Characteristic two-dimensional Fermi surface topology of high-Tc iron-based superconductors.

Unconventional Cooper pairing originating from spin or orbital fluctuations has been proposed for iron-based superconductors. Such pairing may be enhanced by quasi-nesting of two-dimensional electron and hole-like Fermi surfaces (FS), which is considered an important ingredient for superconductivity at high critical temperatures (high-Tc). However, the dimensionality of the FS varies for hole and electron-doped systems, so the precise importance of this feature for high-Tc materials remains unclear. Here we demonstrate a phase of electron-doped CaFe2As2 (La and P co-doped CaFe2As2) with Tc = 45 K, which is the highest Tc found for the AEFe2As2 bulk superconductors (122-type; AE = Alkaline Earth), possesses only cylindrical hole- and electron-like FSs. This result indicates that FS topology consisting only of two-dimensional sheets is characteristic of both hole- and electron-doped 122-type high-Tc superconductors.

Emergence of superconductivity at 45 K by lanthanum and phosphorus co-doping of CaFe2As2.

Co-doping of lanthanum and phosphorus in CaFe2As2 induces superconductivity at 45 K. This superconducting transition temperature is higher than the 38 K transition in Ba1-xKxFe2As2, which is the maximum found thus far among the 122 phases. Superconductivity with a substantial shielding volume fraction was observed at 0.12 ≤ x ≤ 0.18 and y = 0.06 in Ca1-xLaxFe2(As1-yPy)2. The superconducting phase of the present system seems to be not adjacent to an antiferromagnetic phase.