Xiangfeng Wang

We successfully synthesized La- and Sm-doped phenanthrene powder samples and observed superconductivity in them at T(c) around 6 K. The T(c)s are 6.1 K for La(1) phenanthrene and 6.0 K for Sm(1) phenanthrene, which are enhanced by about 1 and 0.5 K compared to those in A(3) phenanthrene (A = K and Rb) and in Ae(1.5) phenanthrene (Ae = Sr and Ba) superconductors, respectively. The superconductive shielding fractions for La(1) phenanthrene and Sm(1) phenanthrene are 46.1% and 49.8% at 2 K, respectively. The small effect of doping with the magnetic ion Sm(3+) on T(c) and the positive pressure dependence coefficient of T(c) strongly suggest unconventional superconductivity in the doped phenanthrene superconductors. The charge transfer to organic molecules from dopants of La and Sm induces a redshift of 7 cm(-1) per electron for the mode at 1441 cm(-1) in the Raman spectra, which is almost the same as those observed in A(3) phenanthrene (A = K and Rb) and Ae(1.5) phenanthrene (Ae = Sr and Ba) superconductors.

Sizable single-crystalline samples of Ca10(Pt3As8)((Fe1$-${}xPtx)2As2)5 (the 10-3-8 phase) with 0$\leqslant${}x<0.1 have been grown and have been systematically characterized via x-ray diffraction, magnetic, and transport measurements. The unsubstituted sample is a heavily doped semiconductor with no sign of magnetic order down to 2~K. With increasing Pt content, the metallic behavior appears, and superconductivity is realized for x$\geqslant${}0.023. Tc rises to its maximum of approximately 13.6~K at the doping level of x$\sim${}0.06 and then decreases for higher x values. The electronic phase diagram of the 10-3-8 phase was mapped out based on the magnetic and transport measurements. The mass anisotropy parameter $\Gamma${}$\sim${}10, obtained from resistive measurements in magnetic fields, indicates a relatively large anisotropy in the iron-based superconductor family. This strong two-dimensional character may lead to the absence of magnetic order. A linear T dependence of susceptibility at high temperatures is observed, indicating that spin fluctuations exist in the underdoped region as in most of the Fe-pnictide superconductors.

We measured the transport properties and susceptibility of single-crystal Ca1$-${}xLaxFe2As2 (x=0, 0.05, 0.1, 0.15, 0.19, and 0.25). Large in-plane resistivity anisotropy similar to that in Co-doped 122 iron pnictides is observed although no transition metals were introduced in the FeAs plane. The in-plane resistivity anisotropy gradually increases with La doping below TSDW, being different from the hole-doped 122 superconductors. The susceptibilities of the samples show that La doping leads to suppression of the spin-density wave (SDW) and induces a Curie-Weiss-like behavior at low temperature, which is much stronger than the other 122 iron-based superconductors.

We establish the phase diagram of the Eu1$-${}xLaxFe2As2 system as a function of doping level x and pressure by measuring the resistivity and magnetic susceptibility. Increased pressure can suppress the spin-density-wave (SDW) and structural transitions very efficiently, while enhancing the antiferromagnetic (AFM) transition temperature TN of Eu2+. Superconductivity coexists with SDW order at low pressure and always coexists with Eu2+ AFM order. The results suggest that the Eu2+ spin dynamics is unconnected with the superconducting pairing taking place in the two-dimensional Fe-As plane, but it can strongly affect superconducting coherence along the c axis.