Brillouin scattering studies on charge-ordered manganites

Brillouin scattering studies were carried out for the first time on charge-ordered manganites, namely, Nd0.5Ca0.5MnO3 and Pr0.63Ca0.37MnO3. We observe surface and bulk magnetic excitations (magnons) in the Brillouin spectra with the characteristics of ferromagnetic magnons. The frequency and intensity of these magnons in the temperature range 300 to 25 K, covering both charge-ordering and antiferromagnetic transition, follow exactly the temperature dependence of the DC magnetic susceptibility. The study strongly suggests the presence of ferromagnetic inhomogeneities in the charge-ordered as well as antiferromagnetic phases

Elastic and structural instability of cubic Sn3N4 and C3N4 under pressure

We use in-situ high pressure angle dispersive x-ray diffraction measurements to determine the equation of state of cubic tin nitride Sn3N4 under pressure up to about 26 GPa. While we find no evidence for any structural phase transition, our estimate of the bulk modulus (B) is 145 GPa, much lower than the earlier theoretical estimates and that of other group IV-nitrides. We corroborate and understand these results with complementary first-principles analysis of structural, elastic and vibrational properties of group IV-nitrides, and predict a structural transition of Sn3N4 at a higher pressure of 88 GPa compared to earlier predictions of 40 GPa. Our comparative analysis of cubic nitrides shows that bulk modulus of cubic C3N4 is the highest (379 GPa) while it is structurally unstable and should not exist at ambient conditions.Comment: 5 pages, 4 figure

Conformational Analysis of Molecules: Combined Vibrational Spectroscopy and Density Functional Theory Study

Vibrational spectroscopy can be broadly classified into Raman and infrared (IR). These two techniques are complementary to each other as the mechanisms behind these are different. Vibrational spectroscopy provides detail information about the structure of molecules. The advantage of this technique over X‐ray diffraction is that it can be used to probe molecules in solid, liquid or gas phase. This is especially helpful for studying biomolecules as those molecules can be probed in their physiological environment. Over the last few decades, quantum mechanical calculation has become important tool to assign bands from vibrational spectra. Combination of these two techniques has been used widely in the field of chemistry and biochemistry. In this chapter, we review some of the works that combine both of these techniques. A brief theoretical background is given for understanding the principle of these two techniques

Brillouin Scattering Studies of La_{0.77}Ca_{0.23}MnO_3 Across Metal-Insulator Transition

Temperature-dependent Brillouin scattering studies have been carried out on La_{0.77}Ca_{0.23}MnO_3 across the paramagnetic insulator - ferromagnetic metal (I-M) transition. The spectra show a surface Rayleigh wave (SRW) and a high velocity pseudo surface acoustic wave (HVPSAW) besides bulk acoustic waves (BAW). The Brillouin shifts associated with SRW and HVPSAW show blue-shifts, where as the frequencies of the BAW decrease below the I-M transition temperature (T_C) of 230 K. These results can be understood based on the temperature dependence of the elastic constants. We also observe a central peak whose width is maximum at T_C.Comment: 7 pages, 8 figure

Griffiths phase-like behaviour and spin-phonon coupling in double perovskite Tb2_{2}NiMnO6_{6}

The Griffiths phase-like features and the spin-phonon coupling effects observed in Tb$_2$NiMnO$_6$ are reported. The double perovskite compound crystallizes in monoclinic $P2_1/n$ space group and exhibits a magnetic phase transition at $T_c \sim$ 111 K as an abrupt change in magnetization. A negative deviation from ideal Curie-Weiss law exhibited by 1/$\chi(T)$ curves and less-than-unity susceptibility exponents from the power-law analysis of inverse susceptibility are reminiscent of Griffiths phase-like features. Arrott plots derived from magnetization isotherms support the inhomogeneous nature of magnetism in this material. The observed effects originate from antiferromagnetic interactions which arise from inherent disorder in the system. Raman scattering experiments display no magnetic-order-induced phonon renormalization below $T_c$ in Tb$_2$NiMnO$_6$ which is different from the results observed in other double perovskites and is correlated to the smaller size of the rare earth. The temperature evolution of full-width-at-half-maximum for the {\it stretching} mode at 645 cm$^{-1}$ presents an anomaly which coincides with the magnetic transition temperature and signals a close connection between magnetism and lattice in this material.Comment: 17 pages, 8 figures; accepted in J. Appl. Phy

Pressure induced topological and topological crystalline insulators

[EN] Research on topological and topological crystalline insulators (TCIs) is one of the most intense and exciting topics due to its fascinating fundamental science and potential technological applications. Pressure (strain) is one potential pathway to induce the non-trivial topological phases in some topologically trivial (normal) insulating or semiconducting materials. In the last ten years, there have been substantial theoretical and experimental efforts from condensed-matter scientists to characterize and understand pressure-induced topological quantum phase transitions (TQPTs). In particular, a promising enhancement of the thermoelectric performance through pressure-induced TQPT has been recently realized; thus evidencing the importance of this subject in society. Since the pressure effect can be mimicked by chemical doping or substitution in many cases, these results have opened a new route to develop more efficient materials for harvesting green energy at ambient conditions. Therefore, a detailed understanding of the mechanism of pressure-induced TQPTs in various classes of materials with spin-orbit interaction is crucial to improve their properties for technological implementations. Hence, this review focuses on the emerging area of pressure-induced TQPTs to provide a comprehensive understanding of this subject from both theoretical and experimental points of view. In particular, it covers the Raman signatures of detecting the topological transitions (under pressure), some of the important pressure-induced topological and TCIs of the various classes of spin-orbit coupling materials, and provide future research directions in this interesting field.V R and C N would like to dedicate this review to Professor C N R Rao who has been a mentor and inspiration for us. V R and C N acknowledge the Department of Science and Technology (DST) and JNCASR, India, for financial support. FJM acknowledges project MALTA Consolider Team network (RED2018-102612-T), financed by MINECO/AEI/10.13039/501100003329, I+D+i project PID2019-106383GB-42 financed by MCIN/AEI/10.13039/501100011033, as well as projects PROMETEO/2018/123 (EFIMAT) and CIPROM/2021/075 (GREENMAT) financed by Generalitat Valenciana. We sincerely thank Professor Umesh V Waghmare, Theoretical Sciences Unit, JNCASR, Professor Kanishka Biswas, New Chemistry Unit, JNCASR, Professor Sebastian C Peter, New Chemistry Unit, JNCASR, and Dr Boby Joseph, Elettra Sincrotrone Trieste, Italy for the active collaboration and fruitful discussion on these topics of interest.Rajaji, V.; Manjón, F.; Narayana, C. (2022). Pressure induced topological and topological crystalline insulators. Journal of Physics Condensed Matter. 34(42):1-16. https://doi.org/10.1088/1361-648X/ac8906116344

Optical nonlinearity and charge transfer analysis of 4-[(E)-2-(2,4,6-Trinitrophenyl) ethylidene] benzonitrile adsorbed on silver nanoparticles : Computational and experimental investigations

The search for a potential nonlinear optical (NLO) material has led to the investigation of an organic compound 4-[(E)-2-(2,4,6-Trinitrophenyl)ethylidene]benzonitrile (TEB), which has a possibility of enhancing the NLO properties by the charge transfer mechanism if metal atoms are adsorbed on it. The experimental characterization of TEB is done using Fourier Transform Infrared (FT-IR), Fr-Raman, Ultraviolet-visible (UV-Visible), Photoluminescence (PL), Thermogravimetric/Differential Thermal Analysis (TG/DTA) and Z-scan techniques. The third order NLO properties evaluated using Z-scan technique proves that the material can be used as a good optical limiter. TEB is attached with silver atoms and the theoretical studies including geometry optimization, NBO analysis and hyperpolarizability calculations are carried out. The TEB molecule with silver atoms adsorbed is found to have increased hyperpolarizability values. The charge transfer from the metal atom to the nitrile group of the molecule is evident from the Surface Enhanced Raman Scattering (SERS) spectra using the silver nanoparticles. (C) 2018 Elsevier Ltd. All rights reserved.Peer reviewe

Spin-phonon coupling in multiferroic RCrO3_3 (R-Y, Lu, Gd, Eu, Sm): A Raman study

Raman study on a select few orthochromites, RCrO$_3$ (R = Y, Lu, Gd, Eu and Sm) shows that the phonon behavior at TN in compounds with magnetic R-ion (Gd and Sm) is remarkably different from that of non-magnetic R-ion (Y, Lu and Eu). While anomalies in most of the observed phonon frequencies in all these compounds may result from the distortion of CrO$_6$ octahedra due to size effect and magnetostriction arising from Cr-ordering, the anomalous behavior of their linewidths observed at TN for the compounds with only magnetic R-ion suggests spin-phonon coupling. The presence of spin-phonon coupling and the anomalies in the low frequency modes related to R-ion motion in orthochromites (R = Gd and Sm) support the suggestion that the coupling between 4f-3d moments play important role in inducing switchable electric polarization.Comment: 6 pages (two column format), 7 figures; The updated version of the manuscript can be found at Euro. Phys. Lett. 101, 17008 (2013

A low-cost Raman spectrometer design used to study Raman scattering from a single-walled carbon nanotube

The paper discusses the design of a low cost Raman spectrometer. Singlewalled Nanotubes (SWNT) have been studied to demonstrate the reach of such a system. We observe both the Radial-breathing Mode (RBM) and the tangential mode from the SWNT. The tube diameters of the SWNT used in these experiments have been determined using RBM to be predominantly 1.4 and 1.6 nm. These are consistent with the TEM images taken of the same sample. The new method of producing SWNT using Ni-Y catalyst in electric-arc discharge method produces nanotubes with very small dispersion in diameter and high yields. The chirality of the SWNT can be deduced from their radial breathing modes and it suggests that they are metallic in nature