V. Zevin

Thermal desorption spectra of hydrogen from the bulk: ZrV2Hx☆

Thermal desorption of hydrogen from the bulk of the system ZrV2Hx, 0.3 ⩽ x ⩽ 4.27, shows spectra which develop from a single peak, for x < 1, to a spectrum that consists of 3 peaks and a shoulder for x ⩽ 4.27. A model is proposed to explain the origin of these peaks and relates them to a consecutive desorption of the hydrogens from the different interstitial sites, in agreement with neutron diffraction data on the sites occupancy. However, neutron diffraction indicates that up to x ≈ 2.5 the hydrogens occupy the tetrahedral sites formed by 2 Zr and 2 V, nevertheless our results show that there is a large difference in the bonding energy of these sites for hydrogens with x < 1 and hydrogens with 1 < x < 2.5.

Relaxation and exchange in a nuclear cooling agent: NMR and EPR in single crystal PrNi5☆☆☆

Abstract NMR of Pr and ESR of Gd in single crystals of PrNi5 are observed to exhibit a dramatic increase of the resonance linewidth with temperature in the range 1.2K ⩽ T ⩽ 10K. The observations are interpreted, within the framework of a crystal field spin Hamiltonian model calculation, in terms of low frequency fluctuations of the Pr ions induced by Pr-Pr exchange coupling. A fitting procedure of the theory to the experimental data enabled us to extract the various Pr-Pr and Pr-Gd exchange parameters. The former yield independent information concerning the lowest temperature that can be achieved in nuclear cooling experiments.

The crystalline electric field in some non-magnetic dialuminides compounds

Abstract Magnetization measurements performed in powdered LuAl 2 : RE ( RE = Dy, Er) and single crystals of YAl 2 :Er and LaAl 2 :Nd are reported. The results yield information concerning the levels scheme of the paramagnetic impurities in these compounds. The results in YAl 2 :Er are discussed in view of recent measurements.

Electron spin resonance study of Gd in the nuclear cooling agent: PrNi5 single crystals

The ESR of Gd in single crystals of PrNi5 is observed to exhibit significant angular dependence of the resonance position and linewidth at low temperatures. This is interpreted in terms of the axial spin Hamiltonian which takes the anisotropic susceptibility and the Gd-Pr exchange into consideration. A lineshape analysis enables the authors to derive the axial crystal field parameter D=165+or-30 G and an isotropic Gd-Pr exchange of 1.0+or-0.2 meV. The Gd ESR linewidth exhibits a dramatic increase with increasing temperature; the thermal broadening is angularly dependent. This dramatic increase of the linewidth versus temperature is similar to that observed by others for the Pr NMR in PrNi5 single crystals. Both the NMR and the ESR thermal broadenings are attributed to low-frequency fluctuations of the Pr ions induced by the Pr-Pr exchange coupling. A model, originally developed to explain the ionic relaxation rate in cubic Van-Vleck compounds, is adapted to the present case of hexagonal Van-Vleck compounds.

Relaxation phenomena and host exchange parameters in Tm van Vleck compounds

The ESR linewidth of Gd in TmP (Sugawara et al. 1975) TmSb and TmBi (measured in the present work and by Davidov and Baberschke 1975) exhibits an appreciable temperature dependence. This behaviour is attributed to the fluctuation spectra of the host Tm ions. A previous theory (Davidov et al. 1977) for impurity relaxation in weakly coupled van Vleck paramagnets based on the Bloch-Redfield kinetic equation is extended and applied to the interpretation of the ESR linewidth in the Tm pnictides. In particular the second moment calculation of the host fluctuation spectra has been extended to include both pair correlation and autocorrelation contributions. Explicit expressions are given for Tm and Pr cubic van Vleck compounds. The Tm-Tm host exchange has been estimated by fitting theory to the experimental results. The host exchange parameter in TmSb is very small confirming previous studies on this compound.

Exchange and relaxation of rare-earth impurities and conduction electrons in YbAl2:R (R=Nd, Eu, Gd, Dy, Er)

The authors use the bottleneck phenomena in YbAl2:Gd and YbAl2:Eu to extract the conduction electrons spin-flip relaxation to the lattice, delta eL(O). The values observed are compared with those observed in the other dialuminides, XAl2:Gd (X=Sc, Y, La, Lu). A correlation between delta eL(O) and the atomic number of the host X atom is clearly demonstrated, and is similar to that found by others for rare-earth beryllides. A simple model is suggested to explain this observed phenomenon. The ESSR isothermal limit of the R ion in YbAl2 enables one to extract the dependence of the ion-conduction electron exchange coupling of the 4f occupation number. The authors results shed some light on the nature of the Yb ions in YbAl2 at low temperatures.

Cumulant approach to the Anderson impurity model

Abstract The ground-state energy of the Anderson impurity model for finite on-site Coulomb repulsion U and large degeneracy Nf is discussed in the framework of new projection techniques, based on the introduction of cumulants. The cumulant approach reproduces a self-consistent equation for the ground-state energy which was previously obtained variationally. This demonstrates its ability to treat strongly correlated systems.

A new method for contactless conductivity measurement of a semiconductor layer

A semiconducting layer, placed on an insulating substrate which contains paramagnetic impurities, distorts the electron paramagnetic resonance (EPR) lineshape of the impurities. This distortion can be used to monitor the conductivity of the semiconducting layer. The method is demonstrated on p-type InSb crystals on a ruby (Al2O3:Cr3+) substrate. The results are in good agreement with independent direct resistivity measurement.

Frustration and commensurate pinning in the low temperature phases of TTF-TCNQ

Abstract Stimulated by new experimental information about the low temperature phases of TTF-TCNQ we investigated the Landau free energy (LFE) which also includes fourth order interaction terms both between similar and different chains. This LFE accounts both for the appearance of the new “2a” phase at T

Paramagnetic fluctuations in cubic Van-Vleck pnictides

Abstract The lineshape of the paramagnetic fluctuation spectra in cubic Van-Vleck compounds is discussed in conjunction with the relaxation of an impurity or that of the host nucleus in these systems.