FeAs Compounds Research Articles

ANALYSIS OF STUDYING THE STRUCTURE OF EPITAXIAL IRON FILMS ON GALLIUM ARSENIDE Fe/GaAs

The anisotropy in thin films coincides with the crystallographic direction of an increase in the thickness of the iron layer. The value of the cubic anisotropy constant decreases. In this case, the value of the uniaxial anisotropy constant, on the contrary, increases. The dilution of the semiconductor matrix with iron atoms is also noted, and the formation of Fe-Ga or Fe-As compounds is assumed. An analysis of the surface sensitivity of the spectra of As and Ga shows that As is more easily mixed with Fe than with Ga.

Mechanistic insights into red mud, blast furnace slag, or metakaolin-assisted stabilization/solidification of arsenic-contaminated sediment

Elevated level of arsenic (As) in marine sediment via deposition and accumulation presents long-term ecological risks. This study proposed a sustainable stabilization/solidification (S/S) of As-contaminated sediment via novel valorization of red mud waste, blast furnace slag and calcined clay mineral, which were selected to mitigate the increased leaching of As under alkaline environment of S/S treatment. Quantitative X-ray diffraction and thermogravimetric analyses illustrated that stable Ca-As complexes (e.g., Ca5(AsO4)3OH) could be formed at the expense of Ca(OH)2 consumption, which inevitably hindered the hydration process and S/S efficiency. The 29Si nuclear magnetic resonance analysis revealed that incorporation of metakaolin for As immobilization resulted in a low degree of hydration and polymerization, whereas addition of red mud promoted Fe-As complexation and demonstrated excellent compatibility with As. Transmission electron microscopy and elemental mapping further confirmed the precipitation of crystalline Ca-As and amorphous Fe-As compounds. Therefore, red mud-incorporated S/S binder achieved the highest efficiency of As immobilization (99.9%), which proved to be applicable for both in-situ and ex-situ S/S of As-contaminated sediment. These results advance our mechanistic understanding for the design of green and sustainable remediation approach for effective As immobilization.

Reclamation of a waste arsenic-bearing gypsum as a soil conditioner via acid treatment and subsequent Fe(II)[sbnd]As stabilization

As a common hazardous waste in metallurgical industry, arsenic-bearing gypsum (ABG) is a great threat to ecological safety. However, the recycle of ABG has rarely been conducted due to its high arsenic mobility and worthless constituents. In this study, an ABG was successfully reclaimed as a soil conditioner via acid treatment and Fe(II)As stabilization. Firstly, the arsenic content of ABG was decreased below Japan standard (150 mg/kg) regarding safe materials by acid treatment (H2SO4H3PO4). Then upon heating oxidation, Fe2+ was used to stabilize the arsenic remaining in gypsum sludge at weakly acidic condition (pH 4) via forming insoluble FeAs compounds. In toxicity characteristic leaching procedure (TCLP), the arsenic leaching (0.125 mg/L) from reclaimed gypsum was much lower than TCLP limitation (5 mg/L). And in Japan standard leaching test (JSLT), it was suppressed below the Japan environmental criteria (<10 μg/L), remaining stable for three months upon air exposure. Furthermore, the possible influence factors regarding Fe-ABG stabilization including the destabilizing effects of acid treatment were examined. It showed that the oxidation was necessary for Fe(II)As immobilization. While keeping the weakly acidic condition was crucial important for that of Fe(III). Additionally, acid treatment would make troubles for subsequent FeAs fixation (mainly for Fe 3+). Especially, the introduction of free PO43− or Cl− ions would weaken the stability of insoluble FeAs compounds via forming iron phosphates or iron chloride hydrates and lead to the formation of soluble lollingite (FeAs2), causing arsenic leaching and compromising FeAs stabilization.

Study of Band Structure Properties of Pnictide LaO1−xF x FeAs (x = 0, 0.2) Superconducting Compound

This paper reports on the band structure properties and changes in band structure of fluorine-doped LaO1−x Fx FeAs (x = 0, 0.2) compound, measured using X-ray photoemission spectroscopy (XPS). The band structure of the superconducting compound is compared with nonsuperconducting parent compound LaOFeAs. With fluorine doping, a shift of the shallow core level is observed in XPS spectra, which may be a response of the band structure due to fluorine doping in the system. The balance of the chemical potential shift with the screening effect of conduction electrons near the Fe and As ions is discussed using nearly unchanged Fe 2p and As 3d core-level spectra. The La 3d core-level spectra shift towards the high energy, ∼0.36 eV, may be due to the chemical potential shift caused by fluorine doping. In our valence band spectra, a small peak at around 0.2 eV is observed, which disappeared with the fluorine doping in the system, indicating a change of Fe 3d state from low spin to high spin states and also confirming the nature of Fe 3d electrons as itinerant, which is responsible for superconductivity in these compounds.

Electron–phonon coupling in 122 Fe pnictides analyzed by femtosecond time-resolved photoemission

Based on the results from femtosecond time-resolved photoemission, we compare three different methods for the determination of the electron–phonon coupling constant λ in Eu- and Ba-based 122 FeAs compounds. We find good agreement between all three methods, which reveal a small λ < 0.2. This makes simple electron–phonon-mediated superconductivity unlikely in these compounds.

LiFeAs Pnictide Superconductor—A Simple Electrochemical Method of Preparation

Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It has a superconducting transition temperature, Tc, of 18 K with electron-like carriers and a very high 0 K upper critical magnetic field, Bc2(0), of greater than 80 T making the compound suitable for many high magnetic field applications at cryogenic temperatures. Oxypnictide materials are known for a complexity of their methods of preparation. The reported methods of LiFeAs preparation are based on the solid-state reaction at high temperatures (740 °-1050 °C) for long times (24-60 h). In the present work a simple electrochemical route is proposed for the preparation of a pnictide LiFeAs superconductor. During electrolysis, Li ions in the electrolyte become inserted into the FeAs lattice to form LiFeAs on the surface of the FeAs electrode. The proposed method differs from the existing electrochemical method and also from the traditional high temperature solid-state reaction methods. It is promising for the preparation of LiFeAs bulks and large scale LiFeAs films or tapes for various electric power or high magnetic field applications at cryogenic temperatures.

Effect of the Indium Addition on the Superconducting Property and the Impurity Phase in Polycrystalline SmFeAsO1-xFx

We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induces the bringing together the superconducting grains. It means the total contact surfaces of grains are increased. We suggest that the enhancement of the magnetic critical current density is a direct effect of the indium addition.

Ferromagnetism in CuFeSb: Evidence of competing magnetic interactions in iron-based superconductors

We have synthesized a new layered iron-pnictide CuFeSb. This material shares a similar layered tetragonal structure with iron-based superconductors, with Fe square planar sheets forming from the edge-sharing iron antimony tetrahedral network. CuFeSb differs remarkably from Fe-based superconductors in the height of anion ${Z}_{\mathrm{anion}}$ from the Fe plane; ${Z}_{\mathrm{Sb}}$ for CuFeSb is $\ensuremath{\sim}$1.84 \AA{}, much larger than ${Z}_{\mathrm{As}}$ (1.31--1.51 \AA{}) in FeAs compounds and ${Z}_{\mathrm{Te}}$ ($\ensuremath{\sim}$1.77 \AA{}) in Fe${}_{1+y}$Te. In contrast with the metallic antiferromagnetic (AFM) or superconducting state of iron pnictides and chalcogenides under current studies, CuFeSb exhibits a metallic, ferromagnetic (FM) state with ${T}_{\mathrm{c}}=375$ K. This finding suggests that the competition between AFM and FM coupling may exist in Fe-based superconductors and that the nature of magnetic coupling within the Fe plane is indeed dependent on the height of anion as predicted in theories.

Suppressed superconductivity in charge-doped Li(Fe1−xCox)As single crystals

Single crystals of the new unconventional superconductor LiFe${}_{1\ensuremath{-}x}$Co${}_{x}$As with $x$$=$ 0, 0.025, 0.05 were grown by a new approach using the self-flux technique. The superconducting transition temperature was found to decrease upon Co doping at the Fe site. Apparently, in LiFeAs this doping scheme suppresses superconductivity, in contrast to the effects of Co doping in other Fe-As compounds, where it suppresses the spin-density wave and establishes superconductivity. Angle-resolved photoemission spectroscopy shows that the bottom of electron-like bands sinks by about 17 meV upon $5%$ Co doping, which indicates that the chemical substitution of Co for Fe in LiFeAs results in charge doping.

Static magnetic order of Sr4A2O6Fe2As2(A=Sc and V) revealed by Mössbauer and muon spin relaxation spectroscopies

Static magnetic order of quasi two-dimensional FeAs compounds Sr4A2O6-xFe2As2, with A = Sc and V, has been detected by 57Fe Moessbauer and muon spin relaxation ({\mu}SR) spectroscopies. The non-superconducting stoichiometric (x = 0) A = Sc system exhibits a static internal/hyperfine magnetic field both at the 57Fe and {\mu}+ sites, indicating antiferromagnetic order of Fe moments below TN = 35 K with ~ 0.1 Bohr magneton per Fe at T = 2 K. The superconducting and oxygen deficient (x = 0.4) A = V system exhibits a static internal field only at the {\mu}+ site below TN ~ 40 K, indicating static magnetic order of V moments co-existing with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments.

Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO0.85F0.15: Evidence of strong anisotropy in the order parameter

Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 with an unexpectedly low superconducting transition temperature Tc ≈ 11.3 K compared to related FeAs compounds based on Sm and Gd surrounding Eu in lanthanide series. The nearly fivefold lower Tc, compared to the expected value, is attributed to the divalent properties of Eu ions; the weakly magnetic Eu3+ ions may be present and the highly magnetic Eu2+ ions are strong suppressors of superconductivity. Most of the measured spectra have features corresponding to two energy gaps whose values varied, from contact to contact, over 2Δs/kTc = 2.2 −4.7 and 2Δ1/kTc = 5.1−11.7 for small and large gaps, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6−6.4. The relatively large crystallite sizes (at least ∼25 μm) and the large number of measured contacts (several tens) suggest, with a high degree of probability, that these spectra account quite fully for the gap distribution in essentially all the crystallographic directions. These data and the absence of zero gaps in the measured spectra indicate anisotropic s− or s±-symmetry of the order parameter in EuAsFeO0.85F0.15 that has been found in other, similar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides.

Fishtail effect and vortex dynamics in LiFeAs single crystals

We investigate the fishtail effect, critical current density (${J}_{c}$), and vortex dynamics in LiFeAs single crystals. The sample exhibits a second peak (SP) in the magnetization loop only with the field $||$ $c$ axis. We calculate a reasonably high ${J}_{c}$, however, values are lower than in Ba-122- and 1111-type FeAs compounds. Magnetic relaxation data imply a strong pinning which appears not to be due to conventional defects. Moreover, we find that the calculated magnetic relaxation rate is very low in LiFeAs. Our data suggest that the origin of the SP may be related to a vortex lattice phase transition. We have constructed the vortex phase diagram for LiFeAs on the field-temperature plane.

Electron transport and anisotropy of the upper critical magnetic field in Ba0.68K0.32Fe2As2 single crystals

Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite for high-Tc superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, Hc2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of the electrical resistivity, \Gamma(T), in Ba0.5K0.5Fe2As2 and Ba0.68K0.32Fe2As2 single crystals in zero magnetic field and for Ba0.68K0.32Fe2As2 as well in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the \Gamma point in reciprocal space. From our data, we construct an H-T phase diagram for the inter-plane (H || c) and in-plane (H || ab) directions for Ba0.68K0.32Fe2As2. Contrary to published data for underdoped 122 FeAs compounds, we find that Hc2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, {\gamma} = Habc2/Hcc2, is about 2.2 at Tc. For both field orientations we find a concave curvature of the Hc2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism we perfectly can describe Hc2(T) and its anisotropy.

Preparation and Superconducting Properties of F-doped SmO 1- xF xFeAs

Here we report the fabrication and superconductivity of the iron-based arsenic oxide SmO 1- x F x FeAs compound. X-ray diffraction (XRD) results prove that the lattice parameters a and c decrease systematically with increasing x in between 0 x ≤0.35, but when x>0.35 the a and c increase with the decrease of x in the SmO 1- x F x FeAs. The critical temperature ( T c ) increases with increasing x in between 0.15≤ x ≤0.3, while x >0.3 the T c decreases with the increase of x . It is found that at x =0.3 SmO 0.7 F 0.3 FeAs has the highest onset resistivity transition temperature of 55.5 K. The critical current density ( J c ) value at 10 K for the obtained SmO 0.7 F 0.3 FeAs is 1.3×10 5 A/cm 2 (0 T). Meanwhile one can estimates H c2 (0) from the slope of the H c2 ( T ) curve at T = T c ( H c2 is the upper critical field), and for the 90% normal-state resistivity ρ n ) criterion ( T c =55K), H c2 (0) is determined to be ∼253 T.

Upper critical field, critical current density and thermally activated flux flow in fluorinedoped CeFeAsO superconductors

We report our studies on the crystal structures, morphologies, and superconductivity inCeO1 − xFxFeAs compounds which were fabricated by solid state reaction. The crystal structures were refinedusing Rietveld refinement. Superconducting properties such as critical temperature (Tc), critical currentdensity (Jc), andupper critical field (Hc2) were determined using magneto-transport and magnetic measurement over a wide range of temperature belowTc, and in magneticfields up to 13 T. Jc is 2 × 103 A cm − 2 for thex = 0.1 sample.However, the Jc exhibited a weak dependence on magnetic field forB > 1 T andT = 5 and 10 K. A peakeffect in the Jc as a function of field was observed at 20 K in thex = 0.1 sample. Weestimate Hc2ab of 185 T for CeO0.9F0.1FeAs compound. The broadening of the superconducting transition nearTc with increasing field can be well understood using the thermal activated flux flowmodel. The pinning potential scales as with n = 0.2 for B < 3 T and n = 0.71 for B > 3 T in the x = 0.1 sample.

Gutzwiller Density Functional Studies of FeAs-Based Superconductors: Structure Optimization and Evidence for a Three-Dimensional Fermi Surface

The electronic structures of FeAs compounds are sensitive to FeAs bonding, which is described unsuccessfully by the local density approximation (LDA). Treating the multiorbital fluctuations from ab inito LDA+Gutzwiller method, we can now predict the correct FeAs bond length and bonding strength, which will explain the observed "soft phonon." The bands are narrowed by a factor of 2 from their LDA widths. The d{3z{2}-r{2}} orbital is pushed up to cross the Fermi level, forming a three-dimensional Fermi surface, which reduces the anisotropy. The interorbital Hund's coupling J rather than U plays a crucial role in obtaining these results.

High Field ESR Spectroscopy on GdO1−x F x FeAs

In the present work we have studied polycrystalline samples of the GdO1−x F x FeAs superconductor by means of high field electron spin resonance (HF-ESR) spectroscopy. A set of the samples with different levels of fluorine doping was measured in the frequency range from 10 GHz to 400 GHz in magnetic fields up to 15 T. Surprising results have been obtained on the GdO0.85F0.15FeAs sample with the superconducting transition temperature T c =20 K. Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations which set in below ∼80 K and continue to develop even in the superconducting state. This suggests an occurrence of an intimate interplay between magnetism and superconductivity in the FeAs planes which evolves in GdO1−x F x FeAs compound upon the fluorine doping.

Identifying valence structure in LiFeAs and NaFeAs with core-level spectroscopy

Resonant x-ray emission spectroscopy (XES) measurements at FeL2,3 edges and electronic structure calculations for LiFeAs and NaFeAs are presented.Experiment and theory show that in the vicinity of the Fermi energy, the density ofstates is dominated by contributions from Fe 3d states. The comparison of FeL2,3 XESwith spectra of related FeAs compounds reveals similar trends in energy and the ratio of intensities of theL2 andL3 peaks (I(L2)/I(L3) ratio).The I(L2)/I(L3) ratio for all FeAs-based superconductors is found to be closer to that of metallic Fe thanthat of the strongly correlated FeO. We conclude that iron-based superconductors areweakly or, at most, moderately correlated systems.

Effects of magnetic doping and temperature dependence of phonon dynamics inCaFe1−xCoxAsFcompounds (x=0, 0.06, and 0.12)

We report detailed measurements of composition as well as temperature dependence of the phonon density-of-states in a new series of FeAs compounds with composition CaFe1\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12). The composition as well as temperature dependence of phonon spectra for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds have been measured using time of flight IN4C and IN6 spectrometers at ILL, France. The comparison of phonon spectra at 300 K in these compounds shows that acoustic phonon modes up to 12 meV harden in the doped compounds in comparison to the parent CaFeAsF. While intermediate energy phonon modes from 15 meV to 25 meV are also found to shift towards high energies only in the 12 % Co doped CaFeAsF compound. The experimental results for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) are quite different from our previous phonon studies on parent and superconducting MFe2As2 (M=Ba, Ca, Sr) where low-energy acoustic phonon modes do not react with doping, while the phonon spectra in the intermediate range from 15 to 25 K are found to soften in these compounds. We argue that stronger spin phonon interaction play an important role for the emergence of superconductivity in these compounds. The lattice dynamics of CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds is also investigated using the ab-initio as well as shell model phonon calculations. We show that the nature of the interaction between the Ca and the Fe-As layers in CaFeAsF compounds is quite different compared with our previous studies on CaFe2As2.

Correlated Electrons in Fe-As Compounds: A Quantum Chemical Perspective

State-of-the-art quantum chemical methods are applied to the study of the multiorbital correlated electronic structure of a Fe-As compound, the recently discovered LiFeAs. Our calculations predict a high-spin, S=2, ground-state configuration for the Fe ions, which shows that the on-site Coulomb interactions are substantial. Also, orbital degeneracy in the (xz, yz) sector and a three-quarter filling of these levels suggest the presence of strong fluctuations and are compatible with a low metallic conductivity in the normal state. The lowest electron-removal states have As 4p character, in analogy with the ligand hole states in p-type cuprate superconductors.