Localized 5f electrons in superconducting PuCoIn5: consequences for superconductivity in PuCoGa5.

The physical properties of the first In analog of the PuMGa(5) (M = Co, Rh) family of superconductors, PuCoIn(5), are reported. With its unit cell volume being 28% larger than that of PuCoGa(5), the characteristic spin-fluctuation energy scale of PuCoIn(5) is three to four times smaller than that of PuCoGa(5), which suggests that the Pu 5f electrons are in a more localized state relative to PuCoGa(5). This raises the possibility that the high superconducting transition temperature T(c) = 18.5 K of PuCoGa(5) stems from the proximity to a valence instability, while the superconductivity at T(c) = 2.5 K of PuCoIn(5) is mediated by antiferromagnetic spin fluctuations associated with a quantum critical point.

Anisotropic effect of Cd and Hg doping on the Pauli limited superconductor CeCoIn5.

We studied the effect of impurity on the first order superconducting (SC) transition and the high field-low temperature (HFLT) SC state of CeCoIn5 by measuring the specific heat of CeCo(In1-xCdx)_{5} with x=0.0011, 0.0022, and 0.0033 and CeCo(In1-xHgx)_{5} with x=0.000 16, 0.000 32, and 0.000 48 at temperatures down to 0.1 K and fields up to 14 T. Cd substitution rapidly suppresses the crossover temperature T0, where the SC transition changes from second to first order, to T=0 K with x=0.0022 for H parallel[100], while it remains roughly constant up to x=0.0033 for H parallel[001]. The associated anomaly of the proposed FFLO state in Hg-doped samples is washed out by x=0.000 48, while remaining at the same temperature, indicating high sensitivity of that state to impurities. We interpret these results as supporting the nonmagnetic, possibly FFLO, origin of the HFLT state in CeCoIn5.

Antimony-121 Mössbauer spectral study of the Eu14MnSb11 and Yb14MnSb11 Zintl compounds.

The antimony-121 Mössbauer spectra of the Yb14MnSb11 and Eu14MnSb11 Zintl compounds have been measured between 2 or 5 and 300 K. The resulting three-dimensional arrays of the spectral counts, velocity, and temperature have been simultaneously fit with a minimum number of free parameters. These fits yield a 0 Kelvin transferred hyperfine field of 2.9(2) T, a Curie temperature of 57(3) K, and a Mössbauer temperature of 182(2) K for Yb14MnSb11; in this case the transferred field arises solely from the ordering of Mn2+. Because Eu14MnSb11 has both Eu2+ and Mn2+ ions that are magnetically ordered, its antimony-121 Mössbauer spectra are more complex and reveal two magnetic transitions, the first at 92(1) K resulting from the ordering of the Mn2+ ions and the second at 9.5(1.0) K resulting from the ordering of the Eu2+ ions; the corresponding 0 Kelvin transferred hyperfine fields are 1.3(1) and 3.7(1) T. The antimony-121 isomer shifts yield electronic configurations of 5s1.745p4.28 and 5s1.745p4.19 for the average antimony anion in Yb14MnSb11 and Eu14MnSb11, respectively.