264 research outputs found
SHG microscopic observations of polar state in Li-doped KTaO3 under electric field
Incipient ferroelectric KTaO3 with off-center Li impurity of the critical
concentration of 2.8 mol% was investigated in order to clarify the dipole glass
state under electric field. Using optical second-harmonic generation (SHG)
microscope, we observed a marked history dependence of SHG intensity through
zero-field cooling (ZFC), zero-field heating (ZFH), field heating after ZFC
(FH/ZFC) and FH after field cooling (FH/FC). These show different paths with
respect to temperature: In the ZFC/ZFH process, weak SHG was observed at low
temperature, while in the FH/ZFC process, relatively high SHG appears in a
limited temperature range below TF depending on the field strength, and in the
FC and FH/FC processes, the SHG exhibits ferroelectric-like temperature
dependence: it appears at the freezing temperature of 50K, increases with
decreasing temperature and has a tendency of saturation. These experimental
results strongly suggest that dipole glass state or polar nano-clusters which
gradually freezes with decreasing temperature is transformed into
semi-macroscopic polar state under the electric field. However at sufficiently
low temperature, the freezing is so strong that the electric field cannot
enlarge the polar clusters. These experimental results show that the polar
nano-cluster model similar to relaxors would be more relevant in KTaO3 doped
with the critical concentration of Li. Further experiments on the anisotropy of
SHG determine that the average symmetry of the field-induced polar phase is
tetragonal 4mm or 4, which is also confirmed by the X-ray diffraction
measurement.Comment: 26 pages, 8 figures, 1 tabl
First-principles study of the electrooptic effect in ferroelectric oxides
We present a method to compute the electrooptic tensor from first principles,
explicitly taking into account the electronic, ionic and piezoelectric
contributions. It allows us to study the non-linear optic behavior of three
ferroelectric ABO_3 compounds : LiNbO_3, BaTiO_3 and PbTiO_3. Our calculations
reveal the dominant contribution of the soft mode to the electrooptic
coefficients in LiNbO_3 and BaTiO_3 and identify the coupling between the
electric field and the polar atomic displacements along the B-O chains as the
origin of the large electrooptic response in these compounds.Comment: accepted for publication in Phys. Rev. Let
Microwave Spectroscopy
Contains research objectives and reports on two research projects.U.S. Army Signal Corps under Contract DA36-039-sc-87376Lincoln Laboratory, Purchase Order DDL B-00306U. S. ArmyU. S. NavyU. S. Air Force under Air Force Contract AF19(604)-740
Microwave Spectroscopy
Contains reports on four research projects.United States Army Signal Corps (Contract DA36-039-sc-87376)Lincoln Laboratory (Purchase Order DDL B-00368)United States ArmyUnited States NavyUnited States Air Force (Contract AF19(604)-7400
Non-linear optical susceptibilities, Raman efficiencies and electrooptic tensors from first-principles density functional perturbation theory
The non-linear response of infinite periodic solids to homogenous electric
fields and collective atomic displacements is discussed in the framework of
density functional perturbation theory. The approach is based on the 2n + 1
theorem applied to an electric-field-dependent energy functional. We report the
expressions for the calculation of the non-linear optical susceptibilities,
Raman scattering efficiencies and electrooptic coefficients. Different
formulations of third-order energy derivatives are examined and their
convergence with respect to the k-point sampling is discussed. We apply our
method to a few simple cases and compare our results to those obtained with
distinct techniques. Finally, we discuss the effect of a scissors correction on
the EO coefficients and non-linear optical susceptibilities
Evaluation of [C(sp3)/[C(sp2)] ratio in diamondlike films through the use of a complex dielectric constant
The evaluation of the amount of tetrahedral and trigonal cross-linking, that is, the sp3- and sp2-hybridized carbon, is of great importance in understanding the properties of amorphous carbon films. In this paper we report a method for deducing the [sp3]/[sp2] ratio from the experimental values of the complex dielectric constant as obtained by optical transmittance and reflectance measurements. We assume a Gaussian-like distribution of π and π* electronic densities of states in order to fit the contribution of π→π* to the imaginary part, ε2, of the dielectric constant in the low-energy region. Through the Kramers-Kronig relationships we deduce the corresponding values of the real part ε1 of the dielectric constant for such transitions. By subtracting these values from the measured ε1 we deduce the contribution of σ→σ* to ε1. The Wemple-Didomenico model has been used to obtain the dispersion energy and the average excitation energy. Knowing the plasmon energies, we apply the ‘‘f-sum rule'' to deduce the [sp3]/[sp2] ratio. The method applied to a-C:H films deposited by rf diode sputtering provides results in agreement with those obtained by other techniques
Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD
We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap
First-principles calculation of the band offset at BaO/BaTiO and SrO/SrTiO interfaces
We report first-principles density-functional pseudopotential calculations on
the atomic structures, electronic properties, and band offsets of BaO/BaTiO
and SrO/SrTiO nanosized heterojunctions grown on top of a silicon
substrate. The density of states at the junction does not reveal any electronic
induced interface states. A dominant perovskite character is found at the
interface layer. The tunability of the band offset with the strain conditions
imposed by the substrate is studied. Using previously reported theoretical data
available for Si/SrO, Si/BaO and BaTiO/SrRuO interfaces we
extrapolate a value for the band alignments along the whole gate stacks of
technological interest: Si/SrO/SrTiO and Si/BaO/BaTiO/SrRuO
heterostructures.Comment: 12 pages, 6 figures, submitted to Phys. Rev.
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