63 research outputs found
Electric-field-induced monoclinic phase in (Ba,Sr)TiO thin film
We have studied electric-field-induced symmetry lowering in the tetragonal
(001)-oriented heteroepitaxial (BaSr)TiO thin film
deposited on (001)MgO substrate. Polarized micro-Raman spectra were recorded
from the film area in between two planar electrodes deposited on the film
surface. Presence of \textit{c}-domains with polarization normal to the
substrate was confirmed from polarized Raman study under zero field, while
splitting and hardening of the \textit{E}(TO) soft mode and polarization
changes in the Raman spectra suggest monoclinic symmetry under external
electric field
Two-dimensional elasticity determines the low-frequency dynamics of single- and double-walled carbon nanotubes
We develop a continuous theory of low-frequency dynamics for nanotubes with
truly two-dimensional (2D) walls constituted by single-atom monolayer. In this
frame topological bending elasticity of the monolayer is not related to its
vanishing macroscopic thickness. The proposed approach predicts completely new
sound dispersions and radius dependences of non-resonant Raman-active modes
frequencies in single-walled carbon nanotubes (SWCNT). Resulting relations are
suitable for nanotubes identification and more complete or alternative
characterization. The theory is also applied to describe the low-frequency
dynamics of double-walled carbon nanotubes (DWCNT). It establishes a clear-cut
relation between the radial breathing mode in SWCNT and breathing-like modes in
DWCNT and fits the existing Raman data better than previously developed 3D
continuous or discrete models. The results obtained constitute the basis for
new quantitative studies of the low-frequency vibrational spectrum, heat
capacity and heat transfer properties of carbon nanotubes.Comment: 12 pages, 1 figur
Intrinsic dead layer effects in relaxed epitaxial BaTiO3 thin film grown by pulsed laser deposition
MEM acknowledges a support from the Region of Haut de France and IPR the Ministry of Education and Science of the Russian Federation (research project 3.1649.2017/PP).Epitaxial BaTiO3 (BT) thin film of about 400 nm thickness was grown on LaSr0.5Co0.5O3 (LSCO) coated (001)MgO using pulsed laser deposition. Ferroelectric properties of the BT thin film in Pt/BT/LSCO/MgO heterostructure capacitor configuration were investigated. Dynamic P-E hysteresis loops at room temperature showed ferroelectric behavior with Ps = 32 μC/cm2, Pr = 14 μC/cm2 and EC = 65 kV/cm. Static C-V measurements confirmed reversible switching with a coercive field EC = 15 kV/cm. Basing on a model taking into account an interface dead-layer we show that the capacitance-voltage “butterfly” loops imply only 25% switching of dipoles that inferred from dynamic polarization-field loops (~ 4 and ~ 16 kV/cm, respectively). Dielectric permittivity as a function of temperature revealed a first-order ferroelectric-to-paraelectric (FE-PE) phase transition in the BT film characterized by a maximum at TC ~ 130 °C. The very large (~ 126 K at 1 kHz) difference between TC and the extrapolated Curie-Weiss temperature T0 is attributed to the dead-layer effects.PostprintPeer reviewe
Phenomenological theory of phase transitions in epitaxial BaxSr(1-x)TiO3 thin films
A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films
epitaxially grown on cubic substrates is developed using the Landau-Devonshire
approach. The eighth-order thermodynamic potential for BT single crystal and
modified fourth-order potential for ST single crystal were used as starting
potentials for the end-members of the solid solution with the aim to develop
potential of BST- solid solution valid at high temperatures. Several
coefficients of these potentials for BT were changed to obtain reasonable
agreement between theory and experimental phase diagram for BST-x (x > 0.2)
solid solutions. For low Ba content we constructed the specific phase diagram
where five phases converge at the multiphase point (T_N2 = 47 K, x = 0.028) and
all transitions are of the second order. The "concentration-misfit strain"
phase diagrams for BST-x thin films at room temperature and "temperature-misfit
strain" phase diagrams for particular concentrations are constructed and
discussed. Near T_N2 coupling between polarization and structural order
parameter in the epitaxial film is modified considerably and large number of
new phases not present in the bulk materials appear on the phase diagram.Comment: 8 pages 5 figure
Interlayer Dependence of G-Modes in Semiconducting Double-Walled Carbon Nanotubes
A double-walled carbon nanotube (DWNT), a coaxial composite of two single walled carbon nanotubes (SWNT), provides a unique model to study interactions between thetwo constituent SWNTs. Combining high resolution transmission electron microscopy (HRTEM), electron diffraction (ED), and resonant Raman scattering (RRS) experiments on the same individual suspended DWNT is the ultimate way to relate unambiguously its atomicstructure, defined by the chiral indices of the coaxial outer/inner SWNTs, and its Raman-active vibration modes. This approach is used to investigate the intertube distance dependence of theG-modes of individual index-identified DWNTs composed of two semiconducting SWNTs.We state the main features of the dependence of the G-mode frequencies on the distance between the inner and outer layers: (i) When the interlayer distance is larger than the nominal van der Waals distance (close to 0.34 nm), a downshift of the inner-layer G-modes with respectto the G-modes in the equivalent SWNTs is measured. (ii) The amplitude of the downshiftdepends on the interlayer distance, or in other words, on the negative pressure felt by the innerlayer in DWNT. (iii) No shift is observed for an intertube distance close to 0.34 nm
Structural phase transitions in nanosized ferroelectric barium strontium titanate films
International audienceThe lattice parameters of epitaxial barium strontium titanate films with various thicknesses (from 6 to 960 nm) were measured as a function of temperature in the normal and tangential directions with respect to the film plane using x-ray diffraction. The films were grown through the layer-by-layer mechanism by rf cathode sputtering under elevated oxygen pressure. A critical film thickness (similar to 50 nm) was found to exist, below and above which the films are subjected to compressive and tensile stresses, respectively. As the temperature varies from 780 to 100 K, the films undergo two diffuse structural phase transitions of the second order over the entire thickness range. The transitions in the films under tensile stresses are likely to be transformations from the paraelectric tetragonal to aa phase and then to r phase, whereas the transitions under compressive stresses are transformations from the tetragonal paraelectric to ferroelectric c phase and then, with further decreasing temperature, to r phase
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