Influence of substrate temperature on formation of porous structure of stainless steel in case of its joint deposition with NaCl and KCl vapors

Porous condensates were produced in joint deposition on a substrate of vapor flows formed by electron beam evaporation in vacuum of the Kh18N10T steel and halogenides of alkaline metals (chloral sodium and chloral potassium). Influence of the substrate temperature on regularities of the porous structure formation of the stainless steel condensate was studied. It was shown that porous structure of open type formed most intensively at such values of the substrate temperature, when boundaries of columnar crystallites loosed their stability.Получены пористые конденсаты при совместном осаждении на подложку паровых потоков, формируемых электронно-лучевым испарением в вакууме нержавеющей стали Х18Н10Т и галогенидов щелочных металлов (хлористого натрия и хлористого калия). Изучено влияние температуры подложки на закономерности формирования пористой структуры конденсата нержавеющей стали. Показано,что наиболее интенсивно пористая структура открытого типа формируется при таких значениях температуры положки, когда границы столбчатых кристаллитов теряют свою устойчивость

Quantum escape of the phase in a strongly driven Josephson junction

A quantum mechanical analysis of the Josephson phase escape in the presence of both dc and ac bias currents is presented. We find that the potential barrier for the escape of the phase is effectively suppressed as the resonant condition occurs, i.e. when the frequency $\omega$ of the ac bias matches the Josephson junction energy level separation. This effect manifests itself by a pronounced drop in the dependence of the switching current $I_s$ on the power $W$ of the applied microwave radiation and by a peculiar double-peak structure in the switching current distribution $P(I_s)$. The developed theory is in a good accord with an experiment which we also report in this paper. The obtained features can be used to characterize certain aspects of the quantum-mechanical behavior of the Josephson phase, such as the energy level quantization, the Rabi frequency of coherent oscillations and the effect of damping.Comment: 4 pages, 3 figures, to be published in Physical Review B (Rapid Communication

Parametric Polyhedra with at least kk Lattice Points: Their Semigroup Structure and the k-Frobenius Problem

Given an integral $d \times n$ matrix $A$, the well-studied affine semigroup \mbox{ Sg} (A)=\{ b : Ax=b, \ x \in {\mathbb Z}^n, x \geq 0\} can be stratified by the number of lattice points inside the parametric polyhedra $P_A(b)=\{x: Ax=b, x\geq0\}$. Such families of parametric polyhedra appear in many areas of combinatorics, convex geometry, algebra and number theory. The key themes of this paper are: (1) A structure theory that characterizes precisely the subset \mbox{ Sg}_{\geq k}(A) of all vectors b \in \mbox{ Sg}(A) such that $P_A(b) \cap {\mathbb Z}^n$ has at least $k$ solutions. We demonstrate that this set is finitely generated, it is a union of translated copies of a semigroup which can be computed explicitly via Hilbert bases computations. Related results can be derived for those right-hand-side vectors $b$ for which $P_A(b) \cap {\mathbb Z}^n$ has exactly $k$ solutions or fewer than $k$ solutions. (2) A computational complexity theory. We show that, when $n$, $k$ are fixed natural numbers, one can compute in polynomial time an encoding of \mbox{ Sg}_{\geq k}(A) as a multivariate generating function, using a short sum of rational functions. As a consequence, one can identify all right-hand-side vectors of bounded norm that have at least $k$ solutions. (3) Applications and computation for the $k$-Frobenius numbers. Using Generating functions we prove that for fixed $n,k$ the $k$-Frobenius number can be computed in polynomial time. This generalizes a well-known result for $k=1$ by R. Kannan. Using some adaptation of dynamic programming we show some practical computations of $k$-Frobenius numbers and their relatives

Stability of the monoclinic phase in the ferroelectric perovskite PbZr(1-x)TixO3

Recent structural studies of ferroelectric PbZr(1-x)TixO3 (PZT) with x= 0.48, have revealed a new monoclinic phase in the vicinity of the morphotropic phase boundary (MPB), previously regarded as the the boundary separating the rhombohedral and tetragonal regions of the PZT phase diagram. In the present paper, the stability region of all three phases has been established from high resolution synchrotron x-ray powder diffraction measurements on a series of highly homogeneous samples with 0.42 <=x<= 0.52. At 20K the monoclinic phase is stable in the range 0.46 <=x<= 0.51, and this range narrows as the temperature is increased. A first-order phase transition from tetragonal to rhombohedral symmetry is observed only for x= 0.45. The MPB, therefore, corresponds not to the tetragonal-rhombohedral phase boundary, but instead to the boundary between the tetragonal and monoclinic phases for 0.46 <=x<= 0.51. This result provides important insight into the close relationship between the monoclinic phase and the striking piezoelectric properties of PZT; in particular, investigations of poled samples have shown that the monoclinic distortion is the origin of the unusually high piezoelectric response of PZT.Comment: REVTeX file, 7 figures embedde

Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers

In antiferromagnetically coupled superlattices grown on (001) faces of cubic substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or Fe/Cr, the magnetic states evolve under competing influence of bilinear and biquadratic exchange interactions, surface-enhanced four-fold in-plane anisotropy, and specific finite-size effects. Using phenomenological (micromagnetic) theory, a comprehensive survey of the magnetic states and reorientation transitions has been carried out for multilayer systems with even number of ferromagnetic sub-layers and magnetizations in the plane. In two-layer systems (N=2) the phase diagrams in dependence on components of the applied field in the plane include ``swallow-tail'' type regions of (metastable) multistate co-existence and a number of continuous and discontinuous reorientation transitions induced by radial and transversal components of the applied field. In multilayers (N \ge 4) noncollinear states are spatially inhomogeneous with magnetization varying across the multilayer stack. For weak four-fold anisotropy the magnetic states under influence of an applied field evolve by a complex continuous reorientation into the saturated state. At higher anisotropy they transform into various inhomogeneous and asymmetric structures. The discontinuous transitions between the magnetic states in these two-layers and multilayers are characterized by broad ranges of multi-phase coexistence of the (metastable) states and give rise to specific transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio

Structural Mechanism of Close-Packed Polytypes Formation at Displacive Type Phase Transformation in Alloys

Statistics of one-dimensional disordering of polytypes formed under phase transformation of a displacive type of Co-alloys was studied. Discovered polytypes with a short-range order in close-packed atomic layers allowed to propose a mechanism of their formation by noncoherent displacements in the initial polymorphous modification along close-packed planes. Signifiant dependence of polytype structures not only on alioying of an alloy but on the kinetics of a polymorphous transformation is considered as an indirect interaction of atomic layers via the impurities subsystem

Germination et texture du composé supraconducteur Nb3_3Sn

The composite design and/or manufacturing process of the Nb$_3$Sn multifilamentary strands are continuously changed so as to improve the superconducting behaviour. Such an enhancement depends on both the amount and microstructure of the superconducting phase. The study of the parameters and of the mechanisms of the phase transformations is therefore of the higher importance. The stages of nucleation and growth of the Nb$_3$Sn grains are mainly investigated by thin foil transmission electron microscopy and by X-ray diffraction. The results obtained with the various techniques are shown to be in accordance and to complete each other. An example establishes their usefulness in order to explain the evolution of the critical current density. The effect of the grain misorientation on the critical current density is also considered.L'amélioration du comportement supraconducteur des composites multifilantentaires à base de Nb$_3$Sn passe par la définition de nouveaux designs et/ou de nouveaux procédés de fabrication. Le comportement supraconducteur est régi par la quantité et la microstructure de la phase supraconductrice. Ce dernier paramètre impose d'identifier la nature des mécanismes des transformations de phases et leurs paramèt res. La démarche met en oeuvre la microscopie électronique en transmission et la détermination des orientations cristallographiques aux rayons X. Elle est appliquée aux stades de germination et de croissance des grains de la phase Nb$_3$Sn dans le procédé de la source d'étain interne. Les apports complémentaires et concordants des différentes techniques sont dégagés. L'importance des résultats pour l'interprétation des variations de la densité de courant critique est démontrée par un exemple. L'influence de la désorientation entre les grains sur la densité de courant critique est finalement considérée