21 cm^?1, and Δ₃ ≈ 30–35 cm^?1. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe₂As₂ multiband compounds.