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The specific heat coefficient Cp/T was investigated for the optimally doped Ba0.6K0.4Fe2As2 and Sr0.55K0.45Fe2As2. Previously, both the single- and the two-gap pairing have been suggested for Ba0.6K0.4Fe2As2 single crystals. Our analysis reveals that the controversy is mainly caused by the differences in the adopted phonon background. This is especially true in the (Ba,K)Fe2As2 system, in which the phonon contribution below 20 K significantly deviates from the simplified Debye model and, in addition, strongly depends on the doping. The different pairing features reported previously can be reproduced from the same Cp/T data set if the respective phonon baselines are adopted. The soft-phonon shifts, therefore, were examined on the (Ba,K)Fe2As2 and (Sr,K)Fe2As2 systems, as well as the (Ba0.6K0.4)(Fe,Co)2As2 system. The data show that although the K-Ba replacement may change the low-temperature slope β=(dCp/T)/(d(T2)) by 50%, the effects of both K-Sr and Co-Fe replacements are much weaker. The carrier part, Ce/T, is consequently extracted for both Sr0.55K0.45Fe2As2 and Ba0.6K0.4Fe2As2. The two-gap features appear in both cases, but the coupling strength is much stronger for the Ba-based superconductors. [ABSTRACT FROM AUTHOR]

The effects of the possible phase separation on macroscopic transport properties are explored in (Sr,K)Fe2As2. Three probes, the low-field shielding 4πχZFC, the reversible diamagnetization MSC at 5 T, and the specific heat Cp, with different characteristic length scales on the order of 10³, 10, and 100 nm, respectively, were used to examine several (Sr1-xKx)Fe2As2 samples. The data from different probes should be consistent if the sample can be regarded as mesoscopically homogeneous. While this seems to be the case for the optimally doped x?=?0.45 samples, the apparent transition temperature and the effective superfluid density strongly depend on the probes used for both the underdoped (x?=?0.3) and the overdoped (x?=?0.7) samples. Discrepancies of up to a factor of 10 are observed. The data further suggest that both the x?=?0.3 and x?=?0.7 samples are better described as Josephson-junction arrays formed over separated superconductive domains of 10-10² nm sizes. Such a morphology evolution may drastically affect the interpretation of the observed doping effects. [ABSTRACT FROM AUTHOR]

The carrier contribution to the specific heat coefficient, Ce/T, of Sr1-xKxFe2As2 with 0 ⩽ x ⩽ 1 has been determined. The Ce/T at the optimal doping level appears to be T-independent above the superconducting transition temperature Tc. Systematic reductions and increases with cooling below 100 K, on the other hand, characterize the underdoped and overdoped samples, respectively. As the result, the low-T limit of the normal-state Ce/T increases almost linearly with x by a factor of 3 over 0 ⩽ x ⩽ 1. However, the Ce/T suppression across the magnetostructure transition of the underdoped samples is actually negligibly small. In particular, the extracted γH-γL at x = 0, which should include all suppression effects, is comparable to or even smaller than that of x = 0.15, where γH and γL are the high- and low-T limits of the normal-state Ce/T. Therefore, it appears that, while the magnetostructure transition still plays a role, it may not be the main factor behind the Ce/T evolution. [ABSTRACT FROM AUTHOR]

We report the discovery of superconductivity at 2.3 K in Zr5Sb3, the first superconducting member in the large compound family of the Mn5Si3-type structure. Transport, magnetic, and calorimetric measurements clearly demonstrate bulk superconductivity for Zr5Sb3 and suggest it to be a possible phonon-mediated BCS superconductor with a relatively large density of states at the Fermi level associated with the d electrons of Zr and substantially larger electron-phonon coupling compared to the Sn counterpart compound Zr5Sn3 from band structure calculations. More superconductors with even higher transition temperatures are expected to be found in this family of compounds. [ABSTRACT FROM AUTHOR]

To explore the origin of the unusual nonbulk superconductivity with a Tc up to 49 K reported in the rare-earth-doped CaFe2As2, the chemical composition, magnetization, specific heat, resistivity, and annealing effect are systematically investigated on nominal (Ca1-xRx)Fe2As2 single crystals with different x and R = La, Ce, Pr, and Nd. All display a doping-independent Tc once superconductivity is induced, a doping-dependent low field superconducting volume fraction f, and a large magnetic anisotropy ? in the superconducting state, suggesting a rather inhomogeneous superconducting state in an otherwise microscale homogenous superconductor. The wavelength dispersive spectroscopy and specific heat show the presence of defects that are closely related to f, regardless of the R involved. The magnetism further reveals that the defects are mainly superparamagnetic clusters for R=Ce, Pr, and Nd with strong intercluster interactions, implying that defects are locally self-organized. Annealing at 500℃, without varying the doping level x, suppresses f profoundly but not the Tc. The above observations provide evidence for the crucial role of defects in the occurrence of the unusually high Tc~49K in (Ca1-xRx)Fe2As2 and are consistent with the interface-enhanced superconductivity recently proposed. [ABSTRACT FROM AUTHOR]