A new method for detection of exciton Bose condensation using stimulated two-photon emission

Stimulated two-photon emission by Bose-condensed excitons accompanied by a coherent two-exciton recombination, i.e., by simultaneous recombination of two excitons with opposite momenta leaving unchanged the occupation numbers of excitonic states with nonzero momenta, is investigated. Raman light scattering accompanied by a similar two-exciton recombination (or generation of two excitons) is also analyzed. The processes under consideration can occur only if a system contains Bose condensate, therefore, their detection can be used as a new method to reveal Bose condensation of excitons. The recoil momentum, which corresponds to a change in the momentum of the electromagnetic field in the processes, is transferred to phonons or impurities. If the recoil momentum is transmitted to optical phonons with frequency $\omega_0^s$, the stimulated two-photon emission with the coherent two-exciton recombination leads to the appearance of a line at $2\Omega'-\omega$, where $\Omega'=\Omega-\omega_0^s$ and $\Omega$ is the light frequency corresponding to the recombination of an exciton with zero momentum. Formulas for the cross sections at finite temperatures are obtained for the processes under consideration. Our estimates indicate that a spectral line, corresponding to the stimulated two-photon emission accompanied by the coherent optical phonon-assisted two-exciton recombination can be experimentally detected in Cu$_2$O.Comment: 28 pages, 3 Postscript figure

Influence of dynamical factors on safe work with a fertilizer spreader

The efficiency of mineral manure most often depends on the evenness of its distribution. The dynamics of the spread of the particles of the mineral manure in regard to the fixed and variable parameters of the imitative model of the spreader have been analyzed. The programmable location of a particle on soil depends on the movement trajectory of the particle on the disc and in the air, which eventually determine the final distribution of particles. The research considered the flow of the manure particles from the spread opening of the manure-box, the trajectory on the disc and in the air, the place of a particle on the surface of the ground as well as on how these trajectories depend on the characteristics of the spreader and particles. The trajectory of particle in the air is the function of its initial velocity and direction, by which the particle leaves the surface of the disc. It determines the final place of the particle on the surface of soil with respect to disc. One of the accidental factors influencing the trajectory of a particle is the impact of spreader lean on hilly soil on the discharge angle of manure particles and the distance of particle fly in the distribution sector. The dependence of the spreading distance of the manure particles on the initial speed of the particle and the trajectory angle of a disc has been obtained. It was established that when the aggregate moves, the trajectory angle changes due to the roughness of the ground from the one that was set constructively up to the largest value when a particle flies farthest. Under the influence of random factors, the angle between the spreader's disc plane and a plane of soil surface can increase up to 8 degrees; therefore the distance of particle's wafting increasesVytauto Didžiojo universitetasŽemės ūkio akademij