Exotic Kondo-hole band resistivity and magnetoresistance of Ce1−x_{1-x}Lax_{x}Os4_4Sb12_{12} alloys

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$\mu\Omega$cmK$^{-2}$ between H=0 and 7 T and then decreases to 18$\mu\Omega$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

Magnetoresistance of Pr1−x_{1-x}Lax_xOs4_4Sb12_{12}: Disentangling local crystalline-electric-field physics and lattice effects

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

Anomalous low temperature state of CeOs4Sb12: Magnetic field and La-impurity study

Specific heat for single crystalline samples of Ce1-xLaxOs4Sb12 at zero-field and magnetic fields to 14 T is reported. Our results confirm enhanced value of the electronic specific heat coefficient in the paramagnetic state. They provide arguments for the intrinsic origin of the 1.1 K anomaly. This transition leads to opening of the gap at the Fermi surface. This low temperature state of CeOs4Sb12 is extremely sensitive to chemical impurities. 2% of La substituted for Ce suppresses the transition and reduces the electronic specific heat coefficient. The magnetic field response of the specific heat is also anomalous.Comment: 4 pages, 3 figure

Glassy Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) = Gbar(t/H^\gamma) expected from glassy dynamics. At 0.05 K \gamma(x=1.0) = 0.35 \pm 0.1, but \gamma(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic neutron scattering results, which yield \gamma = 0.33 for both concentrations. There is no sign of static magnetism \gtrsim 10^{-3} \mu_B/U ion in either material above 0.05 K. Our results strongy suggest that both alloys are quantum spin glasses.Comment: 4 pages, 4 figures, to be published in Physical Review Letter

Relationship between resistivity and specific heat in a canonical non-magnetic heavy fermion alloy system: UPt_5-xAu_x

UPt_(5-x)Au_x alloys form in a single crystal structure, cubic AuBe_5-type, over a wide range of concentrations from x = 0 to at least x = 2.5. All investigated alloys, with an exception for x = 2.5, were non-magnetic. Their electronic specific heat coefficient $\gamma$ varies from about 60 (x = 2) to about 700 mJ/mol K^2 (x = 1). The electrical resistivity for all alloys has a Fermi-liquid-like temperature variation, \rho = \rho_o + AT^2, in the limit of T -> 0 K. The coefficient A is strongly enhanced in the heavy-fermion regime in comparison with normal and transition metals. It changes from about 0.01 (x = 0) to over 2 micro-ohm cm/K^2 (x = 1). A/\gamma^2, which has been postulated to have a universal value for heavy-fermions, varies from about 10^-6 (x = 0, 0.5) to 10^-5 micro-ohm cm (mol K/mJ)^2 (x > 1.1), thus from a value typical of transition metals to that found for some other heavy-fermion metals. This ratio is unaffected, or only weakly affected, by chemical or crystallographic disorder. It correlates with the paramagnetic Curie-Weiss temperature of the high temperature magnetic susceptibility.Comment: 5 pages, 5 eps figures, RevTe

Order and nFl Behavior in UCu4Pd

We have studied the role of disorder in the non-Fermi liquid system UCu4Pd using annealing as a control parameter. Measurement of the lattice parameter indicates that this procedure increases the crystallographic order by rearranging the Pd atoms from the 16e to the 4c sites. We find that the low temperature properties depend strongly on annealing. Whereas the non-Fermi liquid behavior in the specific heat can be observed over a larger temperature range after annealing, the clear non-Fermi liquid behavior of the resistivity of the unannealed sample below 10 K disappears. We come to the conclusion that this argues against the Kondo disorder model as an explanation for the non-Fermi liquid properties of both as-prepared and annealed UCu4Pd