101 research outputs found
Pinning, flow and plastic deformation of flux vortices in high T(sub c) superconductors
In high temperature superconductor (HTSC) materials the vortices are highly mobile and flexible. This is reflected in different models of melt of a vortex lattice. Another aspect of the problem is stressed: an easy nucleation and high mobility of dislocations in the vortex lattice. Some models of plastic deformation of vortex lattice are considered as a result of its interaction with a real crystal structure. Depinning is interpreted as yield of plastic flow is vortex medium. Effect of macroscopic defects in crystal structures (pores, inclusions, grain and domain boundaries) is being considered in detail. Available experimental facts on magnetization and a critical current in HTSC and conventional superconductors are discussed from the points of view of depinning to vortices vs. plastic flow of vortices vs. plastic flow of vortices medium
Proper ferroelastic phase transitions in thin epitaxial films with symmetry-conserving and symmetry-breaking misfit strains
We study how the ferroelastic domain structure sets in in an epitaxial film
of a material with second order proper ferroelastic transition. The domain
structures considered are similar to either or
structures in perovskite ferroelectrics. If the "extrinsic" misfit
strain, not associated with the transition, does not break the symmetry of the
high-temperature phase, the phase transition in the film occurs at somewhat
lower temperature compared to the bulk. The loss of stability then occurs with
respect to a sinusoidal strain wave, which evolves into the domain structure
with practically the same geometry and approximately the same period. In the
presence of the symmetry-breaking component of the misfit strain ("extrinsic"
misfit) the character of the phase transition is qualitatively different. In
this case it is a {\em topological} transition between single-domain and
multi-domain states, which starts from a low density of the domain walls.Comment: 7 pages, 2 figures, REVTeX 3.
Vortex movement and magnetization of high Tc superconductors
The basic characteristics of the thermoactivated vortex mobility in Y1Ba2Cu3O7 are determined by measurement of the kinetics of magnetization in two time regimes. The analysis of the kinetics of the approach of the equilibrium results in the activation energy, while the measurement of the log-creep rate allows determination of the activated moment. It is shown that the movement of vortices can be regarded as the diffusion process
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Domain evolution and polarization of continuously graded ferroelectric films
A thermodynamic analysis of graded ferroelectric films demonstrates that in the equilibrium state the films are subdivided into a single-domain band and a polydomain band which consists of wedge-shape domains. Polarization under an external electrostatic field proceeds through an inter-band boundary movement due to growth or shrinkage of the wedge domains. It is shown how the domain structure and evolution are determined by the principal characteristics of the film: the distribution of the spontaneous polarization and dielectric constant. Graded films exhibit a sharp increase of polarization with the field for weak fields, with a drop of the dielectric constant when the field is increasing. A general approach to finding the dependence of the displacement and the wedge-domain shape on the field as well as analytical solutions for the p{sup 4} Landau-Devonshire and parabolic potentials are presented
Easy collective polarization switching in ferroelectrics
The actual mechanism of polarization switching in ferroelectrics remains a
puzzle for many decades, since the usually estimated barrier for nucleation and
growth is insurmountable ("paradox of the coercive field"). To analyze the
mechanisms of the nucleation we consider the exactly solvable case of a
ferroelectric film with a "dead" layer at the interface with electrodes. The
classical nucleation is easier in this case but still impossible, since the
calculated barrier is huge. We have found that the {\em interaction} between
the nuclei is, however, long range, hence one has to study an {\em ensemble} of
the nuclei. We show that there are the ensembles of small (embryonic) nuclei
that grow {\em without the barrier}. We submit that the interaction between
nuclei is the key point for solving the paradox.Comment: 5 pages, REVTeX 3.1 with one eps-figure. Corrected discussion of
single stripe and cylindrical nuclei, and their interaction. The estimate for
equilibrium density of embryonic nuclei is added. To appear in Phys. Rev.
Letter
Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures
A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is
presented, which enables the thermodynamic description of dense laminar
polydomain states in epitaxial ferroelectric thin films. The theory explicitly
takes into account the mechanical substrate effect on the polarizations and
lattice strains in dissimilar elastic domains (twins). Numerical calculations
are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic
substrates. The "misfit strain-temperature" phase diagrams are developed for
these films, showing stability ranges of various possible polydomain and
single-domain states. Three types of polarization instabilities are revealed
for polydomain epitaxial ferroelectric films, which may lead to the formation
of new polydomain states forbidden in bulk crystals. The total dielectric and
piezoelectric small-signal responses of polydomain films are calculated,
resulting from both the volume and domain-wall contributions. For BaTiO3 films,
strong dielectric anomalies are predicted at room temperature near special
values of the misfit strain.Comment: 19 pages, 8 figure
Self-reported depression and social support are associated with egocentric network characteristics of HIV-infected women of color
Background: We explore the social network characteristics associated with depressive symptoms and social support among HIV-infected women of color (WOC). Methods: Network data were collected from 87 HIV-infected WOC at an academic Infectious Disease clinic in the United States (US) south. With validated instruments, interviewers also asked about depressive symptoms, social support, and treatment-specific social support. Linear regression models resulted in beta coefficients and 95% confidence intervals for the relationships among network characteristics, depression, and support provision. Results: Financial support provision was associated with lower reported depressive symptoms while emotional support provision was associated with increased reported social support. Talking less than daily to the first person named in her network, the primary alter, was associated with a nearly 3-point decrease in reported social support for respondents. Having people in their social network who knew their HIV status was also important. Conclusions: We found that both functional and structural social network characteristics contributed to perceptions of support by HIV-infected WOC
Effect of magnetic state on the transition in iron: First-principle calculations of the Bain transformation path
Energetics of the fcc () - bcc () lattice transformation by
the Bain tetragonal deformation is calculated for both magnetically ordered and
paramagnetic (disordered local moment) states of iron. The first-principle
computational results manifest a relevance of the magnetic order in a scenario
of the - transition and reveal a special role of the Curie
temperature of -Fe, , where a character of the transformation is
changed. At a cooling down to the temperatures one can expect that
the transformation is developed as a lattice instability whereas for
it follows a standard mechanism of creation and growth of an embryo of the new
phase. It explains a closeness of to the temperature of start of the
martensitic transformation, .Comment: 4 pages, 3 figures, submitted in Phys. Rev. Letter
Coexistence of Ferroelectric Triclinic Phases and Origin of Large Piezoelectric Responses in Highly Strained BiFeO3 films
The structural evolution of the strain-driven morphotropic phase boundary
(MPB) in BiFeO3 films has been investigated using synchrotron x-ray
diffractometry in conjunction with scanning probe microscopy. Our results
demonstrate the existence of mixed-phase regions that are mainly made up of two
heavily tilted ferroelectric triclinic phases. Analysis of first-principles
computations suggests that these two triclinic phases originate from a phase
separation of a single monoclinic state accompanied by elastic matching between
the phase-separated states. These first-principle calculations further reveal
that the intrinsic piezoelectric response of these two low-symmetry triclinic
phases is not significantly large, which thus implies that the ease of phase
transition between these two energetically close triclinic phases is likely
responsible for the large piezoelectric response found in the BiFeO3 films near
its MPB. These findings not only enrich the understandings of the lattice and
domain structure of epitaxial BiFeO3 films but may also shed some light on the
origin of enhanced piezoelectric response near MPB.Comment: 19 pages, 3 figures and 1 tabl
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