3,148 research outputs found
Local conductivity and the role of vacancies around twin walls of (001)-BiFeO3 thin films
BiFeO3 thin films epitaxially grown on SrRuO3-buffered (001)-oriented SrTiO3
substrates show orthogonal bundles of twin domains, each of which contains
parallel and periodic 71o domain walls. A smaller amount of 109o domain walls
are also present at the boundaries between two adjacent bundles. All as-grown
twin walls display enhanced conductivity with respect to the domains during
local probe measurements, due to the selective lowering of the Schottky barrier
between the film and the AFM tip (see S. Farokhipoor and B. Noheda, Phys. Rev.
Lett. 107, 127601 (2011)). In this paper we further discuss these results and
show why other conduction mechanisms are discarded. In addition we show the
crucial role that oxygen vacancies play in determining the amount of conduction
at the walls. This prompts us to propose that the oxygen vacancies migrating to
the walls locally lower the Schottky barrier. This mechanism would then be less
efficient in non-ferroelastic domain walls where one expects no strain
gradients around the walls and thus (assuming that walls are not charged) no
driving force for accumulation of defects
Gravitational lensing by gravitational waves
Gravitational lensing by gravitational wave is considered. We notice that
although final and initial direction of photons coincide, displacement between
final and initial trajectories occurs. This displacement is calculated
analytically for the plane gravitational wave pulse. Estimations for
observations are discussed.Comment: 9 pages, 3 figure
Equilibrium basal-plane magnetization of superconductive YNi(2)B(2)C - the influence of non-local electrodynamics
For a single crystal of YNi(2)B(2)C superconductor, the equilibrium
magnetization M in the square basal plane has been studied experimentally as a
function of temperature and magnetic field. While the magnetization M(H)
deviates from conventional London predictions, a recent extension of London
theory (to include effects of non-local electrodynamics) describes the
experiments accurately. The resulting superconductive parameters are well
behaved. These results are compared with corresponding findings for the case
with M perpendicular to the basal plane.Comment: 7 pages, 5 Postscript Figures, 2 table
On Quantum Nature of Black-Hole Spacetime: A Possible New Source of Intense Radiation
Atoms and the planets acquire their stability from the quantum mechanical
incompatibility of the position and momentum measurements. This incompatibility
is expressed by the fundamental commutator [x, p_x]=i hbar, or equivalently,
via the Heisenberg's uncertainty principle Delta x Delta p_x sim hbar. A
further stability-related phenomenon where the quantum realm plays a dramatic
role is the collapse of certain stars into white dwarfs and neutron stars.
Here, an intervention of the Pauli exclusion principle, via the fermionic
degenerate pressure, stops the gravitational collapse. However, by the
neutron-star stage the standard quantum realm runs dry. One is left with the
problematic collapse of a black hole. This essay is devoted to a concrete
argument on why the black-hole spacetime itself should exhibit a quantum
nature. The proposed quantum aspect of spacetime is shown to prevent the
general-relativistic dictated problematic collapse. The quantum nature of
black-hole spacetime is deciphered from a recent result on the universal
equal-area spacing [=lambda_P^2 4 ln(3)] for black holes. In one interpretation
of the emergent picture, an astrophysical black hole can fluctuate to
sqrt{pi/ln(3)} approx 1.7 times its classical size, and thus allow radiation
and matter to escape to the outside observers. These fluctuations I conjecture
provide a new source, perhaps beyond Hawking radiation, of intense radiation
from astrophysical black holes and may be the primary source of observed
radiation from those galactic cores what carry black hole(s). The presented
interpretation may be used as a criterion to choose black holes from black hole
candidates.Comment: This essay received an "honorable mention" in the 1999 Essay
Competition of the Gravity Research Foundation - Ed. Int. J. Mod. Phys. D
(1999, in press). For Joseph Knech
Nature of 45 degree vortex lattice reorientation in tetragonal superconductors
The transformation of the vortex lattice in a tetragonal superconductor which
consists of its 45 degree reorientation relative to the crystal axes is studied
using the nonlocal London model. It is shown that the reorientation occurs as
two successive second order (continuous) phase transitions. The transition
magnetic fields are calculated for a range of parameters relevant for
borocarbide superconductors in which the reorientation has been observed
Four-fold basal plane anisotropy of the nonlocal magnetization of YNi2B2C
Studies of single crystal YNi2B2C have revealed a four-fold anisotropy of the
equilibrium magnetization in the square crystallographic basal plane. This pi/2
periodicity occurs deep in the superconductive mixed state. In this crystal
symmetry, an ordinary superconductive mass anisotropy (as in usual London
theory) allows only a constant, isotropic response. In contrast, the
experimental results are well described by generalized London theory
incorporating non-local electrodynamics, as needed for this clean,
intermediate-k superconductor.Comment: 4 pages, 4 figure
Josephson junction between anisotropic superconductors
The sin-Gordon equation for Josephson junctions with arbitrary misaligned
anisotropic banks is derived. As an application, the problem of Josephson
vortices at twin planes of a YBCO-like material is considered. It is shown that
for an arbitrary orientation of these vortices relative to the crystal axes of
the banks, the junctions should experience a mechanical torque which is
evaluated. This torque and its angular dependence may, in principle, be
measured in small fields, since the flux penetration into twinned crystals
begins with nucleation of Josephson vortices at twin planes.Comment: 6 page
Spin polarization control by electric stirring: proposal for a spintronic device
We propose a spintronic device to generate spin polarization in a mesoscopic
region by purely electric means. We show that the spin Hall effect in
combination with the stirring effect are sufficient to induce measurable spin
polarization in a closed geometry. Our device structure does not require the
application of magnetic fields, external radiation or ferromagnetic leads, and
can be implemented in standard semiconducting materials
Field-induced axion emission via process in plasma
The annihilation into axion is investigated in a plasma and
an external magnetic field. This process via a plasmon intermediate state has a
resonant character at a particular energy of the emitted axion. The emissivity
by is compared with the axion cyclotron emissivity.Comment: 8 pages, latex, 4 PS figure
Interaction of vortices in thin superconducting films and Berezinskii-Kosterlitz-Thouless transition
The precondition for the BKT transition in thin superconducting films, the
logarithmic intervortex interaction, is satisfied at distances short relative
to , is the London penetration depth of the
bulk material and is the film thickness. For this reason, the search for
the transition has been conducted in samples of the size . It is
argued below that film edges turn the interaction into near exponential
(short-range) thus making the BKT transition impossible. If however the
substrate is superconducting and separated from the film by an insulated layer,
the logarithmic intervortex interaction is recovered and the BKT transition
should be observable.Comment: 4 pages, no figure
- …