Fluctuations about the Fubini-Lipatov instanton for false vacuum decay in classically scale invariant models

For a scalar theory whose classical scale invariance is broken by quantum effects, we compute self-consistent bounce solutions and Green's functions. Deriving analytic expressions, we find that the latter are similar to the Green's functions in the archetypal thin-wall model for tunneling between quasi-degenerate vacua. The eigenmodes and eigenspectra are, however, very different. Large infrared effects from the modes of low angular momentum j = 0 and j = 1, which include the approximate dilatational modes for j = 0, are dealt with by a resummation of one loop effects. For a parametric example, this resummation is carried out numerically

Magnetoresistance in semiconductor structures with hopping conductivity: effects of random potential and generalization for the case of acceptor states

We reconsider the theory of magnetoresistance in hopping semiconductors. First, we have shown that the random potential of the background impurities affects significantly preexponential factor of the tunneling amplitude which becomes to be a short-range one in contrast to the long-range one for purely Coulomb hopping centers. This factor to some extent suppresses the negative interference magnetoresistance and can lead to its decrease with temperature decrease which is in agreement with earlier experimental observations. We have also extended the theoretical models of positive spin magnetoresistance, in particular, related to a presence of doubly occupied states (corresponding to the upper Hubbard band) to the case of acceptor states in 2D structures. We have shown that this mechanism can dominate over classical wave-shrinkage magnetoresistance at low temperatures. Our results are in semi-quantitative agreement with experimental data.Comment: 19 pages, 3 figure

Climate drove the fire cycle and humans influenced fire occurrence in the East European boreal forest

Understanding long-term forest fire histories of boreal landscapes is instrumental for parameterizing climate-fire interactions and the role of humans affecting natural fire regimes. The eastern sections of the European boreal zone currently lack a network of annually resolved and centuries-long forest fire histories. To fill in this knowledge gap, we dendrochronologically reconstructed the 600-year fire history of a middle boreal pine-dominated landscape of the southern part of the Republic of Komi, Russia. We combined the reconstruction of fire cycle (FC) and fire occurrence with the data on the village establishment and climate proxies and discussed the relative contribution of climate versus human land use in shaping historic fire regimes. Over the 1340-1610 ce period, the territory had a FC of 66 years (with the 90% confidence envelope of 56.8 and 78.6 years). Fire activity increased during the 1620-1730 ce period, with the FC reaching 32 years (31.0-34.7 years). Between 1740-1950, the FC increased to 47 years (41.9-52.0). The most recent period, 1960-2010, marks FC's historic maximum, with the mean of 153 years (102.5-270.3). Establishment of the villages, often as small harbors on the Pechora River, was associated with a non-significant increase in fire occurrence in the sites nearest the villages (p = 0.07-0.20). We, however, observed a temporal association between village establishment and fire occurrence at the scale of the whole studied landscape. There was no positive association between the former and the FC. In fact, we documented a decline in the area burned, following the wave of village establishment during the second half of the 1600s and the first half of the 1700s. The lack of association between the dynamics of FC and the dates of village establishments, and the significant association between large fire years and the early and latewood pine chronologies, used as historic drought proxy, indirectly suggests that the climate was the primary control of the landscape-level FCs in the studied forests. Pine-dominated forests of the Komi Republic may hold a unique position as the ecosystem with the shortest history of human-related shifts in fire cycles across the European boreal region

Yang-Mills- and D-instantons

In these lectures, which are written at an elementary and pedagogical level, we discuss general aspects of (single) instantons in SU(N_c) Yang-Mills theory, and then specialize to the case of N = 4 supersymmetry and the large N_c limit. We show how to determine the measure of collective coordinates and compute instanton corrections to certain correlation functions. We then relate this to D-instantons in type IIB supergravity. By taking the D-instantons to live in an $AdS_5\times S^5$ background, we perform explicit checks of the AdS/CFT correspondence.Comment: 62 pages, typos corrected, table of contents and references adde

TALL-HERB BOREAL FORESTS ON NORTH URAL

Background. One of the pressing aims of today’s natural resource management is its re-orientation to preserving and restoring ecological functions of ecosystems, among which the function of biodiversity maintenance plays an indicator role. The majority of today’s forests have not retained their natural appearance as the result of long-standing human impact. In this connection, refugia studies are becoming particularly interesting, as they give us an insight into the natural appearance of forests. Materials and methods. Studies were performed in dark conifer forests of the Pechora–Ilych reserve, in the lower reaches of the Bol’shaya Porozhnyaya River in 2013 yr. Vegetation data sampling was done at 50 temporary square plots of a fixed size (100 m2) randomly placed within a forest type. A list of plant species with species abundance was made for each forest layer. The overstorey (or tree canopy layer) was denoted by the Latin letter A. The understorey layer (indicated by the letter B) included tree undergrowth and tall shrubs. Ground vegetation was subdivided into the layers C and D. Layer C (field layer) comprised the herbaceous species (herbs, grasses, sedges) and dwarf shrubs together with low shrubs, tree and shrub seedlings. The height of the field layer was defined by the maximal height of the herbaceous species, ferns, and dwarf shrubs; the height varied from several cm to more than 200 cm in the ‘tall-herb’ forest types. Layer D (bottom layer) included cryptogamic species (bryophytes and lichens). Species abundance in the each layer was usually assessed using the Braun-Blanquet cover scale (Braun-Blanquet 1928). The nomenclature used follows Cherepanov’s (1995) for vascular plants, and Ignatov & Afonina’s (1992). Results. The present article contains descriptions of unique tall-herb boreal forests of European Russia preserved in certain refugia which did not experience prolonged anthropogenic impact or any other catastrophes. Comparative research into species and ecological diversity of typical (anthropogenically transformed) and unique (tall-herb) boreal forests has been conducted. On the basis of the collected field data, a map of the diffuse area for tall-herb boreal forests has been compiled and a set of species characteristic for these forests has been determined. The obtained data fundamentally change our notions of potential vegetation in boreal forests. Conclusions. Considerable species- and ecological diversity of tall-herb forest flora fundamentally changes our notion of the appearance of European boreal forests and determines their unique role in maintaining the highest possible level of biodiversity. The presence of tall-herb forests in various parts of eastern European taiga together with Eurasian habitats of most tall-herb species lead us to a suggestion that it is exactly this type of forests that represented the prehistoric boreal forests. In this connection, further research into still preserved fragments of tall-herb forests within the boundaries of northern Eurasia acquires huge significance. This research will help put forward systems of forest management aimed at restoring potential biodiversity of boreal forests in general