1,048 research outputs found
Influence of random roughness on cantilever curvature sensitivity
In this work we explore the influence of random surface roughness on the
cantilever sensitivity to respond to curvature changes induced by changes in
surface stress. The roughness is characterized by the out-of-plane roughness
amplitude w, the lateral correlation length x, and the roughness or Hurst
exponent H (0<H<1). The cantilever sensitivity is found to decrease with
increasing roughness (decreasing H and/or increasing ratio w/x) or equivalently
increasing local surface slope. Finally, analytic expressions of the cantilever
sensitivity as a function of the parameters w, x, and H are derived in order to
allow direct implementation in sensing systems.Comment: 10 pages, 3 figure
Cluster evolution in a cold Ising ferromagnet:Disappearance of magic numbers with temperature rise
Decay and growth of clusters at low and intermediate temperatures based on the two-dimensional square-lattice Ising model has been studied with Monte Carlo simulations employing Glauber (or Metropolis) dynamics, exploiting a procedure where the starting configuration is a cluster (that tend to grow in the applied magnetic field) on a relatively small lattice. The behavior of such a cluster is stochastic and only when typical several thousands of identical clusters are analyzed will the underlying deterministic behavior become apparent. At 0.4Tc, the time-dependent cluster size distribution is relatively broad, but smooth, i.e., Gaussian, and the decay and growth behaviors of various relative compact clusters are compared. At lower temperatures, modulations in the size distribution occur with minima at magic sizes corresponding to n=m×m+1 and n=m×(m+1)+1 with m integer values. A quantitative analysis of the amplitude of the modulations as a function of temperature is performed. Also the relation between the distributions of size and of the number of internal cluster bonds (or cluster perimeter) is scrutinized.
Polarization retention loss in PbTiO3 ferroelectric films due to leakage currents
The relationship between retention loss in single crystal PbTiO3 ferroelectric thin films and leakage currents is demonstrated by piezoresponse and conductive atomic force microscopy measurements. It was found that the polarization reversal in the absence of an electric field followed a stretched exponential behavior 1−exp[−(t/k)^d] with exponent d>1, which is distinct from a dispersive random walk process with d<1. The latter has been observed in polycrystalline films for which retention loss was associated with grain boundaries. The leakage current indicates power law scaling at short length scales, which strongly depends on the applied electric field. Additional information of the microstructure, which contributes to an explanation of the presence of leakage currents, is presented with high resolution transmission electron microscopy analysis.
Tuning the atomic and domain structure of epitaxial films of multiferroic BiFeO3
Recent works have shown that the domain walls of room-temperature
multiferroic BiFeO3 (BFO) thin films can display distinct and promising
functionalities. It is thus important to understand the mechanisms underlying
domain formation in these films. High-resolution x-ray diffraction and
piezo-force microscopy, combined with first-principles simulations, have
allowed us to characterize both the atomic and domain structure of BFO films
grown under compressive strain on (001)-SrTiO3, as a function of thickness. We
derive a twining model that describes the experimental observations and
explains why the 71o domain walls are the ones commonly observed in these
films. This understanding provides us with a new degree of freedom to control
the structure and, thus, the properties of BiFeO3 thin films.Comment: RevTeX; 4 two-column pages; 4 color figures. Figure 2b does not seem
to display well. A proper version can be found in the source fil
Switching Casimir forces with Phase Change Materials
We demonstrate here a controllable variation in the Casimir force. Changes in
the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST)
surfaces were achieved upon crystallization of an amorphous sample of AIST.
This material is well known for its structural transformation, which produces a
significant change in the optical properties and is exploited in optical data
storage systems. The finding paves the way to the control of forces in
nanosystems, such as micro- or nanoswitches by stimulating the phase change
transition via localized heat sources.Comment: 7 pages, 3 figures The AFM images for the inset in Fig.2 were
replaced with new ones as obtained with tips having high aspect rati
Continuation of connecting orbits in 3D-ODEs: (I) Point-to-cycle connections
We propose new methods for the numerical continuation of point-to-cycle
connecting orbits in 3-dimensional autonomous ODE's using projection boundary
conditions. In our approach, the projection boundary conditions near the cycle
are formulated using an eigenfunction of the associated adjoint variational
equation, avoiding costly and numerically unstable computations of the
monodromy matrix. The equations for the eigenfunction are included in the
defining boundary-value problem, allowing a straightforward implementation in
AUTO, in which only the standard features of the software are employed.
Homotopy methods to find connecting orbits are discussed in general and
illustrated with several examples, including the Lorenz equations. Complete
AUTO demos, which can be easily adapted to any autonomous 3-dimensional ODE
system, are freely available.Comment: 18 pages, 10 figure
Mg/Ti multilayers: structural, optical and hydrogen absorption properties
Mg-Ti alloys have uncommon optical and hydrogen absorbing properties,
originating from a "spinodal-like" microstructure with a small degree of
chemical short-range order in the atoms distribution. In the present study we
artificially engineer short-range order by depositing Pd-capped Mg/Ti
multilayers with different periodicities and characterize them both
structurally and optically. Notwithstanding the large lattice parameter
mismatch between Mg and Ti, the as-deposited metallic multilayers show good
structural coherence. Upon exposure to H2 gas a two-step hydrogenation process
occurs, with the Ti layers forming the hydride before Mg. From in-situ
measurements of the bilayer thickness L at different hydrogen pressures, we
observe large out-of-plane expansions of the Mg and Ti layers upon
hydrogenation, indicating strong plastic deformations in the films and a
consequent shortening of the coherence length. Upon unloading at room
temperature in air, hydrogen atoms remain trapped in the Ti layers due to
kinetic constraints. Such loading/unloading sequence can be explained in terms
of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by
diffusion calculations on a model multilayered systems. Absorption isotherms
measured by hydrogenography can be interpreted as a result of the elastic
clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces
Multilevel reflectance switching of ultrathin phase-change films
Several design techniques for engineering the visible optical and near-infrared response of a thin film are explored. These designs require optically active and absorbing materials and should be easily grown on a large scale. Switchable chalcogenide phase-change material heterostructures with three active layers are grown here using pulsed laser deposition. Both Fabry-Perot and strong interference principles are explored to tune the reflectance. Robust multilevel switching is demonstrated for both principles using dynamic ellipsometry, and measured reflectance profiles agree well with simulations. We find, however, that switching the bottom layer of a three-layer device does not yield a significant change in reflectance, indicating a maximum in accessible levels. The pulsed laser deposition films grown show promise for optical display applications, with three shown reflectance levels. Published under license by AIP Publishing.</p
Schottky barrier formation at amorphous-crystalline interfaces of GeSb phase change materials
The electrical properties of amorphous-crystalline interfaces in phase change materials, which are important for rewritable optical data storage and for random access memory devices, have been investigated by surface scanning potential microscopy. Analysis of GeSb systems indicates that the surface potential of the crystalline phase is similar to 30-60 mV higher than that of the amorphous phase. This potential asymmetry is explained qualitatively by the presence of a Schottky barrier at the amorphous-crystalline interface and supported also by quantitative Schottky model calculations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3691179
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