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Register a new userExotic Kondo-hole band resistivity and magnetoresistance of Ce$_{1-x}$La$_{x}$Os$_4$Sb$_{12}$ alloys
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Electrical resistivity measurements of non-magnetic single-crystalline Ce$_{1-x}$La$_x$Os$_4$Sb$_{12}$ alloys, $x=0.02$ and 0.1, are reported for temperatures down to 20 mK and magnetic fields up to 18 T. At the lowest temperatures, the resistivity of Ce$_{0.98}$La$_{0.02}$Os$_4$Sb$_{12}$ has a Fermi-liquid-like temperature variation $rho=rho_0+A T^2$, but with negative $A$ in small fields. The resistivity has an unusually strong magnetic field dependence for a paramagnetic metal. The 20 mK resistivity increases by 75% between H=0 and 4 T and then decreases by 65% between 4 T and 18 T. Similarly, the $A$ coefficient increases with the field from -77 to 29$ muOmega$cmK$^{-2}$ between H=0 and 7 T and then decreases to 18$ muOmega$cmK$^{-2}$ for 18 T. This nontrivial temperature and field variation is attributed to the existence of a very narrow Kondo-hole band in the hybridization gap, which pins the Fermi energy. Due to disorder the Kondo-hole band has localized states close to the band edges. The resistivity for $x=0.1$ has a qualitatively similar behavior to that of $x=0.02$, but with a larger Kondo-hole band.
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Resistivity measurements were performed on Pr$_{1-x}$La$_x$Os$_4$Sb$_{12}$ single crystals at temperatures down to 20 mK and in fields up to 18 T. The results for dilute-Pr samples ($x=0.3$ and 0.67) are consistent with model calculations performed assuming a singlet crystalline-electric-field (CEF) ground state. The residual resistivity of these crystals features a smeared step centered around 9 T, the predicted crossing field for the lowest CEF levels. The CEF contribution to the magnetoresistance has a weaker-than-calculated dependence on the field direction, suggesting that interactions omitted from the CEF model lead to avoided crossing in the effective levels of the Pr$^{3+}$ ion. The dome-shaped magnetoresistance observed for $x = 0$ and 0.05 cannot be reproduced by the CEF model, and likely results from fluctuations in the field-induced antiferroquadrupolar phase.
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.
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 and 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 $x$.
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 spontaneous 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.
Measurements of the upper critical field $H_{c2}$ near $T_c$ of Pr$_{1- x}$La$_{x}$Os$_{4}$Sb$_{12}$ were performed by specific heat. A positive curvature in $H_{c2}$ versus $T$ was observed in samples and concentrations exhibiting two superconducting transitions. These results argue against this curvature being due to two-band superconductivity. The critical field slope - $dH_{c2}/dT$ suggests the existence of a crossover concentration $x_{cr} approx 0.25$, below which there is a rapid suppression of effective electron mass with La-alloying. This crossover concentration was previously detected in the measurement of the discontinuity of $C/T$ at $T_c$.
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