Formation of strong-coupling (bi)polarons and related in-gap states in lightly-doped cuprate superconductors

The formation of large polarons and bipolarons and related in-gap states in lightly-doped cuprates is studied in the strong electron–phonon coupling regime. The ground-state energies, binding energies and binding radius of strong-coupling large (bi)polarons are calculated using the continuum model and adiabatic approximation taking into account both the short- and long-range electron–phonon interactions. The obtained results show that the binding energies of such large (bi)polarons in the cuprates are progressively increased with decrease in the high frequency dielectric constant [Formula: see text] and ratio [Formula: see text] (where [Formula: see text] is the static dielectric constant). As a result, large (bi)polarons become nearly small (bi)polarons. It is shown that hole carriers strongly interact with acoustic and optical phonons and their self-trapping leads to the formation of (bi)polaronic states in the charge-transfer (CT) gap of the cuprates. As the cuprate system is doped, the CT gap of the parent compound is filled in with low-energy (bi)polaronic states. The calculated energies of such (bi)polaronic states are in good agreement with the experimentally observed energies of new states appearing in the CT gap of the parent cuprates. The calculated values of the binding radius of large polarons are also well consistent with the existing experimental data.

Estimation of optical parameters of silicon single crystals with different orientations

Optical properties of Si single crystals with different orientations (1 0 0) and (1 1 1) were investigated using spectrophotometric measurements in a spectral range of 200 nm to 2500 nm. The data of optical absorption revealed an indirect allowed transition with energy gap of 1.1 ± 0.025 eV. An anomalous dispersion in refractive index. The normal dispersion of the refractive index was discussed according to Wemple-DiDomenico single oscillator model. The oscillator energy Eo, dispersion energy Ed, high frequency dielectric constant ∈∞, lattice dielectric constant ∈L and electronic polarizability αe were estimated. The real ∈1 and imaginary ∈2 parts of dielectric constant were also determined.

Structural and optical properties of AgIn5S8

Compounds of the chalcogenide family Ag–In–VI (VI = S, Se, Te) are interesting materials due to their stoichiometric stability and potential application in nonlinear optics and solar cells. A polycrystalline ingot of AgIn5S8, an ordered vacancy semiconductor, was prepared by direct fusion of the stoichiometric mixture of the elements in an evacuated quartz ampoule. The presence of a single phase with cubic structure was confirmed by X-ray powder diffraction at room temperature. The lattice parameter, [Formula: see text], was calculated, giving 10.821750 Å. Samples in evacuated quartz ampoules were used to perform Differential Thermal Analysis measurements, showing congruent melting at 1110[Formula: see text]C. Transmittance and reflectivity measurements were used to calculate the absorption coefficient [Formula: see text]. From the plot of ([Formula: see text])2 versus [Formula: see text], two direct transitions are observed at 1.25 eV and 1.88 eV. While the higher energy direct transition has been observed by other authors, the direct nature of the lower energy transition was confirmed from the fitting of the plot of the reflectivity versus 1/[Formula: see text] between 0.53 eV[Formula: see text] (1.89 eV) and 0.55 eV[Formula: see text] (1.82 eV), obtaining a value of 1.29 eV. The real refractive index [Formula: see text] and the high-frequency dielectric constant [Formula: see text] were also obtained from the fit of the reflectivity, resulting to be 2.68 and 7.2, respectively.

Optical Properties of Multilayers TiO2/SnO2:F Thin films

Thin films of TiO2/SnO2:F multilayers were prepared by spray pyrolysis technique on glass substrate. The samples were prepared using titanium (IV) isopropoxide 98%, ammonium fluoride and tin(II) chloride dehydrate extra pure as precursor materials. Thus, a TiO2 thickness dependence optical study of TiO2/SnO2:F/glass system is presented. The optical property was characterized by UV-Visible transmittance spectroscopy. For all the samples, the average transmissions in the visible wavelength region (400-800 nm) were between 69 and 84%. The optical parameters, such as the real part dielectric function ε1 and the imaginary part dielectric function ε2, of TiO2 and SnO2:F in the TiO2/SnO2:F thin films structure, determined by fitting the measured optical transmittance spectra, are presented and analyzed. Among the various classical dispersion relations for the dielectric function, the Drude model combined with the Lorentzian oscillators was used to get a good fit of transmittance in the measured spectral range. Results on related parameters such as high frequency dielectric constant, plasma frequency, film thickness and band gap are presented.

Correlation of resistance switching behaviors with dielectric functions of manganite films: A study by spectroscopic ellipsometry

ABSTRACTPr0.5Ca0.5MnO3 (PCMO) films were deposited on LaAlO3 (100) substrates under pressure from 1.33 to 5.33 Pa by RF magnetron sputtering. Resistance switching and dielectric functions of PCMO films were studied by DC current-voltage characteristic measurements and spectroscopic ellipsometry (SE) measurements. Resistance switching was observed in the devices composed of PCMO films deposited under low pressures of 1.33 and 2.67 Pa. SE measurements revealed that dielectric functions also depended on deposition pressure. PCMO films deposited under lower pressure had larger high-frequency dielectric constant, larger oscillator strength of the electric dipole charge transitions in MnO6 octahedral complexes, and lower oscillator strength of d-d transitions in Mn3+ and Mn4+ ions. SE measurements suggested that oxygen vacancies and MnO6 octahedral complexes play an important role in resistance switching in PCMO films.

Effect of Nano ZnO on the Optical Properties of Poly(vinyl chloride) Films

Optical properties of pure and doped poly(vinyl chloride) (PVC) films, prepared by using casting technique, with different nanosize zinc oxide (ZnO) concentrations (1–20) wt% have been studied. Parameters such as extinction coefficient, refractive index, real and imaginary parts, Urbach energy, optical conductivity, infinitely high frequency dielectric constant, and average refractive index were studied by using the absorbance and transmittance measurement from computerized UV-visible spectrophotometer (Shimadzu UV-1601 PC) in the spectral range 200–800 nm. This study reveals that the optical properties of PVC are affected by the doping of ZnO where the absorption increases and transmission decreases as ZnO concentration increases. The extinction coefficient, refractive index, real and imaginary parts, infinitely high frequency dielectric constant, and average refractive index values were found to increase with increasing impurity percentage. The Urbach energy values are found to be decreasing with increasing ZnO concentration. The optical conductivity increased with photon energy after being doped and with the increase of ZnO concentration.

Pool-Frenkel effect and high frequency dielectric constant determination of semiconducting P2O5-Li2MoO4-Li2O and P2O5-Na2MoO4-Na2O bulk glasses

The ternary 70P2O5-10Li2MoO4-20Li2O and 70P2O5-10Na2MoO4-20Na2O glasses, prepared by the press-melt quenching technique, were studied at temperatures between 298 and 418 K for their high dc electric field properties. For the above purpose, the effect of a strong electric field on the dc conduction of these amorphous bulk samples was investigated using the gap-type electrode configuration. At low electric fields, the current-voltage (I — V) characteristics have a linear shape, while at high electric fields (> 103 V/cm), bulk samples show nonlinear effects (nonohmic conduction). Current-voltage curves show increasing departure from Ohm’s law with increasing current density, leading to critical phenomena at a maximum voltage (threshold voltage), known as switching (switch from a low-conduction state to a higher-conduction state at threshold voltage). The Pool-Frenkel high-field effect was observed at electrical fields of about 103–104 V/cm; then the lowering factor of the potential barrier, the high frequency dielectric constant, and the refractive index of these glasses were determined.

PRESSURE DEPENDENCE OF OPTO-ELECTRONIC PROPERTIES IN ZnSxSe1-x

We present an investigation of the electronic properties and optical constants of zinc-blende ZnS x Se 1-x semiconducting alloys at normal and under hydrostatic pressure up to 20 kbar. For this purpose, we used an empirical pseudopotential method within the virtual crystal approximation. The effects of alloy composition are taken into consideration in the calculation, which improves significantly the bandgap bowing parameters with respect to the experiment. Results regarding the composition and pressure dependences of energy bandgaps, electron valence and conduction charge distributions, optical high-frequency dielectric constant and its linear pressure coefficient are presented and discussed. The information derived from the present study may be useful for the development of opto-electronic devices that operate in the blue/green spectral range.