4,085 research outputs found

    Possibility of Geometric Description of Quasiparticles in Solids

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    New phenomenological approach for the description of elementary collective excitations is proposed. The crystal is considered to be an anisotropic space-time vacuum with a prescribed metric tensor in which the information on electromagnetic crystalline fields is included. The quasiparticles in this space are supposed to be described by the equations structurally similar to the relativistic wave equations for particles in empty space. The generalized Klein-Gordon-Fock equation and the generalized Dirac equation in external electromagnetic field are considered. The applicability of the proposed approach to the case of conduction electron in a crystal is discussed.Comment: 17 pages, latex; to appear in Int. Jnl. Mod. Phy

    Hyperfine frequency shift in two-dimensional atomic hydrogen

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    We propose the explanation of a surprisingly small hyperfine frequency shift in the two-dimensional (2D) atomic hydrogen bound to the surface of superfluid helium below 0.1 K. Owing to the symmetry considerations, the microwave-induced triplet-singlet transitions of atomic pairs in the fully spin-polarized sample are forbidden. The apparent nonzero shift is associated with the density-dependent wall shift of the hyperfine constant and the pressure shift due to the presence of H atoms in the hyperfine state aa not involved in the observed bcb\to c transition. The interaction of adsorbed atoms with one another effectively decreases the binding energy and, consequently, the wall shift by the amount proportional to their density. The pressure shift of the bcb\to c resonance comes from the fact that the impurity aa-state atoms interact differently with the initial bb-state and final cc-state atoms and is also linear in density. The net effect of the two contributions, both specific for 2D hydrogen, is comparable with the experimental observation. To our knowledge, this is the first mentioning of the density-dependent wall shift. We also show that the difference between the triplet and singlet scattering lengths of H atoms, atas=30(5)a_t-a_s=30(5) pm, is exactly twice smaller than the value reported by Ahokas {\it et al.}, Phys. Rev. Lett. {\bf101}, 263003 (2008).Comment: 4 pages, no figure

    Thermal compression of two-dimensional atomic hydrogen to quantum degeneracy

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    We describe experiments where 2D atomic hydrogen gas is compressed thermally at a small "cold spot" on the surface of superfluid helium and detected directly with electron-spin resonance. We reach surface densities up to 5e12 1/cm^2 at temperatures of approximately 100 mK corresponding to the maximum 2D phase-space density of about 1.5. By independent measurements of the surface density and its decay rate we make the first direct determination of the three-body recombination rate constant and get the value of 2e-25 cm^4/s for its upper bound, which is an order of magnitude smaller than previously reported experimental results.Comment: 4 pages, 4 postscript figures, bibliography (.bbl) file, submitted to PR

    Синтез нових 5,5’-(R-діїлбіс(сульфандіїл))біс(3-(тіофен-2-ілметил)- 4H-1,2,4-триазол-4-амінів)

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    Topicality. There is a tendency to increase the number of generics in comparison with the original drugs at the present national pharmaceutical market. A few new drugs registered which contain new substances. Thus, the already wellknown drugs appear under new names, while their pharmacological effect is not stronger and safer.The aim of this work was to synthesize and to study structure confirmation of novel 5,5’-(R-diylbis(sulfanediyl)) bis(3-(thiophen-2-ylmethyl)-4H-1,2,4-triazole-4-amines).Materials and methods. There are synthesized new 5,5’-(R diylbis(sulfanediyl))bis(3-(thiophen-2-ylmethyl)-4H- 1,2,4-triazole-4-amines). Structure of substances was established by the modern physical and chemical methods of analysis (Elemental analysis, 1H-NMR-spectroscopy, HPLC-MS).Results and discussion. There are synthesized 5,5’-(R-diylbis(sulfanediyl))bis(3-(thiophen-2-ylmethyl)-4H-1,2,4-triazole-4-amines) and 5,5’-(R-1,3-diylbis(sulfandiyl))bis (N-R-iden-3-(thiophen-2-ylmethyl)-4H-1,2,4-triazole-4-amines.Conclusions. The structure of synthesized substances was established by the modern physical and chemical methods of analysis (Elemental analysis, 1H-NMR-spectroscopy, HPLC-MS)Актуальность. На сегодня существует тенденция к увеличению количества генериков по сравнению с оригинальными препаратами на современном национальном фармацевтическом рынке. Зарегистрировано только небольшое количество новых препаратов, которые содержат новые вещества. Таким образом, уже известные лекарства появляются под новыми названиями, тогда как их фармакологический эффект не является сильным и безопасным.Целью этой работы был синтез и подтверждение структуры новых 5,5’-(R-диилбис(сульфандиил)бис(3- (тиофен-2-илметил)-4H-1,2,4-триазол-4-аминов).Материалы и методы. Синтезированы новые 5,5’-(R-диилбис(сульфандиил))бис(3-(тиофен-2-илметил)-4H-1,2,4-триазол-4-амины). Структура веществ была установлена современными физико-химическими методами анализа (элементный анализ, 1H-ЯМР-спектроскопия, ВЭЖХ-МС).Результаты и их обсуждение. Синтезированы 5,5’-(R-диилбис(сульфандиил))бис(3-(тиофен-2-илметил)-4H-1,2,4-триазол-4-амины) и 5,5’-(R-1,3-диилбис(сульфандиил))бис(N-R-иден-3-(тиофен-2-илметил)-4H-1,2,4-триазол-4-амины.Выводы. Структура синтезированных веществ была установлена современными физико-химическими методами анализа (элементный анализ, 1H-ЯМР-спектроскопия, ВЭЖХ-МС).Актуальність. На сьогодні існує тенденція до збільшення кількості генериків у порівнянні з оригінальними препаратами на сучасному національному фармацевтичному ринку. Тільки невелика кількість нових зареєстрованих препаратів містить нові речовини. Таким чином, вже відомі ліки з’являються під новими назвами, тоді як їх фармакологічний ефект не є сильнішим і безпечнішим.Метою цієї роботи був синтез та підтвердження структури нових 5,5’-(R-діїлбіс(сульфандіїл)біс(3-(тіофен-2-ілметил)-4H-1,2,4-триазол-4-амінів).Матеріали та методи. Синтезовані нові 5,5’-(R-діїлбіс(сульфандіїл))біс(3-(тіофен-2-ілметил)-4H-1,2,4-триазол-4-аміни). Структура речовин була встановлена сучасними фізико-хімічними методами аналізу (елементний аналіз, 1HЯМР-спектроскопія, ВЕРХ-МС).Результати та їх обговорення. Синтезовані 5,5’-(R-діїлбіс(сульфандіїл))біс(3-(тіофен-2-ілметил)-4H-1,2,4-триазол-4-аміни) та 5,5’-(R-1,3-діїлбіс(сульфандіїл))біс(N-R-іден-3-(тіофен-2-ілметил)-4H-1,2,4-триазол-4-аміни.Висновки. Структура синтезованих речовин була встановлена сучасними фізико-хімічними методами аналізу (елементний аналіз, 1HЯМР-спектроскопія, ВЕРХ-МС)

    Robust computer-aided synthesis and optimization of linear multivariable control systems with varying plant dynamics via AUTOCON

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    AUTOCON is an automated computer-aided design tool for the synthesis and optimization of linear multivariable control systems based upon user-defined control parameter optimization. Violations in stability and performance requirements are computed from constraints on Single Input/Single Output (SISO) open- and closed-loop transfer function frequency responses, and from constraints on the singular-value frequency responses of Multiple Input/Multiple Output (MIMO) transfer functions, for all critical plant variations. Optimum nonlinear programming algorithms are used in the search for local constrained solutions in which violations in stability and performance are caused either to vanish or be minimized for a proper selection of the control parameters. Classical control system stability and performance design can, in this way, be combined with modern multivariable robustness methods to offer general frequency response loop-shaping via a computer-aided design tool. Complete Nichols, Nyquist, Bode, singular-value Bode magnitude and transient response plots are produced, including user-defined boundary responses. AUTOCON is used to synthesize and optimize the lateral/directional flight control system for a typical high-performance aircraft

    Shot Noise in Mesoscopic Transport Through Localised States

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    We show that shot noise can be used for studies of hopping and resonant tunnelling between localised electron states. In hopping via several states, shot noise is seen to be suppressed compared with its classical Poisson value SI=2eIS_I=2eI (II is the average current) and the suppression depends on the distribution of the barriers between the localised states. In resonant tunnelling through a single impurity an enhancement of shot noise is observed. It has been established, both theoretically and experimentally, that a considerable increase of noise occurs due to Coulomb interaction between two resonant tunnelling channels.Comment: 7 pages, 5 figures; Proceedings of the 10th Conference on Hopping and Related Phenomena (Trieste 2003); requires Wiley style files (included
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