arXiv subjects

G. Vaitheeswaran

Publications and source records attributed to G. Vaitheeswaran.

At least 19 recordsLinked to original sources

arXiv2025

Correlation stabilized ferromagnetic MnRuAs with distorted kagome lattice

We present an in-depth analysis of MnRuAs, a compound crystallizing in the P$\bar{6}$2m symmetry with a distorted kagome lattice, revealing its distinctive structural, magnetic, and electronic properties through state-of-the-art ab initio calculations. By incorporating strong correlation effects using the DFT+U approach, we demonstrate the stabilization of MnRuAs, transforming its inherent dynamical instability into a robust ferromagnetic state with significant coupling along the $c$ axis. The calculated magnon dispersion reveals a parabolic profile with a minimum at the $Γ$ point, indicative of ferromagnetic behavior. Furthermore, MnRuAs exhibits intriguing electronic properties, including quasi-one-dimensional Fermi surface and the formation of nodal sphere. Our study also delves into the electronic surface states and constant energy contours, offering valuable insights into the complex physics of this material.

cond-mat.mtrl-sci↗

arXiv2025

Evidences for local non-centrosymmetricity and strong phonon anomaly in EuCu2As2: A Raman spectroscopy and lattice dynamics study

Phonon modes and their association with the electronic states have been investigated for the metallic EuCu$_{2}$As$_{2}$ system. In this work, we present the Raman spectra of this pnictide system which clearly shows the presence of seven well defined peaks above $100$~cm$^{-1}$ that is consistent with the locally non-centrosymmetric {\it P4/nmm} crystal structure, contrary to that what is expected from the accepted symmorphic {\it I4/mmm} structure. Lattice dynamics calculations using the {\it P4/nmm} symmetry attest that there is a commendable agreement between the calculated phonon spectra at the $Γ$ point and the observed Raman mode frequencies, with the most intense peak at $\sim 232$~cm$^{-1}$ being ascribed to the A$_{1g}$ mode. Temperature dependent Raman measurements show that there is a significant deviation from the expected anharmonic behaviour around $165$~K for the A$_{1g}$ mode, with anomalies being observed for several other modes as well, although to a lesser extent. Attempts are made to rationalize the observed anomalous behavior related to the hardening of the phonon modes, with parallels being drawn from metal dichalcogenide and allied systems. Similarities in the evolution of the Raman peak frequencies with temperature seem to suggest a strong signature of a subtle electronic density wave instability below $165$~K in this compound.

cond-mat.str-el↗

arXiv2023

Spin-phonon coupling suppressing the structural transition in perovskite-like oxide

Multifunctional properties in quantum systems require the interaction between different degrees of freedom. As such, spin-phonon coupling emerges as an ideal mechanism to tune multiferroicity, magnetism, and magnetoelectric response. In this letter, we demonstrate and explain, based on theoretical and experimental analyses, an unusual manifestation of spin-phonon coupling, i.e., prevention of a ferroelastic structural transition, and locking of high-temperature R-3m phase in a magnetically frustrated perovskite-like oxide Ba2NiTeO6. We present Ba2NiTeO6 as a prototype example among its family where long-range antiferromagnetic structure couples with a low-frequency Eg mode (at 55 cm-1) that exhibits a large anharmonicity. Our findings establish that spin-phonon coupling clearly suppresses the phonon anharmonicity preventing the structural phase transition from the R-3m to the C2/m phase in Ba2NiTeO6.

cond-mat.mtrl-sci↗

arXiv2022

Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: A Raman and first-principles study

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. With this note, we have investigated the structural transition of perovskite-type trigonal Ba2ZnTeO6, across Tc ~ 150 K, which is also accompanied by a para- to ferroelastic transition. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, here we report new intriguing observations associated with the phase transition in Ba2ZnTeO6 elucidating the lattice dynamics across the Tc. We have observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile in concomitant with the damping of the soft mode near Tc, shedding light on the lattice dynamics during the phase transition. While most of the phonon bands split below Tc confirming the phase transition, we have observed thermal hysteretic behavior of phonon modes that signifies the first-order nature of the transition and presence of coexisting phases, which are corroborated by our temperature-dependent x-ray diffraction and specific heat measurements. Further, an evidence of the concomitant structural transition appears in the form of huge softening in the thermal response of the soft phonon mode at ~ 31 cm-1 which is remarkable compared to the hitherto known behavior of soft modes in well-known ferroelectrics. This is further corroborated by our phonon calculations that show an unstable Eg-mode in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and Zn translation) which later condenses into the C2/m low-symmetry phase.

cond-mat.mtrl-sci↗

arXiv2019

Structural, vibrational, and electronic properties of TlReO4 studied using a first principles approach

Present work provides extensive DFT study of TlReO4 using pseudopotential approach for understanding the structural and vibrational properties and full-potential linearized augmented plane wave method for electronic band structure and associated optical properties. Calculated bulk modulus value 29.77 GPa using LDA functional is found to be close to experimental value 26 GPa. The order of elastic constant along the principal axis clearly indicates the crucial role of Re and O3 atoms in the structural and mechanical properties of this material. Analysis of Born effective charge (BEC) along with gamma point phonon frequencies through density functional perturbation approach (DFPT) have also shown the importance of these atoms. The anisotropic nature of BEC is mainly found to be contributed due to O3 atoms. An asymmetric stretching of Re-O3 bond is found to be mainly responsible for high intense IR peak in the high-frequency range whereas the second most intense peak is due to the symmetric stretching of these bonds along with the asymmetric stretching of the Re-O2 bond. To get exact electronic band structure, spin orbit coupling (SOC) in addition to Tran-Blaha Modified Becke-Johnson (TB-mBJ) potential have been considered. Inclusion of SOC clearly shows a decreased band gap of 4.71 eV from TB-mBJ implemented band gap 4.82 eV which mainly attributed to 0.08 eV splitting of Re d-states. Study of density of states (DOS) reveals that conduction band bottom (CBB) is mainly made up of Re d-states but main contribution to valence band top (VBT) is due to oxygen p-states. The bonding nature of this material has also been addressed using DOS which is further verified through the electron charge density plot. Though the optical properties of this material are found to be anisotropic but optical isotropy can be seen in lower energy value.

cond-mat.mtrl-sci↗

arXiv2018

Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy

In this work, we report the structural, magnetic and electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat, gamma (~2.7 mJ/mol-K2), as well as the linear nature of temperature dependence of Seebeck coefficient indicate a semi-metallic ground state with a pseudo-gap that is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2*10-3) in Ru2NbAl, it is the largest among all the reported non-doped full Heusler alloys.

cond-mat.str-el↗

arXiv2016

Ab-initio study of structural, vibrational and optical properties of solid oxidizers

We report the structural, elastic and vibrational properties of five ionic-molecular solid oxidizers MNO$_3$ (M = Li, Na, K) and MClO$_3$ (M = Na, K). By treating long range electron-correlation effects, dispersion corrected method leads to more accurate predictions of structural properties and phase stability of KNO$_3$ polymorphs. The obtained elastic moduli show soft nature of these materials and are consistent with Ultrasonic Pulse Echo measurements. We made a complete assignment of vibrational modes which are in good accord with available experimental results. From calculated IR and Raman spectra, it is found that the vibrational frequencies show a red-shift from Li -> Na -> K (Na -> K) and N -> Cl for nitrates (chlorates) due to increase in mass of metal and non-metal atoms, respectively. The calculated electronic structure using recently developed Tran-Blaha modified Becke-Johnson potential show that the materials are wide band gap insulators with predominant ionic bonding between M$^+$ (metal) and NO$_3^-$ /ClO$_3^-$ ions and covalent bonding (N-O and Cl-O) within nitrate and chlorate anionic group. From the calculated optical spectra, we observe that electric-dipole transitions are due to nitrate/chlorate group below 20 eV and cationic transitions occur above 20 eV. The calculated reflectivity spectra are consistent with the available experimental spectra.

cond-mat.mtrl-sci↗

arXiv2016

High pressure structural, elastic and vibrational properties of green energetic oxidizer ammonium dinitramide

Ammonium DiNitramide (ADN) is one of the most promising green energetic oxidizers for future rocket propellant formulations. In the present work, we report a detailed theoretical study on structural, elastic, and vibrational properties of the emerging oxidizer under hydrostatic compression using various dispersion correction methods to capture weak intermolecular (van der Waals and hydrogen bonding) interactions. The calculated ground state lattice parameters, axial compressibilities, and equation of state are in good accord with the available experimental results. Strength of intermolecular interactions has been correlated using the calculated compressibility curves and elastic moduli. Apart from this, we also observe discontinuities in the structural parameters and elastic constants as a function of pressure. Pictorial representation and quantification of intermolecular interactions are described by the 3D Hirshfeld surfaces and 2D finger print maps. In addition, the computed infra-red (IR) spectra at ambient pressure reveal that ADN is found to have more hygroscopic nature over Ammonium Perchlorate (AP) due to the presence of strong hydrogen bonding. Pressure dependent IR spectra show blue- and red-shift of bending and stretching frequencies which leads to weakening and strengthening of the hydrogen bonding below and above 5 GPa, respectively. The abrupt changes in the calculated structural, mechanical, and IR spectra suggest that ADN might undergo a first order structural transformation to a high pressure phase around 5-6 GPa. From the predicted detonation properties, ADN is found to have high and low performance characteristics when compared with ammonium based energetic oxidizers and well-known secondary explosives, respectively.

cond-mat.mtrl-sci↗

arXiv2016

Electronic topological transitions in Nb$_3$X (X = Al, Ga, In, Ge and Sn) under compression investigated by first principles calculations

First principles electronic structure calculations of A-15 type Nb$_3$X (X = Al, Ga, In, Ge and Sn) compounds are performed at ambient and high pressures. Mechanical stability is confirmed in all the compounds both at ambient as well as under compression from the calculated elastic constants. We have observed four holes and two electron Fermi surfaces (FS) for all the compounds studied and FS nesting feature is observed at M and along X - $Γ$ in all the compounds. A continuous change in the FS topology is observed under pressure in all the compounds which is also reflected in the calculated elastic constants and density of states under pressure indicating the Electronic topological transitions (ETT). The ETT observed at around 21.5 GPa, 17.5 GPa in Nb$_3$Al and Nb$_3$Ga are in good agreement with the anomalies observed by the experiments.

cond-mat.mtrl-sci↗

arXiv2016

Structural, electronic and optical properties of well-known primary explosive: Mercury fulminate

Mercury Fulminate (MF) is one of the well-known primary explosives since 17th century and it has rendered invaluable service over many years. However, the correct molecular and crystal structures are determined recently after 300 years of its discovery. In the present study, we report pressure dependent structural, elastic, electronic and optical properties of MF. Non-local correction methods have been employed to capture the weak van der Waals interactions in layered and molecular energetic MF. Among the non-local correction methods tested, optB88-vdW method works well for the investigated compound. The obtained equilibrium bulk modulus reveals that MF is softer than the well known primary explosives Silver Fulminate (SF), silver azide and lead azide. MF exhibits anisotropic compressibility (b>a>c) under pressure, consequently the corresponding elastic moduli decrease in the following order: C22>C11>C33. The structural and mechanical properties suggest that MF is more sensitive to detonate along c-axis (similar to RDX) due to high compressibility of Hg...O non-bonded interactions along that axis. Electronic structure and optical properties were calculated including spin-orbit (SO) interactions using full potential linearized augmented plane wave method within recently developed Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. The calculated TB-mBJ electronic structures of SF and MF show that these compounds are indirect bandgap insulators. Also, SO coupling is found to be more pronounced for 4d and 5d-states of Ag and Hg atoms of SF and MF, respectively. Partial density of states and electron charge density maps were used to describe the nature of chemical bonding. Ag-C bond is more directional than Hg-C bond which makes SF to be more unstable than MF. The effect of SO coupling on optical properties has also been studied and found to be significant for both of the compounds.

cond-mat.mtrl-sci↗

arXiv2016

High pressure structural, electronic, and optical properties of polymorphic InVO4 phases

In the present work, we report a detailed density functional theory calculation on polymorphic InVO$_4$ phases by means of projector augmented wave method. The computed first-order structural phase transformation from orthorhombic \emph{(Cmcm)} to monoclinic \emph{(P2/c)} structure is found to occur around 5.6 GPa along with a large volume collapse of 16.6$\%$, which is consistent with previously reported experimental data. This transformation also leads to an increase in the coordination number of vanadium atom from 4 to 6. The computed equilibrium and high pressure structural properties of both InVO$_4$ phases, including unit cell parameters, equation of state, and bulk moduli, are in good agreement with the available experimental data. In addition, compressibility is found to be highly anisotropic and the \emph{b}-axis being more compressible than the other for both the structures. Electronic band structures for both the phases were calculated, and the band gap for orthorhombic and monoclinic InVO$_4$ are found to be 4.02 and 1.67 eV, respectively, within the Tran-Blaha Modified Becke-Johnson potential as implemented in linearized augmented planewave method. We further examined the optical properties such as dielectric function, refractive index, and absorption spectra for both the structures. From the implications of these results, it can be proposed that the high pressure InVO$_4$ phase can be more useful than orthorhombic phase for photo catalytic applications.

cond-mat.mtrl-sci↗

arXiv2015

Phase stability and lattice dynamics of ammonium azide under hydrostatic compression

We have investigated the effect of hydrostatic pressure and temperature on phase stability of hydro-nitrogen solids using dispersion corrected Density Functional Theory calculations. From our total energy calculations, Ammonium Azide (AA) is found to be the thermodynamic ground state of N$_4$H$_4$ compounds in preference to Trans-Tetrazene (TTZ), Hydro-Nitrogen Solid-1 (HNS-1) and HNS-2 phases. We have carried out a detailed study on structure and lattice dynamics of the equilibrium phase (AA). AA undergoes a phase transition to TTZ at around $\sim$ 39-43 GPa followed by TTZ to HNS-1 at around 80-90 GPa under the studied temperature range of 0-650 K. The accelerated and decelerated compression of $a$ and $c$ lattice constants suggest that the ambient phase of AA transforms to a tetragonal phase and then to a low symmetry structure with less anisotropy up on further compression. We have noticed that the angle made by Type-II azides with $c$-axis shows a rapid decrease and reaches a minimum value at 12 GPa, and thereafter increases up to 50 GPa. Softening of the shear elastic moduli is suggestive of a mechanical instability of AA under high pressure. In addition, we have also performed density functional perturbation theory calculations to obtain the vibrational spectrum of AA at ambient as well as at high pressures. Further, we have made a complete assignment of all the vibrational modes which is in good agreement with the experimental observations at ambient pressure. Also the calculated pressure dependent IR spectra show that the N-H stretching frequencies undergo a red and blue-shift corresponding to strengthening and weakening of hydrogen bonding, respectively below and above 4 GPa.

cond-mat.mtrl-sci↗

arXiv2015

Structural stability, vibrational and bonding properties of potassium 1,1$'$-dinitroamino-5,5$'$ bistetrazolate: An emerging green primary explosive

Potassium 1,1$'$-dinitroamino-5,5$'$ bistetrazolate (K$_2$DNABT) is a nitrogen rich (50.3 $\%$ by weight, \ce{K2C2N12O4}) green primary explosive with high performance characteristics namely velocity of detonation (D = 8.33 km/s), detonation pressure (P = 31.7 GPa) and fast initiating power to replace existing toxic primaries. In the present work, we report density functional theory (DFT) calculations on structural, equation of state, vibrational spectra, electronic structure and absorption spectra of K$_2$DNABT. We have discussed the influence of weak dispersive interactions on structural and vibrational properties through the DFT-D2 method. We find anisotropic compressibility (b $<$ a $<$ c) from pressure dependent structural properties. The predicted bulk modulus reveals that the material is harder than cyanuric triazide (\ce{C3N12}) and softer than lead azide (\ce{Pb(N3)2}). A complete assignment of all vibrational modes has been made and compared with the available experimental results. The calculated zone centre IR and Raman frequencies show a blue-shift which leads to a hardening of the lattice upon compression. In addition, we have also calculated the electronic structure and absorption spectra using the recently developed Tran Blaha-modified Becke Johnson potential. It is found that K$_2$DNABT is a direct band gap insulator with a band gap of 3.87 eV and the top of the valence band mainly dominated by $2p$-states of oxygen and nitrogen atoms. K$_2$DNABT exhibits mixed ionic (between potassium and tetrazolate ions) and covalent character within tetrazolate molecule. The presence of ionic bonding suggests that the investigated compound is relatively stable and insensitive than covalent primaries. From the calculated absorption spectra, the material is found to decompose under ultra-violet light irradiation.

cond-mat.mtrl-sci↗

arXiv2015

CuAlTe$_2$: A promising bulk thermoelectric material

Transport properties of Cu-based chalcopyrite materials are presented using the full potential linear augmented plane wave method and Boltzmann Semi-classical theory. All the studied compounds appear to be direct band gap semiconductors evaluated based on the Tran-Blaha modified Becke-Johnson potential. The heavy and light band combination found near the valence band maximum (VBM) drive these materials to possess good thermoelectric properties. Among the studied compounds, CuAlTe$_2$ is found to be more promising, in comparison with CuGaTe$_2$, which is reported to be an efficient thermoelectric material with appreciable figure of merit. Another interesting fact about CuAlTe$_2$ is the comparable thermoelectric properties possessed by both $n-$ type and $p-$ type carriers, which might attract good device applications and are explained in detail using the electronic structure calculations.

cond-mat.mtrl-sci↗

arXiv2015

Electronic Structure, Transport and Phonons of SrAg$Ch$F ($Ch$=S, Se, Te): Bulk Superlattice Thermoelectrics

We report calculations of the electronic structure, vibrational properties and transport for the p-type semiconductors, SrAg$Ch$F ($Ch$=S, Se and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below 50 $cm^{-1}$, indicative of a material with low thermal conductivity. The bands at and near the valence band maxima are highly two dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2015

Dispersion Corrected Structural Properties and Quasiparticle Band Gaps of Several Organic Energetic Solids

We have performed {\it ab initio} calculations for a series of energetic solids to explore their structural and electronic properties. To evaluate the ground state volume of these molecular solids, different dispersion correction methods were accounted in DFT, namely the Tkatchenko-Scheffler method (with and without self-consistent screening), Grimme's methods (D2, D3(BJ)) and the vdW-DF method. Our results reveals that dispersion correction methods are essential in understanding these complex structures with van der Waals interactions and hydrogen bonding. The calculated ground state volumes and bulk moduli show that the performance of each method is not unique, and therefore a careful examination is mandatory for interpreting theoretical predictions. This work also emphasizes the importance of quasiparticle calculations in predicting the band gap, which is obtained here with the GW approximation. We find that the obtained band gaps are ranging from 4 to 7 eV for the different compounds, indicating their insulating nature. In addition, we show the essential role of quasiparticle band structure calculations to correlate the gap with the energetic properties.

cond-mat.mtrl-sci↗

arXiv2014

Polymorphism and thermodynamic ground state of Silver fulminate studied from van der Waals density functional calculations

Silver fulminate (AgCNO) is a primary explosive, which exists in two polymorphic phases namely orthorhombic (\emph{Cmcm}) and trigonal (\emph{R$\bar{3}$}) forms at ambient conditions. In the present study, we have investigated the effect of pressure and temperature on relative phase stability of the polymorphs using planewave pseudopotential approaches based on Density Functional Theory (DFT). van der Waals interactions play a significant role in predicting the phase stability and they can be effectively captured by semiempirical dispersion correction methods incontrast to standard DFT functionals. Based on our total energy calculations using DFT-D2 method, the \emph{Cmcm} structure is found to be the preferred thermodynamic equilibrium phase under studied pressure and temperature range. Hitherto \emph{Cmcm} and \emph{R$\bar{3}$} phases denoted as $α$ and $β$-forms of AgCNO, respectively. Also a pressure induced polymorphic phase transition is seen using DFT functionals and the same was not observed with DFT-D2 method. The equation of state and compressibility of both polymorphic phases were investigated. Electronic structure and optical properties were calculated using full potential linearized augmented plane wave method within the Tran-Blaha modified Becke-Johnson potential. The calculated electronic structure shows that $α$, $β$ phases are indirect band gap insulators with a band gap values of 3.51 and 4.43 eV, respectively. The nature of chemical bonding is analyzed through the charge density plots and partial density of states. Optical anisotropy, electric-dipole transitions and photo sensitivity to light of the polymorphs are analyzed from the calculated optical spectra. Overall, the present study provides an early indication to experimentalists to avoid the formation of unstable $β$-form of AgCNO.

cond-mat.mtrl-sci↗

arXiv2014

Density functional study of electronic structure, elastic and optical properties of MNH$_2$ (M=Li, Na, K, Rb)

We report systematic first principles density functional study on the electronic structure, elastic and optical properties of nitrogen based solid hydrogen storage materials LiNH$_2$, NaNH$_2$, KNH$_2$, and RbNH$_2$. The ground state structural properties are calculated by using standard density functional theory and also dispersion corrected density functional theory. We find that van der Waals interactions are dominant in LiNH$_2$ whereas they are relatively weak in other alkali metal amides. The calculated elastic constants show that all the compounds are mechanically stable and LiNH$_2$ is found to be stiffer material among the alkali metal amides. The melting temperatures are calculated and which follows the order RbNH$_2$ $<$ KNH$_2$ $<$ NaNH$_2$ $<$ LiNH$_2$. The electronic band structure is calculated by using the Tran-Blaha modified Becke-Johnson potential and found that all the compounds are insulators with a considerable band gap. The [NH$_2$]$^-$ derived states are completely dominating in the entire valence band region while the metal atom states occupy the conduction band. The calculated band structure is used to analyze the different interband optical transitions occur between valence and conduction bands. Our calculations show that these materials have considerable optical anisotropy.

cond-mat.mtrl-sci↗