THEORETICAL DEPARTMENT

HEAD: Prof. Dimitar I. Pushkarov, Dr.Sci.
tel: 7144-608; e-mail: dipushk@issp.bas.bg

TOTAL STAFF: 9
RESEARCH SCIENTISTS: 8
Prof. N.S. Tonchev, Dr. Sci;
Assist.Prof. H. Chamati, PhD;
Assoc. Prof. N. B. Ivanov Dr. Sci;
Assoc.Prof. P. Ivanov, PhD;
Assist.Prof. R. S. Kamburova
Assoc. Prof. E.R. Korutcheva, PhD;
Assoc. Prof. M.T. Primatarowa, PhD;
Assoc.Prof. K.T. Stoychev, PhD;


RESEARCH ACTIVITIES:

    Further investigations were carried out in the directions traditional for the Theoretical Department: linear and nonlinear elementary excitations in condensed matter, propagation of solitary pulses in low-dimensional structures, quantum and classical models of complex magnetic systems, fundamental problems of phase transitions in finite systems, theory of neuron sets etc. Important results were obtained in the theory of phase transitions, soliton propagation in optically active media and molecular crystals, electrodynamics of crystalline structures, defecton kinetics in quantum crystals and mass diffusion in metals. The results obtained were published in 1 book and 13 scientific papers, and were reported to international conferences and invited talks in European universities and institutes.

    The behaviour of defectons in quantum crystals was investigated at low and very low temperatures. The coherent quantum diffusion was considered and improved. A detailed comparison with the experimental data available was carried out. The phase separation of quantum solids was considered with respect to some new experiments giving additional information to the problem of quantum diffusion. A special attention was paid to the investigation of the Andreev-Pushkarov nanoclasters in quantum solid solutions.

    The interaction of nonlinear Schroedinger (NLS) solitons with potential wells with variable shapes was investigated numerically. For fixed initial velocities below the threshold for transmission, the outcome pattern as a function of the width of the potential yields periodically repeating regions of trapping, transmission and reflection. The observed effects are explained by an excitation and a following resonant deexcitation of amplitude (shape) oscillations of the solitons at the boundaries of the well, associated with radiation modes.

    Problems of finite-size scaling due to the presence of long-range interaction decaying at large distance as r-(d+σ) σ>0 were considered. The attention was focused mainly on the renormalization group results in the framework of O(n)φ4 - theory for systems with fully finite (block) geometry under periodic boundary conditions. Some bulk critical properties and Monte Carlo results also are reviewed. The role of the cutoff effects as well their relation with those originating from the long-range interaction is also discussed. Special attention is paid to the description of the adequate mathematical technique that allows to treat the long-range and short-range interactions on equal ground.

    A phenomenological theory of phase transition in P0.6Ca0.4 MnO3 manganite is developed. It is shown that this is the orbital phase transition and that two electronic states of manganese ion, which are discussed in the literature, result from two different types of condensation of the same orbital order parameter. Thus, the manganese ions in P1-xCaxMnO3 manganites with 0.3<x<0.5 may be in either of the two electronic states, depending on the thermodynamic parameters.

    We present an interatomic potential for Al, Ni and Ni-Al ordered alloys within the second moment approximation of the tight-binding theory. The potential was obtained by fitting to the total energy of these materials computed by first-principles augmented-plane-wave calculations as a function of the volume. The scheme was validated by calculating the bulk modulus and the elastic constants of the pure metals and alloys that were found to be in fair agreement with the experimental measurements. In addition, we performed molecular-dynamics simulations and obtained the thermal expansion coefficient, the temperature dependence of the atomic mean-square displacements and the phonon density of states of the compounds. Despite the simplicity of the model, a satisfactory agreement with the available experimental data was found.


PUBLICATIONS:
  1. E.Korutcheva, Advances in Condensed Мatter and Statistical Physics, Nova Science, New York, 2003D.Bolle, D.Domiguez, R.Erichsen, E.Korutcheva and W.Theumann, Time evolution of the extremely diluted Blume-Emery-Griffiths neural network, Physical Review E 68, 0629-1 (2003)
  2. D.I.Pushkarov, Quantum Diffusion, ArXiv:cond-mat/0310283
  3. H.Chamati and N.Tonchev, Critical Behavior of Systems with Long-Range Interaction in Restricted Geometry, Mod. Phys. Letters B, 17, 1187 (2003)
  4. N.I. Papanicolaou, H. Chamati, G.A. Evangelakis and D.A. Papaconstantopoulos, Second-moment interatomic potential for Al, Ni and Ni-Al alloys and molecular dynamics application, Computational Materials Science, 27, 191-198, (2003)
  5. E.Korutcheva and V.Del Prete, A diagrammatic approach for the information transfer, Modern Trends in Cond. Matter, 5, 1 (2003)
  6. В.С. Шахматов, Н.М. Плакида, Н.С.Тончев, Орбитальный фазовый переход в Pr1-x CaxMnO3 , Письма ЖЭТФ, 77, 15 (2003)
  7. K.T. Stoychev, M.T. Primatarowa and R.S. Kamburova, Resonance Effects in the Interaction of NLS Solitons with Potential Wells, arXiv: nlin. PS/030016.
  8. J.W. Kantelhardt, S. Havlin, and P.Ch. Ivanov, Modeling transient correlations inheartbeat dy-namics during sleep, Europhysics Letters, 62 (2), 147-153 (2003)
  9. B. Suki, A.M. Alencar, U. Frey, P.Ch. Ivanov, S. Buldyrev, A. Majumdar, H. E. Stanley, C.A. Dawson, G.S. Krenz, M. Mishima, Fluctuations, noise and scaling in the cardiopulmonary sy-stem, Fluctuation and Noise Letters, 3(1), R1-R25 (2003)
  10. Y. Ashkenazy, S. Havlin, P.Ch. Ivanov, C-K. Peng, V. Schulte-Frohlinde, H.E. Stanley, Mag-nitude and sign scaling in power-law correlated time series, Physica A, 323: 19 (2003)
  11. K. Ivanova, T.P. Ackerman, E.E. Clothiaux, P.Ch. Ivanov, H.E. Stanley, M. Ausloos, Time correlations and 1/f behavior in backscattering radar reflectivity measurements from cirrus cloud ice fluctuations, J. Geophys. Res., 108 (D9), 4268 (2003)
  12. K. Hu, P.Ch. Ivanov, Z. Chen, M.F. Hilton, H.E. Stanley, S.A. Shea, Novel multiscale regulation in human motor activity, in Fluctuations and Noise in Biological, Biophysical, and Biomedical Systems, edited by S. M. Bezrukov, H. Frauenfelder and F. Moss, SPIE Proceedings, 5110, 235-243 (2003)
  13. Z. Chen, P.Ch. Ivanov, K. Hu, H.E. Stanley, V. Novak, Synchronization patterns in cerebral blood flow and peripheral blood pressure under minor stroke, in Noise in Complex Systems and Stochastic Dynamics, edited by L. Schimansky-Geier, D. Abbott, A. Neiman and C. Van den Broeck, SPIE Proceedings, 5114, 498-506 (2003)
  14. P.Ch. Ivanov, Y. Ashkenazy, J.W. Kantelhardt, H.E. Stanley, Quantifying heartbeat dynamics by magnitude and sign correlations, in "Unsolved Problems of Noise and Fluctuations", UPoN 2002: (Third International Conference), Washington, USA, edited by S.M. Bezrukov American Institute of Physics, AIP Conf. Proc. 665, Melville, New York, 383-391 (2003)

ONGOING RESEARCH PROJECTS:

1. DFG Project: Grant No 436BUL 113/106

Financed by the National Foundation for Scientific Research at the Ministry of Science and Education:
  1. F809: Quantum Magnetic Systems with Low Spatial Dimensions
  2. F810: Soliton Dynamics in Complex Dispersive Systems
  3. F911: Quantum Effects on Transport Phenomena in Solids

INTERNATIONAL COLLABORATION:
  1. Invited visits at the International Centre for Theoretical Physics - Trieste, Italy
  2. Study visit at RWTH - Aachen, Germany
  3. Guest scientist at University of - Augsburg, FRG
  4. Post Doc. Specialization at the university of Ioanina, Greece

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