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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

We report the crystal structure and superconductivity of a new ternary boride Mo5GeB2. Mo5GeB2 crystallizes in a Cr5B3-type structure with a space group of I4/mcm (No. 140). The lattice parameters are a = 6.0721(1) A, and c = 11.1173(2) A. A type-II superconductivity of Tc = 5.8 K is revealed by resistivity, magnetization, and heat capacity measurements. The upper and lower critical fields are 0.52 T and 77.1 mT, respectively. The temperature dependence of low temperature specific heat follows the description of a BCS superconductor with an isotropic s-wave gap (Δ0 = 0.93 meV). However, both the values of the normalized specific heat change (ΔCe/γTc = 1.63) and the coupling strength (Δ0/kBTc = 1.87) exceed the BCS weak-coupling limit, indicating an intermediate coupling in Mo5GeB2.

We report superconductivity in a new ternary molybdenum pnictide Rb2Mo3As3, synthesized via the solid state reaction method. Powder x-ray diffraction analysis reveals a hexagonal crystal structure with space group P 6 ¯ m 2 (No. 187), and the refined lattice parameters are a = 10.431(5) Å, c = 4.460(4) Å. SEM images show rod-like grains with good ductility, confirming a quasi-one-dimensional (Q1D) structure. Electrical resistivity and dc magnetic susceptibility characterizations exhibit superconductivity with an onset of T c = 10.5 K. The upper critical field of Rb2Mo3As3 is estimated to be 28.2T at zero temperature, providing an evidence of possible unconventional superconductivity. Our recent discovery of MoAs-based superconductors above 10 K provides a unique platform for the study of exotic superconductivity in 4d electron systems with Q1D crystal structures.

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.