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One of the fundamental interactions that is found in many functional materials is the spin-phonon coupling (SPC), which is at the heart of many novel functionalities. The simultaneous presence of multi-magnetic phases makes SPC even more intriguing. We have used Raman spectroscopy as well as first-principles methods to investigate the possibility of the appearance of SPC in Gd-doped Y$_2$CoMnO$_6$ (YGCMO) double perovskite oxide and the influence of anti-site disorder on the same. YGCMO is found to exhibit anti-site disorder leading to both ferromagnetic (between Co and Mn) and anti-ferromagnetic interactions (Co-Co, Mn-Mn, Gd-Co/Mn). An analysis of the temperature-dependent phonon frequency for the stretching modes of YGCMO, obtained using RAMAN spectroscopy, indicates that SPC is possibly emerging from simultaneous presence of ferromagnetic and antiferromagnetic interactions. The nature of the phonon linewidth and the insulating state of the material eliminate the role of magnetostriction on the observed anomaly. The spin-phonon coupling strength comes out to be 0.29 cm$^{-1}$. Our experimental findings are corroborated by first-principles DFT calculations which indicate the presence of SPC in ordered YGCMO getting enhanced in the presence of anti-site disorder. This indicates a strong influence of B-site (Co/Mn) ordering on SPC in the bulk double perovskite systems. An analysis of the cause behind the enhanced SPC in the presence of anti-site disorder is also presented., Comment: 23 pages, 14 figures

We report emergence of photoluminescence at room temperature in trigonal $Cs_3Bi_2Br_9$ at high pressures. Enhancement in intensity with pressure is found to be driven by increase in distortion of $BiBr_6$ octahedra and iso-structural transitions. Electronic band structure calculations show the sample in the high pressure phase to be an indirect band gap semiconductor. The luminescence peak profile show signatures related to the recombination of free and self trapped excitons, respectively. Blue shift of the both peaks till about 4.4 GPa are due to the exciton recombination before relaxation due to the decrease in exciton lifetime with scattering from phonons

LaAgSb$_{2}$ is a rare material, which offers the opportunity to investigate the complex interplay between charge density wave (CDW) ordering and topology protected electronic band structure. As both of these phenomena are governed by the structural symmetries, a comprehensive study of the lattice dynamics is highly desirable. In this report, we present the results of temperature and pressure dependent Raman spectroscopy and x-ray diffraction in single crystalline LaAgSb$_{2}$. Our results confirm that Raman spectroscopy is a highly sensitive tool to probe CDW ordering phenomenon, particularly the low-temperature second CDW transition in LaAgSb$_{2}$, which appears as a very weak anomaly in most experiments. The crystal orientation-dependent measurements provide the evolution of Raman modes with crystallographic symmetries and can be further studied through group symmetry analysis. The low-temperature x-ray diffraction data show the emergence of structural modulations corresponding to the CDW instability. The combined high-pressure Raman spectroscopy and synchrotron x-ray diffraction reveal multiple structural phase transitions through lowering of crystalline symmetries, which are also expected to lead to electronic topological transitions.

Using first-principles density functional theory (DFT), we studied the issue of water incorporation in iron bearing wadsleyite (Mg$_2$SiO$_4$)mineral at transition zone pressures and temperature under varying conditions of vacancy concentration and oxygen fugacity. We have considered the effect of varying vacancy concentration by considering single and double vacancy. The affect of varying oxygen fugacity has been modelled through varying ratio of ferric and ferrous ions in the cell. Our first-principles calculations have confirmed that about 30% of the Fe$^{3+}$ can be found at the Si tetrahedral site, which is in accordance with the previous experimental observation by Bolfan-Casanova et al. [2012]. Using different structures of Fe bearing wadsleyite, representing vacancy concentration and oxidizing condition, we calculated elastic properties, such as bulk and shear moduli, phase and shear wave velocity, as a function of pressure and temperature. By comparison of our theoretical prediction with the Preliminary Reference Earth Model (PREM) data, we conclude that the water content of the transition zone decreases with increasing depth, corroborating the previously proposed theories on mantle convection [Bercovici and Karato, 2003]., Comment: one author not agree with results part

Electron- and hole-doped La$_2$CoMnO$_6$(LCMO) are investigated using first principles DFT calculations. Hole and electron doping are achieved respectively by introducing Sr$^{2+}$ at La$^{3+}$ sites and by inducing O-site vacancies in LCMO. Electronic structure calculations suggest that hole doping alters the charge and valence state of Co ions, whereas electron doping influences the Mn ions. Introduction of defects is found to enhance antisite disorder(ASD) at Co/Mn site, which is expected to influence the magnetodielectric properties of the system. Our calculations suggest that while ASD and/or hole doping induces half-metallicity in LCMO, electron doping restores its insulating state. Mean-field calculations performed using exchange constants obtained by mapping DFT total energies of different collinear spin configurations onto the Ising Hamiltonian find that defects tend to reduce the Curie temperature ($T_C$). Interestingly, the calculated linear optical properties of the system suggest that the material becomes optically active with high values of birefringence in the presence of defects, a property that is highly sought-after in the optical communications and laser industry., Comment: 14 pages, 9 figures

Topological materials provide an exclusive platform to study the dynamics of relativistic particles in table-top experiments and offer the possibility of wide-scale technological applications. ZrSiS is a newly discovered topological nodal-line semimetal and has drawn enormous interests. In this report, we have investigated the lattice dynamics and electron-phonon interaction in single crystalline ZrSiS using Raman spectroscopy. Polarization and angle resolved measurements have been performed and the results have been analyzed using crystal symmetries and theoretically calculated atomic vibrational patterns along with phonon dispersion spectra. Wavelength and temperature dependent measurements show the complex interplay of electron and phonon degrees of freedom, resulting in resonant phonon and quasielastic electron scatterings through inter-band transitions. Our high-pressure Raman studies reveal vibrational anomalies, which were further investigated from the high-pressure synchrotron x-ray diffraction (HPXRD) spectra. From HPXRD, we have clearly identified pressure-induced structural transitions and coexistence of multiple phases, which also indicate possible electronic topological transitions in ZrSiS. The present study not only provides the fundamental information on the phonon subsystem, but also sheds some light in understanding the topological nodal-line phase in ZrSiS and other iso-structural systems.

Disorder, primarily in the form of oxygen vacancies, cation stoichiometry and atomic inter-diffusion, appear to play vital roles in the electronic and transport properties of the metallic electron liquid at the oxide hetero-interfaces. Antisite disorder is also understood to be a key player in this context. In order to delineate the roles of two of these key factors, oxygen vacancy and antisite disorder, we have investigated the effect of oxygen vacancy on the antisite disorder at a number of interfaces separating two La-based transition metal oxides using density functional theory. Oxygen vacancy is found to suppress the antisite disorder in some heterostructures and thereby stabilizes the ordered structure, while in some other systems it tends to drive the disorder. Thus by controlling the oxygen partial pressure during the growth, it is possible to engineer the antisite disorder in many oxide heretostructures.

In this article, we report negative thermal expansion and spin frustration in hexagonal GdInO$_{3}$. Rietveld refinement of the XRD patterns reveal that the negative thermal expansion in the temperature range of 50-100K stems from the triangular lattice of Gd$^{3+}$ ions. At low temperature, the downward deviation of the inverse susceptibility ($\chi^{-1}$) vs. $T$ plot from the Curie-Weiss law indicates spin frustration which inhibits long-range magnetic ordering down to 2K. Magnetostriction measurements clearly demonstrate a strong spin-lattice coupling. Low temperature anomalous phonon softening, as obtained from temperature dependent Raman measurements, also reveals the same. Our experimental observations are supported by first principles density functional theory calculations of the electronic and phonon dispersion of GdInO$_3$. The calculations suggest that the GdInO$_3$ lattice is highly frustrated at low temperature. Further, the calculated normal mode frequencies of the Gd related $\Gamma$ point phonons are found to depend on the magnetic structure of the lattice, suggesting significant magneto-elastic coupling., Comment: 16 pages, 10 figures