ABSTRACT:A new iron-based superconductor (Ca,Pr)FeAs 2 was discovered. Plate-like crystals of the new phase were obtained and crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2 1 /m, and is composed of two Ca(Pr) planes, anti-fluorite Fe 2 As 2 layers, and As 2 zigzag chain layers. Plate-like crystals composed of the new phase showed superconductivity with T c ~20 K in both magnetization and resistivity measurements.Several groups of iron-based superconductors, such as REFeAs(O,F) 1 (RE = rare earth elements), AEFeAsF 2 (AE = alkaline earth metals), AEFe 2 As 2 3 , LiFeAs 4 , FeSe 5 , and compounds having perovskite-type oxide layer (ex. Fe 2 P 2 Sr 4 Sc 2 O 6 6 ) have been discovered since 2008. Developments of superconducting tapes and wires have been already attempted for potassium doped AEFe 2 As 2 and fluorine doped REFeAsO, because of their high T c and high H c2 . On the other hand, discovery of new superconductor with high T c and high chemical stability has been still desirable. Since iron-based superconductors are composed of stacking of superconducting layers of Fe 2 Pn 2 or Fe 2 Ch 2 ) (Pn = P, As, Ch = S, Se, Te) and blocking layers, design and search of new blocking layers are promising for discovery of new superconductors. There are several compounds having anti-fluorite blocks and As-based blocks such as UCuAs 2 7 . Recently new iron based superconductors Ca 10 (Pt 3 As 8 )(Fe 2 As 2 ) 5 and Ca 10 (Pt 4 As 8 )(Fe 2 As 2 ) 5 8-10 were reported. These compounds have As-based blocking layers between Fe 2 As 2 layers. The structures of these compounds indicate that there are possibilities to find new iron-based superconductors with arsenide blocking layers. In addition, Saha et al. reported that single crystalline (Ca,RE)Fe 2 As 2 (RE = La, Ce, Pr, Nd) showed superconductivity with high T c exceeding 40 K in resistivity measurement 11 . After this report, superconductivity in RE-doped CaFe 2 As 2 has been reported by several groups 12 . On the other hand, there are several works that report coexistence of two superconducting phases or interface superconductivity in this system 13,14 . Therefore, new superconducting phase is expected in Ca-RE-Fe-As system.In the present study, we have explored new iron-based superconductors in Ca-Pr-Fe-As system and found a new compound (Ca,Pr)FeAs 2 . We report a crystal structure and physical properties of (Ca,Pr)FeAs 2 .All samples were synthesized by the solid-state reaction starting from FeAs(3N), PrAs(3N), Ca(2N), As(4N), and CaO(3N). Since the starting reagents, PrAs and Ca are sensitive to moisture and/or oxygen in air, manipulations were carried out in an argon-filled glove box. Powder mixtures were pelletized, sealed in evacuated quartz ampoules, and reacted at 1000~1200°C for 24 h. Constituent phases were studied by the powder XRD measurements using RIGAKU Ultima-IV diffractometer and intensity date were collected in the 2 range of 5°-80° at a step of 0....

The synthesis of a series of layered iron arsenides Ca1-xRExFeAs2(112) was attempted by heating at 1000 degrees C under a high pressure of 2 GPa. The 112 phase successfully forms with rare earth (RE) = La, Ce, Nd, Sm, Eu, and Gd, while RE-free samples do not contain the 112 phase. While a Ce-doped sample does not show superconductivity, La-, Pr-, Nd-, Sm-, Eu-, and Gd-doped samples show superconducting transition temperatures of 24.5, 13.2, 11.9, 11.6, 9.3, and 12.6 K, respectively, in magnetization measurements. Their T-c and Fe-Fe interlayer distances decreased with decreasing ionic radii of RE, but there are exceptions

Superconducting properties of Co-co-doped (Ca,RE)FeAs 2 ((Ca,RE)112: RE = La, Pr) were investigated. Co-co-doping increased T c of (Ca,Pr)112 while Mn-co-doping suppressed superconductivity of (Ca,RE)112. Co-co-doped (Ca,La)112 showed large diamagnetic screening and sharper superconducting transition than Co-free (Ca,La)112. T c zero observed in resistivity measurements increased from 14 K to 30 K by Co-co-doping, while T c onset was not increased.The critical current density (J c ) of Co-co-doped (Ca,La)112 were approximately 2.1 x 10 4 Acm -2 and 3.2 x 10 3 Acm -2 at 2 K and 25 K, respectively, near zero field. These relatively high J c s and large diamagnetic screening observed in susceptibility measurement as for polycrystalline bulks suggest bulk superconductivity of Co-co-doped (Ca,RE)112 compounds.

Structural and transport properties in the normal and superconducting states are investigated in a Ca0.8La0.2FeAs2 single crystal with Tc = 27 K, belonging to the newly discovered 112 family of iron based superconductors. The transport critical current density Jc for both field directions measured in a focused ion beam patterned microbridge reveals a weakly field dependent and low anisotropic behaviour with a low temperature value as high as Jc(B = 0) ∼ 105 A/cm2. This demonstrates not only bulk superconductivity but also the potential of 112 superconductors towards applications. Interestingly, this superconducting compound undergoes a structural transition below 100 K which is evidenced by temperature-dependent X-ray diffraction measurements. Data analysis of Hall resistance and magnetoresistivity indicate that magnetotransport properties are largely dominated by an electron band, with a change of regime observed in correspondence of the onset of a structural transition. In the low temperature regime, the contribution of a hole band to transport is suggested, possibly playing a role in determining the superconducting state

Dependence of superconducting properties of (Ca,RE)(Fe,TM)As 2 [(Ca,RE)112, TM: Co, Ni)] on RE elements (RE = La-Gd) was systematically investigated. Improvement of superconducting properties by Co or Ni co-doping was observed for all (Ca,RE)112, which is similar to Co-co-doped (Ca,La)112 or (Ca,Pr)112. T c of Co-co-doped samples decreased from 38 K for RE = La to 29 K for RE = Gd with decreasing ionic radii of RE 3+ . However, Co-co-doped (Ca,Eu)112 showed exceptionally low T c = 21 K probably due to the co-existence of Eu 3+ and Eu 2+ suggested by longer interlayer distance d Fe-Fe of (Ca,Eu)112 than other (Ca,RE)112.