Electric-field-induced monoclinic phase in (Ba,Sr)TiO3_3 thin film

We have studied electric-field-induced symmetry lowering in the tetragonal (001)-oriented heteroepitaxial (Ba$_{0.8}$Sr$_{0.2}$)TiO$_3$ 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-$x$ 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