Exchange broadening of EPR line in ZnO:Co

We study the X-band EPR spectra of Co²⁺ in single crystalline Zn₁₋xCoxO (x = 0.001–0.075) thin films grown by plasma-assisted molecular beam epitaxy. By analyzing the EPR linewidth behavior we argue that the exchange-narrowing model, usually applied to Mn-based II-VI DMS, fails here and that a combined effect of exchange and dipolar broadening can explain the linewidth variation with Co content and temperature

Effects of interatomic interaction on cooperative relaxation of two-level atoms

We study effects of direct interatomic interaction on cooperative processes in atom-photon dynamics. Using a model of two-level atoms with Ising-type interaction as an example, it is demonstrated that interparticle interaction combined with atom-field coupling can introduce additional interatomic correlations acting as a phase synchronizing factor. For the case of weakly interacting atoms with $J<\hbar\omega_0$, where $J$ is the interparticle coupling constant and $\omega_0$ is the atomic frequency, dynamical regimes of cooperative relaxation of atoms are analyzed in Born-Markov approximation both numerically and using the mean field approximation. We show that interparticle correlations induced by the direct interaction result in inhibition of incoherent spontaneous decay leading to the regime of collective pulse relaxation which differs from superradiance in nature. For superradiant transition, the synchronizing effect of interatomic interaction is found to manifest itself in enhancement of superradiance. When the interaction is strong and $J>\hbar\omega_0$, one-partice one-photon transitions are excluded and transition to the regime of multiphoton relaxation occurs. Using a simple model of two atoms in a high-Q single mode cavity we show that such transition is accompanied by Rabi oscillations involving many-atom multiphoton states. Dephasing effect of dipole-dipole interaction and solitonic mechanism of relaxation are discussed.Comment: 34 pages, 8 figure

Domain theory of polarization echoes in ferroelectrics

A classification of echo type signals is proposed. It is shown that to classify an echo one must know the following : the physical nature of the particles or the objects that possess phase memory and generate the echo signal, the physical nature of the exciting pulses and of the response signal, the mobility aspects of the particles that generate the echo and the real correspondence between the physical nature of the echo signal and the signal in a receiver. After all of these questions have been answered the nature and type of the echo may be said to be identified These rules are illustrated by an example of the polarization echoes in ferroelectric monocrystals.Une classification des signaux d'écho est proposée. Pour ce faire, il est nécessaire de connaître à la fois la nature physique des particules ou micro-objets qui conservent la mémoire de la phase et peuvent engendrer l'écho, la nature physique des impulsions d'excitation et de réponse, la mobilité des particules qui produisent l'écho et enfin quelle est la transformation qui permet aux signaux dans l'échantillon d'être détectés dans le récepteur. Une telle identification a été faite pour les échos de polarisation dans les monocristaux ferroélectriques