An origin of light induced centers in the visible range in ferroelectric oxides: possible role of the states of charge transfer vibronic excitons

A light induced absorption peculiarity connected with an absorption peak in the visible range for SBN/Ce was detected. The analysis of these data together with investigation results for a visible range center (VIS-center) absorption study in SBN/Ce and Cr, in nominally pure strontium barium niobate (SBN) as well as in Ba_0.77Ca_0.23TiO₃ crystals give support to charge transfer vibronic excitons (CTVEs) as origin of the phenomenon. While the fast (with relaxation time ∼1 ns at T∼300 K) response can be connected with transitions between excited branches of the adiabatic potential of the CTVE phase, the slow (with relaxation time ∼50 s at T∼230 K for SBN/Cr) response is connected with transitions between ground and first excited CTVE phase branches. The additional contribution to the latter effect due to the absorption of recharged oxygen ions which are in the framework of charge transfer induced by the CTVE cannot be disregarded. The explanation of the main results of the experiments shows that the VIS-center phenomena can be induced by the CTVE-states.     

Superconducting gap in iron pnictides studied by optical spectroscopy

We investigated the in-plane optical spectrum of a Co-doped Ba(Fe_0.94Co_0.06)₂As₂ single crystal showing superconductivity below . In the normal state, the low-energy optical conductivity spectrum can be decomposed into a sharp Drude term and a broad “incoherent” term. Below T_c, an s-wave-like superconducting gap appears in both components. We also investigated a magnetic or spin-density-wave gap in the detwinned parent compound.     

The influence of chromium on the defect structure and their mobility in nonstoichiometric cobaltous oxide

Defect structure and the mobility of point defects in pure metal deficient cobalt oxide (Co_1−yO) and in Co_1−yO-Cr₂O₃ solid solutions have been studied as a function of temperature (1223-1573 K) and oxygen pressure (10-10⁵ Pa) using microthermogravimetric techniques. It has been shown that the predominant defects in pure and Cr-doped cobaltous oxide are singly ionized cation vacancies, and 3% at of dopant is high enough to fix the concentration of predominant defects in such solid solutions on a constant level being much higher than in pure Co_1−yO. Re-equilibration rate measurements have demonstrated that the chemical diffusion coefficient and thereby the mobility of point defects in pure Co_1−yO is concentration independent, strongly suggesting that in spite of rather high their concentration no interactions and clustering of defects is to be expected. On the other hand, in Cr-doped cobaltous oxide, re-equilibration rate measurements have shown, that in this case the defect structure is more complicated, although singly ionized cation vacancies seem to be still predominant defects.     

Hydration states and metallic intercalation effects in dry DNA

Temperature dependence of the complex dielectric constant at 16.3 GHz has been measured on dry poly(dA)-poly(dT) DNA with Na⁺ counterions under different relative humidity (RH). The rotational motion of sole water dipoles hydrated in PO₂⁻ is detected at low RH, and the collective motion due to water network in major groove is successfully observed at high RH. From the infrared (IR) spectroscopy in dry DNA introducing metallic ions (M-DNA), the symmetric and antisymmetric stretching bands of PO₂⁻ distinctly depend on RH and type of metallic ions. The IR spectral change indicates that the monovalent ions (Li and Na) are arranged in PO₂⁻ though the divalent and trivalent ions connect to the base molecules.     

Singlet and triplet bipolarons on the triangular lattice

We study the Coulomb-Fröhlich model on a triangular lattice, looking in particular at states with angular momentum. We examine a simplified model of crab bipolarons with angular momentum by projecting onto the low energy subspace of the Coulomb-Fröhlich model with large phonon frequency. Such a projection is consistent with large long-range electron-phonon coupling and large repulsive Hubbard U. Significant differences are found between the band structure of singlet and triplet states: The triplet state (which has a flat band) is found to be significantly heavier than the singlet state (which has mass similar to the polaron). We test whether the heavier triplet states persist to lower electron-phonon coupling using continuous time quantum Monte Carlo (QMC) simulation. The triplet state is both heavier and larger, demonstrating that the heavier mass is due to quantum interference effects on the motion. We also find that retardation effects reduce the differences between singlet and triplet states, since they reintroduce second order terms in the hopping into the inverse effective mass.     

Effect of electrostatic screening due to non-equilibrium carriers on the lattice scattering rate at low temperatures

The intravalley acoustic scattering rate has been calculated here taking the screening by non-equilibrium electrons into account under the condition when the lifetime of the electrons is controlled by shallow attractive traps at low lattice temperature. The scattering rates now turn out to be field dependent and the characteristics are significantly different from what follows when the electron ensemble is in equilibrium with the lattice. The results indicate the possibility of interesting non-ohmic transport characteristics under these conditions. Numerical results are obtained for high purity samples of Si.     

Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations

The electronic structures of titanium dioxide (TiO₂) doped with 3d transition metals (V, Cr, Mn, Fe, Co and Ni) have been analyzed by ab initio band calculations based on the density functional theory with the full-potential linearized-augmented-plane-wave method. When TiO₂ is doped with V, Cr, Mn, Fe, or Co, an electron occupied level occurs and the electrons are localized around each dopant. As the atomic number of the dopant increases the localized level shifts to lower energy. The energy of the localized level due to Co is sufficiently low to lie at the top of the valence band while the other metals produce midgap states. In contrast, the electrons from the Ni dopant are somewhat delocalized, thus significantly contributing to the formation of the valence band with the O p and Ti 3d electrons. Based on a comparison with the absorption and photoconductivity data previously reported, we show that the t_2g state of the dopant plays a significant role in the photoresponse of TiO₂ under visible light irradiation.     

Looking at the superconducting gap of iron pnictides

THz and infrared spectroscopies are widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron-phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides.     

The effect of a large SWNT diameter distribution on cesium intercalation

Vapor-phase intercalation of a single-walled carbon nanotube sample with Cs was carried out and monitored in situ by Raman spectroscopy. Results indicate that the endpoint of the intercalation was limited by small interstitial gaps in the nanotube bundles. These small-diameter gaps are present because of the significant number of small-diameter nanotubes (0.9-1.0 nm, as calculated from Raman radial breathing mode frequencies) present in the sample. It is not possible to determine from our Raman spectra whether the early endpoint is the result of diffusion limitation or the equilibrium energetics at the endpoint, although some diffusion limitation is observed near the beginning of the reaction. A simple geometric model for expansion of the nanotube bundles under intercalation is presented; this model reproduces, reasonably well, measured expansions reported by others and explains both diffusion- and equilibrium-limited mechanisms in terms of the larger lattice expansion required for smaller-diameter nanotubes. Staging of the intercalation process, in analogy with the staged intercalation of graphite intercalation compounds, is not observed. Instead, the transverse mode peaks undergo a gradual decrease in intensity and a gradual charge transfer- and electronic coupling-induced downshift.     

Phonon spectra of stoichiometric rare-earth compound of EuGa2S4

Investigations of EuGa₂S₄ have been done on the photoluminescence (PL) related to the transition between 4f⁶5d and 4f⁷ configuration of the Eu²⁺ ion and its excitation (PLE) spectra, Raman scattering and infrared absorption. The energies of phonons coupled to the ground and the excited states of the transition are analyzed to be 34 and 19 meV from the shapes of the PL and PLE bands, respectively. The former corresponds to the energy of the Raman line showing the highest intensity. The latter is close to the value obtained from analysis of the temperature dependence of the half width of the PL band. These correspondences indicate that the relevant emission of EuGa₂S₄ surely has phonon-terminated character.     

Effects of Fe doping on crystalline and optical properties of yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) samples with different Fe concentrations were prepared to study the effects of Fe doping on crystalline and optical properties of YSZ. The former properties were determined by X-ray diffraction, while the latter properties were determined by diffuse reflectance (DR) and photoluminescence (PL) spectroscopies. Lattice contraction of YSZ caused by the Fe doping was observed. We revealed that the DR spectra of the 3 and 6 mol% Fe-doped YSZ samples originate from the Fe ions dissolved and undissolved in the YSZ, respectively. Moreover, two PL bands centered around 440 and 530 nm were observed for the YSZ sample, whereas one PL band centered around 440 nm was observed for the Fe-doped YSZ samples. The Fe doping reduced the PL intensity of YSZ and quenched the PL band around 530 nm. This could be explained by considering that the concentration of Fe ions near the surface of YSZ is much larger than that in the bulk of YSZ or by considering that the Fe doping enhances surface band bending of YSZ.     

Relationships between lattice energy and electronic polarizability of AB crystals

The correlations between the electronic polarizability, determined from Clausius-Mosotti equation based on dielectric constant ε, and the lattice energy density u have been established for A^NB^8-N crystals, such as the systems of rock salt crystals (group I-VII, II-VI) and tetrahedral coordinated crystals (group II-VI, III-V). For the A^NB^8-N crystals systems, our present conclusions suggest that lattice energy density u decreases exponentially with increasing electronic polarizability, and the normal mathematical expression between lattice energy density u and electronic polarizability is u = pα^q, p and q depend on the type of crystals. For the same cation binary A^NB^8-N crystals systems, curve fitting equations have been obtained, and the relevant squares of the correlation coefficient R² are larger than 0.99, which show all lattice energy density u are in good exponential relation with electronic polarizability. These empirical equations will give more information on calculating lattice energy or electronic polarizability. New data of lattice energy have been calculated on the above equation u = pα^q, and a good linear trend in the calculating values along with the Zhang’s values has been obtained.     

Effects of non-equilibrium polar optic phonons and the band non-parabolicity on small signal high-frequency hot electrons ac mobility in narrow gap semiconductors in high magnetic fields

The small signal high-frequency ac mobility of hot electrons in n-HgCdTe in the extreme quantum limit at low and high temperatures have been calculated considering the non-equilibrium phonon distribution as well as the thermal phonon distribution .The energy loss rate has been calculated considering only optical phonon scattering while the momentum loss rate has been calculated considering acoustic phonon scattering and piezoelectric scattering together with polar optical phonon scattering and separately considering only the polar optical scattering. The results have been discussed and compared. It has been observed that at 20 K, the normalized mobility considering all the three scattering mechanisms differs appreciably from that considering only the polar optical phonon scattering. However, at 77 K, there is no difference in the normalized mobility. This establishes the fact that at higher temperature, the effect of acoustic phonon scattering and piezoelectric coupling is negligible, compared to the polar optical phonon scattering. So the ac mobility considering only polar optical phonon scattering has been studied at 77 and 20 K. The ac mobility is found to remain constant up to 100 GHz and thereafter it started decreasing at higher frequencies at 77 K whereas the ac mobility reduces at much lower frequencies at lower temperature at lower field. The non-parabolicity of the band structure enhances the normalized mobility.     

The influence of oxygen vacancies on the electronic and magnetic properties of perovskite-like SrFeO3-x

The electronic, magnetic properties and lattice relaxations of oxygen-deficient cubic strontium ferrite, SrFeO_2.875, in ferromagnetic configuration are studied by means of the density functional theory using LCAO basis (SIESTA code) calculations. It is shown that Fe and Sr atoms are displaced from oxygen vacancies while oxygen anions are attracted to the vacancies. The DOS distributions, magnetic moments and atomic effective charges are analyzed in comparison with vacancy free SrFeO₃; these parameters are found to change weakly with appearance of oxygen vacancies, in contrast to conventional ionic picture. Some strengthening of Fe-O covalent bonds in the vicinity of the oxygen vacancy is found. The formation energy of oxygen vacancies and divacancies are evaluated.     

Oxygen non-stoichiometry and defect thermodynamics in La2Ni0.9Fe0.1O4+δ

The oxygen hyperstoichiometry of K₂NiF₄-type La₂Ni_0.9Fe_0.1O_4+δ, studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10⁻⁵-0.7 atm at 923-1223 K, is considerably higher than that of undoped lanthanum nickelate. The p(O₂)-T-δ diagram of iron-doped lanthanum nickelate can be adequately described by introducing point-defect interaction energy in the concentration-dependent part of defect chemical potentials and accounting for the site-exclusion effects. The critical factors affecting the equilibrium oxygen incorporation process include coulombic repulsion of interstitial anions, trapping of the p-type electronic charge carriers by iron, and interaction between Fe³⁺ and holes localized on nickel cations. Due to low chemical expansion of La₂Ni_0.9Fe_0.1O_4+δ lattice, the thermodynamic functions governing oxygen intercalation, site-blocking factors and hole mobility are all independent of the defect concentrations. The predominant 3+ state of iron cations under oxidizing conditions was confirmed by the Mössbauer spectroscopy. The stability of La₂NiO₄-based phase in reducing atmospheres is essentially unaffected by doping.     

Evaluation of stoichiometric rare-earth molybdate and tungstate compounds as laser materials

Photoluminescence spectra, photoluminescence decay curves and Raman scattering spectra have been investigated for stoichiometric rare-earth molybdate and tungstate compounds. NaNd(MoO₄)₂ and NaNd(WO₄)₂ show emissions due to the transition ⁴F_3/2→⁴I_9/2 in Nd³⁺. A possibility of laser oscillation in NaNd(MoO₄)₂ is pointed from comparisons of the emission intensity and the decay time constant with NaNd(WO₄)₂ where laser oscillations have been reported. In NaLa(MoO₄)₂ and NaLa(WO₄)₂, observed emissions which are not related to La³⁺ are probably due to the transitions in MoO₄^2- and WO₄^2- molecular ions, respectively, in scheelite crystal. Raman spectra of these compounds are similar, probably related to the same crystal structure. LiEr(MoO₄)₂ shows the emissions due to transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 in Er³⁺, respectively, which are believed to be observed for the first time.     

Ab initio study of phonons in the rutile structure of TiO2

The local-density approximation is used to find the phonon dispersion relations, total and partial phonon density of states for TiO₂ crystal of rutile structure. For that the Hellmann-Feynman forces were computed and direct method applied. Some thermodynamic quantities are also presented. Calculated results are analyzed and compared with the experimental neutron scattering and optic data. Good agreement has been achieved. A giant LO/TO splitting is observed for A_2u and E_u modes.     

Incident-angle dependence of the longitudinal optic mode and the Doppler effect in silica glass

The infrared reflection spectra of silica glass were monitored at different incident angles of the wave. The density of states (DOS) and frequency ω_LO of the longitudinal optic (LO) mode were found to be the functions of the incident angle. The DOS of the LO mode increases with increasing incident angle as a whole. However, two regions can be divided in which one is below ∼30° and the other is above 30°. The frequency ω_LO of the LO mode increases linearly with incident angle. The earlier proposed relationship between the shifted frequency produced by the Doppler effect and the source frequency of the main transverse optic (TO) mode was investigated by studying the infrared spectra of the annealed glass capillaries. The shifted frequency was found to be linearly proportional to the source frequency of the TO mode, as depicted by the Doppler effect.     

Schottky defects in cubic lattice of SrTiO3

Structural and electronic properties produced by formation of Schottky defects in cubic structure of SrTiO₃ crystal are investigated by means of a quantum-chemical simulation based on the Hartree-Fock methodology. The occurrence of Sr partial Schottky defect (V_Sr+V_O) and two types of Ti partial Schottky defects (V_Ti+2V_O) is modeled using a supercell containing 135 atoms. Vacancy-induced changes in the positions of their neighboring atoms are analyzed in light of the computed electron density redistribution in the defective region of supercell. The observed local one-electron energy levels in the gap between the upper valence band and the conduction band can be attributed to the presence of anion and cation vacancies.     

Structure-related infrared optical properties of BaTiO3 thin films grown on Pt/Ti/SiO2/Si substrates

BaTiO₃ thin films with different thickness have been grown on Pt/Ti/SiO₂/Si substrates by a modified sol-gel method. X-ray diffraction analyses show that the BaTiO₃ thin films are polycrystalline. The crystalline quality of the films is improved with increasing thickness. The infrared optical properties of the BaTiO₃ thin films have been investigated using an infrared spectroscopic ellipsometry in the wave number range of 800-4000 cm⁻¹ (2.5-12.5 μm). By fitting the measured pseudodielectric functions with a three-phase model (Air/BaTiO₃/Pt), and a derived classical dispersion relation for the thin films, the optical constants and thicknesses of the thin films have been simultaneously obtained. The refractive index of the BaTiO₃ thin films increases and on the other hand, the extinction coefficient does not change with increasing thickness in the entirely measured wave number range. The dependence of the refractive index on the film thickness has been discussed in detail and was mainly due to both the crystalline quality of the films and packing density. Finally, the absorption coefficient was calculated in the infrared region for applications in the pyroelectric IR detectors.