19 research outputs found
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 . The
dependence of the low-energy Andreev reflection probability on
reveals the existence of a characteristic length scale beyond which
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 ,
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