Variation off-electron localization in diluted US and UTe

Abstract

The temperature dependence of the magnetization, the electrical resistivity, and the Hall effect has been measured for 11 different uranium concentrations in single crystals of U x La 1Ϫx S and is compared with corresponding data for U 0.2 ͑La 0.15 Y 0.85 ͒ 0.8 Te and UTe. While the electrical transport data for diluted UTe show maxima related to crystal-field splittings, such structures are absent in all sulfide compounds. The magnetic susceptibility of the sulfides, measured up to 1200 K, is fitted by the sum of a Curie-Weiss and a Pauli paramagnetism term in second-order approximation. In a quasi-free-electron model the variation with the uranium concentration of the Fermi energy, the carrier concentration and the effective mass is derived. It is found that the degree of localization varies nonmonotonically and, in a certain range, contrary to the U-U separation. It is concluded that hybridization and magnetic exchange play dominant roles. ͓S0163-1829͑96͒00921-6͔

Application of theS=1underscreened Anderson lattice model to Kondo uranium and neptunium compounds

“…3b) and 3c) can give a description of the experimentally observed variation of T C with pressure in UTe compound, which is passing through a maximum and then decreasing with applied pressure. 5,16 To summarize our paper, the present work improves the previous S = 1 UKL model of Ref. are of the same order of magnitude: a possible disappearance of the Kondo effect at low temperature, which is a direct consequence of the underscreened Kondo effect, and a maximum of T C as a function of J K .…”

“…For example, magnetic moments observed in UTe are substantially smaller than the freeion values for either the 5f 2 or the 5f 3 configurations. 16 On the same side, in the series of uranium monochalcogenides, US lies closest to the itinerant side for the 5f -electrons, USe is in the middle and the 5f -electrons are more localized in UTe, as evidenced by magnetization measurements. 16 Moreover, the Curie temperature of UTe is passing through a maximum and is then decreas-ing with applied pressure, which is interpreted as a weak delocalization of the 5f -electrons under pressure.…”

“…Indeed, this phenomenon was observed in several uranium compounds, like UTe, [14][15][16] UCu 0.9 Sb 2 , 17 or UCo 0.5 Sb 2 18 in which a ferromagnetic order with large Curie temperatures (equal, respectively, to T C = 102 K, 113 K and 64.5 K) and a logarithmic Kondo-type decrease of the resistivity above T C has been experimentally detected. A similar behavior has been observed in the neptunium compounds, NpNiSi 2 19 and Np 2 PdGa 3 , 20 with Curie temperatures equal to, respectively, T C =51.5 K and 62.5 K.…”

“…16 Moreover, the Curie temperature of UTe is passing through a maximum and is then decreas-ing with applied pressure, which is interpreted as a weak delocalization of the 5f -electrons under pressure. 5,16 The dual nature of the 5f -electrons, assuming two localized 5f -electrons and one delocalized electron, has been also studied by Zwicknagl et al 8,9 who have accounted, by using band calculations, for the mass enhancement factor experimentally observed in some uranium compounds.…”

Coexistence of Kondo effect and ferromagnetism in the Underscreened Kondo Lattice model

“…In the case of actinide compounds with 5 f electrons, it is known that some of them exhibit a coexistence of Kondo effect and ferromagnetism. Some uranium and neptunium compounds such as UTe [2,3,4], UCu 0.9 Sb 2 [5], UCo 0.5 Sb 2 [6], NpNiSi 2 [7] and Np 2 PdGa 3 [8] present this behavior, with Curie temperatures of order T C ∼ 50 − 100K, which are higher than the magnetic transition temperatures observed in the rare-earth compounds. Another interesting behavior is the variation of T C as a function of applied pressure in UTe [9,10], where T C increases firstly with pressure, goes then through a maximum and finally decreases at very high pressures.…”