Superconductivity in Pt- and La-Doped BaFe2As2 Compounds Prepared by Solid-State Reaction

We report magnetization, resistivity, and heat capacity measurements on samples of BaFe₂As₂, BaFe_1.9Pt_0.1As₂, and Ba_0.7La_0.3Fe_1.9Pt_0.1As₂ that have been prepared by solid-state reaction. Standard four-probe transport measurements in the temperature range 4.2-300 K showed that the substitution of non-isoelectronic Pt for Fe produced an electron-doped situation resulting in a superconductive phase transition at T_c = 24 K. Furthermore, we observe that the coupled spin-density-wave/antiferromagnetic transition which occurs for the BaFe₂As₂ sample at 140 K, as expected, also occurs in the Pt-doped BaFe_1.9Pt_0.1As₂ sample with only a small decrease in critical temperature to 138 K. We have also investigated in detail the superconducting state with magnetization measurements in the temperature range 5-400 K, up to a field of 9 T. From the magnetization data for BaFe_1.9Pt_0.1As₂, we observe that T_c = 21.7 K, which is in good agreement with the resistivity measurement. The zero-field cooled results correspond to a large Meissner fraction. In Ba_0.7La_0.3Fe_1.9Pt_0.1As₂, we also see a large diamagnetic response, with a reduction in T_c to 19.6 K. In addition, we analyzed heat capacity measurements for the electronic and phonon behavior of these materials. The analysis shows an excess entropy associated with the change in electronic behavior that is consistent with what is expected for Fe local moments, pointing to an electronic density of states that is based on a spin-fluctuation mechanism. We discuss the results in relation to a varying electronic density of states present in the samples and a possible pseudogap induced by addition of Pt.