Doping - dependent superconducting gap anisotropy in the two-dimensional 10-3-8 pnictide Ca10_{10}(Pt3_3As8_8)[(Fe1−x_{1-x}Ptx_{x})2_2As2_2]5_5

The characteristic features of Ca$_{10}$(Pt$_3$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ("10-3-8") superconductor are relatively high anisotropy and a clear separation of superconductivity and structural/magnetic transitions, which allows studying the superconducting gap without complications due to the coexisting order parameters. The London penetration depth, measured in underdoped single crystals of 10-3-8 ($x =$ 0.028, 0.041, 0.042, and 0.097), shows behavior remarkably similar to other Fe-based superconductors, exhibiting robust power-law, $\Delta \lambda(T) = A T^n$. The exponent $n$ decreases from 2.36 ($x =$ 0.097, close to optimal doping) to 1.7 ($x =$ 0.028, a heavily underdoped composition), suggesting that the superconducting gap becomes more anisotropic at the dome edge. A similar trend is found in low-anisotropy superconductors based on BaFe$_2$As$_2$ ("122"), implying that it is an intrinsic property of superconductivity in iron pnictides, unrelated to the coexistence of magnetic order and superconductivity or the anisotropy of the normal state. Overall this doping dependence is consistent with $s_{\pm}$ pairing competing with intra-band repulsion

57-Fe Mossbauer study of magnetic ordering in superconducting K_0.85Fe_1.83Se_2.09 single crystals

The magnetic ordering of superconducting single crystals of K_0.85Fe_1.83Se_2.09 has been studied between 10K and 550K using 57-Fe Mossbauer spectroscopy. Despite being superconducting below T_sc ~30K, the iron sublattice in K_0.85Fe_1.83Se_2.09 clearly exhibits magnetic order from well below T_sc to its N\'eel temperature of T_N = 532 +/- 2K. The iron moments are ordered perpendicular to the single crystal plates, i.e. parallel to the crystal c-axis. The order collapses rapidly above 500K and the accompanying growth of a paramagnetic component suggests that the magnetic transition may be first order, which may explain the unusual temperature dependence reported in recent neutron diffraction studies.Comment: 6 pages, 4 figures Submitted to Phys.Rev.

Control of magnetic, nonmagnetic, and superconducting states in annealed Ca(Fe1-xCox)(2)As-2

We have grown single-crystal samples of Co substituted CaFe2As2 using an FeAs flux and systematically studied the effects of annealing/quenching temperature on the physical properties of these samples. Whereas the as-grown samples (quenched from 960 degrees C) all enter the collapsed tetragonal phase upon cooling, annealing/quenching temperatures between 350 and 800 degrees C can be used to tune the system to low-temperature antiferromagnetic/orthorhomic or superconducting states as well. The progression of the transition temperature versus annealing/quenching temperature (T-T-anneal) phase diagrams with increasing Co concentration shows that, by substituting Co, the antiferromagnetic/orthorhombic and the collapsed tetragonal phase lines are separated and bulk superconductivity is revealed. We established a 3D phase diagram with Co concentration and annealing/quenching temperature as two independent control parameters. At ambient pressure, for modest x and T-anneal values, the Ca(Fe1-xCox)(2)As-2 system offers ready access to the salient low-temperature states associated with Fe-based superconductors: antiferromagnetic/orthorhombic, superconducting, and nonmagnetic/collapsed tetragonal

Physical and magnetic properties of Ba(Fe1-xMnx)(2)As-2 single crystals

Single crystals of Ba(Fe1−xMnx)2As2, 00.1–0.2. Our measurements show that whereas the structural/magnetic phase transition found in pure BaFe2As2 at 134 K is initially suppressed by Mn substitution, superconductivity is not observed at any substitution level. Although the effect of hydrostatic pressure up to 20 kbar in the parent BaFe2As2 compound is to suppress the structural/magnetic transition at the approximate rate of 0.9 K/kbar, the effects of pressure and Mn substitution in the x=0.102 compound are not cumulative. Phase diagrams of transition temperature versus substitution concentration x based on electrical transport, magnetization, and thermopower measurements have been constructed and compared to those of the Ba(Fe1−xTMx)2As2 (TM= Co and Cr) series