374 research outputs found
Charge density waves enhance the electronic noise of manganites
The transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin
films in the temperature range from room temperature to 160 K are reported. It
is shown that both the broadband 1/f noise properties and the dependence of
resistance on electric field are consistent with the idea of a collective
electrical transport, as in the classical model of sliding charge density
waves. On the other hand, the observations cannot be reconciled with standard
models of charge ordering and charge melting. Methodologically, it is proposed
to consider noise-spectra analysis as a unique tool for the identification of
the transport mechanism in such highly correlated systems. On the basis of the
results, the electrical transport is envisaged as one of the most effective
ways to understand the nature of the insulating, charge-modulated ground states
in manganites.Comment: 6 two-column pages, 5 figure
Analytical Study of Sub-Wavelength Imaging by Uniaxial Epsilon-Near-Zero Metamaterial Slabs
We discuss the imaging properties of uniaxial epsilon-near-zero metamaterial
slabs with possibly tilted optical axis, analyzing their sub-wavelength
focusing properties as a function of the design parameters. We derive in closed
analytical form the associated two-dimensional Green's function in terms of
special cylindrical functions. For the near-field parameter ranges of interest,
we are also able to derive a small-argument approximation in terms of simpler
analytical functions. Our results, validated and calibrated against a full-wave
reference solution, expand the analytical tools available for
computationally-efficient and physically-incisive modeling and design of
metamaterial-based sub-wavelength imaging systems.Comment: 25 pages, 9 figures (modifications in the text; two figures and
several references added
Multiple double-exchange mechanism by Mn-doping in manganite compounds
Double-exchange mechanisms in REAEMnO manganites (where
RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies
on the strong exchange interaction between two Mn and Mn ions
through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions
has ever been considered "silent" with respect to the DE conducting mechanisms.
Here we show that a new path for DE-mechanism is indeed possible by partially
replacing the RE-AE elements by Mn-ions, in La-deficient
LaMnO thin films. X-ray absorption spectroscopy demonstrated
the relevant presence of Mn ions, which is unambiguously proved to be
substituted at La-site by Resonant Inelastic X-ray Scattering. Mn is
proved to be directly correlated to the enhanced magneto-transport properties
because of an additional hopping mechanism trough interfiling Mn-ions,
theoretically confirmed by calculations within the effective single band model.
The very idea to use Mn both as a doping element and an ions
electronically involved in the conduction mechanism, has never been foreseen,
revealing a new phenomena in transport properties of manganites. More
important, such a strategy might be also pursed in other strongly correlated
materials.Comment: 6 pages, 5 figure
On discretization in time in simulations of particulate flows
We propose a time discretization scheme for a class of ordinary differential
equations arising in simulations of fluid/particle flows. The scheme is
intended to work robustly in the lubrication regime when the distance between
two particles immersed in the fluid or between a particle and the wall tends to
zero. The idea consists in introducing a small threshold for the particle-wall
distance below which the real trajectory of the particle is replaced by an
approximated one where the distance is kept equal to the threshold value. The
error of this approximation is estimated both theoretically and by numerical
experiments. Our time marching scheme can be easily incorporated into a full
simulation method where the velocity of the fluid is obtained by a numerical
solution to Stokes or Navier-Stokes equations. We also provide a derivation of
the asymptotic expansion for the lubrication force (used in our numerical
experiments) acting on a disk immersed in a Newtonian fluid and approaching the
wall. The method of this derivation is new and can be easily adapted to other
cases
Partial Regularity of solutions to the Four-dimensional Navier-Stokes equations at the first blow-up time
The solutions of incompressible Navier-Stokes equations in four spatial
dimensions are considered. We prove that the two-dimensional Hausdorff measure
of the set of singular points at the first blow-up time is equal to zero.Comment: 19 pages, a comment regarding five or higher dimensional case is
added in Remark 1.3. accepted by Comm. Math. Phy
Cancellation of vorticity in steady-state non-isentropic flows of complex fluids
In steady-state non-isentropic flows of perfect fluids there is always
thermodynamic generation of vorticity when the difference between the product
of the temperature with the gradient of the entropy and the gradient of total
enthalpy is different from zero. We note that this property does not hold in
general for complex fluids for which the prominent influence of the material
substructure on the gross motion may cancel the thermodynamic vorticity. We
indicate the explicit condition for this cancellation (topological transition
from vortex sheet to shear flow) for general complex fluids described by
coarse-grained order parameters and extended forms of Ginzburg-Landau energies.
As a prominent sample case we treat first Korteweg's fluid, used commonly as a
model of capillary motion or phase transitions characterized by diffused
interfaces. Then we discuss general complex fluids. We show also that, when the
entropy and the total enthalpy are constant throughout the flow, vorticity may
be generated by the inhomogeneous character of the distribution of material
substructures, and indicate the explicit condition for such a generation. We
discuss also some aspects of unsteady motion and show that in two-dimensional
flows of incompressible perfect complex fluids the vorticity is in general not
conserved, due to a mechanism of transfer of energy between different levels.Comment: 12 page
The Stokes and Poisson problem in variable exponent spaces
We study the Stokes and Poisson problem in the context of variable exponent
spaces. We prove the existence of strong and weak solutions for bounded domains
with C^{1,1} boundary with inhomogenous boundary values. The result is based on
generalizations of the classical theories of Calderon-Zygmund and
Agmon-Douglis-Nirenberg to variable exponent spaces.Comment: 20 pages, 1 figur
Existence of global strong solutions to a beam-fluid interaction system
We study an unsteady non linear fluid-structure interaction problem which is
a simplified model to describe blood flow through viscoleastic arteries. We
consider a Newtonian incompressible two-dimensional flow described by the
Navier-Stokes equations set in an unknown domain depending on the displacement
of a structure, which itself satisfies a linear viscoelastic beam equation. The
fluid and the structure are fully coupled via interface conditions prescribing
the continuity of the velocities at the fluid-structure interface and the
action-reaction principle. We prove that strong solutions to this problem are
global-in-time. We obtain in particular that contact between the viscoleastic
wall and the bottom of the fluid cavity does not occur in finite time. To our
knowledge, this is the first occurrence of a no-contact result, but also of
existence of strong solutions globally in time, in the frame of interactions
between a viscous fluid and a deformable structure
Global regularity criterion for the 3D Navier-Stokes equations involving one entry of the velocity gradient tensor
In this paper we provide a sufficient condition, in terms of only one of the
nine entries of the gradient tensor, i.e., the Jacobian matrix of the velocity
vector field, for the global regularity of strong solutions to the
three-dimensional Navier-Stokes equations in the whole space, as well as for
the case of periodic boundary conditions
Gapped collective charge excitations and interlayer hopping in cuprate superconductors
We use resonant inelastic x-ray scattering (RIXS) to probe the propagation of
plasmons in the electron-doped cuprate superconductor
SrLaCuO (SLCO). We detect a plasmon gap of ~120 meV
at the two-dimensional Brillouin zone center, indicating that low-energy
plasmons in SLCO are not strictly acoustic. The plasmon dispersion, including
the gap, is accurately captured by layered -- model calculations. A
similar analysis performed on recent RIXS data from other cuprates suggests
that the plasmon gap is generic and its size is related to the magnitude of the
interlayer hopping . Our work signifies the three-dimensionality of the
charge dynamics in layered cuprates and provides a new method to determine
.Comment: 17 pages, 10 figures, includes Supplemental Material. Accepted for
publication in Physical Review Letter
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