Iron Arsenide Compounds Research Articles

Unconventional Multi-gap Superconductivity and Antiferromagnetic Spin Fluctuations in New Iron-arsenide LaFe2As2 in Heavily Electron-doped Regime

We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental results indicate the close relationship between the AFMSFs and the SC from the hole-doped state to heavily electron-doped state, which shed light on a unique SC phase emerged in the heavily electron-doped regime being formally equivalent to non-SC compound Ba(Fe0.5Co0.5)Fe2As2.

Direct study of structural phase transformation in single crystalline bulk and thin film BaFe2As2

The ternary iron arsenide compound BaFe2As2 exhibits a structural phase transition from tetragonal to orthorhombic at a temperature of about 140 K. The twin lamellae arising below this transition temperature were studied in undoped single crystalline bulk and epitaxial thin film samples using electron backscatter diffraction in a scanning electron microscope equipped with a helium cryostat. Applying this technique on bulk single crystals a characteristic twin lamella size in the range of 0.1 μm up to a few μm was observed. In contrast, in epitaxially strained thin films the phase transition is not observed at temperatures above 19 K.

Magnetic ordering and electronic structure of the ternary iron arsenide BaFe2As2

In this paper, we report structural parameters, magnetic ordering and the electronic structure of the tetragonal and orthorhombic ternary iron arsenide compound [Formula: see text]. Ab initio calculations are performed by the means of full-potential linearized augmented plane waves plus local orbitals method for both spin states. The local spin density plus Hubbard potential ([Formula: see text]) and generalized gradient plus Hubbard potential ([Formula: see text]) approximations are used to treat the exchange and correlation potentials. The results mainly show that orthorhombic [Formula: see text] compound has an antiferromagnetic spin ordering. Both spin-up and spin-down polarizations are considered to investigate the band structure, density of states and electron charge density. High density of states population at the Fermi level reveals metallic character of this material. The magnetism property arises essentially from the [Formula: see text] orbital of iron atom.

Effects of isovalent substitution and pressure on the interplane resistivity of single-crystalBa(Fe1−xRux)2As2

Temperature-dependent interplane resistivity ${\ensuremath{\rho}}_{c}(T)$ was measured in an isovalent substituted iron-arsenide compound $\text{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}){}_{2}{\mathrm{As}}_{2}$ over a substitution range from parent compound to slightly below optimal doping $x=0.29$. The feature of interest in the ${\ensuremath{\rho}}_{c}(T)$, a broad resistivity crossover maximum found in the parent compound at ${T}_{\text{max}}\ensuremath{\approx}200$ K, shifts to higher temperatures with Ru substitution, $\ensuremath{\sim}340$ K for $x=0.161$ and goes out of the 400 K range for $x=0.29$. Nearly $T$-linear dependence of interplane resistivity is found at the highest substitution level $x=0.29$. This temperature-dependent ${\ensuremath{\rho}}_{c}$ and its evolution with substitution bear close similarity to another type of isovalent substituted system ${\mathrm{BaFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}){}_{2}$. Similarly to the isovalent substitutions, the measurements of interplane resistivity in the parent ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ compound under pressures up to 20 kbar also revealed a rapid rise in ${T}_{\text{max}}$.

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.

Pressure-induced phase coexistence in BaFe1.8Co0.2As2

We report high-pressure powder synchrotron X-ray diffraction measurements of overdoped BaFe1.8Co0.2As2 under quasi-hydrostatic pressure up to 40.1 GPa. Our results indicate that a tetragonal (T) to collapsed tetragonal (CT) phase transition occurs at 16.8 GPa and the two phase coexist until 30 GPa, which has not been previously observed in iron arsenide compounds. Both the lattice parameters a and c show discontinuous change for the T and CT phases. The decrease of the c lattice parameter is as large as 12.2% owing to the uniaxial pressure effect. The axial ratio c/a of the T phase exhibits similar features to the other 122-type compounds below 16.8 GPa, whereas there is a very small increase with increasing pressure in the two phase coexistence region. Because of the relationship between the axial ratio and superconductivity, the abnormal expansion may be related to the sudden increase of the strength of antiferromagnetic spin fluctuations.

Influence of Cr doping on the magnetic structure of the FeAs-strips compound CaFe4As3: A single-crystal neutron diffraction study

We have studied the magnetic structure of a Cr-doped iron-arsenide compound CaFe${}_{4}$As${}_{3}$ by means of single crystal neutron diffraction. The neutron data reveal that below 90 K, an antiferromagnetic structure with propagation vector $k=0$ is adopted. Refinement of the magnetic structure using one of the modes allowed by symmetry analysis indicates that two of the four Fe sites, including the one where the selective substitution Fe/Cr happens, bear reduced magnetic moments. Density functional theory calculations confirm the stability of such a magnetic arrangement.

Angular dependent 75As NMR study of the electric field gradient in CaFe2As2

The electric field gradient (EFG) at the As nuclei in the BaFe2As2 and SrFe2As2 iron arsenide compounds becomes axially asymmetric in the low temperature orthorhombic phase, but little has been reported about the EFG in the CaFe2As2 analog. We present As nuclear magnetic resonance (NMR) spectra as a function of orientation in this phase and extract the EFG tensor. The EFG asymmetry is significantly less in this material, which is a result of the unusually strong c-axis hybridization.

A Bayesian Approach to Magnetic Moment Determination Using μSR

A significant challenge in zero-field μSR experiments arises from the uncertainty in the muon site. It is possible to calculate the dipole field (and hence precession frequency v) at any particular site given the magnetic moment μ and magnetic structure. One can also evaluate f(v), the probability distribution function of v assuming that the muon site can be anywhere within the unit cell with equal probability, excluding physically forbidden sites. Since v is obtained from experiment, what we would like to know is g(μjv), the probability density function of μ given the observed v. This can be obtained from our calculated f(v/μ) using Bayes’ theorem. We describe an approach to this problem which we have used to extract information about real systems including a low-moment osmate compound, a family of molecular magnets, and an iron-arsenide compound.

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.

The stable structures of iron pnictides AFeAs (A = alkali and alkaline-earth metals): A first principles study

We have investigated the structural stabilities of iron arsenide compounds Ax(FeAs)1−x (A = alkali and alkaline-earth metals) by first principles calculations. We find that (i) all of the experimental “122” type structures with composition x=1/3 are stable; (ii) all of the “111” type structures with composition x=1/2 except CsFeAs are stable; (iii) K3FeAs with composition x=3/4 is stable. The predicted stable KFeAs, RbFeAs, SrFeAs, BaFeAs, K3FeAs have the As–Fe–As bond angles close to the ideal tetrahedral angles, indicating that they may be superconductors.

Structure–property relationships of iron arsenide superconductors

Iron based superconductors sent material scientists into a renewed excitement reminiscent of the time when the first high-Tc superconductors were discovered 25 years ago. This feature article reviews relationships between structural chemistry and magnetic as well as superconducting properties of iron arsenide compounds, which are outstandingly rich and uniquely coupled. Particular attention is paid to the nature of the structural phase transitions of the parent compounds and their possible origins, on effects of doping on the crystal structures and on the coexistence of magnetic ordering and superconductivity. In spite of the many fascinating insights that have already enriched the research on superconductivity, many questions are still open and prove iron based superconductors to be a good recipe for future discoveries in this lively field.

Pressure versus Concentration Tuning of the Superconductivity in Ba(Fe1-xCox)2As2

In the iron arsenide compound BaFe2As2, superconductivity can be induced either by a variation of its chemical composition, e.g., by replacing Fe with Co, or by a reduction of the unit-cell volume through the application of hydrostatic pressure p. In contrast to chemical substitutions, pressure is expected to introduce no additional disorder into the lattice. We compare the two routes to superconductivity by measuring the p dependence of the superconducting transition temperature Tc of Ba(Fe(1-x)Cox)2As2 single crystals with different Co content x. We find that Tc(p) of underdoped and overdoped samples increases and decreases, respectively, tracking quantitatively the Tc(x) dependence. To clarify to which extent the superconductivity relies on distinct structural features we analyze the crystal structure as a function of x and compare the results with that of BaFe2As2 under pressure.

Antiferromagnetic ordering in the absence of structural distortion inBa(Fe1−xMnx)2As2

Neutron and x-ray diffraction studies of $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Mn}}_{x})}_{2}{\text{As}}_{2}$ for low doping concentrations $(x\ensuremath{\le}0.176)$ reveal that at a critical concentration, $0.102<x<0.118$, the tetragonal-to-orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of $(\frac{1}{2}\text{ }\frac{1}{2}\text{ }1)$ persists. Among all of the iron arsenides this observation is unique to Mn doping, and unexpected because all models for ``stripelike'' antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}T{M}_{x})}_{2}{\text{As}}_{2}$ compounds $(TM=\text{transition}\text{ }\text{metal})$, and models for magnetic ordering in the iron arsenide compounds.

Critical current density properties in polycrystalline Sr0.6K0.4Fe2As2 superconductors

The recent discovery of superconductivity in iron arsenide compounds REFeAsO (RE=rare earth) or AFe2As2 (A=alkaline earth) has attracted great attention due to the high Tc that raised above 50K. In this study, the effect of Ag addition (20wt.%) on the critical current density Jc of polycrystalline Sr0.6K0.4Fe2As2 was investigated for improvement of the weak links in polycrystalline iron-based superconductors. Although effect of Ag addition on the intra-grain current density Jclocal was not confirmed, the connectivity among grains is speculated to be improved by filling voids among grains by Ag.

Incommensurate spin-density wave and magnetic lock-in transition inCaFe4As3

The magnetic structure for the recently synthesized iron-arsenide compound ${\text{CaFe}}_{4}{\text{As}}_{3}$ has been studied by neutron-powder diffraction. Long-range magnetic order is detected below 85 K, with an incommensurate modulation described by the propagation vector $k=(0,\ensuremath{\delta},0)$, $\ensuremath{\delta}\ensuremath{\sim}0.39$. Below $\ensuremath{\sim}25\text{ }\text{K}$, our measurements detect a first-order phase transition where $\ensuremath{\delta}$ locks into the commensurate value $\frac{3}{8}$. A model of the magnetic structure is proposed for both temperature regimes, based on Rietveld refinements of the powder data and symmetry considerations. The structures correspond to longitudinal spin-density waves with magnetic moments directed along the $b$ axis. A Landau analysis captures the change in thermodynamic quantities observed at the two magnetic transitions, in particular, the drop in resistivity at the lock-in transition.

Superconductivity in Ti-doped iron-arsenide compound Sr4Cr0.8Ti1.2O6Fe2As2

Superconductivity was achieved in Ti-doped iron-arsenide compound Sr4Cr0.8Ti1.2O6Fe2As2 (abbreviated as Cr-FeAs-42622). The X-ray diffraction measurement shows that this material has a layered structure with the space group of P4/nmm, and with the lattice constants a = b = 3.9003 Å and c = 15.8376 Å. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2 K, which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase.

Superconductivity of “1 1 1”-type iron arsenide compounds

“111”-type iron arsenide compounds AFeAs (A=Li, Na) were synthesized. These compounds crystallize into Cu2Sb type tetragonal layered structure with space group P4/nmm and show superconductivity with the transition temperature ∼18K for LiFeAs and ∼23K for NaFeAs. The effect of pressure on LiFeAs sample was studied. It was found that the Tc decreased almost linearly with increasing pressure and the pressure derivative dTc/dP was about −1.38K/GPa.

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 BaFe 2As 2 (‘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.

The superconductivity at 18 K in LiFeAs system

The recent discovery of superconductivity in iron arsenide compounds RFeAsO (R=rare earth) or AFe 2As 2 (A=alkaline earth) has attracted great attention due to the unexpected high T c in the system containing ferromagnetic elements like Fe. Similar to high T c cuprates, the superconductivity in iron arsenide is related to a layered structure. Searching for new superconductors with [FeAs] layer, but of simpler structure will be of scientific significance either to build up new multilayered superconductors that may reach higher T c or to study the mysterious underlined superconducting mechanism in iron arsenide compounds. Here we report that a new superconducting iron arsenide system LiFeAs was found. The compound crystallizes into a structure containing [FeAs] conducting layer that is interlaced with Li charge reservoir. Superconductivity was observed with T c up to 18 K in the compounds.