Method for analyzing the ground state of intermediate-valence systems: Application to metallic SmS (original) (raw)
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The electronic structure of Tm and Sm monochalcogenides, SmB 6 and Yb 4 As 3 is theoretically investigated from the first principles, using the fully relativistic Dirac LMTO band structure method. The electronic structure is obtained using the local spin-density approximation (LSDA), as well as the so-called LSDA+U approach. While the standard LSDA approach is incapable of correctly describing the electronic structure of such materials due to the strong on-site Coulomb repulsion, the LSDA+U approach is remarkably accurate in providing a detailed agreement with experiment for a number of properties.
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We propose a first-principles based method for calculating the electronic structure and total energy of solids in an intermediate-valence configuration. The method takes into account correlation effects (d ÿ f Coulomb interaction) and many-body renormalization of the effective hybridization parameter of the f system. As an example, the formation of a pressure-induced intermediate-valence state in Yb is considered and its electronic structure and equation of state are calculated and compared to experimental data. The agreement is found to be excellent for both properties, and we argue that the developed method, which applies to any element or compound, provides for the first time a quantitative theoretical treatment of intermediate-valence materials.
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Zeitschrift f�r Physik B Condensed Matter and Quanta, 1979
Symmetry properties and phonon phenomena of mixed valence compounds are discussed within the framework of the periodic Anderson model which was extended to include the interaction of 4 f electrons with longitudinal optical phonons. The temperature anomaly in the thermal expansion found in CeSn 3 (positive thermal expansion coefficient e) and YbCuA1 (negative c~) is correctlY described. Within the model the anomaly is a consequence of the particle hole symmetry of the underlying Hamiltonian. Moreover, the theory gives the positive slope of the phase boundary in the pressure-temperature phase diagram (dP/dT > 0), for example for Ce, and predicts a negative slope (dP/dT< 0) for Yb compounds. Furthermore, the quite unusual low temperature features of the pressure-temperature phase diagram have been calculated. It is found that the lattice vibrational contribution renormalizes the two essential parameters of the periodic Anderson model. The hybridization energy V 0 of 4 f and 5 d -6 s states is changed to V = V o -a q5-b (q)2) whereas the energy of the 4 f state E o with respect to the 5 d band becomes E = E o -a ' 0 -b ' (qo2). qS, being proportional to the lattice constant, is determined by minimizing the Gibbs free energy, while ((p2) is proportional to the mean square displacement of the rare earth ions. The strong temperature dependence of ~b and ((p2) determines the behaviour of the phase boundary and for large enough coefficients b and b' the phase boundary terminates at two critical points. An argument is given why the unusual low temperature features are more expressed in dirty mixed valence compounds as Sml_xGdxS than in the pure compound SINS. Furthermore, the theory predicts a quite unusual behaviour of the plasma-like phonon mode in the mixed valence phase: It softens at the critical temperature and at an intermediate temperature.
Intermediate valence: Phase diagram and Kondo behaviour in a simple model
Zeitschrift f�r Physik B Condensed Matter and Quanta, 1979
We have studied an infinitely narrow band version of the Anderson Hamiltonian. Including a Coulomb repulsion between localized and band states in the mean field approximation, we have been able to describe metal-insulator phase transitions and different properties characteristic of intermediate valence in both phases. These include: non-integer occupation, specific heat, saturating static magnetic succeptibility and dynamical properties such as M/Sssbauer spectra and spin flip neutron spectra. We discuss for what range of values of the parameters some results can be reproduced by a narrow band version of the Kondo Hamiltonian.