Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (TP). The values of various optical parameters such as band gap (Eg), Urbach energy (Eu), and Urbach focus (E0) were estimated in the temperature range of 300 K to 480 K. It was observed that with increasing temperature, Eg decreases and shows a sharp anomaly at TP. First principle studies were employed in order to understand the observed change in Eg due to the structural phase transition. Near TP, there exist two values of E0, suggesting the presence of electronic heterogeneity. Further, near TP, Eu shows metastability, i.e., the value of Eu at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of Eu (t=0)/Eu (t = tm), almost remains constant at 300 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of Eu, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across TP. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample.

Room temperature optical absorption spectroscopy (OAS) and Raman spectroscopy measurements have been carried out in order to understand the effect of structural disorder on the electronic and phononic states. For this purpose, polycrystalline samples of Cr doped PrFeO3 have been prepared via wet chemical route. OAS analysis suggests the systematic scaling of electronic disorder with Cr doping; whereas, shifting in Raman line shapes and an increase in Raman line width has been observed with Cr doping. X-ray diffraction analysis clearly suggests the increase in structural disorders in the form of crystallographic strain with Cr doping, which is consistent with the broadening in Raman line shapes. The major contribution to Raman line width has been understood in terms of temperature independent terms i.e. structural disorder induced by doping. The generation of a new phonon mode at ~510 cm−1 has been observed and understood as a disorder phonon mode due to strain induced structural disorder. Moreover, a systematic correlation between crystallographic strain, Raman line width, disordered parameter (σ) and Urbach energy has been observed, which implies that structural disorder affects phononic as well as electronic states of the system. Such comparative study allows us to find the correlation between densities of tail states, structural disorders and anharmonic effects probed by Raman spectroscopy.

A combined effect of doping (type and species) and size on Raman scattering from silicon (Si) nanowires (NWs) has been presented here to study interplay between quantum confinement and Fano effects. The SiNWs prepared from low doping Si wafers show only confinement effect, as evident from the asymmetry in the Raman line‐shape, irrespective of the doping type. On the other hand SiNWs prepared from wafer with high doping shows the presence of electron–phonon interaction in addition to the phonon confinement effect as revealed from the presence of asymmetry and antiresonence in the corresponding Raman spectra. This combined effect induces an extra asymmetry in the lower energy side of Raman peak for n‐type SiNWs whereas the asymmetry flips from lower energy side to the higher energy side of the Raman peak in p‐type SiNWs. Such an interplay can be represented by considering a general Fano‐Raman line‐shape equation to take care of the combined effect in SiNWs. Copyright © 2015 John Wiley & Sons, Ltd.

Quantification of the short-range order in amorphous silicon has been formulized using Raman scattering by taking into account established frameworks for studying the spectral line-shape and size dependent Raman peak shift. A theoretical line-shape function has been proposed for representing the observed Raman scattering spectrum from amorphous-Si-based on modified phonon confinement model framework. While analyzing modified phonon confinement model, the term "confinement size" used in the context of nanocrystalline Si was found analogous to the short-range order distance in a-Si thus enabling one to quantify the same using Raman scattering. Additionally, an empirical formula has been proposed using bond polarizability model for estimating the short-range order making one capable to quantify the distance of short-range order by looking at the Raman peak position alone. Both the proposals have been validated using three different data sets reported by three different research groups from a-Si samples prepared by three different methods making the analysis universal.

Abstract. A step-by-step evolution of an asymmetric Raman line-shape function from a Lorentzian line-shape is presented here for low dimensional semiconductors. The evolution reported here is based on the phonon confinement model which is successfully used in literature to explain the asymmetric Raman line-shape from semiconductor nano-structures. Physical significance of different terms in the theoretical asymmetric Raman line-shape has been explained here. Better understanding of theoretical reasoning behind each term allows one to use the theoretical Raman line-shape without going into details of theory from first principle. This will enable one to empirically derive a theoretical Raman line-shape function for any material if information about its phonon dispersion, size dependence etc is known. ‡ Corresponding Author: rajeshkumar@iiti.ac.in arXiv:1309.5180v1 [cond-mat.mes-hall]