Hydrobiological service for observations and control of surface waters in the USSR

Technological progress, having reached in our time an unprecedented speed, is still increasing the rate of mineral extraction, industrial construction, and the mastering of new kinds of energy is growing. Correspondingly the anthropogenic load on the biosphere is increased and that requires the comprehensive development of monitoring the anthropogenic changes in the natural environment. Among problems resulting from the scientific-technological development, a noticeable place is given to the problem of pure water. Surface land waters proved to be a sensitive link in the natural environment. The hydrobiological service for observations and control of the surface waters is one of the subsystems of the State/Federal Service for Observations and Control of pollution levels in environmental objects, conducted by the USSR State Committee for Hydrometeor- ology and Control of the Natural Environment. This paper summarises the the main principles of the organisation and goals of the national system of monitoring of the state of the natural environment in the USSR

Theory of transient spectroscopy of multiple quantum well structures

A theory of the transient spectroscopy of quantum well (QW) structures under a large applied bias is presented. An analytical model of the initial part of the transient current is proposed. The time constant of the transient current depends not only on the emission rate from the QWs, as is usually assumed, but also on the subsequent carrier transport across QWs. Numerical simulation was used to confirm the validity of the proposed model, and to study the transient current on a larger time scale. It is shown that the transient current is influenced by the nonuniform distribution of the electric field and related effects, which results in a step-like behavior of the current. A procedure of extraction of the QW emission time from the transient spectroscopy experiments is suggested.Comment: 5 pages, 4 figures, to be published in J. Appl. Phy

Frustrated square lattice with spatial anisotropy: crystal structure and magnetic properties of PbZnVO(PO4)2

Crystal structure and magnetic properties of the layered vanadium phosphate PbZnVO(PO4)2 are studied using x-ray powder diffraction, magnetization and specific heat measurements, as well as band structure calculations. The compound resembles AA'VO(PO4)2 vanadium phosphates and fits to the extended frustrated square lattice model with the couplings J(1), J(1)' between nearest-neighbors and J(2), J(2)' between next-nearest-neighbors. The temperature dependence of the magnetization yields estimates of averaged nearest-neighbor and next-nearest-neighbor couplings, J(1) ~ -5.2 K and J(2) ~ 10.0 K, respectively. The effective frustration ratio alpha=J(2)/J(1) amounts to -1.9 and suggests columnar antiferromagnetic ordering in PbZnVO(PO4)2. Specific heat data support the estimates of J(1) and J(2) and indicate a likely magnetic ordering transition at 3.9 K. However, the averaged couplings underestimate the saturation field, thus pointing to the spatial anisotropy of the nearest-neighbor interactions. Band structure calculations confirm the identification of ferromagnetic J(1), J(1)' and antiferromagnetic J(2), J(2)' in PbZnVO(PO4)2 and yield J(1)'-J(1) ~ 1.1 K in excellent agreement with the experimental value of 1.1 K, deduced from the difference between the expected and experimentally measured saturation fields. Based on the comparison of layered vanadium phosphates with different metal cations, we show that a moderate spatial anisotropy of the frustrated square lattice has minor influence on the thermodynamic properties of the model. We discuss relevant geometrical parameters, controlling the exchange interactions in these compounds, and propose a new route towards strongly frustrated square lattice materials.Comment: 14 pages, 9 figures, 5 table

Soil formation in technogenic landscapes: trends, results, and representation in the current classifications (Review)

For hundreds of years, humans have been a soil formation factor. With the recent industrial development of vast territories, the formation of soils in technogenic and postanthropogenic conditions requires more attention. This study reviews the literature on the soils of human-transformed or human-made landscapes (technogenic landscapes),in which soil formation starts on a new technogenic substrate. Such soils may occur in different bioclimatic conditions. We focused on processes that govern soil morphology and the subsequent transformation of these soils. Often, the soils of technogenic landscapes are characterized by high bulk density values and by the presence of dense contact. Their properties are affected mainly by organic matter accumulation (humus, litter, and peat). The paper also covers approaches to the reclamation of technogenic landscapes, the main stages, and partly the reclamation options. It is noted that the efficiency of reclamation activities depends on the available resources and timely decision-making. We assessed the efficiency of soil reclamation methods and suggested technogenic landscape survey techniques. The major approaches to soil classification in technogenic landscapes in national and international soil classification systems are briefly discussed, and an approximate correlation of soil names used in different systems is suggested. All considered classifications provide the opportunity to assess the soil properties and specifics of soil formation in technogenic landscapes. However, in most studies, the soil diagnostics are limited to top-order taxa only

Interplay of atomic displacements in the quantum magnet (CuCl)LaNb2O7

We report on the crystal structure of the quantum magnet (CuCl)LaNb2O7 that was controversially described with respect to its structural organization and magnetic behavior. Using high-resolution synchrotron powder x-ray diffraction, electron diffraction, transmission electron microscopy, and band structure calculations, we solve the room-temperature structure of this compound [alpha-(CuCl)LaNb2O7] and find two high-temperature polymorphs. The gamma-(CuCl)LaNb2O7 phase, stable above 640K, is tetragonal with a(sub) = 3.889 A, c(sub) = 11.738 A, and the space group P4/mmm. In the gamma-(CuCl)LaNb2O7 structure, the Cu and Cl atoms are randomly displaced from the special positions along the {100} directions. The beta-phase [a(sub) x 2a(sub) x c(sub), space group Pbmm] and the alpha-phase [2a(sub) x 2a(sub) x c(sub), space group Pbam] are stable between 640 K and 500 K and below 500 K, respectively. The structural changes at 500 K and 640 K are identified as order-disorder phase transitions. The displacement of the Cl atoms is frozen upon the gamma --> beta transformation, while a cooperative tilting of the NbO6 octahedra in the alpha-phase further eliminates the disorder of the Cu atoms. The low-temperature alpha-(CuCl)LaNb2O7 structure thus combines the two types of the atomic displacements that interfere due to the bonding between the Cu atoms and the apical oxygens of the NbO6 octahedra. The precise structural information resolves the controversy between the previous computation-based models and provides the long-sought input for understanding the magnetic properties of (CuCl)LaNb2O7.Comment: 12 pages, 10 figures, 5 tables; crystallographic information (cif files) include

Capture of carriers to screened charged centres and low temperature shallow impurity electric field break down in semiconductors

Free carrier capture by a screened Coulomb potential in semiconductors are considered. It is established that with decreasing screening radius the capture cross section decreases drastically, and it goes to zero when $$. On the basis of this result a new mechanism of shallow impurity electric field break down in semiconductors is suggested.Comment: 8 pages, latex, 1 figure in gif format, to be submitted to "Journal of Condensed Matter

Tunneling spin-galvanic effect

It has been shown that tunneling of spin-polarized electrons through a semiconductor barrier is accompanied by generation of an electric current in the plane of the interfaces. The direction of this interface current is determined by the spin orientation of the electrons, in particular the current changes its direction if the spin orientation changes the sign. Microscopic origin of such a 'tunneling spin-galvanic' effect is the spin-orbit coupling-induced dependence of the barrier transparency on the spin orientation and the wavevector of electrons.Comment: 3 pages, 2 figure

Spiral ground state against ferroelectricity in the frustrated magnet BiMnFe2O6

The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are investigated by low-temperature neutron powder diffraction and density functional theory band structure calculations. In spite of the random distribution of the Mn3+ and Fe3+ cations, this compound undergoes a transition into an incommensurate antiferromagnetically ordered state below TN ~ 220 K. The magnetic structure is characterized by the propagation vector k=[0,beta,0] with beta ~ 0.14 and the P22_12_11'(0 \beta 0)0s0s magnetic superspace symmetry. It comprises antiferromagnetic helixes propagating along the b-axis. The magnetic moments lie in the ac plane and rotate about pi*(1+beta) ~ 204.8 deg angle between the adjacent magnetic atoms along b. The spiral magnetic structure arises from the peculiar frustrated arrangement of exchange couplings in the ab plane. The antiferromagnetic coupling along the c-axis leads to the cancellation of electric polarization, and results in the lack of ferroelectricity in BiMnFe2O6.Comment: 11 pages, 8 figures, 8 table

Molecular and elemental composition of humic acids isolated from selected soils of the Russian Arctic

Humic substances, isolated from selected soils of the Russian Arctic, were ,+investigated in terms of molecular composition and stabilization rate. The degree of polar soil organic matter stabilization was assessed with the use of modern instrumental spectroscopy methods. The analysis of humic acid (HAs) preparations showed that aliphatic fragments prevail in the organic matter isolated in polar soils. The predominance of aliphatic fragments was revealed in HAs from soils located in the coastal zone, which could be caused by regular refreshment of organic matter during sin-lithogenic process and processes of hydrogenation in HAs. Breaking of the C-C bonds and formation of chains with a high hydrogen content, which leads to the formation of aliphatic fragments in HAs, were noted. Data on the calculated atomic ratios of the elements in HAs are given and graphs show the main regularities in the formation of HAs and their properties. The integrated indicators of the molecular composition of humic acids of soils of the Russian Arctic are presented. The paper contains 4 Tables, 4 Figures and 44 References