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The sphere of tourism can be safely attributed to the category of unique in efficiency industrial complexes that can help solve economic anti-crisis problems. Tourist activity has a significant impact on economic growth, which is manifested in the improvement of entrepreneurship, the scale of production of products and services, the growth of effective demand. The article considers the concept of strategic management in the tourism industry. In the course of research the stages of a cycle of strategic management of the tourist company are defined on the basis. The constituent elements of strategic planning as a basis of strategic management are considered. Peculiarities of strategic management at tourist enterprises are determined. The article identifies current problems of management of tourist enterprises in Ukraine. It is noted that the success of the tourist enterprises of Ukraine depends on an effective system of management and coordination of all components of tourist attractiveness: tourist resources, infrastructure, investment and quality service. It is offered to consider the management system of the tourist enterprise in dynamics which interacts with such important elements, as: tourist resources, services, production, external environment and the factors influencing development of tourist sphere, the purposes of development of system of management of tourism. The key characteristics of the process of realization of the tourist enterprise management system are given. In today’s global economic change and the crisis in the tourism industry, tourism companies should apply strategic management, which includes a program of action to identify specific goals and means of implementing the chosen path of development. The strategic management of tourism enterprises should be based on strategic planning, which will provide an opportunity to predict the future development of production of goods and services, promising projects, personnel and financial activities on a planned basis. Key words: strategic management, management system, planning, strategy, tourism company.

Based on the study of existing practice and specific examples of the use of innovative blockchain technology in real estate transactions and its management, the article gives the basic concepts of this technology, analyzes foreign and domestic experience of its application, examines its sphere of use in the real estate market, substantiates main advantages and shortcomings, as well as the process of various mechanisms for financing real estate transactions. It was revealed that when using the blockchain system, all real estate transactions proceed faster, safer and cheaper. Thus, the objectives of the article include studying and summarizing the Russian and foreign practices of using blockchain technology, as well as determining the main ways and options for its further development and improvement, as a highly effective and promising financial and economic mechanism for participants in real estate transactions.

Defect signatures, such as deep level positions, hyperfine parameters, local vibrational modes and optical transitions characterize a defect and enable the identification of defect centers. This identification is a key to an understanding of complex phenomena like the defect kinetics. Albeit density functional theory enabled the identification of several defects and their kinetic properties, a new approach is needed to address the optical excitation of defect. Within a quasiparticle theory and taking into account excitonic effects we analyze the excited states of VC +.

In SiC, mobile point defects may form thermally stable clusters and aggregates, such as di-vacancies or carbon interstitial complexes. Although predicted by theory, experimental evidence of such clusters became available only recently. Combining theoretical and experimental approaches, the unique identification of the di-vacancy, the carbon vacancy-antisite complex with the spin resonance centers P6/P7 and SI5 was recently achieved. In this way also the di-carbon and tri-carbon antisites with the photoluminiscence centers P–T and U, HT3 and HT4, respectively were identified. The two identified vacancy complexes show distinct properties: while the di-vacancy, like the silicon vacancy possesses a high-spin ground state, the carbon vacancy–antisite complex, like the carbon vacancy, is a Jahn–Teller center. These effects consistently explain the complex properties of the spin resonance spectra and are discussed in detail for the isolated vacancies. The aggregation of vacancies proved to be relevant in the explantation of the kinetic deactivation of nitrogen in co-implanted SiC. This and further evidence for defect aggregates underline the relevance of this notion. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Despite the ongoing “graphene boom” of the last three years our understanding of epitaxial graphene grown on SiC substrate is only beginning to emerge. Along with experimental methods such as low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES), ab initio calculations help to uncover the geometric and electronic structure of the graphene/SiC interface. In this chapter we describe the density-functional calculations we performed for single and double graphene layers on Si- and C-terminated 6H-SiC surfaces. Experimental data reveal a pronounced difference between the two surface terminations. On a Si-terminated surface the interface adopts a 6√3 × 6√3 unit cell whereas the C-face supports misoriented (turbostratic) graphene layers. It has been recently realized that, on the Si-face, the large commensurate cell is subdivided into patches of coherently matching to the substrate carbon atoms. In our calculations we assumed the “coherent match” geometry for the whole interface plane. This reduces the periodic unit to the √3 × √3R 30° cell but requires a substantial stretching of the graphene sheet. Although simplified, the model provides a qualitative picture of the bonding and of the interface electron energy spectrum. We find that the covalent bonding between the carbon layer and the substrate destroys the massless “relativistic” electron energy spectrum, the hallmark of a freestanding graphene. Hence the first carbon layer cannot be responsible for the graphene-type electron spectrum observed by ARPES and rather plays a role of a buffer between the substrate and the subsequent carbon sheets. The “true” graphene spectrum appears with the second carbon layer which exhibits a weak van der Waals bonding to the underlying structure. For Si-terminated substrate, we find that the Fermi level is pinned by the interface state at 0.45 eV above the graphene Dirac point, in agreement with experimental data. This renders the interface metallic. On the contrary, for a C-face the “coherent match” model predicts the Fermi level exactly at the Dirac point. However, this does not necessarily apply to the turbostratic graphene layers that normally grow on the C-terminated substrate. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Employing density functional theory we investigate the model interface between 1 × 1-6H-SiC{0001} surfaces and graphene layers. We find that the first graphene layer is covalently bonded to the SiC substrate, opposing the earlier assumption of a weak van-der-Waals bonding. The interface at the Si-face is metallic, while on the C-face it remains semiconducting. Further graphene layers are then only weakly bound and the typical graphitic properties of the electronic structure appear.

Using density functional theory, we investigate the 6H-SiC{0001} surfaces in the unreconstructed 1 × 1 and the H-passivated configuration. The strong correlation effects of the dangling bonds at the surface are treated by spin-polarised calculations including the Hubbard-U parameter. We find that the clean surfaces are semiconducting with surface states in good agreement with experimental data. The impact of the Hubbard-U is stronger on the C-terminated face. For the H-passivated surfaces we find resonances in the valence band. The antibonding C−H state is located in the upper part of the bandgap around the ¯􀀀-point.

Nitrogen (N) donors in SiC are partially deactivated either by Si+-/N+-co-implantation or by irradiation with electrons of 200 keV energy and subsequent annealing at temperatures above 1450°C; simultaneously the compensation is decreased. The free electron concentration and the formation of energetically deep defects in the processed samples are determined by Hall effect and deep level transient spectroscopy. A detailed theoretical treatment based on the density functional theory is conducted; it takes into account the kinetic mechanisms for the formation of N interstitial clusters and (N-vacancy)-complexes. This analysis clearly indicates that the (NC)4-VSi complex, which is thermally stable up to high temperatures and which has no level in the band gap of 4HSiC, is responsible for the N donor deactivation.

Kinetic mechanisms for the deactivation of nitrogen are investigated by ab initio theory. We find that the interaction of nitrogen with self-interstitials can lead to a deactivation of nitrogen, yet it cannot explain the experimentally observed nitrogen deactivation at high temperatures in silicon co-implanted samples. Our analysis suggests the aggregation of vacancies at high temperatures and the subsequent formation of passive nitrogen-vacancy complexes as a likely explanation.

We observe new photoluminescence centers in electron-irradiated 6H-SiC with phonon replicas up to 250 meV and clear threefold isotope splitting of the highest energy mode. Based on ab initio calculations, we discuss the tri-carbon anti-site (C3)Si and the di-interstitial (C2)Hex as models for these centers.

Using an ab initio method we analyze the mechanisms of the boron diffusion with emphasis on the role of the intrinsic interstitials. It is shown that the boron diffusion is dominated by a kick-out mechanism. The different effect of silicon and carbon interstitials gives rise to kinetic effects. A preference for a kick-in of the boron interstitial into the carbon lattice sites is found. Kinetic effects reported in co-implantation experiments and in-diffusion experiments are explained by our findings.

The key factor determining nucleation processes and faceting in homoepitaxial growth as well as texture competition is the mobility of adatoms and small clusters across step edges and facets. Using a combination of molecular dynamics and ab initio calculations, we investigate the mechanisms of small clusters (dimer and trimer) diffusion down the aluminum (1 1 1) surface. In this paper we report results of molecular dynamics studies. Our study shows that the clusters dissociate at the step-edge of compact islands. As a result, the clusters diffuse down the step by an exchange mechanism with a small or medium Schwoebel barrier. The mechanism of this down-diffusion/dissociation is discussed and the corresponding energetics are calculated using the molecular statics method. We find a large anisotropy between the barriers at the two types of 〈1 1 0〉 oriented steps.

In photo luminescence experiments, the DII center shows strong carbon–like vibrational modes above the highest bulk phonon mode. We have performed ab initio DFT calculations of the localized vibrational modes (LVM) of defects in SiC possessing carbon–carbon bonds. Among these defects, only the carbon split interstitial–antisite complex is found to exhibit a LVM spectrum compatible with that of the DII center. The formation energy of this complex compares to the formation energy of the most abundant interstitial defects, namely the carbon split interstitial and the carbon–silicon split interstitial. We find a high binding energy of 3.6 eV and above.

Vacancy-mediated diffusion of substitutional impurities in Si drastically depends on the character of the impurity-vacancy interaction. The attractive interaction which extends to at least the third nearest neighbors allows a vacancy to move along a ring of Si sites around the impurity. This process enables the impurity-vacancy pair to move and underlies the ``ring mechanism'' of diffusion. To evaluate the importance of this mechanism for As impurity in Si we performed density functional calculations of the As-vacancy interaction. We also calculated the vacancy migration barriers on the circular path around the impurity. The binding energy of the As-$V$ pair was found to be 1.17 eV. This energy decreases down to 0.36 eV when the vacancy moves onto the third-nearest-neighbor site, which is the most remote site on the ring path. We found that the migration barriers for the vacancy hops around the impurity are an order of magnitude smaller than in a bulk Si. The effective barrier along the ring path is therefore mostly determined by the difference of the binding energies on the first- and the third-nearest-neighbor sites and was found to be about 0.81 eV, in a fairly good agreement with experiment. Using ab initio binding energies and migration barriers we carried out the Monte Carlo simulation of As diffusion in silicon. We found that in a temperature range between 700 K and 1300 K the vacancy-assisted As diffusion is totally dominated by the ring mechanism.

The n-type dopants phosphorus and nitrogen, and their complexes with intrinsic point defects are investigated in 4H-SiC by first-principles theory. The solubility and electrical activation of the dopants in thermodynamic equilibrium are calculated. For nitrogen, a saturation of the electrical activation above a certain critical concentration is found that is driven by a preferential incorporation of nitrogen into electrically passive nitrogen-vacancy complexes. This explains the observations of recent experiments. An almost complete phosphorus activation is found up to the solubility limit. We suggest that the low phosphorus doping achieved by sublimation growth is related to the growth kinetics.

We apply the optimized effective potential (OEP) implementation of density functional theory (DFT) to the model system of interacting spinless electrons on a quantum ring. The ring encircles a magnetic flux that induces a persistent current. In a perfect rotationally invariant system the current does not depend on the electron-electron interaction (the latter is characterized by a standard dimensionless parameter r(S)) and hence is not sensitive to the microscopic structure of the electron correlated state. This changes, however, if a symmetry-breaking external potential is introduced or, in a realistic system, due to the crystal lattice potential (Hamer et al 1987 J. Phys. A: Math. Gen. 20 5677-93). In our model, we calculate the persistent current as a function of r(S) in the presence of a weak Gaussian-shaped 'impurity' potential. We find that while below a threshold value r(S)r(S)(c)≈2.05 the current is independent of r(S), it decays exponentially for r(S)r(S)(c). This signals the formation of an electron Wigner crystal pinned by the impurity potential. The electron density, homogeneous below r(S)(c), indeed shows a periodic modulation at r(S)r(S)(c). The modulation amplitude follows a (r(S)-r(S)(c))((1)/(2)) behaviour which is characteristic for a second-order phase transition, as expected in the mean-field-type DFT-OEP approach. Our calculation shows that the macroscopic current, which is a quantity directly accessible in DFT, can serve as an indicator of the formation of a correlated electron state.

The performance of three serological tests manufactured in Belarus for the diagnosis of syphilis, i.e. a microprecipitation reaction (MPR) and two enzyme-linked immunosorbent assays (ELISAs) were compared with internationally recognized assays, namely the rapid plasma reagin test and the Treponema pallidum passive particle agglutination assay (TPPA). Sera from 392 consecutive patients attending Brest (Belarus) regional dermatovenereological dispensaries were tested. The sensitivity of the MPR test was low (77.3%) compared with the rapid plasma reagin test, while the specificity was high (100%). In contrast, both Belarusian ELISAs performed well when compared with the TPPA (sensitivities of 99.2% and 100%, specificities of 98.7% and 99.0%, respectively). There is a clear need to improve the sensitivity of the existing Belarusian MPR test or to use a more sensitive screening test in order to improve diagnosis of the disease in Belarus.

4 páginas, 3 figuras, 2 tablas.-- PACS numbers: 71.35.-y, 71.55.-i, 76.30.Mi, We show that electron correlations control the photophysics of defects in SiC through both renormalization of the quasiparticle band structure and excitonic effects. We consider the carbon vacancy with two possible excitation channels that involve conduction and valence bands. Corrections to the Kohn-Sham ionization levels strongly depend on the defect charge state. Excitonic effects introduce a redshift of 0.23 eV. The analysis reassigns excitation mechanism at the thresholds in photoinduced paramagnetic resonance measurements, We acknowledge funding by the Deutsche Forschungsgemeinschaft (BO1851/2), Spanish MEC (FIS2007-65702-C02-01), ACI-promciona project (ACI2009-1036), and ‘‘Grupos Consolidados UPV/EHU del Gobierno Vasco’’ (IT-319-07), the European Community through e-I3 ETSF project (Contract No. 211956), and support by the Barcelona Supercomputing Center, ‘‘Red Espanola de Supercomputacion.’’

With the aim of describing real-time electron dynamics, we introduce an adiabatic approximation for the equation of motion of the one-body reduced-density matrix (one-matrix). The eigenvalues of the one-matrix, which represent the occupation numbers of single-particle orbitals, are obtained from the constrained minimization of the instantaneous ground state energy functional rather than from their dynamical equations. To clarify the motivation for this minimization condition, we discuss a sequence of adiabatic energy functionals, each obeying a minimum principle. The performance of the approximation vis-a`-vis nonadiabatic effects is assessed in real-time simulations for a two-site Hubbard model. Due to the presence of Landau-Zener-type transitions, the system evolves into a nonstationary state with persistent oscillations in the observables. The amplitude and phase of the oscillations exhibit resonance behavior both with respect to the strength of the electron-electron interaction and the rate of variation of the external potential. Both types of resonances have the same origin -- the interference of dynamical and scattering phases., Comment: 18 pages, 8 figures; thoroughly revised

We prove that if the two-body terms in the equation of motion for the one-body reduced density matrix are approximated by ground-state functionals, the eigenvalues of the one-body reduced density matrix (occupation numbers) remain constant in time. This deficiency is related to the inability of such an approximation to account for relative phases in the two-body reduced density matrix. We derive an exact differential equation giving the functional dependence of these phases in an interacting Landau-Zener model and study their behavior in short- and long-time regimes. The phases undergo resonances whenever the occupation numbers approach the boundaries of the interval [0,1]. In the long-time regime, the occupation numbers display correlation-induced oscillations and the memory dependence of the functionals assumes a simple form., Comment: 6 pages, revised, Fig. 2 added

A system of electrons in a local or nonlocal external potential can be described with one-matrix functional theory (1MFT), which is similar to density-functional theory (DFT) but takes the one-particle reduced density matrix (one-matrix) instead of the density as its basic variable. Within 1MFT, Gilbert derived [Phys. Rev. B 12, 2111 (1975)] effective single-particle equations analogous to the Kohn-Sham (KS) equations in DFT. The self-consistent solution of these 1MFT-KS equations reproduces not only the density of the original electron system but also its one-matrix. While in DFT it is usually possible to reproduce the density using KS orbitals with integer (0 or 1) occupancy, in 1MFT reproducing the one-matrix requires in general fractional occupancies. The variational principle implies that the KS eigenvalues of all fractionally occupied orbitals must collapse at self-consistency to a single level. We show that as a consequence of the degeneracy, the iteration of the KS equations is intrinsically divergent. Fortunately, the level-shifting method, commonly introduced in Hartree-Fock calculations, is always able to force convergence. We introduce an alternative derivation of the 1MFT-KS equations that allows control of the eigenvalue collapse by constraining the occupancies. As an explicit example, we apply the 1MFT-KS scheme to calculate the ground state one-matrix of an exactly solvable two-site Hubbard model.

We explore the consequences of a rotation between graphene layers for the electronic spectrum. We derive the commensuration condition in real space and show that the interlayer electronic coupling is governed by an equivalent commensuration in reciprocal space. The larger the commensuration cell, the weaker the interlayer coupling, with exact decoupling for incommensurate rotations and in the theta-->0 limit. Furthermore, from first-principles calculations we determine that even for the smallest possible commensuration cell the decoupling is effectively perfect, and thus graphene layers will be seen to decouple for all rotation angles.

We study the Kohn-Sham scheme for the calculation of the steady state linear response to a harmonic perturbation that is turned on adiabatically. Although in general the exact time dependent exchange-correlation potential cannot be expressed as the functional derivative of a universal functional due to the so-called causality paradox, we show that for a harmonic perturbation the exchange-correlation part of the first-order Kohn-Sham potential $v_s^{(1)}(r) \cos(\omega t)$ is given by $v_{xc}^{(1)}(r) = \delta K_{xc}^{(2)}/\delta n^{(1)}(r)$. $K_{xc}^{(2)}$ is the exchange-correlation part of the second-order quasienergy $K_v^{(2)}$. The Frenkel variation principle implies a stationary principle for the second-order quasienergy. We also find an analogous stationary principle and KS scheme in the time dependent extension of one-matrix functional theory, in which the basic variable is the one-matrix (one-body reduced density matrix)., Comment: 11 pages; minor corrections; details added

Employing density-functional calculations we study single and double graphene layers on Si- and C-terminated 1x1-6H-SiC surfaces. We show that, in contrast to earlier assumptions, the first carbon layer is covalently bonded to the substrate, and cannot be responsible for the graphene-type electronic spectrum observed experimentally. The characteristic spectrum of free-standing graphene appears with the second carbon layer, which exhibits a weak van der Waals bonding to the underlying structure. For Si-terminated substrate, the interface is metallic, whereas on C-face it is semiconducting or semimetallic for single or double graphene coverage, respectively., 4 pages, 3 figures, 1 table; Submitted to Physical Review Letters on 01/08/07

Recent ab initio calculations [Mattausch et al., Phys. Rev. B 70, 235211 (2004)] of carbon clusters in SiC reveal a possible connection between the tricarbon antisite (C_3)_Si and the U photoluminescence center in 6H-SiC [Evans et al., Phys. Rev. B 66, 35204 (2002)]. Yet, some of the predicted vibrational modes were not observed experimentally. Here we report experiments which indeed confirm the existence of a low-energy mode for the U-center (as well as for the HT3- and HT4-centers with spectral details similar to the U-center). We calculated the isotope splitting for the (C_3)_Si-defect and found near-perfect agreement with our data. In addition, we discuss the carbon di-interstitial (C_2)_Hex as a model for the Z- and HT5-centers. The isotope splitting is also well reproduced, but the absolute values of the local mode energies show a discrepancy of about 10 meV., 4 pages, 3 figures, to appear in Phys. Rev. B

The electronic, structural and vibrational properties of small carbon interstitial and antisite clusters are investigated by ab initio methods in 3C and 4H-SiC. The defects possess sizable dissociation energies and may be formed via condensation of carbon interstitials, e.g. generated in the course of ion implantation. All considered defect complexes possess localized vibrational modes (LVM's) well above the SiC bulk phonon spectrum. In particular, the compact antisite clusters exhibit high-frequency LVM's up to 250meV. The isotope shifts resulting from a_{13}C enrichment are analyzed. In the light of these results, the photoluminescence centers D_{II} and P-U are discussed. The dicarbon antisite is identified as a plausible key ingredient of the D_{II}-center, whereas the carbon split-interstitial is a likely origin of the P-T centers. The comparison of the calculated and observed high-frequency modes suggests that the U-center is also a carbon-antisite based defect., 15 pages, 6 figures, accepted by Phys. Rev. B

The interstitial and vacancy mediated boron diffusion in silicon carbide is investigated with an ab initio method. The boron interstitials in $p$-type and $n$-type materials are found to be far more mobile than the boron-vacancy complexes. A kick-out mechanism and an interstitialcy mechanism govern the diffusion in $p$-type/intrinsic and $n$-type material, respectively. A comparison of activation energies demonstrates that the equilibrium diffusion is dominated by the positive hexagonal interstitial for typical experimental conditions. The activation energy and the charge state is in agreement with experimental findings. The analysis of the kick-out reaction demonstrates that silicon and carbon interstitials have different effects on boron acceptors on the silicon and carbon sublattice (${\mathrm{B}}_{\mathrm{Si}}$ and ${\mathrm{B}}_{\mathrm{C}}$). While silicon interstitials mediate the diffusion of both acceptors, carbon interstitials are only relevant for ${\mathrm{B}}_{\mathrm{C}}$. A larger kick-in barrier into silicon sites is found than into carbon sites. This implies a dominant formation of ${\mathrm{B}}_{\mathrm{C}}$ in the extended diffusion tails of boron profiles. The stable boron complexes with carbon or boron interstitials are found that potentially reduce the boron diffusion. This and the characteristics of the kick-out mechanism facilitate an explanation of recent co-implantation experiments.

The annealing kinetics of mobile intrinsic defects in cubic SiC is investigated by an ab initio method based on density-functional theory. The interstitial-vacancy recombination, the diffusion of vacancies, and interstitials to defect sinks (e.g., surfaces or dislocations) as well as the formation of interstitial clusters are considered. The calculated migration and reaction barriers suggest a hierarchical ordering of competing annealing mechanisms. The higher mobility of carbon and silicon interstitials as compared to the vacancies drives the annealing mechanisms at lower temperatures including the vacancy-interstitial recombination and the formation of interstitial carbon clusters. These clusters act as a source of carbon interstials at elevated temperatures. In p-type material the transformation of the silicon vacancy into the more stable vacancy-antisite complex constitutes an annealing mechanism which is activated before the vacancy migration. Recent annealing studies of vacancy-related centers in irradiated 3C-SiC and 4H-SiC and semi-insulating 4H-SiC are interpreted in terms of the proposed hierarchy of annealing mechanisms.

The photoluminescence center ${D}_{\mathrm{II}}$ is a persistent intrinsic defect which is common in all SiC polytypes. Its fingerprints are the characteristic phonon replicas in luminescence spectra. We perform ab initio calculations of vibrational spectra for various defect complexes and find that carbon antisite clusters exhibit vibrational modes in the frequency range of the ${D}_{\mathrm{II}}$ spectrum. The clusters possess very high binding energies which guarantee their thermal stability---a known feature of the ${D}_{\mathrm{II}}$ center. The dicarbon antisite $({\mathrm{C}}_{2}{)}_{\mathrm{Si}}$ (two carbon atoms sharing a silicon site) is an important building block of these clusters.

The diffusion and the annealing of defects represent two fundamental mechanisms of mass transport and defect reactions in semiconductors including SiC. They play a pivotal role in the device processing, for instance in the ion-implantation. Ion-implantation is the main technology to produce doped regions. It provides spatially well-defined dopant-profiles that are required to separate regions of p- and n-type conductivity in modern devices. Yet, the implantation creates various defects in the crystal that affect the electron mobility. The subsequent annealing seeks to remove the implantation-induced defects and to achieve an electrical activation of the dopant. However, the heat treatment not only anneals unfavorable defects, it also invokes a dopant diffusion which broadens the previously implanted profiles. Understanding the annealing processes and the dopant diffusion is therefore pivotal for the success of a semiconductor material.

To reveal the microscopic origin of the spin-resonance centers in 3C- and 4H-SiC, we perform first-principles calculations of the hyperfine tensors for vacancy-related defects and interstitials. The calculations for the silicon vacancy corroborates the earlier experimental identification. The signature of the carbon vacancy in 4H-SiC, in contrast to the silicon vacancy, is found to discriminate between cubic and hexagonal sites. The EI5 center in 4H-SiC can be attributed to a carbon vacancy at the cubic site. The assignment of the T5 center in 3C-SiC to a carbon vacancy or a complex of a carbon vacancy with hydrogen is not supported. The EI6 center in 4H-SiC originally identified as a silicon antisite is not compatible with the HF signature of this defect.

Following the collapse of the Soviet Union, the manage-ment of sexually transmitted infections (STIs) became fragmented, with a consequent increase in STI morbidity. In Belarus, reliable data are generally lacking as a result of poor access to effective diagnostic facilities, while the quality of laboratory testing for STIs in recognised centres is questionable. As part of the Belarusian-Swedish project “Optimization of the prevention and control of STI/HIV in Belarus” (1) we surveyed STI diagnostic facilities for the methods they used (including their range, availability and quality) and their adherence to international evidence-based guidelines. MATerIAlS AND MeTHODS

Electron-hole excitations in the surface bands of GaAs(110) are analyzed using constrained density-functional theory calculations. The results show that Frenkel-type autolocalized excitons are formed. The excitons induce a local surface unrelaxation which results in a strong exciton-exciton attraction and makes complexes of two or three electron-hole pairs more favorable than separate excitons. In such microscopic exciton "droplets" the electron density is mainly concentrated in the dangling orbital of a surface Ga atom whereas the holes are distributed over the bonds of this atom to its As neighbors thus weakening the bonding to the substrate. This finding suggests the microscopic mechanism of a laser-induced emission of neutral Ga atoms from GaAs and GaP (110) surfaces., Comment: submitted to PRL, 10 pages, 4 figures available upon request from: oleg@theo24.rz-berlin.mpg.de

We apply exact-exchange spin-density functional theory in the Krieger-Li-Iafrate approximation to interacting electrons in quantum rings of different widths. The rings are threaded by a magnetic flux that induces a persistent current. A weak space and spin symmetry breaking potential is introduced to allow for localized solutions. As the electron-electron interaction strength described by the dimensionless parameter r(S) is increased, we observe-at a fixed spin magnetic moment-the subsequent transition of both spin sub-systems from the Fermi liquid to the Wigner crystal state. A dramatic signature of Wigner crystallization is that the persistent current drops sharply with increasing r(S). We observe simultaneously the emergence of pronounced oscillations in the spin-resolved densities and in the electron localization functions indicating a spatial electron localization showing ferrimagnetic order after both spin sub-systems have undergone the Wigner crystallization. The critical r(S)(c) at the transition point is substantially smaller than in a fully spin-polarized system and decreases further with decreasing ring width. Relaxing the constraint of a fixed spin magnetic moment, we find that on increasing r(S) the stable phase changes from an unpolarized Fermi liquid to an antiferromagnetic Wigner crystal and finally to a fully polarized Fermi liquid.

The energetics of the translational and rotational degrees of freedom of graphene layers are investigated using density functional theory. It is found that the sliding (translation) energy of a bilayer depends dramatically on whether such layers are mutually rotated or not. While for unrotated layers the sliding energy is large, with the AB stacked bilayer lowest in energy, for mutually rotated layers the sliding energy is zero. Turning to the rotational degree of freedom, we find that dependence of energy on the relative rotation between layers is considerable, and that the lowest energy structure is that generated by 30° ± 2.208°. The impact of a perpendicular electric field on mutually rotated graphene layers is explored. The electronic decoupling of such layers ensures that the Dirac cones simply shift relative to each other to accommodate the charge transfer between the layers. Interestingly, this shift is approximately the same in magnitude as that of the field induced gap opened when an electric field is applied to an AB stacked bilayer.