Experimental evidence about the temperature dependence of phonon frequencies in V3Si is used to demonstrate that anharmonicity is a major factor in causing the resistance anomalies recently observed in many high-Tc superconductors. © 1976.

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.

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li_2.15(3)Os_0.85(3)O₃ (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μ_B, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T^(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)-Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.