Replacing U for Ca in semiconducting CaB6 at the few at.% level induces metallic behaviour and Kondo-type phenomena at low temperatures, a rather unusual feature for U impurities in metallic hosts. For Ca 0.992U0.008B6, the resistance minimum occurs at T =17 K. The subsequent, characteristic logarithmic increase of the resistivity with decreasing temperature merges into the expected T 2-dependence below 0.8 K. Data of the low-temperature specific heat and the magnetization are analyzed by employing a simple resonance-level model. Analogous measurements on LaB6 with a small amount of U revealed no traces of Kondo behavior, above 0.4 K.

Replacing U for Ca in semiconducting CaB6 at a few atomic percent level induces metallic behavior and Kondo-type phenomena at low temperatures, a rather unusual feature for U impurities in metallic hosts. For Ca0.992U0.008B6, the resistance minimum occurs at T≈17K. The subsequent characteristic logarithmic increase of the resistivity with decreasing temperature merges into a T2 dependence below 0.8 K. Correspondingly, the low-temperature specific heat indicates the formation of a ground state with a strongly enhanced electronic specific heat parameter. © 2005 Elsevier B.V. All rights reserved.

The electronic band structure of Yb Rh2 Si2 is calculated in a relativistic framework including correlation corrections and magnetization of the Yb ion and compared to detailed angle-resolved photoemission spectra. The photoemission spectra for Lu Rh2 Si2 are used as reference to identify electronic bands with no f symmetry. The calculated band structure manifests a 4 f13 spin-polarized configuration leaving the unoccupied state at 1.4 eV above the Fermi energy. At the band theory level, the 4f bands are located far below the Fermi level and the anisotropic Coulomb interaction within the 4f shell spreads the multilevel into broader 4f complexes below -2.5 eV. The photoemission spectra obtained on Yb Rh2 Si2 show a clear f -multilevel splitting into j=7 2 and 5 2 excitations. The interaction of the 4 f7 2 levels close to the Fermi energy with two conduction bands shows visible hybridization gaps of 45 and 80 meV, respectively. We discuss the origin of these excitations and provide an analysis according to Anderson's single-impurity model with parameters suggested by the band-structure calculation and the photoemission spectra. Both experiment and theory indicate nearly identical Fermi surfaces for Lu Rh2 Si2 and Yb Rh2 Si2. The valency of Yb in Yb Rh2 Si2 is estimated to be close to +3. © 2007 The American Physical Society.

We report the results of measurements of the electrical resistivity, the magnetization, and the specific heat of PrB4 below room temperature. The crystal electric field (CEF) splits the 4f2 multiplet of the Pr ions into nine singlets. In its ground state, PrB4 is an induced-moment ferromagnet with a saturation magnetization as high as 2.1μB (Prion). Contrary to previous experiments, we observe a first-order ferromagnetic to antiferromagnetic transition at TC≈15.9±0.1K with a latent heat ΔQ=8.6J mol. The antiferromagnetic long-range order is quenched above TN=19.5K. Susceptibility and specific heat data, as well as the enormous magnetic anisotropy, can quantitatively be reproduced by a mean-field calculation which takes into account the CEF, obtained from a point-charge model using effective charges obtained from a self-consistent electronic band-structure calculation and a Zeeman term, as well as an anisotropic dipolar magnetic exchange and an effective quadrupole-quadrupole interaction term. © 2005 The American Physical Society.

We report the results of measurements of the low-temperature specific heat Cp(T) and the ac susceptibility χac(T) in low applied magnetic fields for a series of samples of Eu1-xCaxB 6. The anomalies in Cp(T), together with the results for χac(T) and M(H), confirm the onset of phase transitions to long range magnetic order for x < 0.7 and provide evidence that for x ≥ 0.7, the Eu moments, which are captured in large magnetic clusters with magnetic moments of the order of 260 μB, adopt a spin-glass type ground state. The data set allows to establish the low-temperature [ T,x] phase diagram of this alloy series. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2005.