1,201 research outputs found
Electron-Polarization Coupling in Superconductor-Ferroelectric Superlattices
We present a phenomenological model of periodic ferroelectric-superconductor
(FE-S) heterostructures containing two alternating ferroelectric and
superconducting layers. The interaction at the FE-S contacts is described as a
coupling of the local carrier density of the superconductor with the
spontaneous ferroelectric polarization near the FE-S interface. We obtain a
stable symmetric domain-type phase exhibiting a contact-induced polarization
and the ferroelectric domain structure at temperatures above the bulk
ferroelectric transition temperature. With an increasing coupling energy, we
find the appearance of the ferroelectric phase coexisting with the suppressed
superconductivity in the S-film. The system is analyzed for different
thicknesses of the FE- and S-films demonstrating the dramatic change of the
topology of the phase diagrams with a variation of the layers thickness. The
results are expected to shed light on the processes occurring in
high-temperature superconducting films grown on perovskite alloy-substrates
exhibiting ferroelectric properties at lower temperatures.Comment: 12 pages, 8 figure
Space of State Vectors in PT Symmetrical Quantum Mechanics
Space of states of PT symmetrical quantum mechanics is examined. Requirement
that eigenstates with different eigenvalues must be orthogonal leads to the
conclusion that eigenfunctions belong to the space with an indefinite metric.
The self consistent expressions for the probability amplitude and average value
of operator are suggested. Further specification of space of state vectors
yield the superselection rule, redefining notion of the superposition
principle. The expression for the probability current density, satisfying
equation of continuity and vanishing for the bound state, is proposed.Comment: Revised version, explicit expressions for average values and
probability amplitude adde
Size effects and depolarization field influence on the phase diagrams of cylindrical ferroelectric nanoparticles
Ferroelectric nanoparticles of different shape and their nanocomposites are
actively studied in modern physics. Because of their applications in many
fields of nanotechnology, the size effects and the possible disappearance of
ferroelectricity at a critical particle volume attract a growing scientific
interest. In this paper we study the size effects of the cylindrical
nanoparticle phase diagrams allowing for effective surface tension and
depolarization field influence. The Euler-Lagrange equations were solved by
direct variational method. The approximate analytical expression for the
paraelectric-ferroelectric transition temperature dependence on nanoparticle
sizes, polarization gradient coefficient, extrapolation length, effective
surface tension and electrostriction coefficient was derived. It was shown that
the transition temperature could be higher than the one of the bulk material
for nanorods and nanowires in contrast to nanodisks, where the decrease takes
place. The critical sizes and volume of ferroelectric-paraelectric phase
transition are calculated. We proved that among all cylindrical shapes a
nanobar reveals the minimal critical volume. We predicted the enhancement of
ferroelectric properties in nanorods and nanowires. Obtained results explain
the observed ferroelectricity enhancement in nanorods and could be very useful
for elaboration of modern nanocomposites with perfect polar properties.Comment: 22 pages, 7 figures, 1 tabl
Metric Features of a Dipolar Model
The lattice spin model, with nearest neighbor ferromagnetic exchange and long
range dipolar interaction, is studied by the method of time series for
observables based on cluster configurations and associated partitions, such as
Shannon entropy, Hamming and Rohlin distances. Previous results based on the
two peaks shape of the specific heat, suggested the existence of two possible
transitions. By the analysis of the Shannon entropy we are able to prove that
the first one is a true phase transition corresponding to a particular melting
process of oriented domains, where colored noise is present almost
independently of true fractality. The second one is not a real transition and
it may be ascribed to a smooth balancing between two geometrical effects: a
progressive fragmentation of the big clusters (possibly creating fractals), and
the slow onset of a small clusters chaotic phase. Comparison with the nearest
neighbor Ising ferromagnetic system points out a substantial difference in the
cluster geometrical properties of the two models and in their critical
behavior.Comment: 20 pages, 15 figures, submitted to JPhys
symmetric non-selfadjoint operators, diagonalizable and non-diagonalizable, with real discrete spectrum
Consider in , , the operator family . \ds
H_0= a^\ast_1a_1+... +a^\ast_da_d+d/2 is the quantum harmonic oscillator with
rational frequencies, a symmetric bounded potential, and a real
coupling constant. We show that if , being an explicitly
determined constant, the spectrum of is real and discrete. Moreover we
show that the operator \ds H(g)=a^\ast_1 a_1+a^\ast_2a_2+ig a^\ast_2a_1 has
real discrete spectrum but is not diagonalizable.Comment: 20 page
Radioactive 26Al and massive stars in the Galaxy
Gamma-rays from radioactive 26Al (half life ~7.2 10^5 yr) provide a
'snapshot' view of ongoing nucleosynthesis in the Galaxy. The Galaxy is
relatively transparent to such gamma-rays, and emission has been found
concentrated along the plane of the Galaxy. This led to the conclusion1 that
massive stars throughout the Galaxy dominate the production of 26Al. On the
other hand, meteoritic data show locally-produced 26Al, perhaps from spallation
reactions in the protosolar disk. Furthermore, prominent gamma-ray emission
from the Cygnus region suggests that a substantial fraction of Galactic 26Al
could originate in localized star-forming regions. Here we report high spectral
resolution measurements of 26Al emission at 1808.65 keV, which demonstrate that
the 26Al source regions corotate with the Galaxy, supporting its Galaxy-wide
origin. We determine a present-day equilibrium mass of 2.8 (+/-0.8) M_sol of
26Al. We use this to estimate that the frequency of core collapse (i.e. type
Ib/c and type II) supernovae to be 1.9(+/- 1.1) events per century.Comment: accepted for publication in Nature, 24 pages including Online
Supplements, 11 figures, 1 tabl
A bird's-eye view of chromosomic evolution in the Class Aves
Birds (Aves) are the most speciose of terrestrial vertebrates, displaying Class-specific characteristics yet incredible external phenotypic diversity. Critical to agriculture and as model organisms, birds have adapted to many habitats. The only extant examples of dinosaurs, birds emerged ~150 mya and >10% are currently threatened with extinction. This review is a comprehensive overview of avian genome (âchromosomicâ) organization research based mostly on chromosome painting and BAC-based studies. We discuss traditional and contemporary tools for reliably generating chromosome-level assemblies and analyzing multiple species at a higher resolution and wider phylogenetic distance than previously possible. These results permit more detailed investigations into inter- and intrachromosomal rearrangements, providing unique insights into evolution and speciation mechanisms. The âsignatureâ avian karyotype likely arose ~250 mya and remained largely unchanged in most groups including extinct dinosaurs. Exceptions include Psittaciformes, Falconiformes, Caprimulgiformes, Cuculiformes, Suliformes, occasional Passeriformes, Ciconiiformes, and Pelecaniformes. The reasons for this remarkable conservation may be the greater diploid chromosome number generating variation (the driver of natural selection) through a greater possible combination of gametes and/or an increase in recombination rate. A deeper understanding of avian genomic structure permits the exploration of fundamental biological questions pertaining to the role of evolutionary breakpoint regions and homologous synteny blocks
Birdâs-eye view of chromosomic evolution in Class Aves
Birds (Aves) are the most speciose of terrestrial vertebrates, displaying Class-specific characteristics yet incredible external phenotypic diversity. Critical to agriculture and as model organisms, birds have adapted to many habitats. The only extant examples of dinosaurs, birds emerged ~150 mya and >10% are currently threatened with extinction
Ordering of dipolar Ising crystals
We study Ising systems of spins with dipolar interactions. We find a simple
approximate relation for the interaction energy between pairs of parallel
lattice columns of spins running along the Ising spin direction. This relation
provides insight into the relation between lattice geometry and the nature of
the ordered state. It can be used to calculate ground state energies. We have
also obtained ground state energies and ordering temperatures T_0 from Monte
Carlo simulations. Simple empirical relations, that give T_0 for simple and
body centered tetragonal lattices in terms of lattice parameters are also
established. Finally, the nature of the ordered state and T_0 are determined
for Fe_8 clusters, which crystallize on a triclinic lattice.Comment: 13 pages, 4 eps figures, to be published in PRB. For related work,
see http://pipe.unizar.es/~jf
- âŠ