Research area:

Dynamics and Relaxation of Electronic Excitations in Wide-Band Dielectric Crystals

Main directions and results consist in what follows.

1. Essential role of two-site polarons with intrinsic vibrational degree of freedom in relaxation processes.

   Traditionally, a single relaxation path of holes and excitons along the band dispersion branch was taken into account. It has been shown that in fact the relaxation process is drastically affected by another relaxation path through vibrational levels of two-site polarons covering the total energy range of the corresponding band. Two relaxation channels act simultaneously with a partial exchange of excitations. A possibility was found to populate each channel separately. The efficiency of energy transport is strongly dependent on the entry chosen. The developed notion explained a series of spectroscopically observed anomalies, in particular, a sharp anomaly in energy transport.

2. Coherent directional motion of two-site polarons with a large binding energy.

   It has been shown that vibrational degrees of freedom make hopping barrier small compared to binding energy and lead to a possibility of a coherent manner of motion at a low temperature. This explains an anomalous rate of polaronic transport, observed in alkali halides at low temperatures, and its sharp decrease with an increase of temperature. The existing notion on a hopping motion, accelerated by an increase of temperature, is restricted to polarons devoid of intrinsic structure.

3. Dynamics of two-site oxygen holes in garnets.

   The mechanism of recombination and reorientation of two-site oxygen holes, strongly influenced by random intrinsic electric fields of photoproduced charges, was derived from experimental data on photoinduced absorption and dichroism.

4. Structure and optical properties of excitons in small dielectric crystals in the vicinity of allowed transition.

   A deep size rearrangement of excitonic states, not reducible to size quantization, was established. This new notion explains anomalous polaritonic phenomena observed experimantally in rare-gas solids consisting of small crystalline grains.

List of the most important papers:

1. A.M. Ratner, Free Atoms and the Simplest Atomic Crystals, Physics Reports 269, 197-332 (1996).
2. A.M. Ratner, Long-Distance Potential Inherent in Copper-Oxide Superconductors and its Manifestations in Optical and Magnetic Properties. Phys. Lett. A 224, 383 - 388 (1997).
3. S.L. Gnatchenko, A.M. Ratner, M. Baran, R. Szymczak, and H.Szymczak, Formation and Interaction of Ferromagnetic Clusters in YBCO Antiferromagnetic Dielectric Films, Phys. Rev. B 55, 3876 - 3885 (1997).
4. A.M. Ratner, M.E. Globus, Threshold Properties of Quasi-One-Dimensional Self-Trapping, Phys. Lett. A 249, 335 - 341 (1998).
5. A.M. Ratner, Local Two-Dimensional Ferromagnetic Ordering as an Origin of the Metal Semiconductor Transition in YBCO Crystals, Phys. Lett. A 239, 313?320 (1998).
6. A.M. Ratner, Spectral and Relaxation Features of Excitons Mixed with Excimer Quasi-Molecular States, Journal of Luminescence, 81, 271 - 283 (1999).
7. G.S. Katrich, K. Kemnitz, Yu.V. Malyukin, A.M. Ratner, Distinctive Features of Exciton Self-Trapping in Quasi-One-Dimensional Molecule Chains, Journal of Luminescence, 90, 55 - 71 (2000).
8. V.V. Eremenko, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, A.M. Ratner, and V.V. Shapiro, Photoproduction and Relaxation of Long-Lived Active Charges in Ca₃Mn₂Ge₃O₁₂ Garnet Revealed Through Optical Absorption, Phys. Rev. B 61, 10670 - 10676 (2000).
9. A.N. Ogurtsov, A.M. Ratner, E.V. Savchenko, and S. Vielhauer, Branched Relaxation of Electronic Excitations in Rare-Gas Crystals, Journ. Phys.: Condensed Matter, 12, 2769-2781 (2000).
10. A.M. Ratner, Coherent Motion of Two-Site Self-Trapped Excitons and Energy Transport in Wide-Band Dielectrics, Phys. Lett. 291, No 2-3, p. 165-174 (2001).
11. A.M. Ratner, Small-Radius Hole Polarons with Intrinsic Structure: Continuous Motion and Anomalous Mobility, Phys. Lett. A 298, N 5-6, pp. 422-431 (2002).
12. A. M. Ratner, Coherent Motion and Anomalous Transport Properties of Exciton and Hole Polarons with Intrinsic Vibrational Structure, Low Temp. Phys., 29, 174-184 (2003).
13. V.V. Eremenko, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, A.M. Ratner, M.B. Kosmyna, B.P. Nazarenko and V.M. Puzikov, Photoproduction of Long-Lived Holes and Electronic Processes in Intrinsic Electric Fields Seen Through Photoinduced Absorption and Dichroism in Ca₃Ga_2-xMn_xGe₃O₁₂ Garnets, Journ. Phys.: Condensed Matter, 15, N 23, pp. 4025-4044 (2003).