16 research outputs found
Diffusion in a strongly correlated anisotropic overlayer
We study the collective diffusion in chain structures on anisotropic
substrates like (112) bcc and (110) fcc surfaces with deep troughs in the
substrate potential corrugation. These chain structures are aligned normal to
the troughs and can move only along the troughs. In a combination of
theoretical arguments and of numerical simulations, we study the mass transport
in these anisotropic systems. We find that a mechanism similar to soliton
diffusion, instead of single particle diffusion, is still effective at
temperatures well above the melting temperature of the ordered chain
structures. This mechanism is directly correlated with the ordered phases that
appear at much lower temperatures. As a consequence, also the influence of
frozen disorder is still visible above the melting temperature. Theoretically
we predict a strong dependence of the pre-exponential factor and weak
dependence of the activation energy on the concentration of frozen surface
defects. These predictions are confirmed by the simulations.Comment: Latex file, 18 pages and 9 eps figures include
Magnetization Controlled Superconductivity in a Film with Magnetic Dots
We consider a superconducting film with Magnetic Dots Array (MDA) placed on
it. Magnetic moments of these dots are supposed to be normal to the film and
strong enough to create vortices in the superconducting film. Magnetic
interaction between dots is negligible. Below the superconducting transition
temperature of the film in zero magnetic field the MDA with randomly magnetized
dots produces resistive state of the film. Paradoxically, in a finite magnetic
field the film with MDA is superconductive.Comment: RevTex, 4 page
Faceting via correlated disorder of a stochastically growing interface or domain boundary
Journals published by the American Physical Society can be found at http://journals.aps.org/We consider a stochastically growing or evaporating interface in the presence of disorder which is correlated in the direction normal to the interface. The growth or evaporation rate at randomly distributed disorder points is assumed to be different from that of the rest of the interface. This model is of relevance not only to island growth in overlayers, but also to the domain growth in an ultrathin magnetic film after reversal of the magnetization. For a growing one-dimensional interface or a moving domain wall in a magnetic film on a crystal surface, this type of correlated disorder simulates the effect of, e.g., surface steps or grain boundaries on the growth process while, for a growing or evaporating crystal surface, it describes the effect of screw dislocations or of grain boundaries again. We show that, for interface dimensions d = 1,2 during the growth (or evaporation) e-scale faceting develops, although on a small scale the interface is rough. Exploiting the formal connection between the interface model and the model used in the problem of flux line localization in a superconductor we show that correlated disorder localizes the flux line in the presence of point disorder
Interaction of mesoscopic magnetic textures with superconductors
Journals published by the American Physical Society can be found at http://journals.aps.org/Here we report a method to calculate the vortex and magnetization arrangement for a system of interacting superconductors and ferromagnets separated in space. The method is based on static London-Maxwell equations and the corresponding energy. Possible superconducting vortices are included in this system. Using this method we analyze screening currents in a superconducting film induced by magnetic textures in a thin magnetic film. We assume that the two films are parallel and positioned close to each to other, but interact exclusively via magnetic fields. We also consider possible vortices within this superconducting film and their interactions with magnetic texture. As an example of such magnetic texture we use a single magnetic dot with magnetization either perpendicular or parallel to the film. We derive a condition where spontaneous formation of one, two, or more vortices and antivortices is energetically favorable. We prove that, in the case of such a circular magnetic dot with perpendicular magnetization, when the vortex emerges in the superconducting film the normal component of magnetic field near the superconducting film changes sign outside of the dot range
Phase transitions in the adsorption system Li/Mo(112)
Journals published by the American Physical Society can be found at http://journals.aps.org/Experimental studies of the phase transitions in the adsorption system Li/Mo(112) are presented. This system is a model system for highly anisotropic interactions. From measurements of the half-widths of the low-energy electron diffraction spot profiles a phase diagram is derived for the whole submonolayer region of coverage in the temperature range 100-500 K. The commensurate low-coverage phases below theta=0.6 form chains normal to the troughs of the substrate. The commensurate p(4X1) phase, which is completed at a coverage, theta, of 0.25 monolayers (ML), seems to he truly long range ordered, whereas the p(2x1) phase at theta=0.5 still contains domain boundaries even at the lowest temperature of 100 K. Both undergo temperature driven order-disorder phase transitions. In contrast, the incommensurate phases existing in the coverage range theta=0.66-0.90 form chains along the troughs, which are only weakly coupled normal to the troughs of the substrate. These phases exhibit two coverage-driven phase transitions from rectangular to oblique units cells and back at critical coverages of 0.66 and 0.85, respectively, and represent floating solids. As a function of temperature, they undergo a two-dimensional melting transition. Close to the critical coverages, the melting temperatures show a sharp drop below the temperature range accessible in our experiments. Both functional dependences of the angular deviation from 90 degrees and of the melting temperature on coverage are in good agreement with a phenomenological theoretical model, assuming an instability of the shear modulus of the adsorbate unit cell at the critical coverages
Theory of Hysteresis Loop in Ferromagnets
We consider three mechanisms of hysteresis phenomena in alternating magnetic
field: the domain wall motion in a random medium, the nucleation and the
retardation of magnetization due to slow (critical) fluctuations. We construct
quantitative theory for all these processes. The hysteresis is characterized by
two dynamic threshold fields, by coercive field and by the so-called reversal
field. Their ratios to the static threshold field is shown to be function of
two dimensionless variables constituted from the frequency and amplitude of the
ac field as well as from some characteristics of the magnet. The area and the
shape of the hysteresis loop are found. We consider different limiting cases in
which power dependencies are valid. Numerical simulations show the domain wall
formation and propagation and confirm the main theoretical predictions. Theory
is compared with available experimental data.Comment: RevTex, 13 pages, 8 figures (PostScript), acknowledgements adde
Deconstruction Via Adsorbate-Driven Ordering - O-W(001)
Journals published by the American Physical Society can be found at http://journals.aps.org
Domain-Wall Lattices in the Te-W(211) System
Journals published by the American Physical Society can be found at http://journals.aps.org