Positive-muon ($mu^+$) Knight shifts have been measured in the paramagnetic states of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloys, where $x =$ 0, 0.25, 0.45, 0.50, 0.55, 0.75, and 1.00. In Pr-substituted NdOs$_4$Sb$_{12}$ ($x le$ 0.75), but not in NdOs$_4
$Sb$_{12}$, Clogston-Jaccarino plots of $mu^+$ Knight shift~$K$ versus magnetic susceptibility~$chi$ exhibit an anomalous saturation of $K(chi)$ at $sim-$0.5% for large susceptibilities (low temperatures), indicating a reduction of the coupling strength between $mu^+$ spins and $4f$ paramagnetism for temperatures $lesssim$ 15~K. We speculate that itinerant Pr$^{3+}$ quadrupolar excitations, invoked to mediate the superconducting Cooper-pair interaction, might modify the $mu^+$-$4f$ ion indirect spin-spin interaction.
Muon spin rotation and relaxation ($mu$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloy series. In the ferromagnetic end compound NdOs$_4$Sb$_{12}$ the spont
aneous local field at positive-muon ($mu^+$) sites below the ordering temperature $T_C$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd$^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For 0.45 $le x le$ 0.75, $mu^+$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at $mu^+$ sites below $T_C$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd$^{3+}$ moments, and speculate that the Nd$^{3+}$-$mu^+$ interaction is suppressed and disordered by Pr doping. In an $x$ = 0.25 sample, which is superconducting below $T_c$ = 1.3 K, there is no sign of spin freezing (static Nd$^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic $mu^+$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The $mu^+$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below $T_c$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For 0.25 $le x le$ 0.75, the $mu$SR data provide evidence against phase separation; superconductivity and Nd$^{3+}$ magnetism coexist on the atomic scale.
Superconductivity, magnetic order, and quadrupolar order have been investigated in the filled skutterudite system Pr$_{1-x}$Nd$_{x}$Os$_4$Sb$_{12}$ as a function of composition $x$ in magnetic fields up to 9 tesla and at temperatures between 50 mK an
d 10 K. Electrical resistivity measurements indicate that the high field ordered phase (HFOP), which has been identified with antiferroquadruoplar order, persists to $x$ $sim$ 0.5. The superconducting critical temperature $T_c$ of PrOs$_4$Sb$_{12}$ is depressed linearly with Nd concentration to $x$ $sim$ 0.55, whereas the Curie temperature $T_{FM}$ of NdOs$_4$Sb$_{12}$ is depressed linearly with Pr composition to ($1-x$) $sim$ 0.45. In the superconducting region, the upper critical field $H_{c2}(x,0)$ is depressed quadratically with $x$ in the range 0 $<$ $x$ $lesssim$ 0.3, exhibits a kink at $x$ $approx$ 0.3, and then decreases linearly with $x$ in the range 0.3 $lesssim$ $x$ $lesssim$ 0.6. The behavior of $H_{c2}(x,0)$ appears to be due to pair breaking caused by the applied magnetic field and the exhange field associated with the polarization of the Nd magnetic moments, in the superconducting state. From magnetic susceptibility measurements, the correlations between the Nd moments in the superconducting state appear to change from ferromagnetic in the range 0.3 $lesssim$ $x$ $lesssim$ 0.6 to antiferromagnetic in the range 0 $<$ $x$ $lesssim$ 0.3. Specific heat measurements on a sample with $x$ $=$ 0.45 indicate that magnetic order occurs in the superconducting state, as is also inferred from the depression of $H_{c2}(x,0)$ with $x$.
The M{o}ssbauer Effect(ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, $B_{hyperfine}$, at the ME active site. This is the case in the ruthenates, whe
re the temperature dependence of $B_{hyperfine}$ at $^{99}$Ru sites tracks the temperature dependence of the ferromagnetic or antiferromagnetic order. However this does not happen in the rare-earth intermetallics, GdRu$_2$ and HoRu$_2$. Specific heat, magnetization, magnetic susceptibility, M{o}ssbauer effect, and neutron diffraction have been used to study the nature of the magnetic order in these materials. Both materials are found to order ferromagnetically at 82.3 and 15.3 K, respectively. Despite the ferromagnetic order of the rare earth moments in both systems, there is no evidence of a correspondingly large $B_{hyperfine}$ in the M{o}ssbauer spectrum at the Ru site. Instead the measured spectra consist of a narrow peak at all temperatures which points to the absence of magnetic order. To understand the surprising absence of a transferred hyperfine magnetic field, we carried out {it ab initio} calculations which show that spin polarization is present only on the rare-earth site. The electron spin at the Ru sites is effectively unpolarized and, as a result, $B_{hyperfine}$ is very small at those sites. This occurs because the 4$d$ Ru electrons form broad conduction bands rather than localized moments. These 4$d$ conduction bands are polarized in the region of the Fermi energy and mediate the interaction between the localized rare earth moments.
X-ray diffraction, electrical resistivity, magnetization, specific heat, and thermoelectric power measurements are presented for single crystals of the new filled skutterudite compound {CeOsAs}, which reveal phenomena that are associated with f - ele
ctron - conduction electron hybridization. Valence fluctuations or Kondo behavior dominates the physics down to $T$ $sim$ 135 K. The correlated electron behavior is manifested at low temperatures as a hybridization gap insulating state. The small energy gap $Delta$$_1$/k$_B$ $sim$ 73 K, taken from fits to electrical resistivity data, correlates with the evolution of a weakly magnetic or nonmagnetic ground state, which is evident in the magnetization data below a coherence temperature $T$$_{coh}$ $sim$ 45 K. Additionally, the low temperature electronic specific heat coefficient is small, $gamma$ $sim$ 19 mJ/mol K$^2$. Some results for the nonmagnetic analogue compound {LaOsAs} are also presented for comparison purposes.
One of the most notorious non-Fermi liquid properties of both archetypal heavy-fermion systems [1-4] and the high-Tc copper oxide superconductors [5] is an electrical resistivity that evolves linearly with temperature, T. In the heavy-fermion superco
nductor CeCoIn5 [5], this linear behaviour was one of the first indications of the presence of a zero-temperature instability, or quantum critical point. Here, we report the observation of a unique control parameter of T-linear scattering in CeCoIn5, found through systematic chemical substitutions of both magnetic and non-magnetic rare-earth, R, ions into the Ce sub-lattice. We find that the evolution of inelastic scattering in Ce1-xRxCoIn5 is strongly dependent on the f-electron configuration of the R ion, whereas two other key properties -- Cooper-pair breaking and Kondo-lattice coherence -- are not. Thus, T-linear resistivity in CeCoIn5 is intimately related to the nature of incoherent scattering centers in the Kondo lattice, which provides insight into the anomalous scattering rate synonymous with quantum criticality [7].
Electrical resistivity $rho$, specific heat C, and magnetic susceptibility $chi$ measurements made on the filled skutterudite CeRu_4As_{12} reveal non-Fermi liquid (NFL) T - dependences at low T, i.e., $rho$(T) $sim$ T^{1.4} and weak power law or log
arithmic divergences in C(T)/T and $chi$(T). Measurements also show that the T - dependence of the thermoelectric power S(T) deviates from that seen in other Ce systems. The NFL behavior appears to be associated with fluctuations of the Ce valence between 3^+ and 4^+ rather than a typical Kondo lattice scenario that would be appropriate for an integral Ce valence of 3^+.
