4,961 research outputs found
Experimental Study of the Effect of External Signal on Microwave Oscillations in a Nonrelativistic Electron Beam with Virtual Cathode
The effect of an external harmonic signal on the characteristics of microwave
generation in a nonrelativistic electron beam with virtual cathode (VC) formed
in a static retarding electric field (low-voltage vircator system) has been
experimentally studied. A significant increase in the vircator generation power
is observed when the frequency of the external signal is close to the frequency
of VC oscillations. At large detunings, a broadband chaotic generation is
observed.Comment: 3 pages, 2 figure
Dynamic Behavior in Piezoresponse Force Microscopy
Frequency dependent dynamic behavior in Piezoresponse Force Microscopy (PFM)
implemented on a beam-deflection atomic force microscope (AFM) is analyzed
using a combination of modeling and experimental measurements. The PFM signal
comprises contributions from local electrostatic forces acting on the tip,
distributed forces acting on the cantilever, and three components of the
electromechanical response vector. These interactions result in the bending and
torsion of the cantilever, detected as vertical and lateral PFM signals. The
relative magnitudes of these contributions depend on geometric parameters of
the system, the stiffness and frictional forces of tip-surface junction, and
operation frequencies. The dynamic signal formation mechanism in PFM is
analyzed and conditions for optimal PFM imaging are formulated. The
experimental approach for probing cantilever dynamics using frequency-bias
spectroscopy and deconvolution of electromechanical and electrostatic contrast
is implemented.Comment: 65 pages, 15 figures, high quality version available upon reques
Materials Contrast in Piezoresponse Force Microscopy
Piezoresponse Force Microscopy contrast in transversally isotropic material
corresponding to the case of c+ - c- domains in tetragonal ferroelectrics is
analyzed using Green's function theory by Felten et al. [J. Appl. Phys. 96, 563
(2004)]. A simplified expression for PFM signal as a linear combination of
relevant piezoelectric constant are obtained. This analysis is extended to
piezoelectric material of arbitrary symmetry with weak elastic and dielectric
anisotropies. This result provides a framework for interpretation of PFM
signals for systems with unknown or poorly known local elastic and dielectric
properties, including nanocrystalline materials, ferroelectric polymers, and
biopolymers.Comment: 20 pages, 3 figures, 1 table, accepted to Appl. Phys. Lett. (without
Appendices), algebraic errors were correcte
Experimental and Theoretical Investigation into the Effect of the Electron Velocity Distribution on Chaotic Oscillations in an Electron Beam under Virtual Cathode Formation Conditions
The effect of the electron transverse and longitudinal velocity spread at the
entrance to the interaction space on wide-band chaotic oscillations in intense
multiple-velocity beams is studied theoretically and numerically under the
conditions of formation of a virtual cathode. It is found that an increase in
the electron velocity spread causes chaotization of virtual cathode
oscillations. An insight into physical processes taking place in a virtual
cathode multiple velocity beam is gained by numerical simulation. The
chaotization of the oscillations is shown to be associated with additional
electron structures, which were separated out by constructing charged particle
distribution functions.Comment: 9 pages, 8 figure
Role of Single Defects in Electronic Transport through Carbon Nanotube Field-Effect Transistors
The influence of defects on electron transport in single-wall carbon nanotube
field effect transistors (CNFETs) is probed by combined scanning gate
microscopy (SGM) and scanning impedance microscopy (SIM). SGM reveals a
localized field effect at discrete defects along the CNFET length. The
depletion surface potential of individual defects is quantified from the
SGM-imaged radius of the defect as a function of tip bias voltage. This
provides a measure of the Fermi level at the defect with zero tip voltage,
which is as small as 20 meV for the strongest defects. The effect of defects on
transport is probed by SIM as a function of backgate and tip-gate voltage. When
the backgate voltage is set so the CNFET is "on" (conducting), SIM reveals a
uniform potential drop along its length, consistent with diffusive transport.
In contrast, when the CNFET is "off", potential steps develop at the position
of depleted defects. Finally, high-resolution imaging of a second set of weak
defects is achieved in a new "tip-gated" SIM mode.Comment: to appear in Physical Review Letter
Defect driven flexo-chemical coupling in thin ferroelectric films
Using Landau-Ginzburg-Devonshire theory, we considered the impact of the
flexoelectro-chemical coupling on the size effects inpolar properties and phase
transitions of thin ferroelectric films with a layer of elastic defects. We
investigated a typical case, when defects fill a thin layer below the top film
surface with a constant concentration creating an additional gradient of
elastic fields. The defective surface of the film is not covered with an
electrode, but instead with an ultra-thin layer of ambient screening charges,
characterized by a surface screening length. This geometry is typical for the
scanning probe piezoelectric force microscopy. Obtained results revealed an
unexpectedly strong effect of the joint action of Vegard stresses and
flexoelectric effect (shortly flexo-chemical coupling) on the ferroelectric
transition temperature, distribution of the spontaneous polarization and
elastic fields, domain wall structure and period in thin PbTiO3 films
containing a layer of elastic defects. A nontrivial result is the
ferroelectricity persisting at film thicknesses below 4 nm, temperatures lower
than 350 K and relatively high surface screening length (~0.1 nm). The origin
of this phenomenon is the re-building of the domain structure in the film
(namely the cross-over from c-domain stripes to a-type closure domains) when
its thickness decreases below 4 nm, conditioned by the flexoelectric coupling
and facilitated by negative Vegard effect. For positive Vegard effect, thicker
films exhibit the appearance of pronounced maxima on the thickness dependence
of the transition temperature, whose position and height can be controlled by
the defect type and concentration. The revealed features may have important
implications for miniaturization of ferroelectric-based devices.Comment: 25 pages, 9 figure
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