Superconductivity in LaFeAs1−xPxO: effect of chemical pressures and bond covalency

We report the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO. X-ray diffraction shows that, with the partial substitution of P for As, the Fe2As2 layers are squeezed while the La2O2 layers are stretched along the c-axis. Electrical resistance and magnetization measurements show emergence of bulk superconductivity at ∼10 K for the optimally-doped LaFeAs1−xPxO (x = 0.25 ∼ 0.3). The upper critical fields at zero temperature is estimated to be 27 T, much higher than that of the LaFePO superconductor. The occurrence of superconductivity is discussed in terms of chemical pressures and bond covalency. Introduction. – Superconductivity can be induced by carrier doping in an insulator, semiconductor, and even metal. Representative examples are shown in hole-doped La2CuO4 [1], electron-doped BaBiO3 [2], and electrondoped TiSe2 [3]. Recently, superconductivity at 26 K was discovered in LaFeAsO by either electron doping with florine [4] or hole doping with strontium [5]. Subsequent replacements of La with other rare earth elements raised the critical temperatures (Tc) over the McMillan limit (39 K). [6–8] By electron doping with thorium in GdFeAsO, Tc has reached 56 K. [9] The discovery of high superconducting transition temperatures in these Fe-based compounds has generated great interest in the scientific community. [10] As a prototype parent compound of the new class of high-temperature superconductors, LaFeAsO crystallizes in ZrCuSiAs-type structure, [11] which consists of insulating [La2O2] 2+ layers and conducting [Fe2As2] 2− layers. In addition to the carrier doping in [La2O2] 2+ layers, partial substitution of Fe with Co [12,13] and Ni [14] also leads to superconductivity. Although the valence of the doped Co and Ni seems to remain 2+, electron carriers were believed to be induced owing to the itinerant character of the 3d (a)Electronic address: [email protected] (b)Electronic address: [email protected] electrons. [13] That is to say, the Fe-site substitution by Co/Ni still belongs to the scenario of carrier doping. Apart from chemical doping, superconductivity was also observed via applying hydrostatic pressure in the parent compounds such as AFe2As2 (A=Ca, Sr, Ba and Eu) [15– 17] and LaFeAsO [18]. As ”chemical pressures” may be produced by an isovalent substitution with smaller ions, we have tried the substitution of As by P in EuFe2As2. [19] As a result, superconductivity appears below 26 K. Nevertheless, the superconductivity is then influenced by the subsequent ferromagnetic ordering of Eu moments, and diamagnetic Meissner effect cannot be observed. LaFeAsO is a prototype parent compound of ferroarsenide superconductors, showing spin-density-wave (SDW) antiferromagnetic ground state. [20] In contrast, the other end member LaFeAs1−xPxO (x=1) is a superconductor of ∼4 K, showing non-magnetic behavior in the normal state. [21] According to a recent theory, [22] partial substitution of P for As in the ferroarsenides may induce a quantum criticality, which could induce superconductivity. Therefore, the effect of P doping in LaFeAsO is of great interest. In this Letter, we demonstrate bulk superconductivity in LaFeAs1−xPxO at ∼10 K with the evidences of both zero resistance and Meissner effect. This result establishes a stronger evidence that ”chemical pressures” and/or bond covalency may stabilize superconductivity in