Lack Of Temperature Dependence Research Articles

Theory of collective magnetic excitations in strong crystal fields. II. Application toS=1singlet ground-state ferromagnet

The theory of collective magnetic excitations based upon the set of Green's functions of angular momentum spherical tensor operators is applied to the exchange-coupled ($S=1$) magnet with a singlet crystal-field ground state. The spin Hamiltonian is brought to an approximately diagonal form via a unitary transformation by a spin operator functional, the best single-site approximation being located variationally. This enables the set of equations of motion for the Green's tensor to be linearized by conventional techniques. The essential features of induced-moment systems are clarified and displayed in the theory and numerical calculations, respectively. In particular, soft-mode behaviors at the second-order phase transition to ferromagnetism, mode-mode interactions, and a lack of temperature dependence for the excitation energies except at small wave vectors are evident. The nature of the phase transition is examined in detail and a relationship between the divergence of the static susceptibility and the soft-mode behavior is derived from the unitary transformation to pseudospace.

A transient irradiation creep in non-fissile metals

Abstract Relaxation strains between 2 × 10−5 and 10−4 have been observed in iron, high carbon steel, stainless steel and zirconium, after fast neutron doses between 2 × 1019 and 3 × 1020 cm−2, at stresses between 1 and 3 kg mm−2, and at temperatures about one-fifth of the melting temperature. In iron, the relaxation appears complete at a neutron dose of 2 × 1019 cm−2, and its lack of temperature dependence indicates that it is not an acceleration of thermal creep. In the steels, correlation with other results shows that the relaxation is proportional to stress. Vacancies and interstitials created by irradiation give rise to a chemical stress on dislocations. Dislocation climb under this stress, the external stress, and the line tension of the dislocations gives a final creep strain which is independent of temperature and proportional to stress. For the stresses used here, a final strain of 5 × 10−5 is predicted in a material containing a random array of dislocations. The order of magnitude of the calculated initial strain rate is also correct.