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Register a new userSuperconductivity and magnetism in platinum-substituted SrFe2As2 single crystals
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Single crystals of SrFe2-xPtxAs2 (0 < x < 0.36) were grown using the self flux solution method and characterized using x-ray crystallography, electrical transport, magnetic susceptibility, and specific heat measurements. The magnetic/structural transition is suppressed with increasing Pt concentration, with superconductivity seen over the range 0.08 < x < 0.36 with a maximum transition temperature Tc of 16 K at x = 0.16. The shape of the phase diagram and the changes to the lattice parameters are similar to the effects of other group VIII elements Ni and Pd, however the higher transition temperature and extended range of superconductivity suggest some complexity beyond the simple electron counting picture that has been discussed thus far.
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Single crystals of Ba_{1-x}Rb_{x}Fe_2As_2 with x=0.05-0.1 have been grown from Sn flux and are bulk superconductors with T_c up to 23 K. The crystal structure was determined by X-ray diffraction analysis, and Sn is found to be incorporated for 9% Ba, shifted by 1.1 Angstroem away from the Ba site towards the (Fe_2As_2)-layers. The upper critical field deduced from resistance measurements is anisotropic with slopes of 7.1(3) T/K (H || ab-plane) and 4.2(2) T/K (H || c-axis), sufficiently far below T_c. The extracted upper critical field anisotropy of 3 close to T_c, is in good agreement with the estimate from magnetic torque measurements. This indicates that the electronic properties in the doped BaFe_2As_2 compound are significantly more isotropic than those in the LnFeAsO family. The in-plane critical current density at 5 K exceeds 10^6 A/cm^2, making Ba_{1-x}Rb_xFe_2As_2 a promising candidate for technical applications.
The electrical resistivity rho(T) and heat capacity C(T) on single crystals of SrNi2As2 and EuNi2As2 are reported. While there is no evidence for a structural transition in either compound, SrNi2As2 is found to be a bulk superconductor at T_c=0.62 K with a Sommerfeld coefficient of gamma= 8.7 mJ/mol K^2 and a small upper critical field H_{c2} sim 200 Oe. No superconductivity was found in EuNi2As2 above 0.4 K, but anomalies in rho and C reveal that magnetic order associated with the Eu^{2+} magnetic moments occurs at T_m = 14 K.
The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.
We have studied the intrinsic normal and superconducting properties of the oxypnictide LaFePO. These samples exhibit bulk superconductivity and the evidence suggests that stoichiometric LaFePO is indeed superconducting, in contrast to other reports. We find that superconductivity is independent of the interplane residual resistivity $rho_0$ and discuss the implications of this on the nature of the superconducting order parameter. Finally we find that, unlike $T_c$, other properties in single-crystal LaFePO including the resistivity and magnetoresistance, can be very sensitive to disorder.
To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $tau(theta)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $gamma$ extracted from $tau(theta)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $gamma$ depends on both $T$ and $H$, saturating at $gamma simeq 9$ at lower temperatures. Neither the London penetration depth nor the superfluid density is affected by high fields fields up to the upper critical field. Our results indicate two strongly-coupled superconducting gaps of nearly equal magnitudes.
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