| Research Abstract |
This project has been performed to study the origin of valence transition and the nature of valence fluctuation of Eu-based compounds. In the previous study, we have found that the Eu valence transition is induced by strong magnetic field for the first time. We have discussed this field-indued valence transition in terms of the interconfigurational fluctuation (ICF) model. In order to clarify the microscopic origin of the ICF model, the pressure effect of the valence trasition has been studied. The following results were obtained.
1) Annealing EuPd_2Si_2 at 900℃ strongly improves the quality of the sample. This suppresses the uptum of magnetic susceptibility at low temperatures, which was observed in all the previous studies more or less. We concluded that the uptum of magnetic susceptibility comes not from the oxide but from disordering of Pd/Si site, which is recovered by annealing at higher temperatures than 900℃.
2) In the ICF model, the excitation energy between the ground state of
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Eu^<3+> and the excited state of Eu^<2+> is given by E_<ex>=E_0 (1-αp_2), where p_2 is the occupation probability of Eu^<2+> state E_0 and α are constants. The nature of valence transition of EuNi_2 (Ge_<1-x>Si_x)_2) is explained by assuming that E_0 varies linearly with the concentraiton. On the other hand, the pressure effects have revealed that hydrostatic pressure is equivalent to doping Si for EuNi_2Ge_2. These results suggest that E_0 is strongly depends on volume. Noting the above equation, we believe that the origin of α, hence the valence transition is anomalously large volume dependence of the excitation energy in the framework of the ICF model..
3) The temperature dependence of electrical resistivity, ρ, is discussed by regarding the system as a virtual alloy of Eu^<2+>_<p2>Eu^<3+>_<p3>, where p_2 and p_2 are the occupation probabilities of Eu^<2+> and Eu^<3+> states, respectively. We have shown that impurity scattering due to randomness in a virtual alloy is responsible for the temperature dependence of ρ.
4) Large single crystals of EuNi_2P_2 and EuRu_2P_2 were obtained by the flux method. Less
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