Rare-earth Solids Research Articles

Electron correlation effects reflected in thermodynamic properties of light actinides

ABSTRACTExperimentally available thermodynamic data on the light actinides is used to display the ratio from Th through to Cm. Dramatic variation through the series by a factor of 4 contrasts strongly with that found for the five heavy rare earth solids, also involving f-electrons, where the same ratio is approximately 50 to within a few percent.

Theory and experiment of coherent wave packet dynamics in rare earth solids: Absorption spectrum vs femtosecond fringe-resolved interferogram

Coherent dynamic property of neodymium yttrium aluminum garnet (Nd:YAG) crystal at 77 K is studied via the conventional absorption, the femtosecond fringe-resolved wave packet interferometry, and the related difference-phase spectrum. The recorded interferogram exhibits beatings in subpicosecond time scale arising from the interferences among various weakly split 4f-electronic states and the coupled vibronic optical phonon sidebands. The electron–phonon coupling in Nd:YAG can be well described by multiple Brownian oscillators model involving in each individual electronic transition. The parameters for characterizing material coherence and relaxation are determined via the theoretical modelings of both the frequency and the time-domain experimental signals.

Fluorescence-Detected Ultrafast Free-Induction Decay in Powdered Rare Earth Solids

Fluorescence interferometry is developed and applied to study ultrafast amplitude and phase dynamics for free-induction decay in powdered rare earth solids. The time-resolved phase dynamics of free-induction decay through-out the decaying process is accurately determined by using a novel dual-channel correlation technique and subpicosecond dephasing time is measured for Nd3+ solids at room temperature. The phase dynamics is well simulated with linear coherent polarization theory.

Self-interaction-corrected local-spin-density calculations for rare earth materials

The ab initio self-interaction-corrected (SIC) local-spin-density (LSD) approximation is discussed with emphasis on the ability to describe localized f -electron states in rare earth solids. Two methods for minimizing the SIC-LSD total energy functional are discussed, one using a unified Hamiltonian for all electron states, thus having the advantages of Bloch's theorem, the other one employing an iterative scheme in real space. Results for cerium and cerium compounds as well as other rare earths are presented. For the cerium compounds the onset of f -electron delocalization can be accurately described, including the intricate isostructural phase transitions in elemental cerium and CeP. In Pr and Sm the equilibrium lattice constant and zero temperature equation of state is greatly improved in comparison with the LSD results. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 799-813, 2000

Electron properties and surface effects of rare-earth systems

Recent experimental results concerning the surface-related electronic structure of rare-earth solids are presented. Special attention is paid to the occurrence of surface-valence transitions in trivalent or mixed-valent compounds of Sm, Eu, Tm, and Yb that are due to energetically lowering of unoccupied 4f-states caused by the reduced atomic coordination at the surface. Similar phenomena are encountered in Ce-systems leading to a decrease of 4f-hybridization in the outermost atomic surface layer. The importance of this phenomenon for the correct interpretation of electron-spectroscopic data of Ce-systems is discussed. Experimental evidence for transitions from a localized to an itinerant behavior of the 4f-states is obtained for CeRh 3 .

Bichromatic excitation of coherent Raman beats in rare-earth solids.

In coherent Raman beat experiments a laser field coherently scatters off a material excitation that evolves under the internal Hamiltonian of the system. The Raman signal, which is observed as a beat signal between the test laser and the scattered Raman field, provides information on energy-level splittings and dephasing rates. We propose a modification of this technique, which allows, in contrast to the conventional method, the excitation of coherent Raman beats also in systems with large sublevel splittings and small oscillator strengths. For this purpose, we use a bichromatic laser field to excite the coherent superposition in the medium. We calculate the evolution of the coherence during the laser excitation analytically and discuss its dependence on various experimental parameters. We compare the theoretical results with experimental data from ${\mathrm{Pr}}^{3+}$:${\mathrm{YAlO}}_{3}$.

Excitation of coherent Raman beats in rare earth solids with a bichromatic laser field

A new method is presented for measuring coherent Raman beats in systems with a wide spectral range that works even for weakly allowed optical resonance lines. A bichromatic pump laser field is used to resonantly pump a coherent excitation in the material, and a monochromatic test laser field, whose frequency coincides with that of one component of the pump laser beam.

Electrical conduction in non-metallic rare-earth solids

Schematic energy band diagrams for the genesis of charge carriers in non-metallic rare-earth solids have been presented. It has been shown that positions of 4f bands have significant effect on the genesis and nature of charge carriers, their conduction mechanism and magnitude of electrical conductivity (σ) and Seebeck coefficient (S) of the solid. Relevant relations have been given for bothσ and S in different situations. Experimental data on rare-earth sesquioxides (R2O3), rare-earth tungstates [R2(WO4)3] and rare-earth molybdates [R2(MoO4)3] in the intrinsic range have been explained as examples for the validity of energy band diagrams.

Electron-spectroscopic manifestations of the 4f states in light rare-earth solids.

The consequences of the degeneracy between configurations with different populations of atomiclike f states are studied systematically for a number of light rare-earth materials (Ba, ${\mathrm{LaF}}_{3}$, ${\mathrm{La}}_{2}$${\mathrm{O}}_{3}$, La, ${\mathrm{CeO}}_{2}$, ${\mathrm{CeCo}}_{2}$, CeN, \ensuremath{\alpha}-Ce, and \ensuremath{\gamma}-Ce) using high-energy spectroscopies. The Anderson impurity model applied to this problem (Gunnarsson-Sch\onhammer model) is found to describe convincingly a variety of electron-spectroscopic excitations, such as, x-ray photoemission and bremsstrahlung isochromat spectroscopy. We have extended this many-body formalism to account for electron-energy loss spectroscopy and ${L}_{\mathrm{III}}$ absorption edges. The parameters resulting from the calculation are analyzed within the framework of a simplified Hamiltonian containing no coupling terms between f and band states. This approach reveals in a natural way the importance of the conventional concept of hybridization. The energy degeneracy and the wave-function overlap of quantum states give rise to a mixing and to a continuous energy distribution of excitations. For electronic transitions described within the sudden approximation the impurity model provides a sound and unified description of the ``satellite complex'' encountered in the high-energy excitation spectra of such systems.

Bistability and collective effects in the ground state of cerium and cerium based compounds (invited; abstract)

The properties of rare earth solids are determined by strongly localized 4f electrons in the valence shell. In the limits of strong or weak binding of the 4f electrons this leads to Kondo—or mixed valence—behavior, respectively. The two limiting cases both exhibit ‘‘normal’’ spectral properties, i.e., one main UPS (XPS) photoemission peak and one BIS electron-addition peak separated from each other by U≊5–6 eV. Furthermore, for ‘‘normal’’ mixed valence materials the UPS—peak occurs at the Fermi level EF. In contrast to this, Ce and many of its compounds exhibit anomalous features.1 In addition to the UPS/BIS peaks around −2 and +4 eV there exists also structure near EF (in both UPS and BIS) with varying strength for different compounds. We have recently proposed a model which is designed to account for these findings.2 The model describes the groundstate of Ce as a boundstate of a localized 4f electron and a somewhat more delocalized f exciton. The new state is stabilized as a result of dynamic interactions induced by strong core charge polarizations which lead to a breakdown of the usual description of the groundstate in terms of one-electron states. As a result of the excitonic instability spectral strength appears in UPS experiments both at the Fermi level as well as 2 eV below it. Similar modifications occur in the BIS spectra. The modified groundstate should replace the usual simple localized moment picture in the Anderson Hamiltonian for the study of magnetic or mixed valence properties.

Observation of very-short-lived radiation effects in rare-earth solids following Coulomb excitation

We have observed changes in the M\ossbauer recoilless fraction $f$ associated with short-lived radiation effects for a number of different rare earths with short-lifetime M\ossbauer transitions. A simple model is used to interpret these effects in terms of transient thermal spikes.

Book reviews

Book reviews

Rare Earth Solids

Physics of Rare Earth Solids. By K. N. R. Taylor and M. I. Darby. Pp. xi + 308. (Chapman and Hall: London, April 1972.) £7.