The propagating front of the particle population in branching random walk
Plusnin Juri M.
A new “modus vivendi” against the megapolis
We reinvestigate the applicability of the concept of trap-free carrier transport in molecularly doped polymers and the possibility of realistically describing time-of-flight (TOF) current transients in these materials using the classical convection–diffusion equation (CDE). The problem is treated as rigorously as possible using boundary conditions appropriate to conventional time of flight experiments. Two types of pulsed carrier generation are considered. In addition to the traditional case of surface excitation, we also consider the case where carrier generation is spatially uniform. In our analysis, the front electrode is treated as a reflecting boundary, while the counter electrode is assumed to act either as a neutral contact (not disturbing the current flow) or as an absorbing boundary at which the carrier concentration vanishes. As expected, at low fields transient currents exhibit unusual behavior, as diffusion currents overwhelm drift currents to such an extent that it becomes impossible to determine transit times (and hence, carrier mobilities). At high fields, computed transients are more like those typically observed, with well-defined plateaus and sharp transit times. Careful analysis, however, reveals that the non-dispersive picture, and predictions of the CDE contradict both experiment and existing disorder-based theories in important ways, and that the CDE should be applied rather cautiously, and even then only for engineering purposes.
This chapter discusses the features of the development of the Russian population and the results of population projections to 2025
Many important demographic indices have changed dramatically in Russia during the last 10 years. The total fertility rate decreased from 2.20 to 1.28 between 1987 and 1996. The life expec-tancy for males fell by more than 7 years between 1987 and 1994, and for females by more than 3 years; after this, life expectancy began to rise again, but by 1997 the loss for males was re-stored by only 45% and for females by 60%. The natural increase of population became negative in 1992 and despite positive net migration the population of Russia began to decline. During 1992-1997 it decreased by 1.6 million persons. The population decline will continue and, accord-ing to varying forecasts, the total losses can reach from 2.4 to 12.8 million persons by 2010.
What all this means is that Russia is experiencing a serious demographic crisis. With respect to the level of mortality and its trends, Russia is in a very poor position compared to all the Western industrial countries.
The growth, or at least the stabilization, of the size of the Russian population during the first dec-ades of the 21st century will be possible only on the condition that net migration be positive for Russia and of significant proportion. But taking into consideration economic and political reali-ties, it is unlikely that this condition will be realized. It is more probable that the size of the Rus-sian population will decline.
The paper looks into the dynamics of the population size of Russia, Ukraine and Belarus after the census of 1989. Regions and cities of these countries were the focus of the research (territorial units level NUTS-3). The analysis addresses the question to what degree the remoteness from the regional centre, i.e. the position in the core-periphery system, influences the dynamics of the population size of the territorial units of the given level. For the analytical purposes the distinction has been made between the regional centres including adjacent suburban areas and internal regional periphery comprising districts and cities. The main indicator employed was the distance between the periphery areas and regional centres. The results of the analysis show that in spite of the depopulation of all three countries and severe transformational crisis, there was a steady growth of the population size in the regional centres, while the periphery areas of the regions continued to lose the population. The mentioned differences are primarily determined by migration flows, since the fertility rates are below the replacement level in all the countries’ territories. Population tends to concentrate in the regional centres, which means urbanisation has not been completed yet. While similar patterns of population decline are observed in the periphery areas of Ukraine and Belarus, in Russia the depopulation rates are negatively influenced by the factor of remoteness of a periphery area from the regional centre. All three countries experienced rural population decline everywhere but suburban areas of the regional centres.