Paul de Villiers du Plessis

Possible Kondo insulating behaviour in U2Ru2Sn

Abstract The compound U2Ru2Sn is known to crystallize in the tetragonal U3Si2-type structure and is believed to be a weak paramagnet. We report electrical resistivity studies on this compound which seem to indicate semiconducting behaviour at low temperatures and hence a gap formation in this compound. This is evidenced from a sharp rise in the resistivity seen at low temperatures similar to that observed in the so-called Kondo insulators.

Magnetotransport in (Ce1-xNdx)Cu6 Kondo alloys

The results of electrical resistivity ρ(T) and magnetoresistivity MR(T, B) for K and T are reported for the (Ce1-xNdx)Cu6 alloy system. For small Nd substitutions in the heavy-fermion CeCu6 parent compound, the Kondo lattice is characterized through coherence effects in ρ(T) at low temperatures. The ρ(T) data above room temperature are used to resolve the electron-phonon scattering, and it is indicated that the single-ion Kondo interaction dominates both ρ(T) and MR(B) for a wide range of intermediate Ce concentrations. Based on this, the MR(B) data for different isotherms and alloy compositions are analysed according to the Bethe- ansatz description. We also discuss the observed deviations of our data from the preceding theoretical description due to the onset of magnetic order in alloys with high Nd content, and to phase coherence at low temperatures.

Magnetoresistance and thermoelectric power in U2Ni2Sn

Abstract Low-temperature magnetoresistance ( MR ) and thermoelectric power ( S ) measurements are presented for antiferromagnetic U 2 Ni 2 Sn. Appreciable remanence in the MR behaviour is observed. S changes its sign at T N = 24 K from small positive values above T N to large negative values at 6 K where it shows a well resolved minimum.

Magnetic, Kondo and non-Fermi-liquid behaviour of U1-xThxPd2Al3

Electrical resistivity (T), magnetoresistance MR, magnetization M(T,B) and magnetic susceptibility (T,B) measurements at temperatures down to 25 mK and applied magnetic fields up to 14 T are presented for Th-diluted U1-xThxPd2Al3 alloys. For U-rich alloys, antiferromagnetic order is observed and TN varies slowly with the Th concentration in the region 0 x0.2. From (T) data we calculate a value of the energy gap = 12 K in the spin-wave spectrum. In the U-rich concentration region a coherent Kondo description applies, but for alloys with 0.4 x0.93 a non-Fermi-liquid (NFL) phase prevails. An approximately linear temperature dependence of (T) is observed at low temperatures for about a decade in temperature, but deviation from this dependence occurs at the lowest temperatures. We show that the MR of the NFL alloys may be described in terms of the Coqblin-Schrieffer Hamiltonian, from which values of the single-ion characteristic Kondo temperature TK40 K are calculated. An applied magnetic field is shown to recover Fermi-liquid dynamics in both (T) and (T). We give a description of the low-temperature magnetization M(T,B) in terms of a two-channel Kondo model.

Magnetoresistivity in uranium-based heavy-fermion compounds

The resistivities ϱ(T) of U2Pt2In and UCu4Pd display NFL characteristics as indicated by linear ϱ(T) behaviour at low temperatures, but this linear relationship is broken down by applying at 7.5 T magnetic field. (U0.9Th0.1)2Pt2ln displays a knee at 19 K in its zero-field resistivity but the knee is smeared out by application of a magnetic field.

Low-temperature transport properties of U2Rh2Sn and U2Fe2Sn

Magnetoresistivity measurements of polycrystalline U2Rh2Sn in an applied magnetic field of 8.29 T reveal a sharp minimum at the antiferromagnetic transition temperature TN≊20 K. The thermoelectric power of U2Rh2Sn exhibits negative values for temperatures T⪯24 K. At T≊4 K, a large decrease in resistivity with lowering of temperature is observed for polycrystalline U2Fe2Sn. This anomaly originates from the occurrence of free tin on the grain boundaries of the sample giving a decrease in resistivity as a result of superconductivity.

Resistivity and magneto-resistivity of antiferromagnetic U2Pt2Sn

Abstract The temperature dependence of the resistivity, ϱ(T) and magnetoresistivity (T ≥ 1.5 K, B ≤ 8 T) of polycrystalline U2Pt2Sn are reported. ϱ(T) shows maxima at 12 K and 45 K and a minimum at 19 K. A minimum is observed in d ϱ(T) d T at the Neel temperature of 15 K. The magnetoresistance at low temperatures is negative and below TN it displays a step-like decrease in an applied field of about 2 T indicating the presence of a phase transition.

Non-Fermi liquid studies in (U,Th)Pd2X3 compounds (X = Al, Ga)☆

Abstract Resistivity ϱ ( T ) and magnetoresistivity results are reported for several U 1− x Th x Pd 2 Al 3 compounds and for U 0.3 Th 0.7 Pd 2 Ga 3 . Non-Fermi liquid behaviour is observed for U 0.4 Th 0.6 Pd 2 Al 3 as evidenced from a linear temperature dependence of ϱ ( T ) at low temperatures. Application of a magnetic field changes the resistivity to a ϱ = ϱ 0 − cT n dependence.

Neutron diffraction and ultrasonic studies of spin-slip structures in holmium

Spin-slip behaviour in high-purity holmium single crystals is characterized by neutron diffraction and ultrasonic velocity and attenuation measurements as a function of temperature and of magnetic field applied along b, c, and a axes. Neutron diffraction measurements of intensity and turn angle give information on wave vector lock-in effects for various spin-slip structures in applied fields. These findings are supplemented with ultrasonic studies of the elastic constants and and corresponding attenuation coefficients and . Various phase diagrams are presented and results compared with experiments by other groups and with some theoretical predictions.

Electrical resistivity of (UxTh1 − x)2Pt2In

Abstract Measurements on the temperature dependence of resistivity (ϱ) of polycrystalline specimens of the heavy-fermion compound U 2 Pt 2 In and of thorium-substituted (U x Th 1 − x ) 2 Pt 2 In are reported. For U 2 Pt 2 In, a linear curve of ϱ ( T ) is observed for 15 K > T > 1.5 K, while the x = 0.9 alloy displays a knee at T = 20 K followed upon cooling by a quadratic decrease of ϱ ( T ).