arXiv subjects

Urmimala Dey

Publications and source records attributed to Urmimala Dey.

18 recordsLinked to original sources

arXiv2026

Near-room-temperature magnetoelectric coupling engineered through inversion-breaking tilts in a bulk perovskite polytype

Systematic strategies to design properties such as ferroelectricity or magnetoelectric coupling are well established in simple perovskite materials, but they remain scarce in more complex framework structures. Using a hexagonal polytype of the ternary manganite AMnO3 (A = Ba, Sr, Ca) as a model system, we introduce a symmetry-guided design principle in which an inversion-breaking rigid-unit mode (RUM) serves as a single structural instability generating both polar and ferromagnetic orders within a bulk material. Symmetry analysis and first-principles calculations reveal that co-operative tilts of the Mn2O9 bioctahedral dimers generate both a spontaneous polarization and a ferromagnetic moment. High-resolution diffraction and magnetic susceptibility measurements show the structural and magnetic orders persist as high as 450 K and 280 K, respectively, highlighting the untapped potential of framework structures which deviate from simple perovskite motifs to be designed to host useful ferroic properties. Our approach establishes a transferable symmetry-based framework to engineer ferroelectric and magnetoelectric states across chemically diverse framework architectures.

cond-mat.mtrl-sci↗

arXiv2025

Chiral soft mode transition driven by strain in ferroelectric bubble domains

Chirality in solids is attracting growing attention as a potential ferroic order, yet virtually no paradigmatic example of a soft-mode achiral-to-chiral phase transition has been firmly established to date. Here we identify ferroelectric bubble domains as a model system that undergoes a strain-driven achiral-to-chiral transition exhibiting the hallmarks of spontaneous symmetry breaking. Using second-principles atomistic simulations, we uncover chiral phonon modes in ferroelectric/dielectric superlattices that soften under epitaxial strain following textbook soft-mode behaviour. The transition is accompanied by a change in topological character, highlighting an interplay between chirality and topology in these systems. This work provides a concrete step towards establishing chirality as a genuine ferroic order in solids.

cond-mat.mtrl-sci↗

arXiv2025

Pressure-induced orbital reordering in Na$_2$CuF$_4$

The high-pressure behaviour of Na$_2$CuF$_4$ is explored by powder neutron diffraction and density functional theory (DFT) calculations. A first-order phase transition is observed to take place between 2.4 - 2.9 GPa, involving a reorientation of the Jahn-Teller (JT) long axes of the (CuF6) octahedra (and therefore the d$_{z^2}$ Cu orbitals), in agreement with our DFT calculations which suggest a transition at 2.8 GPa. The transition can be described as being between a state of ferro-orbital order and one of A-type antiferro-orbital order, reflecting a shift in the associated electronic instability from being in the zone-center to zone boundary of the first Brillouin zone of the parent structure, with pressure. This change results in a decoupling of magnitude of the associated Jahn-Teller distortion of the Cu-F bond lengths from the lattice strain. This scenario is supported by our observations that the compressibility of the pre-transition phase is highly anisotropic, whilst in the post-transition phase it becomes almost isotropic, and that we observed no further decrease of the magnitude the JT distortion up to 5 GPa, or melting of the OO in our DFT calculations up to at least 5 GPa.

cond-mat.str-el↗

arXiv2025

Dielectric softening in the halide double perovskites $A_2$Au$_2X_6$ ($A$: Cs, Rb; $X$: Cl, Br, I) via a strain-mediated pseudotriggered mechanism

Halide perovskites have emerged as promising candidates for next generation photovoltaic applications, attracting significant attention in recent years. Through first-principles calculations combined with group-theoretical analyses, we investigate the structural phase diagram of Pb-free Jahn-Teller-active $A_2$Au$_2X_6$ ($A$: Cs, Rb; $X$: Cl, Br, I) double perovskites. Our study identifies a previously unreported ferroelectric phase, where the softening of the polar mode$, $key to ferroelectricity, is driven by an unconventional and indirect coupling with improper strains originating from Jahn-Teller distortions. The proposed strain mediated \textit{pseudo}triggered mechanism offers an alternative pathway to enhance the static dielectric constant or even promote (photo-)ferroelectricity, addressing challenges such as defects, excitons, and charge scattering that hinder photovoltaic efficiency. More broadly, this unique mechanism could be extended to oxide double perovskites and opens up a new type of ferroelectric phase transition worthy of future investigation.

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↗

arXiv2025

Pressure induced enhancement of polar distortions in a metal, and implications on the Rashba spin-splitting

Polar metals are an intriguing class of materials in which electric polarisation and metallicity can coexist within a single phase. The unique properties of polar metals challenge expectations, making way for the exploration of exotic phenomena such as unconventional magnetism, hyperferroelectric multiferroicity and developing multifunctional devices that can leverage both the materials electric polarization and its asymmetry in the spin conductivity, that arises due to the Rashba effect. Here, via a high pressure single crystal diffraction study, we report the pressure-induced enhancement of polar distortions in such a metal, Ca$_3$Ru$_2$O$_7$. Our DFT calculations highlight that naive assumptions about the linear dependency between polar distortion amplitudes and the magnitude of the Rashba spin splitting may not be generally valid.

cond-mat.str-el↗

arXiv2024

Prediction of Room Temperature Electric Field Reversal of Magnetization in the Family of $A_4B_3\rm{O}_9$ Layered Oxides

The promise of a strong magnetoelectric coupling in a multiferroic material is not only of fundamental interest, but also forms the basis of next generation memory devices where the direction of magnetization can be reversed by an external electric field. Using group-theory led first-principles calculations, we have identified a hitherto unknown polar phase of the $A_4B_3\rm{O}_9$ layered oxides, where the polar mode couples to the magnetic modes through a rare $\Gamma$-point magnetoelectric-multiferroic coupling scheme such that the net magnetization can be directly reversed by an electric field switching of the polar mode. Furthermore, in agreement with previous experimental observations, we predict room temperature magnetism in $A_4B_3\rm{O}_9$ oxides which indicates the promising practical applications of these compounds in the next generation memory devices.

cond-mat.mtrl-sci↗

arXiv2023

First-principles investigation of the magnetoelectric properties of Ba$_7$Mn$_4$O$_{15}$

Type-II multiferroics, in which the magnetic order breaks inversion symmetry, are appealing for both fundamental and applied research due their intrinsic coupling between magnetic and electrical orders. Using first-principles calculations we study the ground state magnetic behaviour of Ba$_7$Mn$_4$O$_{15}$ which has been classified as a type-II multiferroic in recent experiments. Our constrained moment calculations with the proposed experimental magnetic structure shows the spontaneous emergence of a polar mode giving rise to an electrical polarisation comparable to other known type-II multiferroics. When the constraints on the magnetic moments are removed, the spins self-consistently relax into a canted antiferromagnetic ground state configuration where two magnetic modes transforming as distinct irreducible representations coexist. While the dominant magnetic mode matches well with the previous experimental observations, the second mode is found to possess a different character resulting in a non-polar ground state. Interestingly, the non-polar magnetic ground state exhibits a significantly strong linear magnetoelectric coupling comparable to the well-known multiferroic BiFeO$_3$, suggesting strategies to design new linear magnetoelectrics.

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

Electric field and Strain-induced Band-gap Engineering and Manipulation of the Rashba Spin Splitting in Janus van der Waals Heterostructures

The compositional as well as structural asymmetries in Janus transition metal dichalcogenides (J-TMDs) and their van der Waals heterostructures (vdW HSs) induce an intrinsic Rashba spin-splitting. We investigate the variation of band-gaps and the Rashba parameter in three different Janus heterostructures having AB-stacked Mo$XY$/W$XY$ ($X$, $Y$ = S, Se, Te; $X\neq Y$) geometry with a $Y-Y$ interface, using first-principles calculations. We consider the effect of external electric field and in-plane biaxial strain in tuning the strength of the intrinsic electric field, which leads to remarkable modifications of the band-gap and the Rashba spin-splitting. In particular, it is found that the positive applied field and compressive in-plane biaxial strain can lead to a notable increase in the Rashba spin-splitting of the valence bands about the $Γ$-point. Moreover, our \textit{ab-initio} density functional theory (DFT) calculations reveal the existence of a type-II band alignment in these heterostructures, which remains robust under positive external field and biaxial strain. These suggest novel ways of engineering the electronic, optical, and spin properties of J-TMD van der Waals heterostructures holding a huge promise in spintronic and optoelectronic devices. Detailed $\mathbf{k\cdot p}$ model analyses have been performed to investigate the electronic and spin properties near the $Γ$ and K points of the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2022

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↗

arXiv2020

The Berry Phase Rectification Tensor and The Solar Rectification Vector

We introduce an operational definition of the Berry Phase Rectification Tensor as the second-order change of polarization of a material in response to an ideal short pulse of an electric field. Under time-reversal symmetry this tensor depends exclusively on the Berry phases of the Bloch bands and not their energy dispersions, making it an intrinsic property to each material which contains contributions from both the inter-band shift currents and the intra-band Berry Curvature Dipole. We also introduce the Solar Rectification Vector as a technologically relevant figure of merit for a bulk photo-current generation under ideal black-body radiation in analogy with the classic solar cell model of Shockley and Queisser. We perform first-principles calculations of the Berry Phase Rectification Tensor and the Solar Rectification Vector for the Weyl semimetal TaAs and the insulator LiAsSe2 which features large shift currents close to the peak of solar radiation intensity.

cond-mat.mes-hall↗

arXiv2019

Dynamic chiral magnetic effect and anisotropic natural optical activity of tilted Weyl semimetals

We study the dynamic chiral magnetic conductivity (DCMC) and natural optical activity in an inversion-broken tilted Weyl semimetal (WSM). Starting from the Kubo formula, we derive the analytical expressions for the DCMC for two different directions of the incident electromagnetic wave. We show that the angle of rotation of the plane of polarization of the transmitted wave exhibits remarkable anisotropic behavior and is larger along the tilt direction. This striking anisotropy of DCMC which results in anisotropic optical activity and rotary power, can be experimentally observed as a topological magneto-electric effect of inversion-broken tilted WSMs. Finally, using the low energy Hamiltonian, we show that the DCMC follows the universal $\frac{1}{ω^2}$ decay in the high frequency regime. In the low frequency regime, however, the DCMC shows sharp peaks at the tilt dependent effective chemical potentials of the left-handed and right-handed Weyl points. This can serve as a signature to distinguish between the type-I and type-II Weyl semimetals.

cond-mat.mes-hall↗

arXiv2019

Cell damage and mitigation in Swiss albino mice: experiment and modelling

Chronic exposure to inorganic arsenic is a potential cause of carcinogenesis. It elicits its potential by generation of ROS, leading to DNA, protein and lipid damage. Therefore, the deleterious effect of arsenic can be mitigated by quenching ROS using antioxidants. There is a homology between the protein coding regions of mice and human. Effect of these alterations in human can be mimicked in mice. Therefore to understand the underlying mechanism of arsenic toxicity and its amelioration by black tea, studies have been conducted in mice model. Long term exposure to iAs leads to tumour growth, which has been found to be alleviated by black tea. Observations reveal that black tea has two salutary effects on the growth of tumour: the rate of growth of damaged cells was appreciably reduced and an early saturation of the level of damage is achieved. To take the experimental findings further, the experimental data have been modelled with simple dynamical equations. The curves obtained from \textit{in vivo} studies have been fitted with the data obtained from the model. The corresponding steady states and their stabilities are analyzed.

q-bio.TO↗

arXiv2018

Bulk band inversion and surface Dirac cones in LaSb and LaBi : Prediction of a new topological heterostructure

We perform \textit{ab initio} investigations of the bulk and surface band structures of LaSb and LaBi and resolve the existing disagreements about the topological property of LaSb, considering LaBi as a reference. We examine the bulk band structure for band inversion, along with the stability of surface Dirac cones (if any) to time-reversal-preserving perturbations, as a strong diagnostic test for determining the topological character of LaSb, LaBi and LaSb-LaBi multilayer. A detailed \textit{ab initio} investigation of a multilayer consisting of alternating unit cells of LaSb and LaBi shows the presence of band inversion in the bulk and a massless Dirac cone on the (001) surface, which remains stable under the influence of time-reversal-preserving perturbations, thus confirming the topologically non-trivial nature of the multilayer in which the electronic properties can be tailored as per requirement. A detailed $\mathbb{Z}_2$ invariant calculation is performed to arrive at a holistic conclusion.

cond-mat.mtrl-sci↗

arXiv2018

Body centered phase of Cu at high temperature and pressure

The existence of a body centered tetragonal phase of Cu has been investigated in this manuscript when the single crystal FCC Cu is subjected to both high pressure and high temperature. The results have been demonstrated through DFT calculations (which are typically done at 0 K) followed by Helmholtz free energy calculations (for high temperature). The new metastable phase of Cu demonstrates higher thermal conductivity compared to that of the FCC phase and thereby may be beneficial for high temperature engineering applications.

cond-mat.mtrl-sci↗

arXiv2018

Comparative study of the compensated semi-metals LaBi and LuBi : A first-principles approach

We have investigated the electronic structures of LaBi and LuBi, employing the full-potential all electron method as implemented in Wien2k. Using this, we have studied in detail both the bulk and the surface states of these materials. From our band structure calculations we find that LuBi, like LaBi, is a compensated semi-metal with almost equal and sizable electron and hole pockets. In analogy with experimental evidence in LaBi, we thus predict that LuBi will also be a candidate for extremely large magneto-resistance (XMR), which should be of immense technological interest. Our calculations reveal that LaBi, despite being gapless in the bulk spectrum, displays the characteristic features of a $\mathbb{Z}_{2}$ topological semi-metal, resulting in gapless Dirac cones on the surface, whereas LuBi only shows avoided band inversion in the bulk and is thus a conventional compensated semi-metal with extremely large magneto-resistance.

cond-mat.mtrl-sci↗

arXiv2017

Effect of oxygen vacancy on structural, electronic and magnetic properties of La-based oxide interfaces

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.

cond-mat.str-el↗