arXiv subjects

Deepika Gill

Publications and source records attributed to Deepika Gill.

18 recordsLinked to original sources

arXiv2026

All optical ultrafast pure spin current in the altermagnet Cr$_2$SO

All-optical generation of pure spin current -- the flow of spin in the absence of a corresponding charge flow -- relies on a symmetry based compensation of valley charge. The 2d $d$-wave altermagnets, ideal spintronics materials due to a very low spin-orbit coupling, possess a magnetic point group and highly anisotropic valley manifolds that would appear to preclude such current compensation, excluding them as materials for the ultrafast generation of pure spin current. Here we show that infra-red valley excitation combined with a THz pulse envelope allows the generation of large and nearly 100\% pure spin currents in the altermagnet Cr$_2$SO. Our approach is based on a valley selection rule coupling linearly polarized light to spin opposite valleys, along with the intrinsic momentum shift that a co-occurring THz pulse imbues a valley spin excitation with. These results thus provide a practical and all-optical route to the generation of pure spin current in $d$-wave 2d altermagnets, opening a route to lightwave control of spin in an environment with very low intrinsic spin mixing.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast ghost Hall states in a 2d altermagnet

Two-dimensional materials that exhibit optically active spin and valley degrees of freedom represent one of the most fascinating -- and potentially most technologically useful -- platforms for the ultrafast interaction of light and matter. Here we show, via the example of Cr$_2$SO, that two dimensional altermagnets host valley states controllable by femtosecond laser light: linearly polarized light pulses excite charge at one of two inequivalent valleys, with which valley charge is excited at determined by the polarization vector direction. This underpins a rich spin and valley physics including: (i) valleytronics $-$ the generation of nearly 100$\%$ spin polarized valley currents, as well as (ii) a "ghost Hall" effect $-$ the ultrafast creation of states in which spin and charge currents are orthogonal without invoking Hall physics. Our findings establish 2d altermagents as a platform providing a new route for the control of spin- and charge currents at ultrafast times.

cond-mat.mes-hall↗

arXiv2025

Valley polarization of graphene via the saddle point

Graphene, and other members of the monolayer Xene family, represent an ideal materials platform for "valleytronics", the control of valley localized charge excitations. The absence of a gap in these semi-metals, however, precludes valley excitation by circularly polarized light pulses, sharply circumscribing the possibility of a lightwave valleytronics in these materials. Here we show that combining a deep ultraviolet linearly polarized light pulse with a THz envelope can induce highly valley polarized states in graphene. This dual frequency lightform operates by (i) the deep ultraviolet pulse activating a selection rule at the M saddle points and (ii) the THz pulse displacing the M point excitation to one of the low-energy K valleys. Employing both tight-binding and state-of-the-art time dependent density functional theory, we show that such a pulse results in a near perfect valley polarized excitation in graphene, thus providing a route via the saddle point to a lightwave valleytronics in the gapless Xene family.

physics.optics↗

arXiv2025

Generation of pure, spin polarized, and unpolarized charge currents at the few cycle limit of circularly polarized light

In certain members of the transition metal dichalcogenide (TMDC) family, laser pulses of oppositely circularly polarized light excite electrons of opposite spin. Here we show that in the few cycle limit such pulses generate not only a spin density excitation, but also a spin current excitation. Employing the example of the TMDC WSe$_2$ we show that pure spin currents, the flow of spin in the absence of net charge flow, 100% spin polarized currents, and charge currents are all accessible and controllable by tuning the amplitude of ~ 5 femtosecond gap tuned light pulses. Underpinning this physics is a symmetry lowering of the valley charge excitation from C3 at long duration to C2 in the few cycle limit, imbuing the excitation with net current. Our results both highlight the emergence of a rich light-spin current coupling at ultrafast times in the TMDC family, as well presenting a route to the all-optical generation of pure spin currents.

cond-mat.mes-hall↗

arXiv2025

Tunable magnons in a dual-gated 2D antiferromagnet

The layered antiferromagnet CrSBr features magnons coupled to other quasiparticles, including excitons and polaritons, enabling their easy optical accessibility. In this work, we investigate the tunability of magnons in few-layered devices in response to changes in carrier density and the application of a perpendicular electric field. We demonstrate an on-chip tunability of the in- and out-of-phase magnon frequencies by up to 2 GHz. While the frequencies of both modes increase with the electron density, we observe an asymmetric response with respect to the electric field in a dual-gated trilayer device. To understand the mechanism of this disparity, we develop a layer-resolved macrospin model describing the magnetic dynamics in thin, non-uniformly doped devices. Through this model we establish the doping- and electric-field-dependence of the exchange interaction, magnetic anisotropy, and magnetic moment of individual layers. Our results advance the applications of gate-tunable magnonic devices based on 2D materials.

cond-mat.mes-hall↗

arXiv2025

Coupled femto-excitons, free carriers and light

Non-equilibrium quantum matter generated by ultrafast laser light opens new pathways in fundamental condensed matter physics, as well as offering rich control possibilities in "tailoring matter by light". Here we explore the coupling between free carriers and excitons mediated by femtosecond scale laser pulses. Employing monolayer WSe$_2$ and an {\it ab-initio} treatment of pump-probe spectroscopy we find that, counter-intuitively, laser light resonant with the exciton can generate massive enhancement of the early time free carrier population. This exhibits complex dynamical correlation to the excitons, with an oscillatory coupling between free carrier population and exciton peak height that persists. Our results both unveil "femto-excitons" as possessing a rich femtosecond dynamics as well as, we argue, allowing tailoring of early time light-matter interaction via laser pulse design to control simultaneously excitonic and free carrier physics at ultrafast times.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast Saddletronics

Low energy valleys in the band structure of 2d materials represent a potential route to the ultrafast writing of information in quantum matter by laser light, with excited charge at the K or K$^\ast$ valleys representing the fundamental states of 1 and 0. Here we demonstrate that a second electronic feature, the saddle point, is endowed with lightwave control over information states. Linearly polarized light is shown to excite 2 of the 3 inequivalent M point saddles in graphene, generating three possible excited configurations, with which of these are realised determined by the polarization vector direction. We show that saddle excitation is highly robust, with "saddle polarized" states created both in the sub-cycle strong field regime and the long time limit of extended multi-cycle pulses. Our findings, applicable to other members of the graphene family and Xenes such as stanene, point towards a rich and ultrafast light based manipulation of matter based on the saddle point.

cond-mat.mes-hall↗

arXiv2025

Giant moment increase by ultrafast laser light

It is now well established that a few femtosecond laser pulse will induce an ultrafast loss of moment in a magnetic material. Here we show that the opposite effect can also occur: an ultrafast increase in moment. Employing both tight-binding and state-of-the-art time dependent density functional theory we find that laser light tuned to the majority spin conduction band in the 2d magnets CrI$_3$ and CrSBr generates an ultrafast giant moment increase, of up to 33\% in the case of CrI$_3$ (2~$\mu_B$). Underpinning this is spin-orbit induced valence band spin texture that, in combination with a strong field light pulse, facilitates an optical spin flip transition involving both intra- and inter-band excitation. Our findings, that establish a general mechanism by which ultrafast light pulses may enhance as well as decrease the magnetic moment, point towards rich possibilities for light control over magnetic matter at femtosecond times.

cond-mat.mtrl-sci↗

arXiv2024

Creation and control of valley currents in graphene by few cycle light pulses

Well established for the visible spectrum gaps of the transition metal dichalcogenide family, valleytronics - the control of valley charge and current by light - is comparatively unexplored for the THz gaps that characterize graphene and topological insulators. Here we show that few cycle pulses of THz light can create and control a 100% valley polarized current in graphene, with light wave control over the current magnitude and direction. The latter is equal to an emergent pulse property of few cycle circularly polarized pulses, the "global" carrier envelope phase. Our findings both highlight the richness of few cycle light pulses in control over quantum matter, and provide a route towards a "THz valleytronics" in meV gapped systems.

cond-mat.mes-hall↗

arXiv2024

Ultrafast all-optical generation of pure spin and valley currents

Pure currents comprise the flow of a two state quantum freedom -- for example the electron spin -- in the absence of charge flow. Radically different from the charge currents that underpin present day electronics, in two dimensional materials possessing additional two state freedoms such as valley index they offer profound possibilities for miniaturization and energy efficiency in a next generation spin- and valley- tronics. Here we demonstrate a robust multi-pump light wave protocol capable of generating both pure spin and valley currents on femtosecond times. The generation time is determined by the 2d material gap, with the creation of pure spin current in WSe2 at 40 fs and pure valley current in bilayer graphene at ~200 fs. Our all-optical approach demands no special material design, requiring only a gapped valley active material, and is thus applicable to a wide range of 2d materials.

cond-mat.mes-hall↗

arXiv2023

Theoretical evaluation of oxynitride, oxyfluoride and nitrofluoride perovskites with promising photon absorption properties for solar water splitting

Photocatalytic water splitting represents a very promising but at the same time very challenging contribution to a clean and renewable route to produce hydrogen fuel. Developing efficient and cost-effective photocatalysts for water splitting is a growing need. For this purpose, semiconductor photocatalysts have attracted much more attention due to their stability and low manufacturing cost. Here, we have systematically applied several state-of-the-art advanced first-principles-based methodologies, viz., hybrid density functional theory, many-body perturbation theory (G$_0$W$_0$) and density functional perturbation theory (DFPT), to understand the electronic structure properties of ABX$_2$Y perovskites. We have chosen the vast composition space of ABX$_2$Y type perovskites where A and B are cations and X and Y can be nitrogen, oxygen, or fluorine. These perovskites exhibit direct band gaps ranging from 1.6 to 3.3 eV. Further, to evaluate the feasibility of the visible light catalytic performance, we calculate the structural, electronic, and optical properties of ABX$_2$Y perovskites. In addition, from hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) mechanism, BaInO$_2$F, InSnO$_2$N, CsPbO$_2$F and LaNbN$_2$O are found as probable photocatalysts.

cond-mat.mtrl-sci↗

arXiv2022

Coupled spin-valley, Rashba effect and hidden persistent spin polarization in WSi$_2$N$_4$ family

The new two-dimensional materials, MoSi$_2$N$_4$ and WSi$_2$N$_4$ are experimentally synthesized successfully and various similar structures are predicted theoretically. Here, we report the electronic properties with a special focus on the band splitting in WA$_2$Z$_4$ (A=Si, Ge; Z=N, P, As), using state-of-the-art density functional theory and many-body perturbation theory (within the framework of G$_0$W$_0$ and BSE). Due to the broken inversion symmetry and strong spin-orbit coupling effects, we detect coupled spin-valley effects at the corners of the first Brillouin zone (BZ). Additionally, we observe cubically and linearly split bands around the $\Gamma$ and M points, respectively. Interestingly, the in-plane mirror symmetry ($\sigma_h$) and the reduced symmetry of arbitrary $k$-point, enforce the persistent spin textures (PST) to occur in full BZ. We induce the Rashba splitting by breaking the $\sigma_h$ through an out-of-plane external electric field (EEF). The inversion asymmetric site point group of the W atom introduces the hidden spin polarization in centrosymmetric layered bulk counterparts. Therefore, the spin-layer locking effect, namely, energy degenerate opposite spins spatially segregated in the top and bottom W layers, has been identified. Our low energy $k.p$ model demonstrates that the PST along the M-K line is robust to EEF and layer thickness, making them suitable for applications in spintronics and valleytronics.

cond-mat.mtrl-sci↗

arXiv2022

Sn/Ge substitution in ((C$_\textrm{n}$H$_{2\textrm{n}-1}$NH$_3$)$_2$PbI$_4$; n=3): An emerging 2D layered hybrid perovskites with enhanced optoelectronic properties$^\dag$

Two-dimensional (2D) perovskites show higher stability in comparison to their three-dimensional (3D) counterparts. Therefore, 2D perovskites have invoked remarkable attention in basic understanding of their physical properties and optoelectronic applications. Here we present a low-dimensional naturally self-assembled inorganic-organic (IO) hybrid systems based on primary cyclic ammonium-based (C$_{\textrm{n}}$H$_{2\textrm{n}-1}$NH$_{3}$) semiconductor series [viz. ((C$_{\textrm{n}}$H$_{2\textrm{n}-1}$ NH$_3$)$_2$PbI$_4$; n=3-6)]. However, the wide bandgap nature and presence of toxicity due to lead (Pb) prohibit their applications. Therefore, in the present work, we study the role of Ge/Sn substitution and Pb-vacancy (Pb-$\boxtimes$) to reduce concentration of Pb and to enhance solar cell efficiency by the formation of mixed perovskite structures. We have discussed the effect of spin-orbit coupling (SOC) using state-of-the-art hybrid density functional theory (DFT). We find the mixed conformers with Pb-$\boxtimes$ do not possess structural stability. Moreover, they have indirect bandgap, which is not good for solar cell applications. Only those conformers, which have favourable thermodynamics and structural stability, are considered for further study of optical properties. Our results infer that Sn substitution is more favorable than that of Ge in replacing Pb and enhancing the efficiency. Exciton binding energies calculated using Wannier-Mott approach for pristine and substituted conformers are larger than lead halide perovskites, while the electron-phonon coupling is smaller in the former. From computed spectroscopic limited maximum efficiency (SLME), these 2D perovskites show enough promise as alternatives to conventional lead halide perovskites.

cond-mat.mtrl-sci↗

arXiv2021

Origin of Rashba spin-splitting and strain tunability in ferroelectric bulk CsPbF$_3$

Spin-orbit coupling (SOC) in conjunction with broken inversion symmetry acts as a key ingredient for several intriguing quantum phenomena viz. persistent spin textures, topological surface states and Rashba-Dresselhaus (RD) effects. The coexistence of spontaneous polarization and the RD effect in ferroelectric materials enables the electrical control of spin degree of freedom. In light of this, we explore here the ferroelectric lead halide perovskite viz. CsPbF$_3$ as a potential candidate in the field of spintronics by employing state-of-the-art first-principles based methodologies viz. density functional theory (DFT) with semi-local and hybrid functional (HSE06) combined with spin-orbit coupling (SOC) and many-body perturbation theory (G$_0$W$_0$). For a deeper understanding of the observed spin-splitting, we have analyzed the spin textures within the combined framework of DFT and $\textbf{k.p}$ model Hamiltonian. The latter confirms that there is no out of plane spin component indicating that the Rashba splitting dominates over Dresselhaus splitting in this system. Owing to the presence of Pb-6$p$ orbital in conduction band, the large value of Rashba coefficient ($\alpha_R$) at conduction band minimum (CBm) is noticed in comparison to that of at the valence band maximum (VBM). Notably, we also observe that strength of Rashba spin-splitting can be substancially tuned on application of uniaxial strain ($\pm5\%$). This finding will further pave the path for perovskite-based spintronics devices.

cond-mat.mtrl-sci↗

arXiv2021

Exploring Exciton and Polaron Dominated Photo-physical Phenomena in Ruddlesden-Popper Phases of Ban+1ZrnS3n+1 (n=[1-3]) from Many Body Perturbation Theory

Ruddlesden-Popper (RP) phases of Ba$_{\textrm{n+1}}$Zr$_{\textrm{n}}$S$_{\textrm{3n+1}}$ (n=[1-3]) are evolved as new promising class of chalcogenide perovskites in the field of optoelectronics, especially in solar cells. However, detailed studies regarding its optical, excitonic, polaronic and transport properties are hitherto unknown. Here, we have explored the excitonic and polaronic effect in RP phases of Ba$_{\textrm{n+1}}$Zr$_{\textrm{n}}$S$_{\textrm{3n+1}}$ (n=[1-3]) using several first-principles based state-of-the-art methodologies under the framework of Many Body Perturbation Theory. Unlike it's bulk counterpart, the optical and excitonic anisotropy are observed in Ba$_{\textrm{n+1}}$Zr$_{\textrm{n}}$S$_{\textrm{3n+1}}$ (n=[1-3]) RP phases. From Wannier-Mott approach, we show that in the RP phases of this class of chalcogenide perovskites, capturing the ionic contribution to the dielectric constant is important. We report significant ionic contribution and relatively smaller electron-phonon coupling constant for Ba$_{\textrm{n+1}}$Zr$_\textrm{n}$S$_{\textrm{3n+1}}$ in comparison to the bulk BaZrS$_3$. The exciton binding energy is found to be dependent on the presence of large electron-phonon coupling. The charge carrier mobility is maximum in Ba$_2$ZrS$_4$, computed employing deformation potential of the same. As per our analysis, the optical phonon modes are observed to dominate the acoustic phonon modes, leading to decrease in polaron mobility on increasing n in Ba$_{\textrm{n+1}}$Zr$_{\textrm{n}}$S$_{\textrm{3n+1}}$ (n=[1-3]).

cond-mat.mtrl-sci↗

arXiv2021

Optoelectronic Properties of Chalcogenide Perovskites by Many-Body Perturbation Theory

Chalcogenide perovskites have emerged as non-toxic and stable photovoltaic materials, acting as an alternative to lead halide hybrid perovskites having similar optoelectronic properties. In the present work, we report the electronic and optical properties of chalcogenide perovskites AZrS$_3$ (A=Ca, Sr, Ba) by using the density functional theory (DFT) and many-body perturbation theory (MBPT viz. G$_0$W$_0$ and BSE). This study includes excitonic analysis for the aforementioned systems. The exciton binding energy (E$_\textrm{B}$) is found to be larger than that of the halide perovskites, as the ionic contribution to dielectric screening is negligible in the former. We also observe a more stable charge-separated polaronic state as compared to that of the bound exciton. Finally, on the basis of direct gap and absorption coefficient, the estimated spectroscopic limited maximum efficiency (SLME) of the solar cells is large and suggests the applicability of these perovskites in photovoltaics.

cond-mat.mtrl-sci↗

arXiv2020

Capturing Excitonic Effects in Lead Iodide Perovskites from Many-Body Perturbation Theory

Lead iodide perovskites have attracted considerable interest in the upcoming photovoltaic technologies and optoelectronic devices. Therefore, an accurate theoretical description of the electronic and optical properties especially to understand the excitonic effects in this class of materials is of scientific and practical interest. However, despite several theoretical research endeavours in past, the most accurate analysis of the key electronic parameters for solar cell performance, such as optical properties, effective mass, exciton binding energy (E$_B$) and the radiative exciton lifetime are still largely unknown. Here, we employ state-of-the-art first-principles based methodologies viz. hybrid functional(HSE06) combined with spin-orbit coupling (SOC), many-body perturbation theory (GW, BSE), model-BSE (mBSE), Wannier-Mott (WM) and Density Functional Perturbation Theory (DFPT). By taking a prototypical model system viz. APbI$_3$ (A = Formamidinium (FA), methylammonium (MA), and Cs), an exhaustive analysis is presented on the theoretical understanding of the optical, electronic and excitonic properties. We show that tuning of exact exchange parameter ($\alpha$) in HSE06 calculations incorporating SOC, followed by single shot GW, and BSE play a pivotal role in obtaining a reliable predictions for the experimental bandgap. We demonstrate that mBSE approach improves the feature of optical spectra w.r.t experiments. Furthermore, WM approach and ionic contribution to dielectric screening (below 16 meV) ameliorate the E$_B$. Our results reveal that the direct-indirect band gap transition (Rashba splitting) may be a factor responsible for the reduced charge carrier recombination rate in MAPbI$_3$ and FAPbI$_3$. The role of cation ''A'' for procuring the long-lived exciton lifetime is well understood. This proposed methodology allows to design new materials with tailored excitonic properties.

cond-mat.mtrl-sci↗

arXiv2020

High-Throughput Screening for Band gap Engineering by Sublattice Mixing of Cs$_2$AgBiCl$_6$ from First-Principles

The lead-free double perovskite material (viz. Cs$_2$AgBiCl$_6$) has emerged as an efficient and environmentally friendly alternative to lead halide perovskites. To make Cs$_2$AgBiCl$_6$ optically active in the visible region of solar spectrum, band gap engineering approach has been undertaken. Using Cs$_2$AgBiCl$_6$ as a host, band gap and optical properties of Cs$_2$AgBiCl$_6$ have been modulated by alloying with M(I), M(II), and M(III) cations at Ag-/Bi-sites. Here, we have employed density functional theory (DFT) with suitable exchange-correlation functionals in light of spin-orbit coupling (SOC) to determine the stability, band gap and optical properties of different compositions, that are obtained on Ag-Cl and Bi-Cl sublattices mixing. On analyzing the 64 combinations within Cs$_2$AgBiCl$_6$, we have identified 19 promising configurations having band gap sensitive to solar cell applications. The most suitable configurations with Ge(II) and Sn(II) substitutions have spectroscopic limited maximum efficiency (SLME) of 32.08% and 30.91%, respectively, which are apt for solar cell absorber.

cond-mat.mtrl-sci↗