arXiv subjects

Sanghita Sengupta

Publications and source records attributed to Sanghita Sengupta.

13 recordsLinked to original sources

arXiv2026

A geometry-first tutorial for time-resolved morphological analysis with PyPETANA

We present a step-by-step, reproducible tutorial for PyPETANA, an open-source Python framework for geometry-first, time-resolved quantification of evolving morphology from image data. Starting from time-lapse video input, the tutorial demonstrates how to extract binary masks, compute time-resolved geometric observables including area, perimeter, circularity, and effective fractal dimensions, and analyze their temporal evolution. The workflow emphasizes direct reconstruction of morphology from images without assuming microscopic growth mechanisms. In addition to compactness-sensitive geometric descriptors, the framework supports multiscale boundary analysis through supersampled box-counting methods applied to filled morphologies and finite-width boundary bands. The benchmark suite further demonstrates applicability to invasive tumor morphologies and multiscale boundary evolution in time-resolved cancer-growth interfaces. This tutorial accompanies the computational workflow underlying arXiv:2602.05958 and provides a reproducible foundation for geometry-based analysis of evolving non-equilibrium morphologies.

cond-mat.soft↗

arXiv2026

Geometry and dynamical morphology of growing bacterial colonies

We study non-equilibrium bacterial colony growth using a geometry-first, time-resolved analysis of morphology. From time-lapse microscopy data, we track the coupled evolution of area, perimeter, and boundary-sensitive shape descriptors along the full growth history. We find that non-equilibrium growth can exhibit extended intervals of compact area--perimeter scaling with exponent $α\approx 2$, consistent with growth governed by a single effective geometric length scale, as well as time-localized breakdowns of this scaling during ongoing growth. These breakdowns coincide with transient boundary reorganization while bulk area growth remains sustained. Our results demonstrate that visually distinct morphologies can arise within the same geometric growth regime, and that departures from single-scale behavior reflect intrinsic dynamical restructuring rather than growth arrest. More broadly, this work establishes time-resolved geometry as a coarse-grained framework for identifying when non-equilibrium growth departs from single-scale geometric constraints in living systems.

cond-mat.soft↗

arXiv2025

Nonreciprocal Acoustic and Optical Phonon Dispersion Mediated by Berry Curvature in Chiral Weyl Semimetals

We investigate the phonon magnetochiral effect (PMCE) in chiral Weyl semimetals by deriving the nonreciprocal dispersion relations of both acoustic and non-polar optical phonons in the presence of a magnetic field. Using a semiclassical Boltzmann kinetic framework that incorporates Berry curvature, orbital magnetic moment, and node-dependent electronic structure, we obtain analytic expressions for the magnetic-field-induced corrections to the phonon dynamical matrix. Inequivalent Weyl nodes with distinct Fermi velocities, Fermi energies, and relaxation times generate a dynamical chiral imbalance that alters the phonon dispersion. For acoustic phonons, the formalism yields the magnetic-field-dependent corrections to the longitudinal mode, while for optical phonons we identify an optical analogue of the PMCE that produces a corresponding shift in the optical branch. Together, these results provide a unified theoretical description of how band-geometric properties of Weyl fermions influence both acoustic and optical phonon dispersions in chiral Weyl semimetals.

cond-mat.mes-hall↗

arXiv2023

Hyperfine interactions in open-shell planar $sp^2$-carbon nanostructures

We investigate hyperfine interaction (HFI) using density-functional theory for several open-shell planar $sp^2$-carbon nanostructures displaying $π$ magnetism. Our prototype structures include both benzenoid ([$n$]triangulenes and a graphene nanoribbon) as well as non-benzenoid (indene, fluorene, and indene[2,1-b]fluorene) molecules. Our results obtained with ORCA indicate that isotropic Fermi contact and anisotropic dipolar terms contribute in comparable strength, rendering the HFI markedly anisotropic. We find that the magnitude of HFI in these molecules can reach more than 100 MHz, thereby opening up the possibility of experimental detection via methods such as electron spin resonance-scanning tunneling microscopy (ESR-STM). Using these results, we obtain empirical models based on $π$-spin polarizations at carbon sites. These are defined by generic $sp^{2}$ HFI fit parameters which are derived by matching the computed HFI couplings to $π$-spin polarizations computed with methods such as ORCA, SIESTA, or mean-field Hubbard (MFH) models. This approach successfully describes the Fermi contact and dipolar contributions for $^{13}$C and $^{1}$H nuclei. These fit parameters allow to obtain hyperfine tensors for large systems where existing methodology is not suitable or computationally too expensive. As an example, we show how HFI scales with system size in [$n$]triangulenes for large $n$ using MFH. We also discuss some implications of HFI for electron-spin decoherence and for coherent nuclear dynamics.

cond-mat.mes-hall↗

arXiv2020

Phonon magnetochiral effect of band-geometric origin in Weyl semimetals

The phonon magnetochiral effect consists of a nonreciprocity in the velocity or attenuation of acoustic waves when they propagate parallel and antiparallel to an external magnetic field. The first experimental observation of this effect has been reported recently in a chiral magnet and ascribed to the hybridization between acoustic phonons and chiral magnons. Here, we predict a potentially measurable phonon magnetochiral effect of electronic origin in chiral Weyl semimetals. Caused by the Berry curvature and the orbital magnetic moment, this effect is enhanced for longitudinal phonons by the chiral anomaly.

cond-mat.mes-hall↗

arXiv2020

Anomalous Transition Magnetic Moments in two-dimensional Dirac Materials

We show that the magnetic response of atomically thin materials with Dirac spectrum and spin-orbit interactions can exhibit strong dependence on electron-electron interactions. While graphene itself has a very small spin-orbit coupling, various two-dimensional (2D) compounds "beyond graphene" are good candidates to exhibit the strong interplay between spin-orbit and Coulomb interactions. Materials in this class include dichalcogenides (such as MoS$_2$ and WSe$_2$), silicene, germanene, as well as 2D topological insulators described by the Kane-Mele model. We present a unified theory for their in-plane magnetic field response leading to "anomalous", i.e. electron interaction dependent transition moments. Our predictions can be potentially used to construct unique magnetic probes with high sensitivity to electron correlations.

cond-mat.mes-hall↗

arXiv2020

Reply to "Comment on `Theory of Phonon-Assisted Adsorption in Graphene: Many-Body Infrared Dynamics' "

Based on a new self-energy for atom-phonon interaction, preceding Comment argues about the insufficiency of the mathematical techniques within the Independent Boson Model (IBM) to study physisorption in graphene membranes. In this Reply, we show that the new self-energy reported in the Comment is a perturbative expansion approximated for a 2-phonon process, severely divergent for membrane sizes larger than 100 nm and within its current mathematical form, ill-suited for investigating the physics of physisorption in graphene micromembranes. Additionally, we provide with further evidence of the adsorption rate within the IBM that further reinforces the physical soundness of the mathematical techniques reported in Phys. Rev. B 100, 075429 (2019).

cond-mat.mtrl-sci↗

arXiv2020

Theory of Phonon-Assisted Adsorption in Graphene: Many-Body Infrared Dynamics

We devise a theory of adsorption of low-energy atoms on suspended graphene membranes maintained at 10 K based on a model of atom-acoustic phonon interactions. Our primary technique includes a non-perturbative method which treats the dynamics of the multiple phonons in an exact manner within the purview of the Independent Boson Model. We present a study on the effects of the phonons assisting the renormalization as well as decay of the incident atom propagator and discuss results for the many-body adsorption rates for atomic hydrogen on graphene micromembranes. Additionally, we report similarities of this model with other branches of quantum field theories that include long-range interactions like quantum electrodynamics (QED) and perturbative gravity.

cond-mat.mes-hall↗

arXiv2018

Infrared Problem in Cold Atom Quantum Physisorption on 2D Materials

Results from four different approximations to the phonon-assisted quantum adsorption rate for cold atoms on a 2D material are compared and contrasted: (1) a loop expansion (LE) based on the atom-phonon coupling, (2) non-crossing approximation (NCA), (3) independent boson model approximation (IBMA), and (4) a leading-order soft-phonon resummation method (SPR). We conclude that, of the four approximations considered, only the SPR method gives a divergence-free result in the large membrane regime at finite temperature. The other three methods give an adsorption rate that diverges in the limit of an infinite surface.

cond-mat.mes-hall↗

arXiv2018

Theory of liquid film growth and wetting instabilities on graphene

We investigate wetting phenomena near graphene within the Dzyaloshinskii-Lifshitz-Pitaevskii theory for light gases composed of hydrogen, helium and nitrogen in three different geometries where graphene is either affixed to an insulating substrate, submerged or suspended. We find that the presence of graphene has a significant effect in all configurations. In a suspended geometry where graphene is able to wet on only one side, liquid film growth becomes arrested at a critical thickness which may trigger surface instabilities and pattern formation analogous to spinodal dewetting. These phenomena are also universally present in other two-dimensional materials.

cond-mat.mes-hall↗

arXiv2017

Radiative Corrections to Quantum Sticking on Graphene

We study the sticking rate of atomic hydrogen to suspended graphene using four different methods that include contributions from processes with multiphonon emission. We compare the numerical results of the sticking rate obtained by: (1) the loop expansion of the atom self-energy, (2) the non-crossing approximation (NCA), (3) the independent boson model approximation (IBMA), and (4) a leading-order soft-phonon resummation method (SPR). The loop expansion reveals an infrared problem, analogous to the infamous infrared problem in QED. The 2-loop contribution to the sticking rate gives a result that tends to diverge for large membranes. The latter three methods remedy this infrared problem and give results that are finite in the limit of an infinite membrane. We find that for micromembranes (sizes ranging 100 nm to 10 $μ$m), the latter three methods give results that are in good agreement with each other and yield sticking rates that are mildly suppressed relative to the lowest-order golden rule rate. Lastly, we find that the SPR sticking rate decreases slowly to zero with increasing membrane size, while both the NCA and IBMA rates tend to a nonzero constant in this limit. Thus, approximations to the sticking rate can be sensitive to the effects of soft-phonon emission for large membranes.

cond-mat.mes-hall↗

arXiv2017

Infrared Problem in Quantum Acoustodynamics

Quantum electrodynamics (QED) provides a highly accurate description of phenomena involving the interaction of atoms with light. We argue that the quantum theory describing the interaction of cold atoms with a vibrating membrane--quantum acoustodynamics (QAD)--shares many issues and features with QED. Specifically, the adsorption of an atom on a vibrating membrane can be viewed as the counterpart to QED radiative electron capture. A calculation of the adsorption rate to lowest-order in the atom-phonon coupling is finite; however, higher-order contributions suffer from an infrared problem mimicking the case of radiative capture in QED. Terms in the perturbation series for the adsorption rate diverge as a result of massless particles in the model (flexural phonons of the membrane in QAD and photons in QED). We treat this infrared problem in QAD explicitly to obtain finite results by regularizing with a low-frequency cutoff that corresponds to the inverse size of the membrane. Using a coherent state basis for the soft phonon final state, we then sum the dominant contributions to derive a new formula for the multiphonon adsorption rate of atoms on the membrane that gives results that are finite, nonperturbative in the atom-phonon coupling, and consistent with the KLN theorem. For micromembranes, we predict a reduction with increasing membrane size for the low-energy adsorption rate. We discuss the relevance of this to the adsorption of a cold gas of atomic hydrogen on suspended graphene.

cond-mat.mes-hall↗

arXiv2016

Infrared Dynamics of Cold Atoms on Hot Graphene Membranes

We study the infrared dynamics of low-energy atoms interacting with a sample of suspended graphene at finite temperature. The dynamics exhibits severe infrared divergences order by order in perturbation theory as a result of the singular nature of low-energy flexural phonon emission. Our model can be viewed as a two-channel generalization of the independent boson model with asymmetric atom-phonon coupling. This allows us to take advantage of the exact non-perturbative solution of the independent boson model in the stronger channel while treating the weaker one perturbatively. In the low-energy limit, the exact solution can be viewed as a resummation (exponentiation) of the most divergent diagrams in the perturbative expansion. As a result of this procedure, we obtain the atom's Green function which we use to calculate the atom damping rate, a quantity equal to the quantum sticking rate. A characteristic feature of our results is that the Green's function retains a weak, infrared cutoff dependence that reflects the reduced dimensionality of the problem. As a consequence, we predict a measurable dependence of the sticking rate on graphene sample size. We provide detailed predictions for the sticking rate of atomic hydrogen as a function of temperature and sample size. The resummation yields an enhanced sticking rate relative to the conventional Fermi golden rule result (equivalent to the one-loop atom self-energy), as higher-order processes increase damping at finite temperature.

cond-mat.mes-hall↗