Geometrical enhancement of the proximity effect in quantum wires with extended superconducting tunnel contacts

We study Andreev reflection in a ballistic one-dimensional channel coupled in parallel to a superconductor via a tunnel barrier of finite length $L$. The dependence of the low-energy Andreev reflection probability $R_A$ on $L$ reveals the existence of a characteristic length scale $\xi_N$ beyond which $R_A(L)$ is enhanced up to unity despite the low interfacial transparency. The Andreev reflection enhancement is due to the strong mixing of particle and hole states that builds up in contacts exceeding the coherence length $\xi_N$, leading to a small energy gap (minigap) in the density of states of the normal system. The role of the geometry of such hybrid contacts is discussed in the context of the experimental observation of zero-bias Andreev anomalies in the resistance of extended carbon nanotube/superconductor junctions in field effect transistor setups.Comment: 11 pages, 8 figures; minor revisions including added Ref. 7 and inset to Fig. 3b; version as accepted for publication to Phys. Rev.

Nonmonotonic inelastic tunneling spectra due to surface spin excitations in ferromagnetic junctions

The paper addresses inelastic spin-flip tunneling accompanied by surface spin excitations (magnons) in ferromagnetic junctions. The inelastic tunneling current is proportional to the magnon density of states which is energy-independent for the surface waves and, for this reason, cannot account for the bias-voltage dependence of the observed inelastic tunneling spectra. This paper shows that the bias-voltage dependence of the tunneling spectra can arise from the tunneling matrix elements of the electron-magnon interaction. These matrix elements are derived from the Coulomb exchange interaction using the itinerant-electron model of magnon-assisted tunneling. The results for the inelastic tunneling spectra, based on the nonequilibrium Green's function calculations, are presented for both parallel and antiparallel magnetizations in the ferromagnetic leads.Comment: 9 pages, 4 figures, version as publishe

Counting master integrals: integration by parts vs. differential reduction

The techniques of integration by parts and differential reduction differ in the counting of master integrals. This is illustrated using as an example the two-loop sunset diagram with on-shell kinematics. A new algebraic relation between the master integrals of the two-loop sunset diagram that does not follow from the integration-by-parts technique is found.Comment: 6 pages, 1 figure; minor changes to the text, figure added; to appear in Phys. Lett.

Nonquasiparticle states in half-metallic ferromagnets

Anomalous magnetic and electronic properties of the half-metallic ferromagnets (HMF) have been discussed. The general conception of the HMF electronic structure which take into account the most important correlation effects from electron-magnon interactions, in particular, the spin-polaron effects, is presented. Special attention is paid to the so called non-quasiparticle (NQP) or incoherent states which are present in the gap near the Fermi level and can give considerable contributions to thermodynamic and transport properties. Prospects of experimental observation of the NQP states in core-level spectroscopy is discussed. Special features of transport properties of the HMF which are connected with the absence of one-magnon spin-flip scattering processes are investigated. The temperature and magnetic field dependences of resistivity in various regimes are calculated. It is shown that the NQP states can give a dominate contribution to the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity. First principle calculations of the NQP-states for the prototype half-metallic material NiMnSb within the local-density approximation plus dynamical mean field theory (LDA+DMFT) are presented.Comment: 27 pages, 9 figures, Proceedings of Berlin/Wandlitz workshop 2004; Local-Moment Ferromagnets. Unique Properties for Moder Applications, ed. M. Donath, W.Nolting, Springer, Berlin, 200

Towards all-order Laurent expansion of generalized hypergeometric functions around rational values of parameters

We prove the following theorems: 1) The Laurent expansions in epsilon of the Gauss hypergeometric functions 2F1(I_1+a*epsilon, I_2+b*epsilon; I_3+p/q + c epsilon; z), 2F1(I_1+p/q+a*epsilon, I_2+p/q+b*epsilon; I_3+ p/q+c*epsilon;z), 2F1(I_1+p/q+a*epsilon, I_2+b*epsilon; I_3+p/q+c*epsilon;z), where I_1,I_2,I_3,p,q are arbitrary integers, a,b,c are arbitrary numbers and epsilon is an infinitesimal parameter, are expressible in terms of multiple polylogarithms of q-roots of unity with coefficients that are ratios of polynomials; 2) The Laurent expansion of the Gauss hypergeometric function 2F1(I_1+p/q+a*epsilon, I_2+b*epsilon; I_3+c*epsilon;z) is expressible in terms of multiple polylogarithms of q-roots of unity times powers of logarithm with coefficients that are ratios of polynomials; 3) The multiple inverse rational sums (see Eq. (2)) and the multiple rational sums (see Eq. (3)) are expressible in terms of multiple polylogarithms; 4) The generalized hypergeometric functions (see Eq. (4)) are expressible in terms of multiple polylogarithms with coefficients that are ratios of polynomials.Comment: 48 pages in LaTe

Multilevel preconditioning operators on locally modified grids

Systems of grid equations that approximate elliptic boundary value problems on locally modified grids are considered. The triangulation, which approximates the boundary with second order of accuracy, is generated from an initial uniform triangulation by shifting nodes near the boundary according to special rules. This ”locally modified” grid possesses several significant features: this triangulation has a regular structure, the generation of the triangulation is rather fast, this construction allows to use multilevel preconditioning (BPX-like) methods. The proposed iterative methods for solving elliptic boundary value problems approximately are based on two approaches: The fictitious space method, i.e. the reduction of the original problem to a problem in a

Multilevel preconditioning operators on locally modified grids

Systems of grid equations that approximate elliptic boundary value problems on locally modified grids are considered. The triangulation, which approximates the boundary with second order of accuracy, is generated from an initial uniform triangulation by shifting nodes near the boundary according to special rules. This "locally modified" grid possesses several significant features: this triangulation has a regular structure, the generation of the triangulation is rather fast, this construction allows to use multilevel preconditioning (BPX-like) methods. The proposed iterative methods for solving elliptic boundary value problems approximately are based on two approaches: The fictitious space method, i.e. the reduction of the original problem to a problem in an auxiliary (fictitious) space, and the multilevel decomposition method, i.e. the construction of preconditioners by decomposing functions on hierarchical grids. The convergence rate of the corresponding iterative process with the preconditioner obtained is independent of the mesh size. The construction of the grid and the preconditioning operator for the three dimensional problem can be done in the same way

Determination of the efficiency of functioning of the military purpose management system, taking into account of the importance, timeliness, and quality of performance

У статті показано поступове зростання ваги застосування інтегрованих систем управління військового призначення при веденні сучасних конфліктів військами провідних країн світу, що направлене на отримання інформаційної переваги над противником за рахунок використання можливостей єдиного командноінформаційного простору всіма його користувачами. При застосуванні перспективних інтегрованих систем управління військового призначення як у повсякденній діяльності військ, так і при виконанні ними бойових завдань дуже часто у органів управління є потреба здійснювати поточну оцінку ефективності їх функціонування для прийняття ними обґрунтованих рішень щодо необхідності виконання певних заходів для відновлення їх властивостей, нарощування можливостей у відповідних ланках управління з метою забезпечення безперервного управління підпорядкованими об’єктами управління, а також для здійснення прогнозування можливості та якості виконання військами завдань, що поставлені керівництвом. Представлена методика поточної оцінки ефективності функціонування інтегрованої системи управління військового призначення, в основу якої покладена оцінка ефективності її підсистем, що визначаються за певними її функціональними ознаками з врахуванням вагомості, своєчасності та якості виконання завдань в кожній з них при виконанні загального процесу управління підпорядкованими об’єктами. Головна відмінність даної методики оцінки ефективності функціонування інтегрованих систем управління військового призначення від інших полягає в тому, що вона дозволяє відповідним органам управління в стислі терміни визначити її ефективність без застосування складного математичного апарату. Даний матеріал доцільно використовувати органам управління при здійсненні поточної оцінки функціонування інтегрованих систем управління військового призначення при плануванні застосування відповідних сил та засобів для прогнозування якості виконання ними завдань за призначенням.В статье показано постепенное возрастание роли применения интегрированных систем управления военного назначения при ведении современных конфликтов войсками ведущих стран мира, которое направлено на получение информационного преимущества над противником за счет использования возможностей единой командноинформационной среды всеми ее пользователями. Представлена методика текущей оценки эффективности функционирования интегрированной системы управления военного назначения, в основу, которой положена оценка эффективности ее функциональных подсистем, которая определяется с учетом веса, своевременности и качества решения задач в каждой из них при выполнении общего процесса управления подчиненными объектами. Главное отличие данной методики оценки эффективности функционирования интегрированной системы управления военного назначения от других состоит в том, что она позволяет соответствующим органам управления в короткие сроки определить ее эффективность без применения сложного математического аппарата. Данный материал целесообразно использовать органам управления при осуществлении текущей оценки функционирования интегрированных систем управления военного назначения при планировании применения сил и средств для прогнозирования качества выполнения ими задач по назначению.The article shows the gradual increase in the weight of the use of integrated military control systems in the conduct of modern conflicts with the troops of the leading countries of the world, aimed at gaining information advantage over the enemy by using the capabilities of a single command and information space for all its users. When applying advanced integrated military control systems both in the day-to-day operations of the troops and in the performance of their combat missions, it is very often necessary for the governing bodies to carry out an ongoing assessment of the effectiveness of their operation to make informed decisions about the need to take certain measures to restore their properties, build up opportunities in appropriate management units to ensure the continuous management of subordinate management facilities, as well as predicting the capability and quality of troop performance by the leadership. The method of current assessment of the effectiveness of the functioning of the integrated military control system, which is based on the evaluation of the effectiveness of its subsystems, which are determined by its functional features, taking into account the weight, timeliness and quality of the tasks in each of them in the performance of the general process of management of subordinate objects. The main difference between this method of assessing the effectiveness of the functioning of integrated military control systems from others is that it allows the relevant authorities to determine its effectiveness in a short time without the use of a complex mathematical apparatus.It is advisable to use this material for governing bodies in carrying out an ongoing assessment of the functioning of integrated military control systems when planning the use of appropriate forces and means to predict the quality of their intended performance