MIXED VALENCES - STRUCTURE OF PHASE-DIAGRAMS, LATTICE PROPERTIES AND THE CONSEQUENCES OF ELECTRON-HOLE SYMMETRY

被引:12
作者
ENTEL, P
GREWE, N
机构
[1] Institut für Theoretische Physik, Universität zu Köln, Köln, D-5000
来源
ZEITSCHRIFT FUR PHYSIK B-CONDENSED MATTER | 1979年 / 34卷 / 03期
关键词
D O I
10.1007/BF01325617
中图分类号
O469 [凝聚态物理学];
学科分类号
070205 ;
摘要
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 CeSn3 (positive thermal expansion coefficient α) and YbCuAl (negative α) 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 V0 of 4 f and 5 d-6 s states is changed to {Mathematical expression} =V0-aφ-b〈φ{symbol}2〉 whereas the energy of the 4 f state E0 with respect to the 5 d band becomes {Mathematical expression} =E0-a′φ-b′〈φ{symbol}2〉. φ, being proportional to the lattice constant, is determined by minimizing the Gibbs free energy, while 〈φ{symbol}2〉 is proportional to the mean square displacement of the rare earth ions. The strong temperature dependence of φ and 〈φ{symbol}2〉 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 Sm1-xGdxS than in the pure compound SmS. 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. © 1979 Springer-Verlag.
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页码:229 / 241
页数:13
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