One possible mechanism of negative absorption

Sergey G. Rautian, Igor I Sobel'man

Vitaly S. Shumeiko, Lars Jönsson, Robert I Shekhter +1 more

Two-band models (one wide band and one narrow band close to the Fermi energy) and local-pair models (periodic Anderson model) of high- T c superconductors give rise to the same equilibrium thermodynamics despite the different physical pictures underlying the models. The low-energy excitations in these models are, however, not of the same character, which leads to a qualitative difference in their non-equilibrium properties. In particular, this is true for the interband optical absorption at threshold. Using the Golden rule for absorption and a mean-field treatment of the superconducting state, we calculate the threshold absorption at zero temperature for three different cases: a two-band model without additional correlation besides the interband interaction between Cooper pairs; a two-band model with attractive correlation (negative- U ) between electrons on the same site in the narrow band; and a local-pair model with the same correlation as in the previous model, but with a pair interaction due to the hybridization (in second order) of the wide band and the local states. In the two latter models (which are closely related), we assume that the local-pair correlation energy is much higher than T c . In the uncorrelated case the absorption has a square-root singularity at threshold, which is due to the diverging density of states of the one-particle excitations. In the negative- U correlated cases no single-particle transitions are possible at threshold, and only direct two-photon two-particle processes are possible. Further, in the local-pair model the hybridization can act together with a photon, giving an additional one-photon two-electron threshold in the absorption.

Aiping Fang, Wenchao Ge, Meng Wang +2 more

Negative refraction of a probe field is studied in a dense gas consisting of cascade-type four-level atoms. By coupling the magnetic component of the probe field to a $\mathrm{\ensuremath{\Lambda}}$ scheme with initially prepared coherence in the two lower levels, strong negative permeability with minimal absorption can be obtained. The permittivity of the gas to the electric component of the probe field can be made negative by taking into account the local field effect of the dense atoms. Strong negative refraction with zero absorption can be achieved in a wide range of parameters in our scheme. A possible experimental realization is also discussed.

Andreas Wicht, M. Müller, R.-H. Rinkleff +2 more

E. H. Weinberg

M. L. Narchal, William A. Barker

Abstract : A detailed comparison is made of the relative advantages of the Overhauser effect and the Abragam effect for obtaining stimulated emission or resonant absorption of a four level system when all the thermal relaxations are taken into account. The results obtained are applied t F sub 2 negative enters in LiF for the purpose of investigating this system as a possible quantum mechanical amplifier. (Author)