High T(c) electron doped Ca10(Pt3As8)(Fe2As2)5 and Ca10(Pt4As8)(Fe2As2)5 superconductors with skutterudite intermediary layers

Order-disorder phase transition and elastic-to-plastic vortex creep crossover in a triclinic iron pnictide superconductor (Ca[Formula: see text]La[Formula: see text])[Formula: see text](Pt[Formula: see text]As[Formula: see text])(Fe[Formula: see text]As[Formula: see text])[Formula: see text]

Vortex matter in layered high-[Formula: see text] superconductors, including iron-pnictides, undergo several thermodynamic phase transitions due to the complex interplay of pinning energy, thermal energy and elastic energy. Moreover, the presence of anisotropy makes their vortex physics even more intriguing. Here, we report a detailed vortex dynamics study, using dc magnetization measurements, in a triclinic iron-pnictide superconductor (Ca[Formula: see text]La[Formula: see text])[Formula: see text](Pt[Formula: see text]As[Formula: see text])(Fe[Formula: see text]As[Formula: see text])[Formula: see text], with a superconducting transition temperature, T[Formula: see text] [Formula: see text] 31 K. A second magnetization peak (SMP) feature is observed for magnetic field perpendicular (H[Formula: see text]c) and parallel (H[Formula: see text]ab) to the crystal plane. However, its fundamental origin is quite different in both directions. For H[Formula: see text]c, the SMP can be well explained using an elastic-to-plastic vortex creep crossover, using collective creep theory. In addition, a possible rhombic-to-square vortex lattice phase transition is also observed for fields in between the onset-field and peak-field related to the SMP. On the other hand, for H[Formula: see text]ab, a clear signature of an order-disorder vortex phase transition is observed in the isothermal M(H) measurements at T [Formula: see text] 6 K. The disordered phase exhibits the characteristics of entangled pinned vortex-liquid. We construct a comprehensive vortex phase diagram by displaying characteristic temperatures and magnetic fields for both crystal geometries in this unique superconducting compound. Our study sheds light on the intricate vortex dynamics and pinning in an iron-pnictide superconductor with triclinic symmetry.

The breakdown of both strange metal and superconducting states at a pressure-induced quantum critical point in iron-pnictide superconductors

Here we report the first observation of the concurrent breakdown of the strange metal (SM) normal state and superconductivity at a pressure-induced quantum critical point in Ca₁₀(Pt₄As₈)((Fe_0.97Pt_0.03)₂As₂)₅ superconductor. We find that, upon suppressing the superconducting state, the power exponent (α) changes from 1 to 2, and the slope of the temperature-linear resistivity per FeAs layer (A^□) gradually diminishes. At a critical pressure, A^□ and superconducting transition temperature (T_c) go to zero concurrently, where a quantum phase transition from a superconducting state with a SM normal state to a non-superconducting Fermi liquid state occurs. Scaling analysis reveals that the change of A^□ with T_c obeys the relation of T_c ~ (A^□)^0.5, similar to what is seen in other chemically doped unconventional superconductors. These results suggest that there is a simple but powerful organizational principle of connecting the SM normal state with the high-T_c superconductivity.

Observations of Nematicity, Dopants, and Zero-Bias Conductance Peaks for the Ca0.9La0.1FeAs2 Superconductor

Ca_1-xLa_xFeAs₂ (CLFA112) belongs to a new family of Fe-based superconductors (FeSCs) and has a unique crystal structure featuring an arsenic zigzag chain layer, which has been proposed to be a possible two-dimensional topological insulator. This suggests that CLFA112 is a potential topological superconductor-a platform to realize Majorana fermions. Up to now, even a clear superconducting (SC) gap in CLFA112 has never been observed, and the SC properties of CLFA112 remain largely elusive. In this letter, we report the results of an atomic-scale investigation of the electronic structure of CLFA112 crystals using low-temperature scanning tunneling microscopy (STM). We revealed four different types of surfaces exhibiting distinct electronic properties, with all surfaces displaying dominating 2 × 1 surface reconstructions. On a Ca/La layer on top of an FeAs layer, a clear SC gap of ~12 mV was observed only at the crevices (vacancies) where the FeAs layer can be directly accessed. Remarkably, the FeAs termination layer displayed a dispersing nematic modulation both in real and q space. We also present peculiar zero-bias conductance peaks for the very As chain layer that is believed to exhibit a topological edge state as well as the influence of La dopants on the As chain layer.

Multiband effects on the upper critical field angular dependence of 122-family iron pnictide superconductors

Detailed measurements of the in-plane resistivity were performed in a high-quality Ba([Formula: see text])[Formula: see text] ([Formula: see text]) single crystal, in magnetic fields up to 9 T and with different orientations [Formula: see text] relative to the crystal c axis. A significant [Formula: see text] rounding is observed just above the superconducting critical temperature [Formula: see text] due to Cooper pairs created by superconducting fluctuations. These data are analyzed in terms of a generalization of the Aslamazov-Larkin approach, that extends its applicability to high reduced-temperatures and magnetic fields. This method allows us to carry out a criterion-independent determination of the angular dependence of the upper critical field, [Formula: see text]. In spite of the relatively small anisotropy of this compound, it is found that [Formula: see text] presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large [Formula: see text] (typically above [Formula: see text]). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar [Formula: see text] anomalies. Our results are also consistent with an effective anisotropy factor almost temperature independent near [Formula: see text], a result that differs from the ones obtained by using a single-band model.

Chemistry in Superconductors

Superconductors with exotic physical properties are critical to current and future technology. In this review, we highlight several important superconducting families and focus on their crystal structure, chemical bonding, and superconductivity correlations. We connect superconducting materials with chemical bonding interactions based on their structure-property relationships, elucidating our empirically chemical approaches and other methods used in the discovery of new superconductors. Furthermore, we provide some technical strategies to synthesize superconductors and basic but important characterization for chemists needed when reporting new superconductors. In the end, we share our thoughts on how to make new superconductors and where chemists can work on in the superconductivity field. This review is written using chemical terms, with a focus on providing some chemically intuitive thoughts on superconducting materials design.

Synthesis and Structure of a Nonstoichiometric Zr3.55Pt4Sb4 Compound

A nonstoichiometric ternary antimonide, Zr_3.55Pt₄Sb₄, with a new structure type (hP24), has been synthesized via arc-melting. Its crystal structure was determined by single-crystal X-ray diffraction with hexagonal space group P6₃/mmc and lattice parameters a = 4.391(3) Å, c = 30.53(2) Å, and V = 509.7(8) ų. It features the unique Pt₄Sb₄ slab with Pt-Pt bonds and is reminiscent to hexagonal diamond substructures. Three different Zr atoms, occupying three different sites, aid in the close-packing of the Pt and Sb atoms. Electronic structure calculations show the half occupancy of one Zr site creates a pseudogap at the Fermi level and optimizes the Pt-Sb bonding interactions. This enhances the electronic stability and accounts for the very narrow phase width observed for this nonstoichiometric compound. Furthermore, strong Zr-Pt and Zr-Sb interactions play a crucial role in the chemical bonding of the title compound. Electrical transport measurements show metallic behavior of this compound down to 2 K, consistent with the band structure calculations.

Pressure tuning of iron-based superconductor Ca10(Pt3As8) ((Fe0.95Pt0.05)2As2)5

Systematic high pressure transport measurements were performed on underdoped Ca₁₀(Pt₃As₈)((Fe_0.95Pt_0.05)₂As₂)₅ single crystal sample. At ambient pressure, the sample shows a metallic behavior at high temperatures and then increases with further decreasing temperature. The resistivity dip, which is associated with metal to semiconductor transition is monotonically suppressed by increasing pressure. In contrast, the superconducting transition temperature [Formula: see text] first increases with pressure and then decreases with further increasing pressure. Magnetization measurements, which gives the bulk [Formula: see text], show the same trend as the one obtained from resistivity measurements. An upward curvature is observed in the temperature dependence of the upper critical field [Formula: see text], which suggests the multiband nature of the superconductivity. The constructed temperature-pressure (T-P) phase diagram is very similar to the reported temperature-doping (T  -  x) phase diagram, suggesting the similar role played by pressure and chemical doping.

Structure and Transport Properties in Itinerant Antiferromagnet RE2(Ni1- xCu x)5As3O2 (RE = Ce, Sm)

We report the crystal structure and physical properties of two Ni₅As₃-based compounds RE₂Ni₅As₃O₂ (RE = Ce, Sm). The former exhibits structural phase transition from tetragonal (space group I4/ mmm, 139) to orthorhombic (space group Immm, 71) symmetry at 230 K, while the latter undergoes a charge-density-wave-like structural distortion with abrupt change of Ni-As bond length. Both compounds show antiferromagnetic transitions due to RE³⁺ ions ordering at 4.4 and 3.4 K, accompanying with the large enhancement of Sommerfeld coefficients comparing to the nonmagnetic La analogue. Although the Cu substitution for Ni induces structural anomalies and suppression of structural transition like the behaviors in La/Pr/Nd analogues, the superconductivity is not observed in both Cu-doped RE₂Ni₅As₃O₂ (RE = Ce, Sm) above 0.25 K. Our structural refinements reveal that the lacking of superconductivity in RE₂(Ni_{1- x}Cu _x)₅As₃O₂ might relate to the anomalous increase of As height, h₁.

Electrical and Thermal Transport Properties of Layered Superconducting Ca10(Pt₄As8)((Fe0.86Pt0.14)2As₂)₅ Single Crystal

We have synthesized single crystals of iron-based superconducting Ca₁₀(Pt₄As₈)((Fe_0.86Pt_0.14)₂As₂)₅ and performed extensive measurements on their transport properties. A remarkable difference in the behavior and a large anisotropy between in-plane and out-of-plane resistivity was observed. Disorder could explain the in-plane square-root temperature dependence resistivity, and interlayer incoherent scattering may contribute to the out-of-plane transport property. Along the ab plane, the estimated value of the coherence length is 15.5 Å. From measurements of the upper critical magnetic field H_c2 (T ≥ 20 K), we estimate H_c2(0) = 313 T. Thermal conductivity for Ca₁₀(Pt₄As₈)((Fe_0.86Pt_0.14)₂As₂)₅ is relatively small, which can be accounted for by the disorder in the crystal and the low-charge carrier density as verified by the Hall effect.

Temperature dependence of the superconducting energy gaps in Ca9.35La0.65(Pt3As8)(Fe2As2)5 single crystal

We measured the optical reflectivity R(ω) for an underdoped (Ca_0.935La_0.065)₁₀(Pt₃As₈)(Fe₂As₂)₅ single crystal and obtained the optical conductivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{1}(\omega )$$\end{document}σ1(ω) using the K-K transformation. The normal state \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{1}(\omega )$$\end{document}σ1(ω) at 30 K is well fitted by a Drude-Lorentz model with two Drude components (ω_p1 = 1446 cm⁻¹ and ω_p2 = 6322 cm⁻¹) and seven Lorentz components. Relative reflectometry was used to accurately determine the temperature dependence of the superconducting gap at various temperatures below T_c. The results clearly show the opening of a superconducting gap with a weaker second gap structure; the magnitudes for the gaps are estimated from the generalized Mattis-Bardeen model to be Δ₁ = 30 and Δ₂ = 50 cm⁻¹, respectively, at T = 8 K, which both decrease with increasing temperature. The temperature dependence of the gaps was not consistent with one-band BCS theory but was well described by a two-band (hence, two gap) BCS model with interband interactions. The temperature dependence of the superfluid density is flat at low temperatures, indicating an s-wave full-gap superconducting state.

Doping effect on the physical properties of Ca10Pt3As8(Fe2As2)5 single crystals

Ca₁₀Pt₃As₈(Fe₂As₂)₅ is a unique parent compound for superconductivity, which consists of both semiconducting Pt₃As₈ and metallic FeAs layers. We report the observation of superconductivity induced via chemical doping in either Ca site using rare-earth (RE) elements (RE  =  La, Gd) or Fe site using Pt. The interlayer distance and the normal-state physical properties of the doped system change correspondingly. The coupled changes include (1) superconducting transition temperature T _c increases with increasing both doping concentration and interlayer distance, (2) our T _c value is higher than previously reported maximum value for Pt doping in the Fe site, (3) both the normal-state in-plane resistivity and out-of-plane resistivity change from non-metallic to metallic behavior with increasing doping concentration and T _c, and (4) the transverse in-plane magnetoresistance (MR_ab) changes from linear-field dependence to quadratic behavior upon increasing T _c. For La-doped compound with the highest T _c (~35 K), upper critical fields ([Formula: see text], [Formula: see text]), coherence lengths (ξ _ab, ξ _c), and in-plane penetration depth (λ _ab) are estimated. We discuss the relationship between chemical doping, interlayer distance, and physical properties in this system.

Superconductivity at 35 K by self doping in RbGd2Fe4As4O2

We report synthesis, crystal structure and physical properties of a novel quinary compound RbGd₂Fe₄As₄O₂. The new iron oxyarsenide is isostructural to the fluo-arsenide KCa₂Fe₄As₄F₂, both of which contain separate double Fe₂As₂ layers that are self hole-doped in the stoichiometric composition. Bulk superconductivity at [Formula: see text] K is demonstrated by the measurements of electrical resistivity, dc magnetic susceptibility and heat capacity. An exceptionally high value of the initial slope of the upper critical field ([Formula: see text]d[Formula: see text]/d[Formula: see text] [Formula: see text] T K^-1) is measured for the polycrystalline sample.

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.

Discovery of the Pt-Based Superconductor LaPt5As

A novel superconductor, LaPt5As, which exhibits a new crystal structure was discovered by high-pressure synthesis using a Kawai-type apparatus. A superconducting transition temperature was observed at 2.6 K. Depending on the sintering pressure, LaPt5As has superconducting and non-superconducting phases with different crystal structures. A sintering pressure of around 10 GPa is effective to form single-phase superconducting LaPt5As. This material has a very unique crystal structure with an extremely long c lattice parameter of over 60 Å and corner-sharing tetrahedrons composed of network-like Pt layers. Density functional theory calculations have suggested that the superconducting current flows through these Pt layers. Also, this unique layered structure characteristic of LaPt5As is thought to play a key role in the emergence of superconductivity. Furthermore, due to a stacking structure which makes up layers, various structural modifications for the LaPt5As family are conceivable. Since such a high-pressure synthesis using a Kawai-type apparatus is not common in the field of materials science, there is large room for further exploration of unknown phases which are induced by high pressure in various materials.

Phonons of Fe-based superconductor Ca10Pt4As8(Fe1-x Pt x As)10

We report the results of inelastic neutron scattering measurements on particular phonons of a superconducting (SC) Ca10Pt4As8(Fe1-x Pt x As)10 with the onset transition temperature T c ~ 33 K to investigate mainly what roles orbital fluctuation plays in Cooper pairing, where we observed a slight softening of the in-plane transverse acoustic mode corresponding to the elastic constant C 66. This softening starts at temperature T well above the SC T c, as T decreases. An anomalously strong change of the scattering intensity of in-plane optical modes was observed at the M point of the pseudo tetragonal reciprocal space in the range of 35  <  ω  <  40 meV with decreasing T from far above T c. Because this ω region mainly corresponds to the motion of Fe and As atoms in the FeAs planes, the finding presents information on the coupling between the orbital fluctuation of Fe 3d electrons and the lattice system, useful for studying the possible roles of orbital fluctuation in the pairing mechanism and/or the appearance of the so-called nematic phase.

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.

New quaternary arsenide oxides with square planar coordination of gold(I) - structure, (197)Au Mössbauer spectroscopic, XANES and XPS characterization of Nd10Au3As8O10 and Sm10Au3As8O10

The quaternary gold(I) arsenide oxides Nd10Au3As8O10 and Sm10Au3As8O10 were synthesized in sealed quartz ampoules from the rare earth (RE) elements, their appropriate sesquioxides, arsenic, arsenic(III) oxide and finely dispersed gold at maximum annealing temperatures of 1223 K. Both structures were refined from X-ray single crystal diffractometer data at room temperature and at 90 K. Nd10Au3As8O10 and Sm10Au3As8O10 crystallize with a new structure type that derives from the BaAl4 structure through distortions and formation of ordered vacancies. The structures consist of stacked polycationic [RE10O10](10+) layers with oxygen in tetrahedral rare earth coordination and polyanionic [Au(I)3(As2)4](10-) layers with gold in square planar or rectangular planar coordination of four arsenic dumbbells (255 pm As1-As2). In contrast to the well known ionic rare earth oxide layers, the gold arsenide layers rather show covalent bonding and account for the metallic nature of these two new arsenide oxides. This is confirmed by electronic structure calculations and resistivity measurements. The oxidation state of gold was investigated by (197)Au Mössbauer, X-ray absorption near edge structure (XANES) and photoelectron (XPS) spectroscopy. Due to missing comparative gold arsenide compounds, the monovalent gold phosphide oxides RE2AuP2O were measured for comparison. The XANES measurements additionally comprise monovalent gold arsenides REAuAs2. The XPS study contains BaAuAs as reference compound instead. Combination of all data clearly indicates Au(I), which was not observed in square planar coordination up to now. Temperature dependent magnetic susceptibility data show Curie-Weiss paramagnetism for Nd10Au3As8O10 and no magnetic ordering down to 2.5 K. Sm10Au3As8O10 shows the typical Van Vleck type paramagnetism for samarium compounds along with a transition to an antiferromagnetically ordered state at TN = 8.6 K.

Correlation-induced self-doping in the iron-pnictide superconductor Ba2Ti2Fe2As4O

The electronic structure of the iron-based superconductor Ba2Ti2Fe2As4O (Tc(onset)=23.5  K) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the spacer layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect; i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the 3d shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.

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.

Pressure-induced superconductivity and its scaling with doping-induced superconductivity in the iron pnictide with skutterudite intermediary layers

Pressure-induced superconductivity is oberserved in Ca10 (Pt3 As8 )(Fe2 As2 )5 by in situ high-pressure resistance and magnetic susceptibility measurements. Scaling of the pressure-induced and doping-induced superconductivity shows that the electronic phase diagrams of the pressurized and chemically doped 10-3-8 compound are similar in the moderate pressure and doping range but are disparate at higher pressure and heavy doping.

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.

The specific heat of the electron-doped La-1038 compound (Ca0.85La0.15)10(FeAs)10(Pt3As8)

The specific heat of polycrystalline (Ca0.85La0.15)10(FeAs)10(Pt3As8), an electron-doped iron-based superconductor (T(c)(onset) = 34.6 K) with Ca/La ions and Pt3As8 separating the FeAs layers, was measured between 0.4 and 48 K.This compound has been recently reported to represent an electron-doped variant of the non-superconducting 10-3-8 phase, featuring a superconducting transition in the range of that of the 10-4-8 phase. This family of compounds is unique among the iron pnictide superconductors discovered to date due to the second metal pnictide layer, Pt3As8, present in the structure competing with the familiar FeAs layer for the electron from the Ca/La. This superconductor is further unusual in that it has a rather low crystalline symmetry (triclinic) for such a high superconducting transition temperature. The specific heat γ is found to be approximately 26 mJ/(Ca/La mol)K(2), comparable to 122 iron-based superconductors electron-doped on the Fe sites and a factor of two smaller than 122 compounds hole-doped on the cation site, e.g., Ba(1-x)K(x)Fe2As2. The present work also investigates the discontinuity in the specific heat at T(c), ΔC, to compare with the global trend, established by Bud'ko, Ni and Canfield (BNC), of ΔC/T(c) versus T(c) found for essentially all iron-based superconductors. The result is a value lower than the BNC trend by a factor of ten, consistent with a severely broadened superconducting transition.

Antiferromagnetic order in Ca10(Pt3As8)(Fe2As2)5 observed by 75As NMR

75As nuclear magnetic resonance (NMR) measurements carried out on underdoped, non-superconducting Ca10(Pt3As8)(Fe2As2)5 reveal physical properties that are similar but not identical to 122 superconductor parent compounds such as BaFeAs. Results from the single crystal study indicate a phase transition to an antiferromagnetic (AF) state on cooling through T ~ 100 K, albeit nonuniformly. Specifically, the NMR lineshape reflects the presence of staggered hyperfine fields on the As sites associated with a striped AF order. The variation of the internal hyperfine field with temperature suggests that the phase transition to the AF state is discontinuous, and therefore likely coincident with the structural transition inferred from transport experiments.

Structural and magnetic phase transitions in triclinic Ca10(FeAs)10(Pt3As8)

We report the structural and magnetic phase transitions of triclinic Ca10(FeAs)10(Pt3As8), which is the parent compound of the 1038-type iron-arsenide superconductors. High-resolution x-ray diffraction reveals splitting of the in-plane (a,b) lattice parameters at T(s) ≈ 120 K. Platinum-doping weakens the distortion and shifts the transition temperature to 80 K in Ca10(Fe(1-x)Pt(x)As)10(Pt3As8) with x = 0.03. μSR experiments show the onset of magnetic order near T and a broad magnetic phase transition. The structural transition involves no reduction of the space group symmetry in contrast to the other parent compounds of iron-arsenide superconductors; nevertheless the local fourfold symmetry of the FeAs-layers in Ca10(FeAs)10(Pt3As8) is broken.

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.

Growth of single-crystal Ca10(Pt4As8)(Fe(1.8)Pt(0.2)As2)5 nanowhiskers with superconductivity up to 33 K

Single-crystal Ca(10)(Pt(4)As(8))(Fe(1.8)Pt(0.2)As(2))(5) superconducting (SC) nanowhiskers with widths down to hundreds of nanometers were successfully grown in a Ta capsule in an evacuated quartz tube by a flux method. Magnetic and electrical properties measurements demonstrate that the whiskers have excellent crystallinity with critical temperature of up to 33 K, upper critical field of 52.8 T, and critical current density of J(c) of 6.0 × 10(5) A/cm(2) (at 26 K). Since cuprate high-T(c) SC whiskers are fragile ceramics, the present intermetallic SC whiskers with high T(c) have better opportunities for device applications. Moreover, although the growth mechanism is not understood well, the technique can be potentially useful for growth of other whiskers containing toxic elements.