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Register a new userOptical study of archetypical valence-fluctuating Eu-systems
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We have investigated the optical conductivity of the prominent valence fluctuating compounds EuIr2Si2 and EuNi2P2 in the infrared energy range to get new insights into the electronic properties of valence fluctuating systems. For both compounds we observe upon cooling the formation of a renormalized Drude response, a partial suppression of the optical conductivity below 100 meV and the appearance of a mid-infrared peak at 0.15 eV for EuIr2Si2 and at 0.13 eV for EuNi2P2. Most remarkably, our results show a strong similarity with the optical spectra reported for many Ce- or Yb-based heavy fermion metals and intermediate valence systems, although the phase diagrams and the temperature dependence of the valence differ strongly between Eu- and Ce-/Yb-systems. This suggests that the hybridization between 4f- and conduction electrons, which is responsible for the properties of Ce- and Yb-systems, plays an important role in valence fluctuating Eu-systems.
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We investigated the thermoelectric transport properties of EuNi2P2 and EuIr2Si2 in order to evaluate the relevance of Kondo interaction and valence fluctuations in these materials. While the thermal conductivities behave conventionally, the thermopower curves exhibit large values with pronounced maxima as typically observed in Ce- and Yb-based heavy-fermion materials. However, neither the positions of these maxima nor the absolute thermopower values at low temperature are in line with the heavy-fermion scenario and the moderately enhanced effective charge carrier masses. Instead, we may relate the thermopower in our materials to the temperature-dependent Eu valence by taking into account changes in the chemical potential. Our analysis confirms that valence fluctuations play an important role in EuNi2P2 and EuIr2Si2.
X-ray magnetic circular dichroism (XMCD) at the Eu L-edge (2p->5d) in two compounds exhibiting valence fluctuation, namely EuNi2(Si0.18Ge0.82)2 and EuNi2P2, has been investigated at pulsed high magnetic fields of up to 40 T. A distinct XMCD peak corresponding to the trivalent state (Eu3+; f6), whose ground state is nonmagnetic (J=0), was observed in addition to the main XMCD peak corresponding to the magnetic (J=7/2) divalent state (Eu2+; f7). This result indicates that the 5d electrons belonging to both valence states are magnetically polarized. It was also found that the ratio P5d(3+)/P5d(2+) between the polarization of 5d electrons (P5d) in the Eu3+ state and that of Eu2+ is ~ 0.1 in EuNi2(Si0.18Ge0.82)2 and ~ 0.3 in EuNi2P2 at magnetic fields where their macroscopic magnetization values are the same. The possible origin of the XMCD of the Eu3+ state and an explanation of the dependence of P5d(3+)/P5d(2+) on the material are discussed in terms of hybridization between the conduction electrons and the f electrons.
A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise due to the competition of localized and itinerant electronic degrees of freedom. While the respective limits of well-localized or entirely itinerant ground states are well-understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and non-bonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum-mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the long-standing controversy between experiment and theory on plutoniums magnetism, but also suggest an improved understanding of the effects of such electronic dichotomy in complex materials.
In a joint theoretical and experimental study we investigate the pressure dependence of the Eu valence in EuPd_3B_x (0 <= x <= 1). Density functional band structure calculations are combined with x-ray absorption and x-ray diffraction measurements under hydrostatic pressures up to 30 GPa. It is observed that the heterogenous mixed-valence state of Eu in EuPd_3B_x (x >= 0.2) can be suppressed partially in this pressure range. From the complementary measurements we conclude that the valence change in EuPd_3B_x is mainly driven by the number of additional valence electrons due to the insertion of boron, whereas the volume change is a secondary effect. A similar valence change of Eu in Eu_{1-x}La_xPd_3 is predicted for x >= 0.4, in line with the suggested electron count scenario.
CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of $T_{mathrm{K}}sim 480$,K. We report a negative in-plane thermal expansion $alpha/T$ below 2,K, which passes through a broad minimum near 0.75,K. Volume and $a$-axis magnetostriction for $B parallel a$ are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6,T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1,K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at $Tll T_{mathrm{K}}$ result from geometrical frustration.
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