Dispersion Of Elementary Excitations Research Articles

Dispersion of waves in the two-dimensional ultracold fermi gas of atoms and molecules

System of quantum hydrodynamics equations for the polarized Fermi gas is presented. The short-range interaction is considered to within the third order in the interaction radius. The dispersion of elementary excitations in the two-dimensional Fermi gas of atoms and molecules is investigated. The equations of the evolution of the electric polarization together with the continuity and balance equations for the momentum are used for investigations. The dispersion law is analytically derived. It is demonstrated that two waves can exist in the examined system of particles. Without electric dipole moment, a solution is reduced to the well-known concentration wave. The dynamics of polarization contributes additionally to the dispersion of this wave. In addition, the polarization wave arises. Numerical analysis of the dispersion is presented, and stability of the examined system of particles is analyzed. Cases of wave propagation along the direction of equilibrium polarization and perpendicular to it (when the polarization lies in the gas plane or is perpendicular to it) are considered.

Elementary excitations in spinor polariton-electron systems

We consider effective interactions in a two-dimensional (2D) hybrid polariton-electron system and calculate dispersion of elementary excitations by accounting for the spin degrees of freedom of the particles. Due to the crucial role played by the exchange term in polariton-electron interactions the dispersions of this system become spin dependent and show unusual behavior. The coupling of the excitations of the condensate with 2D plasmons can result in the appearance of roton minimum in the dispersion and destruction of the condensate for close enough situated quantum wells with electrons and excitons.

Charge-order-induced sharp Raman peak in Sr14Cu24O41.

In the two-leg S=1/2 ladders of Sr14Cu24O41, a modulation of the exchange coupling arises from the charge order within the other structural element, the CuO2 chains. In general, breaking transla-tional invariance by modulation causes gaps within the dispersion of elementary excitations. We show that the gap induced by the charge order can drastically change the magnetic Raman spectrum, leading to the sharp peak observed in Sr14Cu24O41. This sharp Raman line gives insight into the charge-order periodicity and hence into the distribution of carriers. The much broader spectrum of La6Ca8Cu24O41 reflects the response of an undoped ladder in the absence of charge order.

Optical absorption in a degenerate Bose-Einstein gas

We develop a theory on optical absorption in a dilute Bose-Einstein gas at low temperatures. This theory is motivated by the Bogoliubov theory of elementary excitations for this system, and takes into account explicitly the modification of the nature and dispersion of elementary excitations due to Bose-Einstein condensation. Our results show important differences from existing theories.

Phase Transition and Exciton-Photon Coupling in Frenkel Exciton System

Dynamical and statistical behaviors of Frenkel excitons in a matter system consisting of N two-level-atoms and interacting with an external field are studied with the molecular field and random phase approximations. Existence of a phase transition is suggested both in the exciton system and exciton-photon coupled system. This phase transition is the second order transition whose order parameter is the polarizability. A new type of condensation of the photon-exciton coupled wave, in which a macroscopic number of excitons and photons participate, has been found as a result of appreciable contribution of photons to the phase transition. When the incident light is coherent, the condition of phase transition is modified by photon induced exciton-exciton interaction. The energy dispersion of elementary excitations is derived.

Phonon-Like Excitations in Liquid Helium

An attempt is made to formulate a theory of phonon-like _excitations in 'liquid helium by generalizing the theory of phonons in quantum crystals to quantum. disordered syst~s. By using experimental data for pair correlation functions, dispersion curves of longitudinal phonon modes are calculated, with a. result for their over-all behavior in fairly good agreement with phonon-raton dispersion curves obtained by neutron scattering measurements. It is shown that the temperature-dependence of the calculated raton-minimum is in good .a,greement with experiment for liquid helium I, but not for liquid helium II. A brief discussion is given on the nature of rotons in liquid helium. It is suggested that, within certain restrictions, the rotan spectrum in liquid helium is likely to be a general feature of the dispersion of elementary excitations in liquids, classical or quantum.