J. D. Garrett

Electrical transport in URu2Si2

The temperature dependence of the resistivity, magnetic susceptibility and Hall coefficient have been measured in oriented single crystals of URu2Si2. The susceptibility is Curie-Weiss like at high temperatures and the anisotropy shows that the f moments on the U atoms are highly confined to lie along the c direction. The resistivity is analysed in terms of a Kondo-like behaviour at high temperatures and shows the onset of coherence, modified by electron-spin wave scattering, at low temperatures. There is a sharp superconducting transition at 1.33 K. The Hall coefficient is analysed by using a modification of the recent theory by Fert and Levy (1987) and suggests that the coherence temperature T0 and the Kondo temperature TK need not be sharply distinguished in URu2Si2. There is a large increase in the Hall coefficient as the temperature is lowered below 18 K which is attributed to a Fermi surface restructuring, possibly due to the existence of spin density waves in the antiferromagnetic state.

Anderson-Mott transition induced by hole doping in Nd 1 − x TiO 3

The insulator/metal transition induced by hole doping due to neodymium vacancies of the Mott-Hubbard antiferromagnetic insulator, Nd1�xTiO3, is studied over the composition range 0.0106x 0.24310. Insulating p-type conduction is found for x 0.07110. Anderson localization in the presence of a Mott-Hubbard gap is the dominant localization mechanism for the range of 0.07410x 0.0891 samples. For x 0.0891, n-type conduction is observed and the activation energy extrapolates to zero by x 0.1. The 0.0958x 0.20310 samples are Fermi-liquid metals and the effects of strong electronic correlations are evident near the metal-to-insulator boundaries in features such as large Fermi liquid T 2 coefficients. For 0.0749x 0.1124, a weak negative magnetoresistance is found below 15 K and it is attributed to the interaction of conduction electrons with Nd 3+ magnetic moments. Combining information from our companion study of the magnetic properties of a Nd1�xTiO3 solid solution, a phase diagram is proposed. The main conclusions are that long-range antiferromagnetic order disappears before the onset of metallic behavior, and that the Anderson-Mott transition occurs over a finite range of doping levels. Our results differ from conclusions drawn from a similar study on the hole-doped Nd1�xCaxTiO3 system, which found the coexistence of antiferromagnetic order and metallic behavior and that the Mott transition occurs at a discrete doping level.

Structure and chemical bonding of UAuGe

UAuGe was prepared from the elements by reaction in an arc-melting furnace and subsequent annealing at about 1200 K in a water-cooled silica tube in a high-frequency furnace. UAuGe crystallizes from the melt and is also stable at 920 K. It has the hexagonal YPtAs-type structure: P63/mmc, with a = 435.26(4) pm, c = 1547.4(1) pm, V = 0.2539(1) nm3, wR2 = 0.0785, 144 F2-values, and 12 variables. The structure of UAuGe may be considered as a superstructure with a quadrupled c-axis of the well known AlB2 type. The gold and germanium atoms order on the boron positions and form two-dimensionally infinite puckered layers of Au3Ge3 hexagons with intralayer Au-Ge distances of 257 pm. Between adjacent layers the gold atoms have weak secondary Au-Au interactions with Au-Au distances of 327 pm. Ab initio calculations of the electronic band structure using the tight-binding linear muffin-tin orbital method are presented. The bonding is illustrated by valence charge density and crystal orbital Hamiltonian population plots which are compared with those of ScAuSi which has a similar structure with Au-Au interactions between the layers. The Au-Au bonding is however much weaker in UAuGe than in ScAuSi. Resistivity measurements exhibit a non-metallic temperature dependence. The increase in resistivity towards lower temperatures is uncharacteristic of intermetallic compounds, and may be fitted to a Curie-Weiss-type formula, suggesting a direct correlation to the magnetic ordering. A maximum in the resistivity is observed at T = 26(1) K.

Long range antiferromagnetic order and its coexistence with superconductivity in URu2Si2

Abstract Neutron scattering from a high quality single crystal of the heavy fermion superconductor URu 2 Si 2 shows an abrupt onset of antiferromagnetic order at T N = 17 K, unlike the gradual onset previously seen in lower quality samples. The magnetic peak intensity increases linearly down to 5 K, indicating mean-field behaviour and long range RKKY interactions. The intensity remains constant to within 7% between 3 and 0.2 K with no change at the superconducting transition temperature of 1.3 K. The resolution limited Bragg peak shows the order is long-range. The coexistence of antiferromagnetism and superconductivity seen in a lower quality crystal is confirmed and thus is intrinsic.

Transport properties and phase diagram of UNi2Si2

The resistivity and Hall coefficient of single-crystal UNi2Si2 have been studied in detail for the temperature range 4.2-300 K. The resistivity of UNi2Si2 is largely due to magnetic scattering and the phonon scattering contribution is estimated to be about 14% at room temperature. At low temperatures, the resistivity can be described by a gapped spin-wave model plus a T2 term. The temperature dependence of the Hall coefficient is accounted for by a theoretical model invoking skew scattering of conduction electrons by localized magnetic moments. Among the three magnetic phase transition temperatures, the two lower ones are found to be magnetic field dependent and shift with the field applied along the tetragonal c axis. Using the resistivity measurement in an applied magnetic field, a field-temperature phase diagram of UNi2Si2 is presented.