R. Bhadra

Elastic constants of metal‐insulator superlattices

A Brillouin scattering study of elastic properties of NbN/AlN superlattices is presented. Because the coupling of light to surface waves in this system is very weak, we describe a modification of the technique which enhances the scattering efficiency, thereby allowing measurements of a system which is otherwise inaccessible. This particular multilayer superlattice is one of the very few that does not exhibit any elastic anomalies as a function of layer thickness in accordance with the idea that electron transfer may be the mechanism responsible for elastic anomalies in superlattices.

Crystalline to amorphous transformation in GaAs during Kr ion bombardment: A study of elastic behavior

A marked softening in the shear elastic properties of GaAs following 1.5‐MeV Kr ion irradiation is observed. Ion channeling and Raman scattering are used to monitor the accumulation of damage, which for 1.5‐MeV incident energy extends to a depth of ∼8000 A. Both techniques show that the sample is amorphized by a dose of 2×1014 ions/cm2. Brillouin scattering reveals that a shear elastic modulus of this near‐surface irradiated region decreases by 43% as the material transforms from crystalline to amorphous. The observed results are discussed and compared with similar measurements on Si and intermetallic compounds.

Phase diagram and oxygen stoichiometry of Y‐Ba‐Cu‐O thin films

We have performed an exhaustive study of the phase diagram of Y‐Ba‐Cu‐O thin films using chemical analysis, energy dispersive x‐ray spectroscopy, Auger electron spectroscopy, and x‐ray diffraction. We show that Raman scattering can provide information regarding impurity phases and oxygen stoichiometry in thin films

Relationship between structural phase transitions and elastic anomalies in metallic superlattices

A detailed study of the structural and elastic properties of Fe/Cu superlattices has been performed. These superlattices exhibit a structural phase transition as a function of layer thickness in which bcc α‐Fe transforms into fcc γ‐Fe. This structural phase transition in which the Fe layers become coherent with the Cu layers is signaled by clear‐cut changes in elastic and magnetic properties. The elastic properties studied through this transition indicate that in‐plane coherency plays an important role in the elastic behavior of metallic superlattices.

Irradiation-induced amorphization and elastic shear instability in intermetallic compounds

Previously we reported a substantial ({approximately}50%) decrease in shear modulus prior to amorphization in Kr irradiated Zr{sub 3}Al, and proposed that amorphization is triggered when the crystalline lattice becomes unstable against shear stress. In the present work, the relation between amorphization and shear elastic instability has been investigated in two additional compounds (FeTi and NiAl) during room temperature irradiation with 1.7-MeV Kr{sup +}. A shear modulus was measured using Brillouin scattering; structural information was obtained in situ in a high voltage electron microscope interfaced to a tandem accelerator. During irradiation of FeTi, chemical disordering and a large ({approximately}40%) decrease of shear modulus were observed, and an amorphous phase developed subsequently. In contrast, NiAl remained crystalline and chemically ordered during irradiation, and exhibited only a {approximately}10% decrease in shear modulus. Hence, these two results provide further support that a shear instability triggers irradiation-induced amorphization. The shear instability mechanism may also apply to other solid-state amorphization techniques, e.g., hydrogen charging and mechanical deformation. 22 refs., 3 figs.

Structural and elastic property changes in AgCo superlattices induced by high energy ion irradiation

Abstract The shear elastic modulus and structural properties of a series of Ag Co superlattice films have been characterized using Brillouin scattering and X-ray diffraction. A softening of the shear modulus of the unirradiated Ag Co superlattices with decreasing modulation wavelength is observed and is correlated with an expansion in the average lattice spacing. With increasing irradiation fluence, a reduction in the anomalous shear modulus softening and lattice expansion are observed. These reductions imply that the anomalous elastic properties are related to lattice strains present, which are relieved by the irradiation.

Elastic Instability During Amorphization of Internetallic Compounds by Charged-Particle Beams *

Transmission electron microscopy (TEN) and Brillouin scattering methods were used to determine the lattice dilatation and shear elastic constant as a function of the degree of long-range order during room temperature irradiation of Zr 3 Al with 1.0-MeV Kr + . The results indicate that the onset of amorphization in Zr 3 Al is triggered by an elastic shear instability, and that the instability is directly related to the volume dilatation associated with the destruction of long-range order. It is also shown that the volume dependence of the shear elastic constant associated with radiation-induced disordering and amorphization is virtually identical to that associated with the heating to melting of many metals.