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Register a new userTheory of the thermoelectricity of intermetallic compounds with Ce or Yb ions
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The thermoelectric properties of intermetallic compounds with Ce or Yb ions are explained by the single-impurity Anderson model which takes into account the crystal-field splitting of the 4{it f} ground-state multiplet, and assumes a strong Coulomb repulsion which restricts the number of {it f} electrons or {it f} holes to $n_fleq 1$ for Ce and $n_f^{hole}leq 1$ for Yb ions. Using the non-crossing approximation and imposing the charge neutrality constraint on the local scattering problem at each temperature and pressure, the excitation spectrum and the transport coefficients of the model are obtained. The thermopower calculated in such a way exhibits all the characteristic features observed in Ce and Yb intermetallics. Calculating the effect of pressure on various characteristic energy scales of the model, we obtain the $(T,p)$ phase diagram which agrees with the experimental data on CeRu$_{2}$Si$_2$, CeCu$_{2}$Si$_2$, CePd$_{2}$Si$_2$, and similar compounds. The evolution of the thermopower and the electrical resistance as a function of temperature, pressure or doping is explained in terms of the crossovers between various fixed points of the model and the redistribution of the single-particle spectral weight within the Fermi window.
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The Ce(1-x)LaxCrGe3 (x = 0, 0.19, 0.43, 0.58 and 1) intermetallic compound system has been investigated by magnetization measurements and neutron scattering techniques to determine the effect of La-doping on the magnetic ordering and exchange interaction between Cr ions. The structural and magnetic characterization in this series was first verified by X-ray diffraction and bulk magnetization measurements. The samples exhibit the known hexagonal perovskite structure (P63/mmc space group) and have a single magnetic phase according to magnetization measurements. In this work, the ferromagnetic ordering temperature for Cr evolves smoothly from a range of 68 K to 77 K for CeCrGe3 to a range of 91 K to 96 K for LaCrGe3 as La replaces Ce. Magnetization results indicate the formation of domain walls below the transition temperature for all the Ce(1-x)LaxCrGe3 systems investigated. Neutron results indicate ordered magnetic Cr moments aligned along the c axis for the x = 1 LaCrGe3 system, as well as for x = 0.19, 0.43, and 0.58, which contrasts with the x = 0 CeCrGe3 where the moments order in the ab plane.
Dynamical conductivity spectra s(w) have been measured for a diverse range of heavy-fermion (HF) Ce and Yb compounds. A characteristic excitation peak has been observed in the mid-infrared region of s(w) for all the compounds, and has been analyzed in terms of a simple model based on conduction (c)-f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c-f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c-f hybridized band can generally and quantitatively describe the charge excitation spectra of a wide range of HF compounds.
The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic compounds may display a more local or more itinerant character depending on the interplay of the exchange interactions among the $4f$ electrons and the Kondo coupling between $4f$ and conduction electrons. For the more itinerant case, the materials form heavy-fermions once the Kondo effect is developed at low temperatures. Hence, a temperature variation occurs in the electronic structure that can be traced by investigating the optical conductivity ($sigma(omega)$) spectra. Remarkably, the temperature variation in the $sigma(omega)$ spectrum is still present in the more localized case, even though the Kondo effect is strongly suppressed. Here, we clarify the local and itinerant character in the electronic structure by investigating the temperature dependence in the $sigma(omega)$ spectra of various Ce and Yb compounds with a tetragonal ThCr$_2$Si$_2$-type crystal structure. We explain the temperature change in a unified manner. Above temperatures of about 100 K, the temperature dependence of the $sigma(omega)$ spectra is mainly due to the electron-phonon interaction, while the temperature dependence below is due to the Kondo effect.
We report the magnetic properties strongly varying with the rare-earth elements in the newly found ternary compounds $R$Al$_{0.9}$Si$_{1.1}$, which crystallize in the tetragonal $alpha$-ThSi$_2$-type structure. For $R$ = Ce the alloy has a weak ferromagnetism below 11 K and for $R$ = Pr it orders ferromagnetically at 17 K, while for $R$ = Gd it is antiferromagnetic with $T_{rm N}$ = 30.5 K. In addition, we find no field effect on $T_{rm N}$ of $R$ = Gd because of the large internal mean field, but significant changes in the magnetic properties of $R$ = Ce and Pr.
First-principles calculations of the electronic structure of members of the $R$NiC$_2$ series are presented, and their Fermi surfaces investigated for nesting propensities which might be linked to the charge-density waves exhibited by certain members of the series ($R$ = Sm, Gd and Nd). Calculations of the generalized susceptibility, $chi_{0}({bf q},omega)$, show strong peaks at the same ${bf q}$-vector in both the real and imaginary parts for these compounds. Moreover, this peak occurs at a wavevector which is very close to that experimentally observed in SmNiC$_2$. In contrast, for LaNiC$_2$ (which is a superconductor below 2.7K) as well as for ferromagnetic SmNiC$_2$, there is no such sharp peak. This could explain the absence of a charge-density wave transition in the former, and the destruction of the charge-density wave that has been observed to accompany the onset of ferromagnetic order in the latter.
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