4,337 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
Pyroelectric response of ferroelectric nanoparticles: size effect and electric energy harvesting
The size effect on pyroelectric response of ferroelectric nanowires and
nanotubes is analyzed. The pyroelectric coefficient strongly increases with the
wire radius decrease and diverges at critical radius Rcr corresponding to the
size-driven transition into paraelectric phase. Size-driven enhancement of
pyroelectric coupling leads to the giant pyroelectric current and voltage
generation by the polarized ferroelectric nanoparticles in response to the
temperature fluctuation. The maximum efficiency of the pyroelectric energy
harvesting and bolometric detection is derived, and is shown to approach the
Carnot limit for low temperatures.Comment: 17 pages, 4 figures, 1 Appendi
Quantum Flexoelectricity in Low Dimensional Systems
Symmetry breaking at surfaces and interfaces and the capability to support
large strain gradients in nanoscale systems enable new forms of
electromechanical coupling. Here we introduce the concept of quantum
flexoelectricity, a phenomenon that is manifested when the mechanical
deformation of non-polar quantum systems results in the emergence of net dipole
moments and hence linear electromechanical coupling proportional to local
curvature. The concept is illustrated in carbon systems, including
polyacetylene and nano graphitic ribbons. Using density functional theory
calculations for systems made of up to 400 atoms, we determine the
flexoelectric coefficients to be of the order of ~ 0.1 e, in agreement with the
prediction of linear theory. The implications of quantum flexoelectricity on
electromechanical device applications, and physics of carbon based materials
are discussed.Comment: 15 pages, 3 figure
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
Probing the role of single defects on the thermodynamics of electric-field induced phase transitions
The kinetics and thermodynamics of first order transitions is universally
controlled by defects that act as nucleation sites and pinning centers. Here we
demonstrate that defect-domain interactions during polarization reversal
processes in ferroelectric materials result in a pronounced fine structure in
electromechanical hysteresis loops. Spatially-resolved imaging of a single
defect center in multiferroic BiFeO3 thin film is achieved, and the defect size
and built-in field are determined self-consistently from the single-point
spectroscopic measurements and spatially-resolved images. This methodology is
universal and can be applied to other reversible bias-induced transitions
including electrochemical reactions.Comment: 34 pages,4 figures, high quality figures are available upon request,
submitted to Phys. Rev. Let
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