Phase diagram and transport properties of Sb-doped Ca0.88La0.12Fe2As2 single crystals

The effects of isovalent Sb substitution on the superconducting properties of the Ca0.88La0.12Fe2（As1-ySby）2 system have been studied through electrical resistivity measure- ments. It is seen that the antiferromagnetic or structural transition is suppressed with Sb content, and a high-To superconducting phase, accompanied by a low-Tc phase, emerges at 0.02 ≤y ≤ 0.06. In this intermediate-doping regime, normal-state transport shows non-Fermi-liquid-like behaviors with nearly T-linear resistivity above the high-Tc phase. With further Sb doping, this high-Tc phase abruptly vanishes for y 〉 0.06 and the conventional Fermi liquid is restored, while the low-T,, phase remains robust against Sb inlpurities. The coincidence of the high-Tc phase and non-Fermi liquid transport behaviors in the intermediate Sb-doping regime suggests that AFM fluctuations play an important role in the observed non-Fermi liquid behaviors, which may be intimately related to the unusual nonbulk high-Tc phase in this system.

Anisotropic Magnetoresistivity in Semimetal TaSb_2

We investigate the anisotropic magnetic transports in topological semimetal TaSb2. The compound shows the large magnetoresistance（MR） without saturation and the metal-insulator-like transition no matter whether the magnetic field is parallel to c-axis or a-axis, except that the MR for B‖c is almost twice as large as that of B‖a at low temperatures. The adopted Kohler＇s rule can be obeyed by the MR at distinct temperatures for B‖c,but it is slightly violated as B‖a. The angle-dependent MR measurements exhibit the two-fold rotational symmetry below70 K,consistent with the monoclinic crystal structure of TaSb2. The dumbbell-like picture of angle-dependent MR in TaSb2 suggests a strongly anisotropic Fermi surface at low temperatures. However, it finally loses the two-fold symmetry over 70 K, implying a possible topological phase transition at around the temperature where Tm is related to a metal-insulator-like transition under magnetic fields.

Superconductivity, electronic phase diagram, and pressure effect in Sr_(1-x)Pr_xFBiS_2

Based on a combination of X-ray diffraction, electrical transports, magnetic susceptibility, specific heat, and pressure-effect measurements, we report the results of experiments on a series of BiS_2-based Sr_(1-x)Pr_xFBiS_2 superconductors with the maximum Tcof 2.7 K for x=0.5 and at ambient pressure. Superconductivity appears only for 0.4≤x≤0.7 whereas the normal-state resistivity shows the semiconducting-like behaviors. The magnetic susceptibility χ(T) displays the low superconducting shielding volume fractions and C(T) shows no distinguishable anomaly near Tc, which suggests a filamentary superconductivity in the Pr-doped polycrystalline samples. By varying doping concentrations, an electronic phase diagram is established. Upon applying pressure on the optimally doped Sr_(0.5)Pr_(0.5)FBiS_2 system, Tcis abruptly enhanced, reaches 8.5 K at the critical pressure of P_c=1.5 GPa, and increases slightly to 9.7 K at 2.5 GPa. Accompanied by the enhancement of superconductivity from the low-to the high-Tc phases, the normal state undergoes a semiconductor-to-metal transition when under pressure. This scenario may be linked to enhanced overlap of the Bi-6 p and S-p orbitals, which contributes to the enhanced superconductivity above Pc. The pressuretemperature phase diagram for Sr_(0.5)Pr_(0.5)FBiS_2 is also presented.