arXiv subjects

Rajyavardhan Ray

Publications and source records attributed to Rajyavardhan Ray.

At least 19 recordsLinked to original sources

arXiv2026

Fermi energy Weyl nodes in $\mathbf{AM}$Te$_4$ ($\mathbf{A}$=Ta, Nb, $\mathbf{M}$=Ir, Rh)

Key aspects of the quantum oscillations and magnetoresistance in Weyl semimetals $AM$Te$_4$ ($A$=Nb,Ta, $M$=Rh, Ir) persist as open questions, obscuring the link between their topological electronic structure and practical implementations. Employing a generalised search procedure, we carry out a comprehensive scan of WPs accounting for all the subbands close to the Fermi energy, and show that this dramatically alters the WP landscape in these compounds. In particular, we predict these compounds to feature WPs within a few meV of the Fermi energy which significantly influence their properties. Remarkably, most of the considered compounds host WPs of more than one type, including NbRhTe$_4$ which hosts type-I, II and III Weyl points. Our comparative analysis of structure and fidelity of computational parameters/models not only provides a detailed mapping of the complex electronic structure in these compounds, but also clarifies quantum oscillations and magnetoresistance observations in this family, bridging the gap between theory and experiments and offering a framework for precise tunability of WPs.

cond-mat.mtrl-sci↗

arXiv2025

Electronic structure fingerprints of visible-range excitons in $d^0$ double perovskite oxides

Presence of excitons significantly influence the optoelectronic properties and potential applications of materials. Using combined theoretical and experimental tools, we investigate the absorption spectra of $d^0$ double perovskite oxides Ba$_{2}$Y$B'$O$_6$ ($B'$ = Nb, Ta, Sb), Ba$_{2}$Sc$B'$O$_6$ ($B'$ = Ta, Sb) and $A_{2}$ScSbO$_6$ ($A$=Ca, Sr, Ba), allowing for a systematic variation of composition. We not only show that low-energy excitons possessing large binding energies up to 3 eV are present in the visible range in all the considered wide-gap insulators, but also that the nature and properties of these excitons differs from those in double perovskite halides as well as perovskite oxides. We provide insights on the origin of such differences by a comparative analysis of the electronic structure. Our findings elucidate possible correlations between the exciton properties and the composition, via the electronic structure, towards a comprehensive understanding of correlation effects and rational design principles.

cond-mat.mtrl-sci↗

arXiv2024

Layer dependent topological phases and transitions in TaRhTe$_4$: From monolayer and bilayer to bulk

The recently synthesized ternary quasi-2D material TaRhTe$_4$ is a bulk Weyl semimetal with an intrinsically layered structure, which poses the question how the topology of its electronic structure depends on layers separations. Experimentally these separations may be changed for instance by intercalation of the bulk, or by exfoliation to reach monolayer or few-layer structures. Here we show that in the monolayer limit a quantum spin Hall insulator (QSHI) state emerges, employing density functional calculations as well as a minimal four-orbital tight-binding model that we develop. Even for weak spin-orbit couplings the QSHI is present, which has an interesting edge state that features Rashba-split bands with quadratic band minima. Further we find that a weak topological insulator (WTI) manifests in the bilayer system due to sizable intralayer hopping, contrary to the common lore that only weak interlayer interactions between stacked QSHIs lead to WTIs. Stacked bilayers give rise to a phase diagram as function of the interlayer separation that comprises a Weyl semimetal, WTI and normal insulator phases. These insights on the evolution of topology with dimension can be transferred to the family of layered ternary transition metal tellurides.

cond-mat.mtrl-sci↗

arXiv2023

Correlation between electronic polarization and shift current in cubic and hexagonal semiconductors LiZn$X$ ($X$ = P, As, Sb)

The rectified bulk photovoltaic effect (BPVE) in noncentrosymmetric semiconductors, also called shift current, is considered promising for optoelectronic devices, terahertz emission and possibly solar energy harvesting. A clear understanding of the shift current mechanism and search for materials with large shift current is, therefore, of immense interest. $ABC$ semiconductors LiZn$X$ ($X$ = N, P, As, and Sb) can be stabilized in cubic as well as hexagonal morphologies lacking inversion symmetry$-$an ideal platform to investigate the significant contributing factors to shift current, such as the role of structure and chemical species. Using density-functional calculations properly accounting for the electronic bandgaps, the shift current conductivities in LiZn$X$ ($X$ = P, As, Sb) are found to be approximately an order of magnitude larger than the well-known counterparts and peak close to the maximum solar radiation intensity. Notably, hexagonal LiZnSb shows a peak shift current conductivity of $\sim -75 ~\rm{\rm{\mu}}$A/V$^2$ and Glass coefficient of $ -20$ $\times$ 10$^{-8}$ cm/V, comparable to the highest predicted values in literature. Our comparative analysis reveals a quantitative relationship between the shift current response and the electronic polarization. These findings not only posit Li-Zn-based $ABC$ semiconductors as viable material candidates for potential applications but also elucidates key aspects of the structure-BPVE relationship.

cond-mat.mtrl-sci↗

arXiv2022

Evolution of electronic and magnetic properties of Sr$\mathbf{_2}$IrO$\mathbf{_4}$ under strain

Motivated by properties-controlling potential of the strain, we investigate strain dependence of structure, electronic and magnetic properties of Sr$_2$IrO$_4$ using complementary theoretical tools: {\it ab-initio} calculations, analytical approaches (rigid octahedra picture, Slater-Koster integrals), and extended $t-{\mathcal{J}}$ model. We find that strain affects both Ir-Ir distance and Ir-O-Ir angle, and the rigid octahedra picture is not relevant. Second, we find fundamentally different behavior for compressive and tensile strain. One remarkable feature is the formation of two subsets of bond- and orbital- dependent carriers, a compass-like model, under compression. This originates from the strain-induced renormalization of the Ir-O-Ir superexchange and O on-site energy. We also show that under compressive (tensile) strain, Fermi surface becomes highly dispersive (relatively flat). Already at a tensile strain of $1.5\%$, we observe spectral weight redistribution, with the low-energy band acquiring almost purely singlet character. These results can be directly compared with future experiments.

cond-mat.str-el↗

arXiv2022

Orbital entangled antiferromagnetoc order and spin-orbit-distortion exciton in $\rm Sr_2VO_4$

With electron filling $n=1$ in the $\rm Sr_2VO_4$ compound, the octahedrally coordinated $t_{\rm 2g}$ orbitals are strongly active due to tetragonal distortion induced crystal field tuning by external agent such as pressure. Considering the full range of crystal field induced tetragonal splitting in a realistic three-orbital model, collective spin-orbital excitations are investigated using the generalized self consistent and fluctuation approach. The variety of self consistent states obtained including orbital entangled ferromagnetic and antiferromagnetic orders reflects the rich spin-orbital physics resulting from the interplay between the band, spin-orbit coupling, crystal field, and Coulomb interaction terms. The behavior of the calculated energy scales of collective excitations with crystal field is consistent with that of the transition temperatures with pressure as obtained from susceptibility and resistivity anomalies in high-pressure studies.

cond-mat.str-el↗

arXiv2021

Extended high-harmonic spectra through cascade resonance in confined quantum systems

The study of high-harmonic generation in confined quantum systems is vital to establishing a complete physical picture of harmonic generation from atoms and molecules to bulk solids. Based on a multilevel approach, we demonstrate how intraband resonances significantly influence the harmonic spectra via charge pumping to the higher subbands and, thus, redefine the cutoff laws. As a proof of principle, we consider the interaction of graphene nanoribbons, with zigzag as well as armchair terminations, and resonant fields polarized along the cross-ribbon direction. Here, this effect is particularly prominent due to many nearly equi-separated energy levels. In such a scenario, a cascade resonance effect can take place in high-harmonic generation when the field strength is above a critical threshold, which is completely different from the harmonic generation mechanism of atoms, molecules and bulk solids. We further discuss the implications not only for other systems in a nanoribbon geometry, but also systems where only a few subbands (energy levels) meet this frequency-matching condition by considering a generalized multilevel Hamiltonian. Our study highlights that cascade resonance bears fundamentally distinct influence on the laws of harmonic generation, specifically the cutoff laws based on laser duration, field strength, and wavelength, thus unraveling new insights in solid-state high-harmonic generation.

cond-mat.mes-hall↗

arXiv2021

Multiple low-energy excitons and optical response of $d^0$ double perovskite Ba$_2$ScTaO$_6$

Large bandgap insulators are considered promising for applications such as photocatalysts, dielectric resonators and interference filters. Based on synchrotron X-ray diffraction, diffuse reflectance measurement and density functional theory, we report the crystal structure, optical response, and electronic properties of the synthesized $d^0$ double perovskite Ba$_2$ScTaO$_6$. In contrast to earlier prediction, the electronic bandgap is found to be large, $\sim 4.66$ eV. The optical response is characterized by the presence of multiple exciton modes extending up to the visible range. A detailed investigation of the direct gap excitons based on the Elliot formula is presented. Density functional theory based investigation of the electronic properties within generalized gradient approximation severely underestimates the electronic gap. To reach a quantitative agreement, we consider different available flavors of the modified-Becke-Johnson exchange-correlation potential and discuss their effects on the electronic and optical properties.

cond-mat.mtrl-sci↗

arXiv2021

Finite temperature fluctuation-induced order and responses in magnetic topological insulators

We derive an effective field theory model for magnetic topological insulators and predict that a magnetic electronic gap persists on the surface for temperatures above the ordering temperature of the bulk. Our analysis also applies to interfaces of heterostructures consisting of a ferromagnetic and a topological insulator. In order to make quantitative predictions for MnBi$_2$Te$_4$, and for EuS-Bi$_2$Se$_3$ heterostructures, we combine the effective field theory method with density functional theory and Monte Carlo simulations. For MnBi$_2$Te$_4$ we predict an upwards N\'eel temperature shift at the surface up to $15 \%$, while the EuS-Bi$_2$Se$_3$ interface exhibits a smaller relative shift. The effective theory also predicts induced Dzyaloshinskii-Moriya interactions and a topological magnetoelectric effect, both of which feature a finite temperature and chemical potential dependence.

cond-mat.str-el↗

arXiv2020

Tunable chirality of noncentrosymmetric magnetic Weyl semimetals

Even if Weyl semimetals are characterized by quasiparticles with well-defined chirality, exploiting this experimentally is severely hampered by Weyl lattice-fermions coming in pairs with opposite chirality, typically causing the net chirality picked up by experimental probes to vanish. Here we show this issue can be circumvented in a controlled manner when both time-reversal- and inversion- symmetry are broken. To this end, we investigate chirality-disbalance in the carbide family RMC$_2$ (R a rare-earth and M a transition metal), showing several members to be Weyl semimetals. Using the noncentrosymmetric ferromagnet NdRhC$_2$ as an illustrating example, we show that an odd number of Weyl nodes can be stabilized at its Fermi surface by properly tilting its magnetization. The tilt direction determines the sign of the resulting net chirality, opening up a simple route to control it.

cond-mat.mes-hall↗

arXiv2019

First principles calculation of shift current in chalcopyrite semiconductor ZnSnP$_2$

The bulk photovoltaic effect generates intrinsic photocurrents in materials without inversion symmetry. Shift current is one of the bulk photovoltaic phenomena related to the Berry phase of the constituting electronic bands: photo-excited carriers coherently shift in real space due to the difference in the Berry connection between the valence and conduction bands. Ferroelectric semiconductors and Weyl semimetals are known to exhibit such nonlinear optical phenomena. Here we consider chalcopyrite semiconductor ZnSnP$_2$ which lacks inversion symmetry and calculate the shift current conductivity. We find that the magnitude of the shift current is comparable to the recently measured values on other ferroelectric semiconductors and an order of magnitude larger than bismuth ferrite. The peak response for both optical and shift current conductivity, which mainly comes from P-3$p$ and Sn-5$p$ orbitals, is several eV above the bandgap.

cond-mat.mtrl-sci↗

arXiv2019

Large off diagonal exchange couplings and spin liquid states in $\mathbf{C_3}$ symmetric iridates

Iridate oxides on a honeycomb lattice are considered promising candidates for realization of quantum spin liquid states. We investigate the magnetic couplings in a structural model for a honeycomb iridate K$_2$IrO$_3$, with $C_3$ point group symmetry at the Ir sites, which is an end member of the recently synthesized iridate family K$_x$Ir$_y$O$_2$. Using \textit{ab-initio} quantum chemical methods, we elucidate the subtle relationship between the real space symmetry and magnetic anisotropy and show that the higher point group symmetry leads to high frustration with strong magnetic anisotropy driven by the unusually large off-diagonal exchange couplings ($\Gamma$'s) as opposed to other spin-liquid candidates considered so far. Consequently, large quantum fluctuations imply lack of magnetic ordering consistent with the experiments. Exact diagonalization calculations for the fully anisotropic $K$-$J$-$\Gamma$ Hamiltonian reveal the importance of the off-diagonal anisotropic exchange couplings in stabilizing a spin liquid state and highlight an alternative route to stabilize spin liquid states for ferromagnetic $K$.

cond-mat.str-el↗

arXiv2019

Revised crystal structure and electronic properties of high dielectric Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ ceramics

Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_3$ (BFN) ceramics are considered to be promising for technological applications owing to their high dielectric constant over a wide range of temperatures. However, there exists considerable discrepancy over the structural details. We address this discrepancy through a combined x-ray diffraction at room temperature and neutron powder diffraction measurements in the range from 5K up to room temperature, supplemented by a comparative analysis of the earlier reported structures. Our study reveals a cubic structure with space group Pm$\bar{3}$m at all measured temperatures. Further, the x-ray near edge structure and the extended x-ray absorption fine structure studies on the local environment of the Fe ions is consistent with the cubic symmetry. An appropriate value of $U$ for DFT+$U$ calculations is obtained by comparison with x-ray absorption spectroscopy, which agrees well with the earlier reported electronic properties.

cond-mat.str-el↗

arXiv2019

The X-ray photoemission and Co K -Edge X-ray absorption of Ba2CoWO6

The structural, electronic and optical properties of cubic double perovskite BaCoWO6 have been studied. Neutron powder diffraction data is collected on this sample from 6K to 300K. The crystal structure is face centered cubic, space group being Fm3m (No. 225). We did not find evidence for long range magnetic ordering in this system in this temperature range. The band-gap is estimated using Uv-vis spectroscopy. The Co-K edge X-ray absorption (XAFS) spectra of Ba2CoWO6 was analysed together with those Co-foil, which was used as reference compounds. X-ray photoemission spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies give the insight of the electronic and structural information on the Co local environment for Ba2CoWO6.

cond-mat.mtrl-sci↗

arXiv2019

Topological electronic structure and intrinsic magnetization in MnBi$_4$Te$_7$: a Bi$_2$Te$_3$-derivative with a periodic Mn sublattice

Combinations of non-trivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances material candidates are emerging. Yet, a compound with a band-inverted electronic structure and an intrinsic net magnetization remains unrealized. MnBi$_2$Te$_4$ is a candidate for the first antiferromagnetic topological insulator and the progenitor of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series. For $n$ = 1, we confirm a non-stoichiometric composition proximate to MnBi$_4$Te$_7$ and establish an antiferromagnetic state below 13 K followed by a state with net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topological surface state on the MnBi$_4$Te$_7$(0001) surface, analogous to the non-magnetic parent compound Bi$_2$Te$_3$. Our results render MnBi$_4$Te$_7$ as a band-inverted material with an intrinsic net magnetization and a complex magnetic phase diagram providing a versatile platform for the realization of different topological phases.

cond-mat.mtrl-sci↗

arXiv2018

The influence of lattice termination on the edge states of the quantum spin Hall insulator monolayer $1T'$-WTe$_2$

We study the influence of sample termination on the electronic properties of the novel quantum spin Hall insulator monolayer $1T'$-WTe$_2$. For this purpose, we construct an accurate, minimal 4-orbital tight-binding model with spin-orbit coupling by employing a combination of density-functional theory calculations, symmetry considerations, and fitting to experimental data. Based on this model, we compute energy bands and 2-terminal conductance spectra for various ribbon geometries with different terminations, with and without magnetic field. Because of the strong electron-hole asymmetry we find that the edge Dirac point is buried in the bulk bands for most edge terminations. In the presence of a magnetic field, an in-gap edge Dirac point leads to exponential suppression of conductance as an edge Zeeman gap opens, whereas the conductance stays at the quantized value when the Dirac point is buried in the bulk bands. Finally, we find that disorder in the edge termination drastically changes this picture: the conductance of a sufficiently rough edge is uniformly suppressed for all energies in the bulk gap regardless of the orientation of the edge.

cond-mat.mes-hall↗

arXiv2018

Strong effect of hydrogen order on magnetic Kitaev interactions in H$_3$LiIr$_2$O$_6$

Very recently a quantum liquid was reported to form in H$_3$LiIr$_2$O$_6$, an iridate proposed to be a close realization of the Kitaev honeycomb model. To test this assertion we perform detailed quantum chemistry calculations to determine the magnetic interactions between Ir moments. We find that weakly bond dependent ferromagnetic Kitaev exchange dominates over other couplings, but still is substantially lower than in Na$_2$IrO$_3$. This reduction is caused by the peculiar position of the inter-layer species: removing hydrogen cations next to a Ir$_2$O$_2$ plaquette increases the Kitaev exchange by more than a factor of three on the corresponding Ir-Ir link. Consequently any lack of hydrogen order will have a drastic effect on the magnetic interactions and strongly promote spin disordering.

cond-mat.str-el↗

arXiv2018

Indole moiety induced biological potency in pseudo- peptides derived from 2-amino-2-(1H-indole-2-yl) based acetamides: synthesis, structure and computational investigations

We report the synthesis and theoretical investigations of three novel pseudo-peptide molecules derived from 2-amino-2-(1H-indole-2-yl) acetamides. The compounds were subjected to spectroscopic characterization ($^1$H, $^{13}$C-NMR and MS) and their chemical, electronic, and optical properties have been investigated. To ascertain their potential pharmacological applicability, the prospective reactive centers and molecular sites prone to interaction with water were identified along with possible sensitivity to autoxidation. Further, we have studied the optical response in the presence of different solvents and compared the electronic and optical properties of the pristine molecules. We highlight the subtle dependence of the properties on the structure and composition of these pseudo-peptides. Our results indicate that these molecules have high pharmaceutical potential and could serve as lead components in new drug formulations.

cond-mat.mtrl-sci↗