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Search Results: 1 - 10 of 429150 matches for " B. S. Alexandrov "
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Stored light and released fiction
E. B. Alexandrov,V. S. Zapasskii
Physics , 2003,
Abstract: It is shown that the interpretation of the experimental results reported in the publication Storage of Light in Atomic Vapor" by D.F.Phillips et al., Phys. Rev.Lett. 86, 783 (2001) is incorrect. The experimental observation of this paper can be consistently explained in the framework of standard concepts of the physics of optical pumping and have nothing to do with "storage of light'', or "dynamic reduction of the group velocity'', or "light pulse compression''.
Small adiabatic polaron with a long-range electron-phonon interaction
A. S. Alexandrov,B. Ya. Yavidov
Physics , 2003, DOI: 10.1103/PhysRevB.69.073101
Abstract: Two-site single electron system interacting with many vibrating ions of a lattice via a long-range (Fr\"{o}hlich) electron-phonon interaction is studied in the adiabatic regime. The renormalised hopping integral of small adiabatic Fr\"{o}hlich polarons is calculated and compared with the hopping integral of small adiabatic Holstein polarons.
Fourier Synthesization of Optical Pulses and "Polar'' Light
V. S. Zapasskii,E. B. Alexandrov
Physics , 2002, DOI: 10.1134/1.1481139
Abstract: It is shown that the direct Fourier synthesization of light beams allows one to create polarity-asymmetric waves, which are able, in the process of nonlinear interaction with a medium, to break its inversion symmetry. As a result, these "polar" waves may show the effect of optical rectification in nonlinear centrosymmetric media by generating light-induced dc electric polarization. At the same time, the waves of this type, due to their unusual symmetry properties, can be used for detecting the direction and sign of a dc electric field applied to the medium. The prospects of application of polar waves to data recording and processing are discussed.
Branching Transport Model of Alkali-Halide Scintillators
B. S. Alexandrov,K. D. Ianakiev,P. B. Littlewood
Physics , 2007, DOI: 10.1016/j.nima.2007.12.010
Abstract: We measure the time dependence of the scintillator light-emission pulses in NaI(Tl) crystals at different temperatures, after activation by gamma rays. We confirm that there are two main nonexponential components to the time decay and find that their amplitude ratio shows Arrhenius temperature dependence. We explain these nonexponential components as arising from two competing mechanisms of carrier transport to the Tl activation levels. The total light output of the NaI(Tl) detectors shows a linear temperature dependence explained by our model.
Museum Information Systems: CIDOC data model implementation in the ArchTerra project
V. Vassilev,I. Stoev,B. Gaydarska,S. Alexandrov
Bollettino del CILEA , 1999, DOI: 10.1472/bc.v69isettembre.997
Abstract: The paper summarizes the first stage of the work for development of a virtual archaeological museum carried under the EU Copernicus ArchTerra project. It describes a new implementation of a database with museum information after the CIDOC standard. The system reported here is developed using Oracle CASE methodology and is currently implemented as an Oracle 8i database. The data model it is based on enhances the CIDOC object-oriented data model in several important directions for realistic museum data processing, such as artefact duplication, conservation and Internet browsing. It clearly separates the domain ontology from the specific domain knowledge through the concept of a terminological thesuarus, thus making it applicable to other kind of museum information systems, e.g. in ethnography, natural sciences, etc. The paper discuss also future directions of further development, especially in the direction of data exchange between CIDOC compliant databases containing information related to cultural heritage management.
Feigenbaum Cascade of Discrete Breathers in a Model of DNA
P. Maniadis,B. S. Alexandrov,A. R. Bishop,K. ?. Rasmussen
Physics , 2010, DOI: 10.1103/PhysRevE.83.011904
Abstract: We demonstrate that period-doubled discrete breathers appear from the anti-continuum limit of the driven Peyrard-Bishop-Dauxois model of DNA. These novel breathers result from a stability overlap between sub-harmonic solutions of the driven Morse oscillator. Sub-harmonic breathers exist whenever a stability overlap is present within the Feigenbaum cascade to chaos and therefore an entire cascade of such breathers exists. This phenomenon is present in any driven lattice where the on-site potential admits sub-harmonic solutions. In DNA these breathers may have ramifications for cellular gene expression.
The effective action and quantum gauge transformations
S. Alexandrov
Physics , 1998, DOI: 10.1103/PhysRevD.59.125016
Abstract: The local symmetry transformations of the quantum effective action for general gauge theory are found. Additional symmetries arise under consideration of background gauges. Together with "trivial" gauge transformations, vanishing on mass shell, they can be used for construction simple gauge generators. For example, for the Yang-Mills theory the classically invariant effective action is obtained, reproducing DeWitt's result. For rank one theories a natural generalization is proposed.
Bubble Statistics and Dynamics in Double-Stranded DNA
B. S. Alexandrov,L. T. Wille,K. O. Rasmussen,A. R. Bishop,K. B. Blagoev
Physics , 2006, DOI: 10.1103/PhysRevE.74.050901
Abstract: The dynamical properties of double-stranded DNA are studied in the framework of the Peyrard-Bishop-Dauxois model using Langevin dynamics. Our simulations are analyzed in terms of two probability functions describing coherently localized separations ("bubbles") of the double strand. We find that the resulting bubble distributions are more sharply peaked at the active sites than found in thermodynamically obtained distributions. Our analysis ascribes this to the fact that the bubble life-times significantly afects the distribution function. We find that certain base-pair sequences promote long-lived bubbles and we argue that this is due to a length scale competition between the nonlinearity and disorder present in the system.
Temperature behavior of NaI (Tl) scintillation detectors
K. D. Ianakiev,B. S. Alexandrov,P. B. Littlewood,M. C. Browne
Physics , 2006, DOI: 10.1016/j.nima.2009.02.019
Abstract: It is a familiar fact that the total measured light yield of NaI (Tl) detectors is a nonlinear function of temperature. Here we present new experimental data for the temperature behavior of doped NaI(Tl) scintillators that instead shows a linear dependence of light output over a wide temperature range- including that for outdoor applications. The shape of the light pulse shows in general two decay processes: a single dominant process above room temperature and two decay time constants below. We show that redistribution of the intensities is temperature-dependent; the second (slow) decay component is negligible at room temperatures, but, by -20C, it contributes up to 40 percents of the total light and has a duration of several microseconds. We discuss the profound effect this new understanding of the light output has on the pulse height analysis instrumentation. We introduce a theoretical model to explain the experimental results. In addition, we describe a unique technique for correcting both amplitude and shape temperature changes inside the NaI(Tl) detector package.
DNA Dynamics Is Likely to Be a Factor in the Genomic Nucleotide Repeats Expansions Related to Diseases
Boian S. Alexandrov,Vlad I. Valtchinov,Ludmil B. Alexandrov,Vladimir Gelev,Yossi Dagon,Jonathan Bock,Isaac S. Kohane,Kim ?. Rasmussen,Alan R. Bishop,Anny Usheva
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0019800
Abstract: Trinucleotiderepeats sequences (TRS) represent a common type of genomic DNA motif whose expansion is associated with a large number of human diseases. The driving molecular mechanisms of the TRS ongoing dynamic expansion across generations and within tissues and its influence on genomic DNA functions are not well understood. Here we report results for a novel and notable collective breathing behavior of genomic DNA of tandem TRS, leading to propensity for large local DNA transient openings at physiological temperature. Our Langevin molecular dynamics (LMD) and Markov Chain Monte Carlo (MCMC) simulations demonstrate that the patterns of openings of various TRSs depend specifically on their length. The collective propensity for DNA strand separation of repeated sequences serves as a precursor for outsized intermediate bubble states independently of the G/C-content. We report that repeats have the potential to interfere with the binding of transcription factors to their consensus sequence by altered DNA breathing dynamics in proximity of the binding sites. These observations might influence ongoing attempts to use LMD and MCMC simulations for TRS–related modeling of genomic DNA functionality in elucidating the common denominators of the dynamic TRS expansion mutation with potential therapeutic applications.
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