Enhanced Superconductivity up to 43 K by P/Sb Doping of Ca 1− x La x FeAs 2

A number of iron-based superconductors have been discovered,1–3) which include LaFeAsO (1111-type structure),4) BaFe2As2 (122-type),5) LiFeAs (111-type),6) and FeSe (11-type),7) as well as compounds with complex oxide spacer layers8–11) and arsenide spacer layers such as Ca10(Pt4As8)(Fe2As2)5. The maximum superconducting transition temperature Tc is 55K of the 1111-type structure.18) In order to further increase Tc, an exploration of novel structure types should be performed. Very recently, Katayama et al.19) and Yakita et al.20) have reported superconductivity in Ca1 xLaxFeAs2 and Ca1 xPrxFeAs2, respectively, with a novel 112-type structure. Ca1 xLaxFeAs2 crystalizes in a monoclinic structure with the space group P21 (No. 4) and consists of alternately stacked Fe2As2 and arsenic zigzag bond layers.19) Although pure CaFeAs2 was not obtained, Katayama et al. found that the substitution of a small amount of La for Ca stabilizes the 112 phase and induces superconductivity at Tc 1⁄4 34K for x 1⁄4 0:16. Interestingly, Katayama et al.19) suggested that the trace superconductivity of Ca1 xLaxFeAs2 could exhibit Tc 1⁄4 45K. In this paper, we report that a large increase in Tc occurs with the phosphorus or antimony doping of Ca1 xLaxFeAs2. P-doped Ca0.84La0.16FeAs2 and Sb-doped Ca0.85La0.15FeAs2 exhibited Tc values of 41 and 43K, respectively, while P/Sbfree Ca0.85La0.15FeAs2 exhibited Tc 1⁄4 35K. Single crystals of Ca1 xLaxFe(As1 yPny)2 (Pn = P and Sb) were grown by heating a mixture of Ca, La, FeAs, As, P, and Sb powders. A stoichiometric amount of the mixture was placed in an aluminum crucible and sealed in an evacuated quartz tube. The preparation was carried out in a glove box filled with argon gas. Ampules were heated at 700 °C for 3 h, heated to 1100 °C at a rate of 46 °C/h, and cooled to 1050 °C at a rate of 1.25 °C/h, followed by furnace cooling. The obtained samples were characterized by powder X-ray diffraction (XRD) analysis, performed using a Rigaku RINT-TTR III X-ray diffractometer with Cu K radiation. The Ca1 xLaxFe(As1 yPny)2 was obtained together with a powder mixture of LaAs, FeAs, FeAs2, and CaFe2As2. We separated platelike single crystals of the present system with typical dimensions of 0:4 0:4 0:02mm3 from the mixture. The La content x was analyzed by energy-dispersive X-ray spectrometry (EDS). The synthesis for nominal x 1⁄4 0:07{0:50 yielded samples with x 1⁄4 0:15{0:25. On the other hand, the P and Sb contents could not be determined because the nominal amounts of P (y 1⁄4 0:005) and Sb (y 1⁄4 0:01) were very small. In the rest of this paper, we assume the nominal values of y. Electrical resistivity ab (parallel to the ab-plane) measurements were carried out by a standard DC four-terminal method in a Quantum Design PPMS. The magnetization M was measured using a Quantum Design MPMS. We demonstrate the bulk superconductivity at 35K and the trace superconductivity at 45K in Ca1 xLaxFeAs2 using the temperature dependences of the electrical resistivity ab and magnetization M shown in Figs. 1(a) and 1(d), respectively. The electrical resistivity ab of Ca1 xLaxFeAs2 with x 1⁄4 0:15 exhibits a sharp drop below 40K, and zero resistivity is observed at 35K. The diamagnetic behavior below Tc 1⁄4 35K clearly supports the emergence of bulk superconductivity. On the other hand, the ab of Ca1 xLaxFeAs2 with x 1⁄4 0:19 exhibits the onset of superconductivity at 45K, as shown in Fig. 1(a), but no diamagnetic signal is observed at 345K, as shown in Fig. 1(d). We found that a small amount of isovalent doping converts this trace superconductivity into bulk superconductivity. As shown in Fig. 1(b), the P-doped Ca1 xLaxFe(As0.995P0.005)2 shows the onset of superconductivity at 44K and zero resistivity at 40K for x 1⁄4 0:16. Evidence for bulk superconductivity is found in the magnetization M, which clearly shows diamagnetic behavior below Tc 1⁄4 41K, as shown in Fig. 1(e). The shielding volume fraction at 5K corresponds to 44% for perfect diamagnetism, supporting the emergence of bulk superconductivity. In a composition with x 1⁄4 0:18, although the Tc of 39K is somewhat lower than that with 1.5