arXiv subjects

Himadri S. Chakraborty

Publications and source records attributed to Himadri S. Chakraborty.

At least 19 recordsLinked to original sources

arXiv2025

Imaging atomic scattering potential in centroidal diffraction of elastic electrons

The Fraunhofer diffraction of quantum particles from materials with sharp electron-density edges or symmetric bond structures is ubiquitous. In contrast, diffraction from atoms with characteristic asymptotically-diffused electron distribution is far less intuitive, although known for many years. The current study unravels an unusual diffraction mechanism of elastic electrons from diffused atomic diffractors. Consequently, the fringe pattern converted to the Fourier reciprocal space maps out the effective scattering potential, which is not accessible in direct measurements. This may benefit benchmarking theory models, advances in atom-holography, plasma and astrophysical diagnostics, and accessing time-resolved potential landscapes. The study employs relativistic partial wave analysis with atoms modeled in the Dirac-Fock formalism and performs e-Cd measurements in absolute scale. Analysis for Mg, Ba, and Ra targets demonstrates the universality of the mechanism.

physics.atom-ph↗

arXiv2024

Hybridization and coherence in subshell differential intercluster plasmonic decay in $Na_{20}@C_{240}$

We study the ground state structure and aspects of photoionization dynamics of the $Na_{20}@C_{240}$ endofullerene. The structure shows effects from the electronic coupling between the nested cluster and the fullerene cage. They include the (i) alterations of the overall potential, and thus, the force field, (ii) electron transfer from the cluster to the fullerene forming ionic units, and (iii) hybridization from the admixture of free $Na_{20}$ occupied levels with experimentally known super-atom molecular orbital (SAMO) type empty levels of $C_{240}$ accessible in the jellium-DFT model. These modifications influence the photoionization dynamics of the endofullerene. For the high energy ionization of $Na_{20}$-type levels, a significant overall enhancement of the cross section is noted from additional ionizing force that $C_{240}$ offers. More remarkably, the photoexcited plasmons, both the giant plasmon and the higher energy plasmon, in decay in parts through $Na_{20}$ ionization continuum via the resonant intercluster Coulombic decay (ICD) process. These lead to dramatic enhancements in the ionization of individual $Na_{20}$-type levels resulting enhancements in the cluster's total ionization yield. Based on hybridization, this enhancement incorporates a coherent mixing of the ICD and SAMO-induced Auger-decay amplitude, in which the ICD contribution is dominant.

physics.atm-clus↗

arXiv2024

Diffraction patterns in attosecond photoionization time delay

Upon absorbing a photon, the ionized electron sails through the target force field in attoseconds to reach free space. This navigation probes details of the potential landscape that get imprinted into the phase of the ionization amplitude. The Eisenbud-Wigner-Smith (EWS) time delay, the energy derivative of this phase, provides the navigation time relative to the time of the electron's ``free'' exit. This time is influenced by the diffraction of the electron from the potential landscape, offering structural and dynamical information about interactions. If the potential has an intrinsic symmetry, a regular pattern in the time delay, including subpatterns of delays and advances, may occur from the diffraction process. The recent synthesis of a polyhedral fluorocarbon instigates the current study of photoionization from a cubic molecule. Our simulation of the EWS delay unravels rich diffraction motifs within $\pm$100 attoseconds in both energy and angular distributions. Averaging over the Euler angles from the laboratory to the molecular frame and over the photoelectron azimuthal direction indicates that the pattern should be discernible in ultrafast chronoscopy. The study benchmarks diffraction in molecular photoionization as a fundamental process which can be experimentally accessed through ultrafast time delay.

physics.atom-ph↗

arXiv2024

Heavy-hole spin relaxation in quantum dots: Isotropic versus anisotropic effects

Non-charge based logic in single-hole spin of semiconductor quantum dots (QDs) can be controlled by anisotropic gate potentials providing a notion for making next generation solid-state quantum devices. In this study, we investigate the isotropic and anisotropic behavior of phonon mediated spin relaxation of heavy-hole spin hot spots in QDs. For the electron spin in isotropic QDs, hot spots are known to be always present due to the Rashba spin-orbit coupling. But for heavy holes in isotropic dots, we show that the occurrences of spin hot spots are sensitive to the bulk g-factor. The hot spot for Rashba coupling in InAs and GaSb dots arises because these materials possess negative bulk g-factor, while that for the Dresselhaus coupling in GaAs and InSb dots is found due to their positive bulk g-factor. For anisotropic QDs, on the other hand, the spin hot spot is universally present due to their broken in-plane rotational symmetry. Further, the increasing electric field, that strengthens the Rashba coupling, is shown to cover a wide range of magnetic field by the hot spots. Results demonstrate that the magnetic field, choice of dot materials and size anisotropy can act as effective control parameters which can be experimentally used to design the device for detecting the phonon mediated heavy-hole spin-relaxation behavior of III-V semiconductor QDs.

cond-mat.mes-hall↗

arXiv2024

Simultaneous real and momentum space electron diffraction from a fullerene molecule

Plane-wave electrons undergo momentum transfer as they scatter off a target in overlapping spherical waves. The transferred momentum leads to target structural information to be encoded in angle and energy differential scattering. For symmetric, periodic or structured targets this can engender diffraction in the electron intensity both in real (angular) and in momentum space. With the example of elastic scattering from C60 we show this simultaneous manifestation of diffraction signatures. The simulated angle-momentum diffractograms can be imaged in experiments with a two-dimensional detector and an energy-tunable electron gun. The result may inspire invention of technology to extend scopes of electron diffraction from molecules and nanostructures, open a direction of electron crystallography using the momentum-differential diffraction, and motivate an approach to control the time delay between the pump laser-pulse and the probe electron-pulse by tuning the electron impact-speed in ultrafast electron diffraction experiments.

physics.atm-clus↗

arXiv2024

EWS time delay in low energy e C60 elastic scattering

Time delay in a projectile-target scattering is a fundamental tool in understanding their interactions by probing the temporal domain. The present study focuses on computing and analyzing the Eisenbud-Wigner-Smith (EWS) time delay in low energy elastic e C60 scattering. The investigation is carried out in the framework of a non-relativistic partial wave analysis (PWA) technique. The projectile-target interaction is described in (1) Density Functional Theory (DFT) and (2) Annular Square Well (ASW) static model, and their final results are compared in details. The impact of polarization on resonant and non-resonant time delay is also investigated.

math.NA↗

arXiv2023

Ultrafast nonadiabatic electron dynamics in photoexcited C60: A comparative study among DFT exchange-correlation functionals

The non-radiative electron-relaxation dynamics in C$_{60}$ molecule is studied after selective initial photoexcitations. The methodology includes nonadibabtic molecular simulation combined with time-dependent density functional theory (DFT) and semi-classical surface hopping approach. Results of treating the DFT exchange-correlation (xc) interaction by the non-empirical Perdew-Burke-Ernzerhof (PBE), hybrid PBE0, and hybrid Becke 3-parameter Lee-Yang-Parr (B3LYP) functional are compared. Even though some differences in the details are found, all three functionals produce qualitatively similar unoccupied band structures in the ground state. The model-dependent differences in the ultrafast population dynamics, including the occurrences of transient entrapment of population, are studied systematically. The trend of the results demonstrates a universal dependence on the structure of unoccupied band offering a spectroscopic route to probe this structure. Results can be verified, as well as the best xc model for quantitative accuracy can be determined, by comparing with ultrafast transient absorption or time-resolved photoelectron spectroscopy measurements. From the computational standpoint, the study facilitates method optimization to simulate nonadiabatic relaxation dynamics in technologically important fullerene derivatives.

physics.atm-clus↗

arXiv2022

Plasmonic Resonant Intercluster Coulombic Decay

Light-induced energy confinement in nanoclusters via plasmon excitations influences applications in nanophotonics, photocatalysis, and the design of controlled slow electron sources. The resonant decay of these excitations through the cluster's ionization continuum provides a unique probe of the collective electronic behavior. However, the transfer of a part of this decay amplitude to the continuum of a second conjugated cluster may offer control and efficacy in sharing the energy nonlocally to instigate remote collective events. With the example of a spherically nested dimer Na20@C240 of two plasmonic systems we find that such a transfer is possible through the resonant intercluster Coulomb decay (RICD) as a fundamental process. This plasmonic RICD signal can be experimentally detected by the photoelectron velocity map imaging technique.

physics.atm-clus↗

arXiv2021

Attosecond correlated electron dynamics at C$_{60}$ giant plasmon resonance

Fullerenes have unique physical and chemical properties that are associated with their delocalized conjugated electronic structure. Among them, there is a giant ultra-broadband - and therefore ultrafast - plasmon resonance, which for C$_{60}$ is in the extreme-ultraviolet energy range. While this peculiar resonance has attracted considerable interest for the potential downscaling of nanoplasmonic applications such as sensing, drug delivery and photocatalysis at the atomic level, its electronic character has remained elusive. The ultrafast decay time of this collective excitation demands attosecond techniques for real-time access to the photoinduced dynamics. Here, we uncover the role of electron correlations in the giant plasmon resonance of C$_{60}$ by employing attosecond photoemission chronoscopy. We find a characteristic photoemission delay of up to 200 attoseconds pertaining to the plasmon that is purely induced by coherent large-scale correlations. This result provides novel insight into the quantum nature of plasmonic resonances, and sets a benchmark for advancing nanoplasmonic applications.

physics.atom-ph↗

arXiv2021

Density functional study of the variants of inter-Coulombic decay resonances in the photoionization of Cl@C60

Inter-Coulombic decay (ICD) resonances in the photoionization of Cl@C60 endofullerene molecule are calculated using a perturbative density functional theory (DFT) method. This is the first ICD study of an open shell atom in a fullerene cage. Three classes of resonances are probed: (i) Cl inner vacancies decaying through C60 outer continua, (ii) C60 inner vacancies decaying through Cl outer continua, and (iii) inner vacancies of either system decaying through the continua of Cl-C60 hybrid levels, the hybrid Auger-ICD resonances. Comparisons with Ar@C60 results reveal that the properties of hybrid Auger-ICD resonances are affected by the extent of level hybridization.

physics.atm-clus↗

arXiv2021

Electron vacancy-level dependent hybrid photoionization of F-@C60+ molecule

Our previous studies [J. Phys. B 53, 125101 (2020); Euro. Phys. J. D 74, 191 (2020)] have predicted that the atom-fullerene hybrid photoionization properties for X = Cl, Br and I endohedrally confined in C60 are different before and after an electron transfers from C60 to the halogen. It was further found as a rule that the ionization dynamics is insensitive to the C60 level the electron originates from to produce X-@C60+. In the current study, we report an exception to this rule in F@C60. It is found that when the electron vacancy is situated in the C60 level that participates in the hybridization in F-@C60+, the mixing becomes dramatically large leading to strong modifications in the photoionization of the hybrid levels. But when the vacancy is at any other pure level of C60, the level-invariance is retained showing weak hybridization. Even though this case of F@C60 is an anomaly in the halogen@C60 series, the phenomenon can be more general and can occur with compounds of other atoms caged in a variety of fullerenes. In addition, possible experimental studies are suggested to benchmark the present results.

physics.atm-clus↗

arXiv2020

Ultrafast transfer and transient entrapment of photoexcited Mg electron in Mg@C60

Electron relaxation is studied in endofullerene Mg@C60, after an initial localized photoexcitation in Mg, by nonadiabtic molecular dynamics simulations. To ensure reliability, two methods are used: i) an independent particle approach with a DFT description of the ground state and ii) HF ground state with many-body effects for the excited state dynamics. Both methods exhibit similar relaxation times leading to an ultrafast decay and charge transfer from Mg to C60 within tens of femtoseconds. Method (i) further elicits a robust transient-trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption spectroscopy in gas phase, in solution, or as thin films.

physics.atm-clus↗

arXiv2020

A density functional theory based comparative study of hybrid photoemissions from Cl@C60, Br@C60 and I@C60

Photoionization from atom-C60 hybrid levels in halogen endufullerene molecules, Cl@C60, Br@C60 and I@C60, are calculated using a linear response density functional method. Both the ordinary electron-configuration where the open shell halogen is at the center of C60 and the stable configuration after the atom receives an electron from C60 to form a closed shell anion are considered. Similar ground state hybridization is found for all three systems while, in general, a slight weakening of the effect is noticed after the electron transfer. At lower photon energies, cross sections of the outer hybrid levels attain identical shapes from enhancements driven by the C60 plasmon resonances, while the higher energy emissions remain distinguishable from the differences in atomic responses. These results further show near insensitivity to the choice of a configuration. The inner hybrid cross sections in general exhibit similar overall structures, although differ in details between molecules. However, for these states the results significantly differ before and after the electron transfer -- a feature that can be useful to experimentally determine the real configuration of the molecules via photoelectron spectroscopy.

physics.atm-clus↗

arXiv2019

Photoemission from hybrid states of Cl@$C_{60}$ before and after a stabilizing charge transfer

Photoionization calculations of the endofullerene molecule Cl@$C_{60}$ with an open-shell chlorine atom are performed in the time-dependent local density approximation (TDLDA) based on a spherical jellium model. Cross sections for atom-fullerene hybrid photoemission studied show the effects of the hybridization symmetry, the giant plasmon and the molecular cavity. Comparisons with the results of Ar@$C_{60}$ provide insights in the role of a shell-closing electron and its influence on the dynamics. The results for Cl@$C_{60}$ are further compared with those of a more stable, lower energy configuration that results after a $C_{60}$ electron transfers to Cl forming Cl$^-$@$C_{60}^+$. This comparison reveals noticeable differences in the ionization properties of the antibonding hybrid state while the bonding hybrid remains nearly unaltered showing a magnification covering the entire giant plasmon energy range.

physics.atm-clus↗

arXiv2019

Resonant charge-transfer in grazing collisions of H$^-$ with vicinal nanosurfaces on Cu(111), Au(100) and Pd(111) substrates: A comparative study

We compare the electron dynamics at monocrystalline Cu(111), Au(100) and Pd(111) precursor substrates with vicinal nanosteps. The unoccupied bands of a surface superlattice are populated \textit{via} the resonant charge transfer (RCT) between the surface and a H$^-$ ion that flies by at grazing angles. A quantum mechanical wave packet propagation approach is utilized to simulate the motion of the active electron where time-evolved wave packet densities are used to visualize the dynamics through the superlattice. The survived ion fraction in the reflected beam generally exhibits modulations as a function of the vicinal terrace size and shows peaks at those energies that access the image state subband dispersions. However, differences in magnitudes of the ion-survival as a function of the particular substrate selection as well as the ion-surface interaction time based on the choice of two ion-trajectories are examined. A square well model producing standing waves between the steps on the surface explains well the energies of the maxima in the ion survival probability for all the metals considered, indicating that the primary process of confinement induced subband formation is rather robust. The work may motivate measurements and applications of shallow-angle ion-scattering spectroscopy to access electronic substructures in periodically nanostructured surfaces.

physics.atom-ph↗

arXiv2019

Molecular size effects on diffraction resonances in positronium formation from fullerenes

We previously predicted [P.A. Hervieux et al., Phys. Rev. A \textbf{95}, 020701 (2017)] that owing to predominant electron capture by incoming positrons from the molecular shell, C$_{60}$ acts like a spherical diffractor inducing resonances in the positronium (Ps) formation as a function of the positron impact energy. By extending the study for a larger C$_{240}$ fullerene target, we now demonstrate that the diffraction resonances compactify in energy in analogy with the shrinking fringe separation for larger slit size in classical single-slit experiment. The result brings further impetus for conducting Ps spectroscopic experiments with fullerene targets, including target- and/or captured-level differential measurements. The ground states of the fullerenes are modeled in a spherical jellium frame of the local density approximation (LDA) method with the exchange-correlation functional based on the van Leeuween and Baerends (LB94) model potential, while the positron impact and Ps formation are treated in the continuum distorted-wave final state (CDW-FS) approximation.

physics.atm-clus↗

arXiv2019

Ion survival in grazing collisions of H$^-$ with vicinal nanosurfaces probes subband electronic structures

We study the electron dynamics at a monocrystalline Pd(111) surface with stepped vicinal nanostructures modeled in a simple Kronig-Penney scheme. The unoccupied bands of the surface are resonantly excited \textit{via} the resonant charge transfer (RCT) interaction of the surface with a hydrogen anion reflected at grazing angles. The interaction dynamics is simulated numerically in a quantum mechanical wave packet propagation approach. Visualization of the wave packet density shows that, when the electron is transferred to the metal, the surface and image subband states are the most likely locations of the electron as it evolves through the superlattice. The survival probability of the interacting ion exhibits strong modulations as a function of the vicinal-terrace size and shows peaks at those energies that access the image state subband dispersions. A simple square well model producing standing waves between the steps on the surface suggests the application of such ion-scattering at shallow angles to map electronic substructures in vicinal surfaces. The work also serves as the first proof-of-principle in the utility of our computational method to address, via RCT, surfaces with nanometric patterns.

physics.atom-ph↗

arXiv2016

Ubiquitous diffraction resonances in positronium formation from fullerenes

Due to the dominant electron capture by positrons from the molecular wall and the spatial dephasing across the wall-width, a powerful diffraction effect universally underlies the positronium (Ps) formation from fullerenes. This results into trains of resonances in the Ps formation cross section as a function of the positron beam energy, producing uniform structures in recoil momenta in analogy with classical single-slit diffraction fringes in the configuration space. The prediction opens a hitherto unknown avenue of Ps spectroscopy with nanomaterials.

physics.atm-clus↗