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The discovery of EuFeAs2, currently the only charge-neutral parent phase of the 112-type iron-pnictide system, provides a new platform for the study of elemental doping effects on magnetism and superconductivity (SC). In this study, a series of polycrystalline EuFe1-yCoyAs2 and Eu0.9Pr0.1Fe1-yCoyAs2 samples are synthesized through solid-state reaction, and the evolutions of SC and magnetism with Co doping in EuFeAs2 and Eu0.9Pr0.1FeAs2 are investigated by electrical transport and magnetic susceptibility measurements. For EuFe1-yCoyAs2, the Eu-related antiferromagnetic (AFM) transition around 40 K is barely affected by Co doping, while the Fe-related spin density wave (SDW) transition temperature drops rapidly. Meanwhile, SC is induced by a trace amount of Co doping, with a highest transition temperature Tc ~ 28 K found in EuFe0.9Co0.1As2. For the Eu0.9Pr0.1Fe1-yCoyAs2 series, the magnetism and superconductivity show similar evolutions upon Co doping, and the highest Tc is enhanced to 30.6 K with an optimum doping level y ~ 0.07. Our results shed light on the competition between SC and SDW with Co doping in the 112-type EuFeAs2 system., Comment: 10 pages, 5 figures

Materials with exceptional magnetism and superconductivity usually conceive emergent physical phenomena. Here, we investigate the physical properties of the (Eu,La)FeAs2 system with double magnetic sublattices. The parent EuFeAs2 shows anisotropy-associated magnetic behaviors, such as Eu-related moment canting and exchange bias. Through La doping, the magnetic anisotropy is enhanced with ferromagnetism of Eu2+ realized in the overdoped region, and a special exchange bias of the superposed ferromagnetic/superconducting loop revealed in Eu0.8La0.2FeAs2. Meanwhile, the Fe-related antiferromagnetism shows unusual robustness against La doping. Theoretical calculation and 57Fe M\"ossbauer spectroscopy investigation reveal a doping-tunable dual itinerant/localized nature of the Fe-related antiferromagnetism. Coexistence of the Eu-related ferromagnetism, Fe-related robust antiferromagnetism, and superconductivity is further revealed in Eu0.8La0.2FeAs2, providing a platform for further exploration of potential applications and emergent physics. Finally, an electronic phase diagram is established for (Eu,La)FeAs2 with the whole superconducting dome adjacent to the Fe-related antiferromagnetic phase, which is of benefit for seeking underlying clues to high-temperature superconductivity., Comment: 13 pages, 6 figures for the main text

We report the synthesis and characterization of a new ternary molybdenum pnictide superconductor, Cs2Mo3As3. The powder x-ray diffraction analysis reveals the quasi-one-dimensional (Q1D) hexagonal crystal structure formed by Cs+ and infinite (Mo3As3)2− chains as indicated by the wire-like grain morphology. Electrical resistivity and magnetic susceptibility characterizations exhibit superconductivity with the onset transition temperature at 11.5 K, which is the highest in all Q1D superconductors reported so far. An upper critical magnetic field of about 61.7 T at zero temperature was extrapolated from the resistivity measurement under a magnetic field, which is much higher than the Pauli paramagnetic limit, and the reason for such a high upper critical field may lie in its unconventional nature of superconducting pairing symmetry. The discovery of Cs2Mo3As3 inspires the search for new superconductors for future high field applications.

A Cr-based quasi-one-dimensional superconductor $\mathrm{N}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{r}}_{3}\mathrm{A}{\mathrm{s}}_{3}$ was synthesized by an ion-exchange method in a sodium naphthalenide solution. The crystals are threadlike and the structure was analyzed by x-ray diffraction with a noncentrosymmetric hexagonal space group $P\ensuremath{-}6m2$ (No. 187), in which the ${{(\mathrm{C}{\mathrm{r}}_{3}\mathrm{A}{\mathrm{s}}_{3})}^{2}}^{\ensuremath{-}}$ linear chains are separated by $\mathrm{N}{\mathrm{a}}^{+}$ ions, and the refined lattice parameters are $a=9.239(2)\phantom{\rule{0.16em}{0ex}}\AA{}$ and $c=4.209(6)\phantom{\rule{0.16em}{0ex}}\AA{}$. The measurements for electrical resistivity, magnetic susceptibility, and heat capacity reveal a superconducting transition with unconventional characteristic at 8.6 K, which exceeds that of all previously reported Cr-based superconductors.

Here we report the discovery of the first ternary molybdenum pnictide based superconductor K2Mo3As3. Polycrystalline samples were synthesized by the conventional solid state reaction method. X-ray diffraction analysis reveals a quasi-one-dimensional hexagonal crystal structure with (Mo3As3)2- linear chains separated by K+ ions, similar as previously reported K2Cr3As3, with the space group of P-6m2 (No. 187) and the refined lattice parameters a = 10.145(5) {\AA} and c = 4.453(8) {\AA}. Electrical resistivity, magnetic susceptibility, and heat capacity measurements exhibit bulk superconductivity with the onset Tc at 10.4 K in K2Mo3As3 which is higher than the isostructural Cr-based superconductors. Being the same group VIB transition elements and with similar structural motifs, these Cr and Mo based superconductors may share some common underlying origins for the occurrence of superconductivity and need more investigations to uncover the electron pairing within a quasi-one-dimensional chain structure.

Here we report the preparation and superconductivity of the 133-type Cr-based quasi-one-dimensional (Q1D) RbCr3As3 single crystals. The samples were prepared by the deintercalation of Rb+ ions from the 233-type Rb2Cr3As3 crystals which were grown from a high-temperature solution growth method. The RbCr3As3 compound crystallizes in a centrosymmetric structure with the space group of P63/m (No. 176) different with its non-centrosymmetric Rb2Cr3As3 superconducting precursor, and the refined lattice parameters are a = 9.373(3) {\AA} and c = 4.203(7) {\AA}. Electrical resistivity and magnetic susceptibility characterizations reveal the occurrence of superconductivity with an interestingly higher onset Tc of 7.3 K than other Cr-based superconductors, and a high upper critical field Hc2(0) near 70 T in this 133-type RbCr3As3 crystals.

We report the synthesis, crystal structure, superconductivity and physical property characterizations of the ternary equiatomic compound ScRuSi. Polycrystalline samples of ScRuSi were prepared by an arc-melting method. The as-prepared samples were identified as the orthorhombic Co2P-type o-ScRuSi by the powder X-ray diffraction analysis. Electrical resistivity measurement shows o-ScRuSi to be a metal which superconducts below a Tc of 3.1 K, and the upper critical field {\mu}0Hc2(0) is estimated to be 0.87 T. The magnetization and specific heat measurements confirm the bulk type-II superconductivity in o-ScRuSi, with the specific heat jump within the BCS weak coupling limit. o-ScRuSi is the first Co2P-type superconductor containing scandium. After annealing at 1273 K for a week, o-ScRuSi transforms into the hexagonal Fe2P-type h-ScRuSi, and the latter is a Pauli-paramagnetic metal with no superconductivity observed above 1.8 K.

In the iron-based high-Tc bulk superconductors, Tc above 50K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for the highest T-c in these materials. To introduce more electron carriers and less crystal lattice distortions, we study the Th and F codoping effects into the Sm-O layers with heavy electron doping. Dozens of Sm1-x Th-x FeAsO1-y F-y samples are synthesized through the solid state reaction method, and these samples are carefully characterized by the structural, resistive, and magnetic measurements. We find that the codoping of Th and F clearly enhances the superconducting T-c more than the Th or F single-doped samples, with the highest record T-c up to 58.6K when x= 0.2 and y= 0.225. Further element doping causes more impurities and lattice distortions in the samples with a weakened superconductivity.

In this report, we studied the effects of isovalent Na-doping on the recently discovered quasi-one-dimensional Cr-based unconventional superconductor K2Cr3As3. A series of polycrystalline samples with nominal component (K1-x Na x )2Cr3As3 (x = 0-1) were synthesized by the solid state reaction method. From crystal structure and chemical phase characterizations, we found two distinct chemical phases with the same hexagonal structure but distinguished by different site occupancy of Na+ ions at the two kinds of K-site in the K2Cr3As3 lattice structure. When x ⩽ 0.4, the doped samples form a continuous sosoloid phase of (K1-x Na x )2Cr3As3 with the Na+ ions randomly doping at the K-sites (denoted as α-phase); when x ⩾ 0.5, a novel individual phase of (K0.25Na0.75)2Cr3As3 emerges, in which the Na+ ions selectively occupy all the '3k' sites and the K+ ions occupy the '1c' sites (denoted as β-phase). No chemical phase of Na2Cr3As3 was detected. Superconductivity in these samples was studied by electrical transport and magnetic susceptibility measurements, and it evolves in a much sophisticated manner. In the α-phase, the superconducting T c decreases quickly upon Na-doping. All these α-phase samples have surprisingly low superconducting volume fraction and relatively low T c compared with the undoped K2Cr3As3. However, the β-phase has a clearly enhanced T c up to 7.6 K which locates between the values of K2Cr3As3 and Na2Cr3As3, and exhibits a full superconducting shielding signal.

We report the discovery and characterization of a novel 112-type iron pnictide EuFeAs2, with La-doping induced superconductivity in a series of Eu1-xLaxFeAs2. The polycrystalline samples were synthesized through solid state reaction method only within a very narrow temperature window around 1073 K. Small single crystals were also grown from a flux method with the size about 100 um. The crystal structure was identified by single crystal X-ray diffraction analysis as a monoclinic structure with space group of P21/m. From resistivity and magnetic susceptibility measurements, we found that the parent compound EuFeAs2 shows a Fe2+ related antiferromagnetic/structural phase transition near 110 K and a Eu2+ related antiferromagnetic phase transition near 40 K. La doping suppressed the both phase transitions and induced superconducting transition with a Tc ~ 11 K for Eu0.85La0.15FeAs2.

Here we report the discovery of superconductivity in the ternary LaRu2As2 compound. The polycrystalline LaRu2As2 samples were synthesized by the conventional solid state reaction method. Powder X-ray diffraction analysis indicates that LaRu2As2 crystallizes in the ThCr2Si2-type crystal structure with the space group I4/mmm (No. 139), and the refined lattice parameters are a = 4.182(6) {\AA} and c = 10.590(3) {\AA}. The temperature dependent resistivity measurement shows a clear superconducting transition with the onset Tc (critical temperature) at 7.8 K, and zero resistivity happens at 6.8 K. The upper critical field at zero temperature m0Hc2(0) was estimated to be 1.6 T from the resistivity measurement. DC magnetic susceptibility measurement shows a bulk superconducting Meissner transition at 7.0 K, and the isothermal magnetization measurement indicates that LaRu2As2 is a type-II superconductor., Comment: Science Bulletin,2016

Here we report the synthesis and discovery of superconductivity in a novel ternary iridium-arsenide compound BaIr2As2. The polycrystalline BaIr2As2 sample was synthesized by a high temperature and high pressure method. Crystal structural analysis indicates that BaIr2As2 crystallizes in the ThCr2Si2-type layered tetragonal structure with space group I4/mmm (No. 139), and the lattice parameters were refined to be a = 4.052(9) {\AA} and c = 12.787(8) {\AA}. By the electrical resistivity and magnetic susceptibility measurements we found type-II superconductivity in the new BaIr2As2 compound with a Tc (critical temperature) of 2.45 K, and an upper critical field u0Hc2(0) about 0.2 T. Low temperature specific heat measurements gave a Debye temperature about 202 K and a distinct specific jump with delta Ce/{\gamma}Tc = 1.36, which is close to the value of BCS weak coupling limit and confirms the bulk superconductivity in this new BaIr2As2 compound.

Single crystals of undoped CaFe2As2 are grown by an FeAs self-flux method, and the crystals are quenched in ice-water rapidly after high-temperature growth. The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature-dependent x-ray diffraction measurements. Superconductivity below 25 K is observed in the collapsed phase by resistivity and magnetization measurements. The isothermal magnetization curve measured at 2 K indicates that this is a typical type-II superconductor. For comparison, we systematically characterize the properties of the furnace-cooled, quenched, and post-annealed single crystals, and find strong internal crystallographic strain existing in the quenched samples, which is the key for the occurrence of superconductivity in the undoped CaFe2As2 single crystals.

In this article, the Sm-doping single crystals Ca1 − x Sm x Fe2As2 (x = 0 ~ 0.2) were prepared by the CaAs flux method, and followed by a rapid quenching treatment after the high temperature growth. The samples were characterized by structural, resistive, and magnetic measurements. The successful Sm-substitution was revealed by the reduction of the lattice parameter c, due to the smaller ionic radius of Sm3+ than Ca2+. Superconductivity was observed in all samples with onset T c varying from 27 K to 44 K upon Sm-doping. The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples. Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x > 0.10. The doping dependences of the c-axis length and onset T c were summarized. The high-T c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.

The single crystal of ThCr2Si2-type BaPd2As2 was successfully prepared by a self-flux method, and the crystal structure was characterized by X-ray diffraction method with the lattice parameters and (space group , No. 139). Bulk superconductivity below a T c (critical temperature) of 3.85 K was revealed in this compound by the measurements of resistivity, magnetic susceptibility and specific heat, and this T c is much higher than those of other isostructural Pd-based superconductors.