Low-temperature kinetics of exciton-exciton annihilation of weakly localized one-dimensional Frenkel excitons

We present results of numerical simulations of the kinetics of exciton-exciton annihilation of weakly localized one-dimensional Frenkel excitons at low temperatures. We find that the kinetics is represented by two well-distinguished components: a fast short-time decay and a very slow long-time tail. The former arises from excitons that initially reside in states belonging to the same localization segment of the chain, while the slow component is caused by excitons created on different localization segments. We show that the usual bi-molecular theory fails in the description of the behavior found. We also present a qualitative analytical explanation of the non-exponential behavior observed in both the short- and the long-time decay components.Comment: Published in J. Chem. Phys. 114, 1 April (2001

Molecular analysis of sensory axon branching unraveled a cGMP-dependent signaling cascade

Axonal branching is a key process in the establishment of circuit connectivity within the nervous system. Molecular-genetic studies have shown that a specific form of axonal branching - the bifurcation of sensory neurons at the transition zone between the peripheral and the central nervous System - is regulated by a cyclic guanosine monophosphate (cGMP)-dependent signaling cascade which is composed of C-type natriuretic peptide (CNP), the receptor guanylyl cyclase Npr2, and cGMP-dependent protein kinase Iα (cGKIα). In the absence of any one of these components, neurons in dorsal root ganglia (DRG) and cranial sensory ganglia no longer bifurcate, and instead turn in either an ascending or a descending direction. In contrast, collateral axonal branch formation which represents a second type of axonal branch formation is not affected by inactivation of CNP, Npr2, or cGKI. Whereas axon bifurcation was lost in mouse mutants deficient for components of CNP-induced cGMP formation; the absence of the cGMP-degrading enzyme phosphodiesterase 2A had no effect on axon bifurcation. Adult mice that lack sensory axon bifurcation due to the conditional inactivation of Npr2-mediated cGMP signaling in DRG neurons demonstrated an altered shape of sensory axon terminal fields in the spinal cord, indicating that elaborate compensatory mechanisms reorganize neuronal circuits in the absence of bifurcation. On a functional level, these mice showed impaired heat sensation and nociception induced by chemical irritants, whereas responses to cold sensation, mechanical stimulation, and motor coordination are normal. These data point to a critical role of axon bifurcation for the processing of acute pain perception

CEMENT SELECTION IN DENTAL PRACTICE

The cementation procedure is considered to be the most important stage of fixed prosthodontics, while the correct selection of cement is the guarantee of successful restoration which is conditioned by the its durability. Over the recent years, numerous cementing substances have been introduced to the dental practice that differ greatly from conventional cements with their properties and application methods and that is why even experienced dentists often have certain difficulties in the variety of cements to choose the one that is right for each clinical case. The selection of cement depends on a number of factors, such as the type of resorption substance, the shape of prepared tooth, the possibility to isolate the area, subject to cementation in the oral cavity as well as the patient’s aesthetic requirements. Thus, the objective of the article is to analyze currently used dental cements in order to help the dentists make the right selection of cement for different clinical cases.The cementation procedure is considered to be the most important stage of fixed prosthodontics, while the correct selection of cement is the guarantee of successful restoration which is conditioned by the its durability. Over the recent years, numerous cementing substances have been introduced to the dental practice that differ greatly from conventional cements with their properties and application methods and that is why even experienced dentists often have certain difficulties in the variety of cements to choose the one that is right for each clinical case. The selection of cement depends on a number of factors, such as the type of resorption substance, the shape of prepared tooth, the possibility to isolate the area, subject to cementation in the oral cavity as well as the patient’s aesthetic requirements. Thus, the objective of the article is to analyze currently used dental cements in order to help the dentists make the right selection of cement for different clinical cases

Sequential superradiant scattering from atomic Bose-Einstein condensates

We theoretically discuss several aspects of sequential superradiant scattering from atomic Bose-Einstein condensates. Our treatment is based on the semiclassical description of the process in terms of the Maxwell-Schroedinger equations for the coupled matter-wave and optical fields. First, we investigate sequential scattering in the weak-pulse regime and work out the essential mechanisms responsible for bringing about the characteristic fan-shaped side-mode distribution patterns. Second, we discuss the transition between the Kapitza-Dirac and Bragg regimes of sequential scattering in the strong-pulse regime. Finally, we consider the situation where superradiance is initiated by coherently populating an atomic side mode through Bragg diffraction, as in studies of matter-wave amplification, and describe the effect on the sequential scattering process.Comment: 9 pages, 4 figures. Submitted to Proceedings of LPHYS'06 worksho

Investigation of pulse shape neutron-gamma discrimination

The role of neutron beam investigation is significant not only for fundamental science but also for various fields of applied science. This work is dedicated to the formation of neutron beams using the external 18-MeV proton beam of IBA cyclotron C18/18 with a beam current of up to 100 µA. The facility is located at the A. Alikhanyan National Science Laboratory (Yerevan Physics Institute). The possibility to obtain thermal or epithermal neutron beams using the external proton beam of the cyclotron is studied using Geant4 simulations. In this case, a quasimonoenergetic neutron source 9Be (p, n)9B reaction is chosen. As a result of the simulations, the optimal thickness of the 9Be beryllium isotope target is determined. The induced neutron beam is accompanied by a gamma ray background. To decrease the number of accompanying gamma rays, the lead absorber is considered. As a method of separating neutrons from gamma rays, the pulse shape discrimination (PSD) technique is developed. This study shows the possibility of neutron-gamma PSD and its applicability using the EJ-299-33A plastic scintillator

Modeling of laser-induced plasmon effects in GNS-DLC-based material for application in X-ray source array sensors

An important direction in the development of X-ray computed tomography sensors in systems with increased scanning speed and spatial resolution is the creation of an array of miniature current sources. In this paper, we describe a new material based on gold nanostars (GNS) embedded in nanoscale diamond-like carbon (DLC) films (thickness of 20 nm) for constructing a pixel current source with photoinduced electron emission. The effect of localized surface plasmon resonance in GNS on optical properties in the wavelength range from UV to near IR, peculiarities of localization of field and thermal sources, generation of high-energy hot electrons, and mechanisms of their transportation in vacuum are investigated. The advantages of the proposed material and the prospects for using X-ray computed tomography in the matrix source are evaluated

A Femtosecond Neutron Source

The possibility to use the ultrashort ion bunches produced by circularly polarized laser pulses to drive a source of fusion neutrons with sub-optical cycle duration is discussed. A two-side irradiation of a thin foil deuterated target produces two countermoving ion bunches, whose collision leads to an ultrashort neutron burst. Using particle-in-cell simulations and analytical modeling, it is evaluated that, for intensities of a few $10^{19} W cm^{-2}$, more than $10^3$ neutrons per Joule may be produced within a time shorter than one femtosecond. Another scheme based on a layered deuterium-tritium target is outlined.Comment: 15 pages, 3 figure

Statistics of low-energy levels of a one-dimensional weakly localized Frenkel exciton: A numerical study

Numerical study of the one-dimensional Frenkel Hamiltonian with on-site randomness is carried out. We focus on the statistics of the energy levels near the lower exciton band edge, i. e. those determining optical response. We found that the distribution of the energy spacing between the states that are well localized at the same segment is characterized by non-zero mean, i.e. these states undergo repulsion. This repulsion results in a local discrete energy structure of a localized Frenkel exciton. On the contrary, the energy spacing distribution for weakly overlapping local ground states (the states with no nodes within their localization segments) that are localized at different segments has zero mean and shows almost no repulsion. The typical width of the latter distribution is of the same order as the typical spacing in the local discrete energy structure, so that this local structure is hidden; it does not reveal itself neither in the density of states nor in the linear absorption spectra. However, this structure affects the two-exciton transitions involving the states of the same segment and can be observed by the pump-probe spectroscopy. We analyze also the disorder degree scaling of the first and second momenta of the distributions.Comment: 10 pages, 6 figure

Results of Field and Laboratory Studies of Carriers and Vectors of Natural-Focal Infections on the Territory of the Republic of Armenia

The purpose of the study was to assess the current epizootic potential of the Transcaucasian high-mountain and Pre-Araks low-mountain natural plague foci on the territory of the Republic of Armenia using GIS technologies. Materials and methods. We used the data from an epizootiological survey, records of the abundance and species composition, spatial distribution of rodents and ectoparasites in the plague-enzootic territories of the Republic of Armenia in 2021. Results and discussion. Based on the results of the research, an electronic database of carriers and vectors of pathogens of natural-focal zoonotic infections in the plague-enzootic territories of the Republic of Armenia has been created. Applying GIS technologies, an assessment of the spatial distribution of carriers and vectors of plague has been made and areas of circulation of tularemia and leptospirosis pathogens identified. The results obtained serve as the basis for improving the efficiency of planning and carrying out preventive measures aimed at ensuring the epidemiological welfare as regards natural-focal infectious diseases in the territory of the Republic of Armenia

Superradiance from an ultrathin film of three-level V-type atoms: Interplay between splitting, quantum coherence and local-field effects

We carry out a theoretical study of the collective spontaneous emission (superradiance) from an ultrathin film comprised of three-level atoms with $V$-configuration of the operating transitions. As the thickness of the system is small compared to the emission wavelength inside the film, the local-field correction to the averaged Maxwell field is relevant. We show that the interplay between the low-frequency quantum coherence within the subspace of the upper doublet states and the local-field correction may drastically affect the branching ratio of the operating transitions. This effect may be used for controlling the emission process by varying the doublet splitting and the amount of low-frequency coherence.Comment: 15 pages, 5 figure