In-Situ Nuclear Magnetic Resonance Investigation of Strain, Temperature, and Strain-Rate Variations of Deformation-Induced Vacancy Concentration in Aluminum

Critical strain to serrated flow in solid solution alloys exhibiting dynamic strain aging (DSA) or Portevin–LeChatelier effect is due to the strain-induced vacancy production. Nuclear magnetic resonance (NMR) techniques can be used to monitor in situ the dynamical behavior of point and line defects in materials during deformation, and these techniques are nondestructive and noninvasive. The new CUT-sequence pulse method allowed an accurate evaluation of the strain-enhanced vacancy diffusion and, thus, the excess vacancy concentration during deformation as a function of strain, strain rate, and temperature. Due to skin effect problems in metals at high frequencies, thin foils of Al were used and experimental results correlated with models based on vacancy production through mechanical work (vs thermal jogs), while in situ annealing of excess vacancies is noted at high temperatures. These correlations made it feasible to obtain explicit dependencies of the strain-induced vacancy concentration on test variables such as the strain, strain rate, and temperature. These studies clearly reveal the power and utility of these NMR techniques in the determination of deformation-induced vacancies in situ in a noninvasive fashion.

Effect of sharp structure in the electronic density of states on the electron--photon coupling in high T/sub c/ superconductors

The evidence for sharp structure in the electronic density of states N(epsilon) of certain classes of high T/sub c/ superconductors is based on the normal state temperature dependence of the spin susceptibility chi, electronic specific-heat coefficient $gamma$, electrical resistivity rho, and nuclear spin- lattice relaxation rate 1/T$sub 1$. Model calculations of chi, $gamma$, rho, and T$sub 1$ in the rigid N(epsilon) approximation and of chi and $gamma$ for a temperature dependent N(epsilon) are presented for V$sub 3$Ga and V$sub 3$Si. The model calculations were used to calculate the explicit temperature dependence of the electron mass enhancement and the renormalized electronic specific heat coefficient. The temperature dependence of the dielectric screening, which can have an important effect on renormalizing the phonon frequencies at low temperatures, is also discussed. (auth

MÖSSBAUER STUDY OF LATTICE SOFTENING AND PHONON MOMENTS IN SUPERCONDUCTING COMPOUNDS

La variation en fonction de la température du déplacement Mössbauer et de la fraction de résonance dans les composés pseudobinaires β-W V3GaxSn1-x (x = 0,9, 0,5, 0,1) a été mesurée dans le but de mettre en contraste les propriétés réticulaires de supraconducteurs à Tc élevées et basses. Pour V3Ga0,9Sn0,1 (Tc ~ 11 K) on a montré qu'un écart du déplacement thermique δ(T) par rapport au comportement de Debye provient d'un ramollissement du réseau. On obtient la variation de la température de Debye en fonction de la température a l'aide d'un modèle quasi harmonique. A basse température, le rapport déplacement/facteur de Debye-Waller est proportionnel au rapport des moments de phonon / ; ces derniers représentent dans la théorie de McMillan des supraconducteurs fortement couplés, la seule contribution des phonons au paramètre de couplage électron-phonon. Une mesure Mössbauer de / entre 4 et 50 K dans le système V3GaxSn1-x indique que la dépendance en fonction de la concentration est plus faible que ne le laisse prévoir le paramètre de couplage électron-phonon déduit de Tc. On utilise un modèle de la densité spectrale de phonon consistant en deux spectres de Debye pour obtenir les déplacements de fréquence en fonction de la température pour le composé harmonique (Tc élevée) ; les résultats sont compatibles avec la règle de somme sur . La contribution électronique au déplacement en fonction de la température est calculée sur la base d'un modèle de densité d'états, qui rend compte de l'influence de la température et de la concentration sur la susceptibilité, la chaleur spécifique électronique et la fréquence de relaxation spin nucléaire-réseau. Lorsque la température augmente, le déplacement dû au réseau est de signe opposé à celui du déplacement isomérique.The temperature dependence of the Mössbauer shift and recoil-free fraction for the β-W pseudobinary compounds V3GaxSn1-x, (x = 0.9, 0.5, 0.1) has been measured in an experiment designed to contrast the lattice properties of high and low Tc superconductors. For V3Ga0.9Sn0.1 (Tc ~ 11 K) a deviation of the thermal shift δ(T) from Debye behavior is shown to be due to lattice softening. Using a quasiharmonic model the temperature dependence of the Debye temperature is obtained. At low temperature the ratio of shift/Debye-Waller factor is proportional to the ratio of phonon-moments / which represent the phonon-only part of the electron-phonon coupling parameter in McMillan's theoretical treatment of strong-coupled superconductors. A Mössbauer measure of / in the temperature region from 4 to 50 K in the V3GaxSn1-x system indicates a weaker dependence on concentration than expected from the electron-phonon coupling parameter derived from Tc. A model of the phonon-spectral density which consists of two Debye spectra is used to obtain the temperature-dependent frequency shifts for the anharmonic (high Tc) compound. The electronic contribution to the temperature-dependent shift is calculated on the basis of a model density-of-states, which accounts for the temperature and concentration dependence of the susceptibility, electronic specific heat and nuclear spin-lattice relaxation rate. The lattice shift with increasing temperature is of opposite sign to that of the isomer shift

ELECTRONIC PROPERTIES OF Er3+ IN ErRh4B4

La configuration électronique de Er3+ le supraconducteur ternaire ErRh4B4 a été étudiée par spectroscopie Mössbauer de 166Ere. Le niveau fondamental de champ cristallin est un doublet comportant une contribution d'environ 90 % de la configuration |Jz = ± 15/2>. Le moment de l'ion Er est 8,1 ± 0,3 µB. Les effets de relaxation paramagnétique observés aux basses températures suggèrent la présence d'autres niveaux de champ cristallin à des énergies faibles.The electronic state of Er3+ in the ternary superconductor ErRh4B4 has been investigated using the Mossbauer effect in 166Er. The crystal field ground state is found to be a doublet, about 90 % of which is the state |Jz = ± 15/2>. The moment on the Er ion is 8.1 ± 0.3 µB. Effects of the paramagnetic relaxation at low temperatures indicate the presence of other low lying crystal-field levels

Mossbauer effect and neutron diffraction study of the anomalous lattice behavior of U/sub 6/Fe

Neutron diffraction and /sup 57/Fe Moessbauer effect measurements have been made on the exchange-enhanced superconductor U/sub 6/Fe. The neutron diffraction results indicate a pronounced anisotropy in the thermal expansion of the unit cell and an anomalous temperature dependence for the c-axis component of the total Debye-Waller factor of Fe below 100K. The Moessbauer effect measurements show non-Debye behavior in the mean-squared displacement of Fe, with phonon hardening occurring below 100K. The absence of a temperature dependence to the relative intensities of the lines in the Fe quadrupole doublet indicates almost isotropic lattice vibrations. Taken together these observations imply that Fe undergoes static displacements along the c-axis that begin below 100K and increase with decreasing temperature. Both the Moessbauer shift and the quadrupole coupling indicate electronic changes in the valence of Fe in the temperature region in which the displacement takes place

HYPERFINE INTERACTION STUDIES OF CHEVREL PHASE SUPERCONDUCTORS

Nous avons effectué des études microscopiques des proprietés électroniques de phases de Chevrel supraconductrices. Nous avons examiné les anomalies de phonons au site de Sn dans SnMo6S8, SnMo6,Se8 et La0.98Mo6Se8. La polarisation des électrons de conduction par les ions Eu a été étudiée dans EuxSn1-x MoS6S8n et les conséquences de cette polarisation pour les propriétés supraconductrices sont discutées.Microscopic studies of the electronic properties of Chevrel phase superconductors are reported. Phonon anomalies at the Sn site in SnMo6Se8, SnMo6Se8 and La0.98Sno.02Mo6Se8 have been investigated. Studies of polarization of conduction electrons by the Eu ion in Eux Snl-xMo6S8 have been performed, and the effects of Such polarization on superconducting properties are discussed

MÖSSBAUER STUDIES OF TERNARY SUPERCONDUCTORS

Des études Mössbauer de phases ternaires de Chevrel et de borures de terres rares et de rhodium supraconducteurs sont discutées. On a étudié les propiétés anormales des phonons au site de Sn dans SnMo6S8, SnMo6S8 et La0, 98Sn0 , 02Mo6Se8. La polarisation des électrons de conduction par les ions Eu a été étudiée dans EuxSn1-xMo6S8 ainsi que les conséquences de cette polarisation pour les propriétés supraconductrices. La configuration électronique Er3+ dans le supraconducteur ternaire ErRh4B4 a été étudiée par spectroscopie Mössbauer de 166Er.Mössbauer studies of the ternary Chevrel phase and rare earth rhodium boride superconductors have been made. Anomalous phonon properties at the Sn site in SnMo6S8, SnMo6Se8, and La0.98Sn0.02 Mo6Se8 have been investigated. Studies of polarization of conduction electrons at the site of the magnetic ion have been made by means of the 151Eu Mössbauer effect in EuxSn1-xMo6S8 and the effects of such polarization on superconducting properties discussed . The Mössbauer effect in 166Er has been used to investigate the electronic ground state in the ternary compound ErRh4B4 both in the superconducting and magnetically ordered States