Evolution of crystalline electric field effects, superconductivity, and heavy fermion behavior in the specific heat of Pr(Os1−x_{1-x}Rux_x)4_4Sb12_{12}

Specific heat $C(T)$ measurements were made on single crystals of the superconducting filled skutterudite series Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ down to 0.6 K. Crystalline electric field fits in the normal state produced parameters which were in agreement with previous measurements. Bulk superconductivity was observed for all values of the Ru concentration $x$ with transition temperatures consistent with previous experiments, confirming a minimum in $T_{c}$ at $x=0.6$. The $C(T)$ data below $T_{c}$ appear to be more consistent with power law behavior for $x=0$ (PrOs$_4$Sb$_{12}$), and with exponential behavior for $0.05 \leq x \leq 0.2$. An enhanced electronic specific heat coefficient $\gamma$ was observed for $x \leq 0.4$, further supporting $x \simeq 0.6$ as a critical concentration where the physical properties abruptly change. Significant enhancement of $\Delta C/T_{c}$ above the weak coupling value was only observed for $x=0$ and $x=0.05$.Comment: 16 pages, 5 figures, submitted to Physical Review B. v2: text added and figures modifie

High-pressure study of non-Fermi liquid and spin-glass-like behavior in CeRhSn

We present measurements of the temperature dependence of electrical resistivity of CeRhSn up to ~ 27 kbar. At low temperatures, the electrical resistivity varies linearly with temperature for all pressures, indicating non-Fermi liquid behavior. Below a temperature Tf ~ 6 K, the electrical resistivity deviates from a linear dependence. We found that the low-temperature feature centered at T = Tf shows a pressure dependence dTf/dP ~ 30 mK/kbar which is typical of canonical spin glasses. This interplay between spin-glass-like and non-Fermi liquid behavior was observed in both CeRhSn and a Ce0.9La0.1RhSn alloy.Comment: 5 pages, 3 figures, accepted for publication to Journal of Physics: Condensed Matte

Eliashberg theory of superconductivity and inelastic rare-earth impurity scattering in filled skutterudite La1−x_{1-x}Prx_{x}Os4_{4}Sb12_{12}

We study the influence of inelastic rare-earth impurity scattering on electron-phonon mediated superconductivity and mass renormalization in (La$_{1-x}$Pr$_{x}$)Os$_{4}$Sb$_{12}$ compounds. Solving the strong coupling Eliashberg equations we find that the dominant quadrupolar component of the inelastic scattering on Pr impurities yields an enhancement of the superconducting transition temperature T$_c$ in LaOs$_{4}$Sb$_{12}$ and increases monotonically as a function of Pr concentration. The calculated results are in good agreement with the experimentally observed T$_c (x)$ dependence. Our analysis suggests that phonons and quadrupolar excitations cause the attractive electron interaction which results in the formation of Cooper pairs and singlet superconductivity in PrOs$_{4}$Sb$_{12}$.Comment: 5 pages,4 figures, revised title suggested by editor, original fig.4 and fig.5 combined together, discussion added before conclusio

Inelastic neutron scattering studies of Crystal Field Levels in PrOs4_4As12_{12}

We use neutron scattering to study the Pr$^{3+}$ crystalline electric field (CEF) excitations in the filled skutterudite PrOs$_4$As$_{12}$. By comparing the observed levels and their strengths under neutron excitation with the theoretical spectrum and neutron excitation intensities, we identify the Pr$^{3+}$ CEF levels, and show that the ground state is a magnetic $\Gamma_4^{(2)}$ triplet, and the excited states $\Gamma_1$, $\Gamma_4^{(1)}$ and $\Gamma_{23}$ are at 0.4, 13 and 23 meV, respectively. A comparison of the observed CEF levels in PrOs$_4$As$_{12}$ with the heavy fermion superconductor PrOs$_4$Sb$_{12}$ reveals the microscopic origin of the differences in the ground states of these two filled skutterudites.Comment: 7 pages, 7 figure

Superconductivity in LnFePO (Ln = La, Pr, and Nd) single crystals

Single crystals of the compounds LaFePO, PrFePO, and NdFePO have been prepared by means of a flux growth technique and studied by electrical resistivity, magnetic susceptibility and specific heat measurements. We have found that PrFePO and NdFePO display superconductivity with values of the superconducting critical temperature T_c of 3.2 K and 3.1 K, respectively. The effect of annealing on the properties of LaFePO, PrFePO, and NdFePO is also reported. The LnFePO (Ln = lanthanide) compounds are isostructural with the LnFeAsO_{1-x}F_x compounds that become superconducting with T_c values as high as 55 K for Ln = Sm. A systematic comparison of the occurrence of superconductivity in the series LnFePO and LnFeAsO_{1-x}F_x points to a possible difference in the origin of the superconductivity in these two series of compounds.Comment: submitted to the New Journal of Physic

The suppression of hidden order and onset of ferromagnetism in URu2Si2 via Re substitution

Substitution of Re for Ru in the heavy fermion compound URu2Si2 suppresses the hidden order transition and gives rise to ferromagnetism at higher concentrations. The hidden order transition of URu(2-x)Re(x)Si2, tracked via specific heat and electrical resistivity measurements, decreases in temperature and broadens, and is no longer observed for x>0.1. A critical scaling analysis of the bulk magnetization indicates that the ferromagnetic ordering temperature and ordered moment are suppressed continuously towards zero at a critical concentration of x = 0.15, accompanied by the additional suppression of the critical exponents gamma and (delta-1) towards zero. This unusual trend appears to reflect the underlying interplay between Kondo and ferromagnetic interactions, and perhaps the proximity of the hidden order phase.Comment: 8 pgs, 5 figs, ICM 2009; please refer to Phys. Rev. Lett. 103, 076404 (2009), arXiv:0908.1809 for details on magnetic scaling and phase diagram (reference added to this version