Superconductivity Induced by Co-Doping in Quaternary Fluoroarsenide CaFeAsF

Synthesis and Characterization of [Fe3 (As3 )3 (As4 )]3- , a Binary Fe/As Zintl Cluster With an Fe3 Core

We report here the synthesis and structural characterization of the first binary iron arsenide cluster anion, [Fe₃ (As₃ )₃ (As₄ )]^3- , present in both [K([2.2.2]crypt)]₃ [Fe₃ (As₃ )₃ (As₄ )] (1) and [K(18-crown-6)]₃ [Fe₃ (As₃ )₃ (As₄ )]⋅en (2). The cluster contains an Fe₃ triangle with three short Fe-Fe bond lengths (2.494(1) Å, 2.459(1) Å and 2.668(2) Å for 1, 2.471(1) Å, 2.473(1) Å and 2.660(1) Å for 2), bridged by a 2-butene-like As₄ unit. An analysis of the electronic structure using DFT reveals a triplet ground state with direct Fe-Fe bonds stabilizing the Fe₃ core.

Bulk and Single Crystal Growth Progress of Iron-Based Superconductors (FBS): 1111 and 1144

The discovery of iron-based superconductors (FBS) and their superconducting properties has generated huge research interest and provided a very rich physics high Tc family for fundamental and experimental studies. The 1111 (REFeAsO, RE = Rare earth) and 1144 (AEAFe4As4, AE = Ca, Eu; A = K, Rb) families are the two most important families of FBS, which offer the high Tc of 58 K and 36 K with doping and without doping, respectively. Furthermore, the crystal growth of these families is not an easy process, and a lot of efforts have been reported in this direction. However, the preparation of high-quality and suitable-sized samples is still challenging. In this short review, we will summarize the growth of materials with their superconducting properties, especially polycrystals and single crystals, for the 1111 and 1144 families, and make a short comparison between them to understand the developmental issues.

Correlation of Tunable CoSi4 Tetrahedron with the Superconducting Properties of LaCoSi

It is known that as the FeAs₄ tetrahedron in the Fe-based superconductor is close to the regular tetrahedron, critical temperature (T_c) can be greatly increased. Recently, a Co-based superconductor of LaCoSi (4 K) with "111" structure was found. In this work, we improve the T_c of LaCoSi through structural regulation. T_c can be increased by the chemical substitution of Co by Fe, while the superconductivity is suppressed by the Ni substitution. The combined analysis of neutron and synchrotron X-ray powder diffractions reveals that the change of the Si-Co-Si bond angles of the CoSi₄ tetrahedron is possibly responsible for the determination of superconducting properties. The Fe chemical substitution is favorable for the formation of the regular tetrahedron of CoSi₄. The present new Co-based superconductor of LaCoSi provides a possible method to enhance the superconductivity performance of the Co-based superconductors via controlling Co-based tetrahedra similar to those well established in the Fe-based superconductors.

Neutron Spin Resonance in a Quasi-Two-Dimensional Iron-Based Superconductor

The neutron spin resonance is generally regarded as a key to understanding the magnetically mediated Cooper pairing in unconventional superconductors. Here, we report an inelastic neutron scattering study on the low-energy spin excitations in a quasi-two-dimensional iron-based superconductor KCa_{2}Fe_{4}As_{4}F_{2}. We have discovered a two-dimensional spin resonant mode with downward dispersions, a behavior closely resembling the low branch of the hourglass-type spin resonance in cuprates. While the resonant intensity is predominant by two broad incommensurate peaks near Q=(0.5,0.5) with a sharp energy peak at E_{R}=16  meV, the overall energy dispersion of the mode exceeds the measured maximum total gap Δ_{tot}=|Δ_{k}|+|Δ_{k+Q}|. These results deeply challenge the conventional understanding of the resonance modes as magnetic excitons regardless of underlining pairing symmetry schemes, and it also points out that when the iron-based superconductivity becomes very quasi-two-dimensional, the electronic behaviors are similar to those in cuprates.

A new oxygen-free cobalt-based compound SmCoAsF with multiple magnetic transitions

A new quaternary oxygen-free cobalt-based compound SmCoAsF, which crystallizes with the tetragonal ZrCuSiAs-type structure, is synthesized via a solid state metathesis reaction.

A Self-Doped Oxygen-Free High-Critical-Temperature (High-Tc) Superconductor: SmFFeAs

A new iron-base superconductor SmFFeAs is synthesized via solid-state metathesis reaction by using SmFCl and LiFeAs as precursors. The compound crystallized in the tetragonal ZrCuSiAs-type structure with the space group P4/nmm and lattice parameters of a = 3.9399(0) Å and c = 8.5034(1) Å. The superconducting diamagnetic transition occurs at 56 K for the parent compound, which confirmed by the resistivity and magnetic susceptibility. The appearance of superconductivity without extrinsic doping could be ascribed to the self-doping owing to the mixed valence of Sm ions. The as-synthesized SmFFeAs serves as a new self-doped parent compound for oxygen-free high-critical-temperature (high-T_c) superconductors.

Pressure effects on iron-based superconductor CaFe0.88Co0.12AsF

Systematic measurements of electrical resistivity and Hall coefficient under high pressure were performed on CaFe_0.88Co_0.12AsF single crystal samples. The superconductivity is suppressed quickly by pressure and can not be detected down to 2 K at above 12.7 GPa, while the magnitude of the Hall coefficient [Formula: see text] shows a very weak pressure and temperature dependence. A comprehensive analysis considering the pressure dependence of [Formula: see text], [Formula: see text], residual resistivity ratio, and the Fermi-liquid term of the resistivity indicates that the electron correlation is an important factor in superconductivity of iron-based superconductors.

New layered chromium chalcogenides CsLiCrSe2, RbLiCrS2 and CsLiCrS2: structures and properties

The synthesis and characterization of the first ThCr₂Si₂-type chromium chalcogenides with intimate correlation between Cr concentration and spin-glass transition temperature.

Phase transition in CaFeAsH: bridging 1111 and 122 iron-based superconductors

Iron-based superconductors can be categorized into two types of parent compounds by considering the nature of their temperature-induced phase transitions; namely, first order transitions for 122- and 11-type compounds and second-order transitions for 1111-type compounds. This work examines the structural and magnetic transitions (ST and MT) of CaFeAsH by specific heat, X-ray diffraction, neutron diffraction, and electrical resistivity measurements. Heat capacity measurements revealed a second-order phase transition that accompanies an apparent single peak at 96 K. However, a clear ST from the tetragonal to orthorhombic phase and an MT from the paramagnetic to the antiferromagnetic phase were detected. The structural (T_s) and Néel temperatures (T_N) were respectively determined to be 95(2) and 96 K by X-ray and neutron diffraction and resistivity measurements. This small temperature difference, T_s-T_N, was attributed to strong magnetic coupling in the inter-layer direction owing to CaFeAsH having the shortest lattice constant c among parent 1111-type iron arsenides. Considering that a first-order transition takes place in 11- and 122-type compounds with a short inter-layer distance, we conclude that the nature of the ST and MT in CaFeAsH is intermediate in character, between the second-order transition for 1111-type compounds and the first-order transition for other 11- and 122-type compounds.

Interrogating the superconductor Ca10(Pt4As8)(Fe2-xPtxAs2)5 Layer-by-layer

Ever since the discovery of high-T_c superconductivity in layered cuprates, the roles that individual layers play have been debated, due to difficulty in layer-by-layer characterization. While there is similar challenge in many Fe-based layered superconductors, the newly-discovered Ca₁₀(Pt₄As₈)(Fe₂As₂)₅ provides opportunities to explore superconductivity layer by layer, because it contains both superconducting building blocks (Fe₂As₂ layers) and intermediate Pt₄As₈ layers. Cleaving a single crystal under ultra-high vacuum results in multiple terminations: an ordered Pt₄As₈ layer, two reconstructed Ca layers on the top of a Pt₄As₈ layer, and disordered Ca layer on the top of Fe₂As₂ layer. The electronic properties of individual layers are studied using scanning tunneling microscopy/spectroscopy (STM/S), which reveals different spectra for each surface. Remarkably superconducting coherence peaks are seen only on the ordered Ca/Pt₄As₈ layer. Our results indicate that an ordered structure with proper charge balance is required in order to preserve superconductivity.

Angular dependent torque measurements on CaFe0.88Co0.12AsF

Out-of-plane angular dependent torque measurements were performed on CaFe0.88Co0.12AsF (Ca1 1 1 1) single crystals. In the normal state, the torque data shows [Formula: see text] angular dependence and H (2) magnetic field dependence, as a result of paramagnetism. In the mixed state, the torque signal is a combination of the vortex torque and paramagnetic torque, and the former allows the determination of the anisotropy parameter γ. At T   =  11.5 K, γ (11.5 K ≃ 0.5 T c)  =  19.1, which is similar to the result of SmFeAsO0.8F0.2, [Formula: see text] at [Formula: see text]. So the 11 1 1 is more anisotropic compared to 11 and 122 families of iron-based superconductors. This may suggest that the electronic coupling between layers in 1 1 1 1 is less effective than in 11 and 122 families.

Suppression of superconductivity and structural phase transitions under pressure in tetragonal FeS

Pressure is a powerful tool to study iron-based superconductors. Here, we report systematic high-pressure transport and structural characterizations of the newly discovered superconductor FeS. It is found that superconductor FeS (tetragonal) partly transforms to a hexagonal structure at 0.4 GPa, and then completely transforms to an orthorhombic phase at 7.4 GPa and finally to a monoclinic phase above 9.0 GPa. The superconducting transition temperature of tetragonal FeS was gradually depressed by pressure, different from the case in tetragonal FeSe. With pressure increasing, the S-Fe-S angles only slightly change but the anion height deviates farther from 1.38 Å. This change of anion height, together with the structural instability under pressure, should be closely related to the suppression of superconductivity. We also observed an anomalous metal-semiconductor transition at 6.0 GPa and an unusual increased resistance with further compression above 9.6 GPa. The former can be ascribed to the tetragonal-orthorhombic structural phase transition, and the latter to the electronic structure changes of the high-pressure monoclinic phase. Finally, a phase diagram of tetragonal FeS as functions of pressure and temperature was mapped out for the first time, which will shed new light on understanding of the structure and physics of the superconducting FeS.

A New ZrCuSiAs-Type Superconductor: ThFeAsN

We report the first nitrogen-containing iron-pnictide superconductor ThFeAsN, which is synthesized by a solid-state reaction in an evacuated container. The compound crystallizes in a ZrCuSiAs-type structure with the space group P4/nmm and lattice parameters a = 4.0367(1) Å and c = 8.5262(2) Å at 300 K. The electrical resistivity and dc magnetic susceptibility measurements indicate superconductivity at 30 K for the nominally undoped ThFeAsN.

Hydrogen-Substituted Superconductors SmFeAsO(1-x)Hx Misidentified As Oxygen-Deficient SmFeAsO(1-x)

We investigated the preferred electron dopants at the oxygen sites of 1111-type SmFeAsO by changing the atmospheres around the precursor with the composition of Sm:Fe:As:O = 1:1:1:1 - x in high-pressure synthesis. Under H2O and H2 atmospheres, hydrogens derived from H2O or H2 molecules were introduced into the oxygen sites as a hydride ion, and SmFeAsO(1-x)Hx was obtained. However, when the H2O and H2 sources were removed from the synthetic process, nearly stoichiometric SmFeAsO was obtained and the maximum amount of oxygen vacancies introduced remained x = 0.05(4). Density functional theory calculations indicated that substitution of hydrogen in the form of H(-) is more stable than the formation of an oxygen vacancy at the oxygen site of SmFeAsO. These results strongly imply that oxygen-deficient SmFeAsO(1-x) reported previously is SmFeAsO(1-x)Hx with hydride ion incorporated unintentionally during high-pressure synthesis.

Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

Spin-reorientation transition in CeMnAsO

High-resolution X-ray and neutron powder diffraction are used to reveal details of the spin-reorientation transition in the layered oxide pnictide CeMnAsO. Above 38 K, the localized moments on Mn(2+) are antiferromagnetically ordered in a checkerboard fashion within the antifluorite-type MnAs planes and are oriented perpendicular to the planes. Below 38 K, reorientation of these moments into the planes commences. This is complete by 34 K and is coincident with long-range ordering of the Ce(3+) moments. The Ce(3+) and Mn(2+) moments have an arrangement that is different in detail from that in the isostructural NdMnAsO and PrMnSbO. There is no evidence for structural distortion, as found for PrMnSbO and related Pr(3+)-containing compounds, although there is evidence for a very slight (0.025%) misfit between the magnetic and structural cells below the spin-reorientation transition. It is clarified that neutron powder diffraction methods are unable to distinguish between collinear and noncollinear arrangements of manganese and lanthanide moments when the moments have a component parallel to the MnAs planes. A proposal from computational analysis that NdMnAsO and CeMnAsO should adopt different magnetic structures on the basis of the different balances between biquadratic and antisymmetric exchange interactions should be tested using alternative methods.

Iron-based high transition temperature superconductors

In a superconductor electrons form pairs and electric transport becomes dissipation-less at low temperatures. Recently discovered iron-based superconductors have the highest superconducting transition temperature next to copper oxides. In this article, we review material aspects and physical properties of iron-based superconductors. We discuss the dependence of transition temperature on the crystal structure, the interplay between antiferromagnetism and superconductivity by examining neutron scattering experiments, and the electronic properties of these compounds obtained by angle-resolved photoemission spectroscopy in link with some results from scanning tunneling microscopy/spectroscopy measurements. Possible microscopic model for this class of compounds is discussed from a strong coupling point of view.

Synthesis and Characterization of Superconducting Ca₁-xNaxFFeAs

A representative of the fluoride-containing iron pnictide high-temperature superconductors, namely CaFFeAs, was doped with sodium up to the composition Ca_0.86Na_0.14FFeAs for the first time. Single crystals with an edge length in the range of 0.1 - 2.0 mm were obtained via solid-state and flux syntheses, respectively. The composition of the crystals was verified by means of single crystal X-ray diffractometry and energy dispersive X-ray spectroscopy (EDX). Measurements of the electrical resistivity, as well as the magnetization on a crystal of Ca_0.89Na_0.11FFeAs both show a transition to the superconducting state on cooling to 34.5 K. Investigations of the upper critical fields reveal an anisotropy ratio of about five. The lattice parameters and molar volumes increase with rising sodium content. This effect is clearly observable for the c-axis and the volume, whereas the increase of the a-axis is rather minor.

A new layered iron arsenide superconductor: (Ca,Pr)FeAs2

A new iron-based superconductor, (Ca,Pr)FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2₁/m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a T(c) of ~20 K in both magnetization and resistivity measurements.

Superconductivity in Ca10(Ir4As8)(Fe2As2)5 with Square-Planar Coordination of Iridium

We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca₁₀(Ir₄As₈)(Fe₂As₂)₅. This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first-principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir₄As₈ layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe₂As₂, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.

Effects of Co and Mn doping in K0.8Fe2-ySe2 revisited

Accumulated evidence indicates that phase separation occurs in potassium intercalated iron selenides, a superconducting phase coexisting with the antiferromagnetic phase K2Fe4Se5, the so-called '245 phase'. Here, we report a comparative study of substitution effects by Co and Mn for Fe sites in K0.8Fe2-ySe2 within the phase separation scenario. Our results demonstrate that Co and Mn dopants have distinct differences in occupancy and hence in the suppression mechanism of superconductivity upon doping of Fe sites. In K0.8Fe2-xCoxSe2, Co prefers to occupy the lattice of the superconducting phase and suppresses superconductivity very quickly, obeying the magnetic pair-breaking mechanism or the collapse of the Fermi surface nesting mechanism. In contrast, in K0.8Fe1.7-xMnxSe2, Mn shows no preferential occupancy in the superconducting phase or the 245 phase. The suppression of superconductivity can be attributed to restraining of the superconducting phase and meanwhile inducing another non-superconducting phase by Mn doping.

Mössbauer studies of the superconducting cobalt/nickel-doped BaFe2As2. Whither go the injected electron(s)?

Mössbauer studies of cobalt- and nickel-doped BaFe(2)As(2) show that the s-electron density at the (57)Fe nuclei, as measured by the isomer shift, is the same as that for the parent BaFe(2)As(2). Apparently, the electron population of the d shell, which shields the s-electron density at the nuclei, remains unchanged. We invoke the involvement of p-orbital hybridization with the d orbital in Fe-As bonding. Furthermore, the shrinkage of the lattice on substitution enhances the As-As sp hybridization, providing a path for the migration of additional electrons. The proposed mechanism is consistent with Hall coefficient and thermoelectric effect measurements.

La2Sb, a layered superconductor with metal-metal bonds

We found that La(2)Sb with a layered structure composed of alternate stacking of La square nets and LaSb layers exhibits bulk superconductivity with a critical temperature of 5.3 K. This suggests that the presence of the square net with strong La-La metal bonding is essential for the emergence of superconductivity.

Coexistence of light and heavy carriers associated with superconductivity and antiferromagnetism in CeNi0.8Bi2 with a Bi square net

We found that the ZrCuSiAs-type crystal CeNi(0.8)Bi(2) with a layered structure composed of alternate stacking of [CeNi(x)Bi(1)](δ+) and Bi(2)(δ-) exhibits a superconductive transition at ∼4  K. The conductivities, magnetic susceptibilities, and heat capacities measurements indicate the presence of two types of carriers with notable different masses, i.e., a light electron responsible for superconductivity and a heavy electron interacting with the Ce 4f electron. This observation suggests that 6p electrons of Bi(2) forming the square net and electrons in CeNi(x)Bi(1) layers primarily correspond to the light and heavy electrons, respectively.

Control of the competition between a magnetic phase and a superconducting phase in cobalt-doped and nickel-doped NaFeAs using electron count

Using a combination of neutron, muon, and synchrotron techniques we show how the magnetic state in NaFeAs can be tuned into superconductivity by replacing Fe by either Co or Ni. The electron count is the dominant factor, since Ni doping has double the effect of Co doping for the same doping level. We follow the structural, magnetic, and superconducting properties as a function of doping to show how the superconducting state evolves, concluding that the addition of 0.1 electrons per Fe atom is sufficient to traverse the superconducting domain, and that magnetic order coexists with superconductivity at doping levels less than 0.025 electrons per Fe atom.

Ir doping-induced superconductivity in the SmFeAsO system

The 5d transition metal Ir is successfully doped for Fe in SmFeAsO to induce superconductivity with T(c) = 16 K at a doping level of approximately 15 atom %. Ir doping decreases the As-Fe-As bond angle, beta; this behavior is different from the change in beta for the system with doping charges in the charge-reservoir layers.

Tetragonal-to-orthorhombic structural phase transition at 90 K in the superconductor Fe(1.01)Se

In this Letter we show that superconducting Fe(1.01)Se undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that nonsuperconducting Fe(1.03)Se does not. High resolution electron microscopy at low temperatures further reveals an unexpected additional modulation of the crystal structure of the superconducting phase that involves displacements of the Fe atoms, and that the nonsuperconducting composition shows a different, complex nanometer-scale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.

Insular superconductivity in a Co-doped iron pnictide CaFe1-xCoxAsF

The presence of a macroscopic phase separation between the superconducting and magnetic phases in CaFe1-xCoxAsF is demonstrated by muon spin rotation measurements conducted across their phase boundaries (x=0.05-0.15). The magnetic phase tends to retain the high transition temperature (Tm>Tc), while Co doping induces strong randomness. The volumetric fraction of the superconducting phase is nearly proportional to the Co content x with a constant superfluid density. These observations suggest the formation of superconducting "islands" (or domains) associated with Co ions in the Fe2As2 layers, indicating a very short coherence length.

Superconductivity at 56 K in samarium-doped SrFeAsF

We synthesized the samples Sr(1-x)Sm(x)FFeAs with a ZrCuSiAs-type structure. These samples were characterized by resistivity and susceptibility. It is found that substitution of rare earth metal for alkaline earth metal in this system suppresses the anomaly in resistivity and induces superconductivity. Superconductivity at 56 K in nominal composition Sr(0.5)Sm(0.5)FFeAs is realized, indicating that the superconducting transition temperatures in the iron arsenide fluorides can reach as high as that in oxypnictides with the same structure.