Low temperature resistivity of rare-earth hexaborides

Low temperature resistivity measurements of rare-earth hexaborides have been done down to 2 K. We observe a T⁴ or T³ temperature dependence of resistivity below the Néel temperature, which is associated with electron-spin wave scattering. At the lowest temperatures the resistivity varies as T². This T² dependence is thought to be due to Baber type electron-electron scattering.     

Components of the low-temperature heat capacity of rare-earth hexaborides

The temperature dependence of heat capacity C _p(T) was studied for nine rare-earth hexaborides MB₆(M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) at temperatures of 5–300 K. Using the correspondence principle for lattice heat capacities of isostructural compounds, the lattice contribution C ₁(T) and the excess contribution ΔC(T) to the heat capacity of the hexaborides were determined. The lattice heat capacity C ₁(T) is represented as the sum of the Debye contributions of the metal and boron sublattices: C ₁(T)=C _M (T)+6C _B(T). The Debye temperatures π_M and π_B of the metal and boron sublattices were determined. The anomalies in the excess heat capacity ΔC(T)=C _p (T)?C ₁(T) are related to the magnetic ordering effects, the Schottky contribution, and the Jahn-Teller effect.     

Synchrotron reflectivity spectra and optical properties of rare-earth oxides

The spectra of the complete set of optical functions for nine sesquioxides of rare-earth elements of the R₂O₃ group are obtained at 300 K in the energy range 0–50 eV. The calculations are carried out on the basis of synchrotron reflectivity spectra and computer programs using the Kramers-Kronig relations. The main features of the optical spectra and their dependence on the nature of rare-earth elements were determined.     

Neutron diffraction study of multipole order in light rare-earth hexaborides

Multipole interactions are known to play a central role in the unconventional properties of light rare-earth hexaborides and especially of CeB₆. Substituting Pr at the Ce sites has the effect of enhancing exchange interactions and changing the symmetry of the local 4f charge distribution, while suppressing the octupole moment. The (T, H) magnetic phase diagrams of the Ce _x Pr_{1 − x} B₆ compounds display a large variety of ordered phases involving magnetic and/or charge degrees of freedom. Here we focus on the compound Ce_0.7Pr_0.3B₆, which is located slightly beyond the Pr concentration where the antiferroquadrupolar phase of pure CeB₆ is suppressed in zero field. The different magnetic structures have been characterized by neutron diffraction and their origin is discussed in connection with recent non-resonant X-ray results by Tanaka et al.      

Phonon spectra of stoichiometric rare-earth compound of EuGa2S4

Investigations of EuGa₂S₄ have been done on the photoluminescence (PL) related to the transition between 4f⁶5d and 4f⁷ configuration of the Eu²⁺ ion and its excitation (PLE) spectra, Raman scattering and infrared absorption. The energies of phonons coupled to the ground and the excited states of the transition are analyzed to be 34 and 19 meV from the shapes of the PL and PLE bands, respectively. The former corresponds to the energy of the Raman line showing the highest intensity. The latter is close to the value obtained from analysis of the temperature dependence of the half width of the PL band. These correspondences indicate that the relevant emission of EuGa₂S₄ surely has phonon-terminated character.     

Mean-square displacements of metal and boron atoms in crystal lattices of rare-earth hexaborides

The intensities of the I₄₁₀ and I₄₁₁ reflections of nine rare-earth hexaborides MB₆ (M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy) are experimentally studied in the temperature range 4.2–300 K. The mean-square displacements of metal and boron atoms are calculated from the temperature dependences of the intensities I₄₁₀(T) and I₄₁₁(T). The characteristic temperatures of the metal (θ_M) and boron (θ_B) sublattices of rare-earth hexaborides are determined in the Debye approximation. It is found that the characteristic temperatures decrease with an increase in the atomic number of the metal.     

Thermionic emission properties of hexaborides

Thermionic emission properties of the single crystal hexaborides LaB₆, CeB₆, PrB₆, NdB₆, SmB₆, EuB₆, (La, Sr)B₆, (La, Ba)B₆, (La, Ce)B₆, (La, Pr)B₆, (La, Sm)B₆, and (La, Dy)B₆ are measured in the temperature range between 1250 and 1700°C. Of these, LaB₆ is shown to have the highest emission current density in the temperature range investigated. The LaB₆-based mixed hexaborides, (La, M)B₆, show current densities similar to LaB₆, but a little lower. Analyses by Auger electron spectroscopy indicate that the surface composition of (La, M)B₆ approaches that of LaB₆ at elevated temperatures and that the thickness of the surface layer whose composition is different from that of the bulk is typically several atomic layers. The formation of the surface layer is considered to be caused by a relatively slow evaporation rate of La compared to that of the other metal.     

Phonon spectra in one-dimensional quasicrystals

The propagation of phonons in one-dimensional quasicrystals is investigated. We use the projection method which has been recently proposed to generate almost periodic tilings of the line. We define a natural Laplace operator on these structures, which models phonon (and also tight-binding electron) propagation. The selfsimilarity properties of the spectrum are discussed, as well as some characteristic features of the eigenstates, which are neither extended nor localized. The long-wavelength limit is examined in more detail; it is argued that one is the lower critical dimension for this type of models.     

Elastic properties and electronic structures of lanthanide hexaborides

The structural, elastic, and electronic properties of a series of lanthanide hexaborides(Ln B6) have been investigated by performing ab initio calculations based on the density functional theory using the Vienna ab initio simulation package.The calculated lattice and elastic constants of Ln B6 are in good agreement with the available experimental data and other theoretical results. The polycrystalline Young's modulus, shear modulus, the ratio of bulk to shear modulus B/G, Poisson's ratios, Zener anisotropy factors, as well as the Debye temperature are calculated, and all of the properties display some regularity with increasing atomic number of lanthanide atoms, whereas anomalies are observed for Eu B6 and Yb B6. In addition, detailed electronic structure calculations are carried out to shed light on the peculiar elastic properties of Ln B6.The total density of states demonstrates the existence of a pseudogap and indicates lower structure stability of Eu B6 and Yb B6 compared with others.     

Phonon spectra of a Fibonacci chain

Based on more realistic physics we study the phonon spectra of the Fibonacci chain by taking into account a nonlinear resistance. It is found that the nonlinear force should be very weak and consequently, the continuity, range and gaps of the phonon spectra would be still controlled dominantly by the relative strength of spring constants and chain length. It means that even if no additional nonlinear resistance was taken into account, the conventional results of phonon spectra are exactly correct. On the other hand, in the framework of a conventional model we investigated the relationship between the biggest gaps of phonon spectra and defects of Fibonacci-like aperiodic chains. By means of numerical calculations one can obtain quantitatively the maximum of the length of a one-dimensional aperiodic chain sensitive to boundaries. This method would be useful for the calculation of quasiperiodic and aperiodic lattices.     

Phonon spectra and lattice dynamics of lithium nitride

Lithium nitride, Li₃N, belongs to the hexagonal structure with the symmetry point group D_6h. The structure is ionic with four atoms per unit cell and N^3- coordinated in a regular way with eight Li⁺ ions. Polarized Raman scattering and infrared reflectivity spectra are reported. The frequencies of zone center phonons are determined from a Kramers-Kronig analysis and the oscillator fits to the reflectivity data. A rigid shell model with eight parameters, including the anisotropic polarizability of nitrogen ions, yields an excellent fit to the observed phonon frequencies.     

Phonon emission spectra of superconducting tunnel junctions

Resonant scattering of phonons by the stress split acceptor ground state in Si:B has been used to investigate the phonon emission spectra of superconducting tunnel junctions. Symmetric Sn, Pb and PbBi alloy junctions have been studied in the single particle tunneling regime. The spectra show many features as expected from relaxation and recombination of quasiparticles via one-phonon emission. The results are compared with the steady state solutions of the kinetic equations for quasiparticles and phonons. Spatially homogeneous distribution functions are assumed. This approach describes the spectra of tin junctions fairly well. However, there are marked deviations for Pb and PbBi junctions which are attributed to a decay of transverse phonons within the junction.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

Phonon emission spectra of thin metallic films

Thin metallic films evaporated on an Al₂O₃-single crystal and cooled to liquid helium temperatures are heated by short electric current pulses. The high frequency part of the emitted phonons is detected by calibrated superconductive tunneling junctions on the opposite surface of the substrate. The observed phonon detector signal amplitude is compared with theoretical models taking account of the boundary conditions for elastic waves in the film. It is found that the phonon spectrum emitted perpendicularly to the substrate-film boundary depends strongly on the thickness of the heater film.     

Phonon spectra and unit cell analysis of 2-cyanothiophene

Vibrational spectra of 2-cyanothiophene in the solid state reveal that the crystals are ordered below the freezing point and have an orthorhombic C_2v unit cell. Intermolecular forces between the two molecules occupying the Bravais unit are not strong enough to promote the separation of all the Davidov doublets of the phonon states.     

De Haas-van Alphen effect in rare-earth hexaborides (RE=Pr,Nd, Gd)

The de Haas-van Alphen (dHvA) effect has been measured for antiferromagnetic PrB₆, NdB₆, and GdB₆. The dHvA data of PrB₆ and NdB₆ show paramagnetic Fermi surfaces which arise from magnetic breakdown through the small antiferromagnetic gaps in high fields very similar to those of LaB₆. In contrast, the antiferromagnetic Fermi surfaces which arise from new magnetic Brillouin zone boundaries introduced by the magnetic order seem to be very different in PrB₆ and NdB₆, respectively. In GdB₆ dHvA oscillations could be observed for the first time. In all three compounds magnetic phase transitions have been observed in the dHvA measurements in the field range of 10–12 T.Dedicated to Professor Hund on the occasion of his 95th birthday