Angle-resolved photoemission and LEED from rare-earth metals

Angle-resolved photoemission and LEED studies of single-crystal surfaces of the rare-earth metals are still relatively few in number. The reasons for this are outlined and comparisons are made between the procedures used to obtain clean and well-ordered surfaces suitable for study. Valence-band photoemission spectra are compared with surface electronic structure and photocurrent calculations in order to identify surface and bulk features. Core-level spectroscopy indicates that the 5p levels of the lanthanides show intensity variations with emission angle that are not explicable in terms of a simple core-level shift. Although the 4p levels of Y have some band character, detailed comparison of the 4p photoemission lineshape with those produced by photocurrent calculations enables the surface core-level shift to be determined. Quantitative LEED studies of close-packed (0001) surfaces show essentially a bulk-terminated structure for Sc, a structural reconstruction of the divalent surface of Sm and a magnetic reconstruction of ferromagnetic Gd. The more open (11 0) and (10 0) surfaces of all the rare-earth metals studied undergo extensive reconstructions into a close-packed structure almost identical to that of a (0001) surface. Angle-resolved photoemission and LEED have yet to distinguish between these structures, indicating a reconstruction depth of at least five atomic layers.     

Adsorption of barium and rare-earth metals on silicon

The change in the work function of the Si(111) surface caused by deposition of a submonolayer film of barium has been calculated with due account of dipole-dipole adatom repulsion and metallization effects. The results of the calculations agree satisfactorily with experimental data. The character of the variation in the model parameters in the Ba-Sm-Eu-Yb series is discussed. The electron tunneling probability from an adatom quasi-level to the substrate conduction band and the energy barrier for interatomic transitions of electrons in an adlayer were estimated.     

Special features of thermal evaporation of rare-earth metals

The process of thermal evaporation of rare-earth metals (Ce-Yb) with an incomplete 4f-shell is discussed in terms of electronic structure of their surface. Evaluation of the surface density of neutral intrinsic atoms determining the evaporation process is made. The fact of detachment of Sm atoms in the lowest excited 4f⁵5d6s²-state from the surface is experimentally demonstrated. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 43–47, June, 2006.     

Localisation of 5d electrons in rare-earth metals

We calculate the excess moment in rare-earth metals due to localised 5d character. Comparison with experimental results shows the localisation predicted to be in good agreement with that found from crystalline electric field and field gradient calculations.     

High-field magnetoresistance of noble metals containing rare-earth impurities

We report on magnetoresistance measurements in longitudinal and transverse magnetic fields up to 320 kG for silver and gold containing rare-earth impurities. We focus mainly on the strong anisotropy of the magnetoresistance related to the scattering of conduction electrons by the 4f quadrupoles (non-S ions) and we derive the magnitude of the electron-quadrupole interaction from the analysis of the results. We also consider the isotropic contribution to the magnetoresistance due to exchange scattering. In a number of alloys this contribution is negative in low fields, as this is usually observed in magnetic alloys, but becomes positive in high fields. This change of spin can be ascribed to crystal-field effects.     

Calculation of dipole fields at interstices of rare-earth metals

An effective method for calculating the sum of dipole magnetic fields of lattice sites is developed using the threefold axial symmetry and performing summation over hexagonal planes. The values of fields in octahedral and tetrahedral interstices of an hcp lattice are determined for spiral structures of heavy RE metals and for samarium. It is shown that the absence of an experimentally observed jump in the dipole field at the point of ferro-antiferromagnetic transition in the disposition is due to the small angle of rotation of the spiral magnetic structure. The problem of interpretation of the Lorentz macroscopic field is discussed on the basis of calculation of microscopic fields in a homogeneous continuous medium.     

Afterglow of GaN wurtzite-structure crystals doped by rare-earth metals

The effect of impurity distribution and total defect concentration on the transport of nonequilibrium carriers has been estimated and specific features of afterglow in GaN and GaN〈Eu, Sm, Er〉 crystals revealed by measuring delayed near-edge photoluminescence spectra. A decrease in the total carrier concentration is shown to correlate with the afterglow in GaN wurtzite-structure crystals. The influence of additional illumination at a wavelength of 5145 Å on the evolution of delayed near-edge photoluminescence spectra was estimated in crystals with different Fermi level positions.     

First-principles study of He point-defects in HCP rare-earth metals

He defect properties in Sc, Y, Gd, Tb, Dy, Ho, Er and Lu were studied using first-principles calculations based on density functional theory. The results indicate that the formation energy of an interstitial He atom is smaller than that of a substitutional He atom in all hcp rare-earth metals considered. Furthermore, the tetrahedral interstitial position is more favorable than an octahedral position for He defects. The results are compared with those from bcc and fcc metals.     

Parameters of the elastodynamic state of dodecaborides of rare-earth metals

The mechanical and thermal properties of the crystal lattice are considered as functions of three Mie-Gruneisen parameters. With the relations derived here a number of parameters of the elastodynamic state can be calculated from two data. This model is shown to be applicable for predicting the elastodynamic properties of metallic compounds on the example of dodecaborides of rare-earth metals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 89–92, May, 1991.     

Two- and single-ion magnetic anisotropy in the rare-earth metals

The strong magnetic anisotropy of the rare-earth metals (basically due to the large oribtal contributions to the total ionic moments) has normally been attributed to the crystalline field acting on the ions. Within the last few years a number of experiments have revealed that anisotropic two-ion couplings are of comparable importance. A review of the experimental evidence for the presence of anisotropic coupling between the rare-earth ions is given. Such indications are found primarily in the excitation spectra of the metals, but are also implicit in the crystal-field parameters determined in dilute systems.     

Change in nature of the conductivity of germanides of rare-earth metals

Conclusions The change in the type of conductivity in REM germanides is due to a change in the energy state of electrons in these compounds.The temperatures at which changes occur in the electrophysical and thermal properties of REM germanides of the compositions MeGe₂, MeGe, and Me₅Ge₃ were determined.The transition temperatures are lowest for phases of the composition MeGe₂ and highest for phases of the composition Me₅Ge₃, and increase in the series of REM germanides with reduction in the atomic radius of the metal.The anomalous behavior of cerium germanides is due to the presence of tetravalent metal ions in addition to the trivalent ions.     

Direct experimental evidence for the Ruderman-Kittel-Kasuya-Yosida interaction in rare-earth metals

We show that the ferromagnetic heavy rare-earth (RE) metals show a transport spin polarization at the Fermi level in the majority spin, whereas in ferromagnetic light rare earths it is in the minority spin. The sign of the polarization is in agreement with what is expected due to the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling formalism. We show that magnetotransport measurements on magnetic multilayer samples containing magnetic REs provide a unique opportunity to verify the RKKY coupling scheme in pure rare-earth metals, allowing us to probe both the sign and temperature dependence of the spin-density oscillation.