Suppression of spin-state transition in epitaxially strained LaCoO_{3}

Epitaxial thin films of LaCoO_{3} (E-LCO) exhibit ferromagnetic order with a transition temperature T_c = 85 K, while polycrystalline thin LaCoO_{3} films (P-LCO) remain paramagnetic. The temperature-dependent spin-state structure for both E-LCO and P-LCO was studied by x-ray absorption spectroscopy at the Co L_{2,3} and O K edges. Considerable spectral redistributions over temperature are observed for P-LCO. The spectra for E-LCO, on the other hand, do not show any significant changes for temperatures between 30 K and 450 K at both edges, indicating that the spin state remains constant and that the epitaxial strain inhibits any population of the low-spin (S = 0) state with decreasing temperature. This observation identifies an important prerequisite for ferromagnetism in E-LCO thin films.Comment: 5 pages, 5 figures, submitted to Physical Review

Peculiarities of two-colour optical superradiance in the LaF 3:Pr3+ crystal

We report the results of detailed investigation of optical superradiance (SR) in LaF3:Pr3+ crystal resonantly excited on 3P0-3H4(0) transition by laser pulse. When the power of the pumping pulse was higher than a certain threshold, the resonant medium emitted an optical SR pulse on 3P 0-3H4(0) transition in the direction of the pumping pulse (as well as in the opposite direction) delayed in time with respect to the pumping pulse. Further increasing of the peak power of the pumping pulse led to the observation of SR on the 3P 0-3H6(1) transition also. Thus, the two-color SR in the employed crystal was observed

Specific features of local data erasure and laser cooling in van vleck paramagnetics

Nonequilibrium population of hyperfine sublevels of the ground electronic state of rare-earth ions is theoretically investigated for Van Vleck paramagnetic crystals subject to optical excitation with laser pulses. We discuss the possibility of using the predicted specific features of the phenomenon under study for the development of methods for the laser cooling of the nuclear spin system and methods of local data erasure in the regime of long-lived photon echo. Copyright © 1997 by MAHK Hayka/Interperiodica Publishing

Multipole claster superfluorescence: The generalized Dicke model

The peculiarities of superfluorescence in systems consisting of a few neighbor particles (clusters), when exchange or/and electrostatic multipole interactions between particles become significant, are investigated theoretically. It is shown that interaction between particles in the cluster leads to superpuassonian statistics of emitted photons and the intensity 1 of quadrupole superfluorescence on the double frequency depends on the number N of active centers as 1-Nα, where α > 2. In particular, α = 3, when N ≫ 1. Possible schemes of cluster construction in nuclei systems are considered and prospects of observation of multipole gamma superfluorescence are discussed

The peculiarities of accumulated long-lived photon echo in van vleck paramagnetics

Theory of accumulated long-lived photon echo in Van Vleck paramagnetics is developed. From comparison with experimental data obtained in LaF 3 : Pr 3+ (the 3H 4- 3P 0 transition) the relaxation rates of population of the 3H 4(0) level hyperfine transitions are estimated. Copyright © 1996 by MAHK Hayka/Interperiodica Publishing

Peculiarities of photon echo in extended crystals of ruby

The signals of primary and backward photon-echo (PE) in extended crystals of ruby were observed. The peculiarities of PE formation in these resonant media were also investigated. Conditions of excitation were not changed for crystals with various length. It was found that duration of echo-signals was increased with increasing of the sample length

Laser cooling of the spin system in Van Vleck paramagnetics

A method of one-pulse laser cooling of the spin system in Van Vleck paramagnetics is proposed. The optimal variants of laser excitation are determined. The limit of the expected spin temperature is obtained. Copyright © 1996 by MAHK Hayκa/Interperiodica Publishing