arXiv subjects

Debasish Mondal

Publications and source records attributed to Debasish Mondal.

At least 19 recordsLinked to original sources

arXiv2026

Feller diffusion in an interval: Inhomogeneous fluctuation-induced asymmetric escape

We present an inhomogeneous fluctuation-induced asymmetric escape event of a Feller diffusion confined to a finite interval with two competing absorbing boundaries. The dynamics correspond to an overdamped Brownian motion in a shifted harmonic potential with state-dependent diffusivity. We set two exit points (equal potential energies), equidistant from the potential minima: an extinction site near the origin and an outbreak point. The multiplicative fluctuations are suppressed near the extinction boundary, biasing trajectories toward the outbreak state. The mean exit time exhibits a non-monotonic dependence on the initial condition and the drift-to-noise strength ratio, attaining a maximum when the particle is initialized toward the extinction (low-noise) site. The outbreak possibility is more likely even if the process started with a small initial bias towards the low-noise boundary. The spatial location of the lowest coefficient of variation (CV) is nicely corroborated by the maximum exit time, which is the hallmark of a stochastic escape from an interval. The fluctuation in escape time is found to dominate its mean as a non-trivial function of the drift-to-noise strength and the initial spatial bias. The observed asymmetries in escape also shape the speed-accuracy trade-off in stochastic decision-making events.

cond-mat.stat-mech↗

arXiv2025

$\mbox{H}$ $\mbox{I}$ 21-cm Absorption Spectra Classification using Machine Learning

$\mbox{H}$ $\mbox{I}$ 21-cm absorption, an extremely useful tool to study the cold atomic hydrogen gas, can arise either from the intervening galaxies along the line-of-sight towards the background radio source or from the radio source itself. Determining whether $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines detected as part of large, blind surveys are `intervening' or `associated' using optical spectroscopy would be unfeasible. We therefore investigate a more efficient, machine learning (ML)-based method to classify $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines. Using a sample of 118 known $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines from the literature, we train six ML models (Gaussian naive Bayes, logistic regression, decision tree, random forest, SVM and XGBoost) on the spectral parameters obtained by fitting the Busy function to the absorption spectra. We found that a random forest model trained on these spectral parameters gives the most reliable classification results, with an accuracy of 89%, a $F_1$-score of 0.9 and an AUC score of 0.94. We note that the linewidth parameter $w_{20}$ is the most significant spectral parameter that regulates the classification performance of this model. Retraining this random forest model only with this linewidth and the integrated optical depth parameters yields an accuracy of 88%, a $F_1$-score of 0.88 and an AUC score of 0.91. We have applied this retrained random forest model to predict the type of 30 new $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines detected in recent blind surveys, viz. FLASH, illustrating the potential of the techniques developed in this work for future large $\mbox{H}$ $\mbox{I}$ surveys with the Square Kilometre Array.

astro-ph.GA↗

arXiv2025

Non-Hermitian band topology in twisted bilayer graphene aligned with hexagonal boron nitride

Utilizing the established Bistritzer-MacDonald model for twisted bilayer graphene (tBLG), we theoretically investigate the non-Hermitian (NH) topological properties of this in the presence of non-reciprocal (NR) hopping on both layers and hexagonal boron nitride (hBN) induced mass term incorporated only on the top layer of the tBLG system. It is well known that the hBN mass term breaks the $C_{2}$ symmetry of tBLG and gaps out the Dirac cones inducing a valley Hall insulating phase. However, when NR hopping is introduced, this system transits into a NH valley Hall insulator (NH-VHI). Our analysis reveals that, in the chiral limit, the bandwidth of the system vanishes under NH effects for a wide range of twist angles. Such range can be visibly expanded as we enhance the degree of non-Hermiticity ($β$). At the magic angle, we observe that enhancement of $β$ inflates the robustness of the gapless Dirac points, requiring a progressively larger mass term to induce a gap in the NH tBLG system. Additionally, for a fixed NH parameter, we identify a range of twist angles where gap formation is significantly obstructed. To explore the topological aspects of the NH tBLG, we analyze the direct band gap in the Moiré Brillouin zone (mBZ) and compute the Chern number for the NH system. We find that the corresponding topological phase transitions are associated with corresponding direct band gap closings in the mBZ.

cond-mat.mes-hall↗

arXiv2025

Geometric Brownian information engine with finite cycle time: Optimisation of output work, power and efficiency

We consider a Geometric Brownian Information Engine to explore the effects of finite cycle time $(τ)$ on the extractable work, power, and efficiency. We incorporate an error-free feedback controller that converts the information obtained about the state of overdamped Brownian particles, confined within a 2-D monolobal geometry, into extractable work. The performance of the information engine depends on the cycle period $(τ)$, measurement distance $(x_m)$, and feedback location $(x_f)$ of the controller. Upon increasing the feedback cycle time, the engine transitions from a high non-equilibrium steady state to a completely relaxed state. We set the measurement distance at an optimum position related to a fully relaxed state ($x_m^* \sim 0.6 σ$). When the cycle time is finite and short ($τ<τ_r$), the best information processing occurs with a shorter distance of the feedback site. While increasing the cycle time towards a fully relaxed state ($τ\gg τ_r$), the maximum extractable work that can be achieved with a feedback location is set to be twice that of $x_m^*$, as expected. When the cycle time ($τ$) is longer than the relaxation time ($τ_r$), the maximum power is achieved when the scaled feedback location is exactly double the optimum measurement distance ($x_f^{*}=2x_m^*$). In contrast, when $τ< τ_r$, the maximum power is achieved when the feedback site is set at a lower value. As the $τ$ increases, the maximum average power decreases. In the limit of a long $τ$, the highest efficiency as well extractable work is attained when $x_f$ is located at $2x_m$, regardless of the level of entropic control. As the dominance of entropic control increases, the extractable work and efficiency in the fully relaxed state decrease due to higher information loss during relaxation.

cond-mat.stat-mech↗

arXiv2025

Global optimisation of the control strategy of a Brownian information Engine: Efficient information-energy exchange in a generalised potential energy surface

An information engine harnesses energy from a single heat bath, utilising the gathered information. This study explores the best control strategy of a Brownian information engine (BIE), confined in a potential energy surface (PES) of arbitrary shape, and experiencing a measurement outcome-based feedback cycle. The feedback site corresponds to an instantaneous shift in the potential centre to an additional feedback distance over the measurement outcome. The strategy for the most efficient information-to-energy conversion is achieved when the position of the global potential minimum corresponds to the additional feedback distance. The BIE acts as a heater if and only if the average potential energy is higher than the energy at the additional feedback distance. Operating under confinement PES of different shapes, the BIE can harness energy beyond the average potential energy, and multiple heater-refrigerator re-entrance events are feasible. The consequences of the best control strategy are explained using sufficient examples.

cond-mat.soft↗

arXiv2025

Performance of a Brownian information engine through potential profiling: Optimum output requisites, Heating-to-Refrigeration transition and their Re-entrance

Brownian Information engine (BIE) harnesses the energy from a fluctuating environment by utilizing the associated information change in the presence of a single heat bath. The engine operates in a space-dependent confining potential and requires an appropriate feedback control mechanism. In general, the feedback controller has three different steps: measurement, feedback, and relaxation. The feedback step is related to a sudden change in the potential energy that is essential for a nonzero work output. BIE utilises the amount of information (surprise) acquired during the measurement step for the energy output. However, due to the relaxation process, a certain amount of acquired information is lost or becomes unavailable. So, controlling information loss during relaxation is crucial for the overall efficiency of the engine. The net (available) information, therefore, can be monitored by tuning the feedback controller and the shape of the confining potential. In this paper, we explore the effect of the shape modulation of the confining potential, which may have multiple stable valleys and unstable hills, on the net available information and, hence, the performance of a BIE that operates under an asymmetric feedback protocol. We examine the optimal performance requirements of the BIE and the amount of maximum work output under different potential profiling. For monostable trapping, a concave shape in confining potential results in a higher work output than a convex one. We also find that hills and valleys in the confining potential may lead to multiple good operating conditions. An appropriate shape modulation can create a heater-refrigerator transition and their reentrance due to non-trivial changes in information loss during the relaxation process.

cond-mat.soft↗

arXiv2025

Achievable Information-Energy Exchange in a Brownian Information Engine through Potential Profiling

The information engine extracts work from a single heat bath using mutual information obtained during the operation cycle. This study investigates the influence of the potential shaping in a Brownian information engine (BIE) in harnessing the information from thermal fluctuations. We have designed a BIE by considering an overdamped Brownian particle inside a confined potential and introducing an appropriate symmetric feedback cycle. We find that the upper bound of the extractable work for a BIE with a monostable centrosymmetric confining potential, with a stable state at the potential centre, depends on the bath temperature and the convexity of the confinement. A concave confinement is more efficient for an information-energy exchange. For a bistable confinement with an unstable centre and two symmetric stable basins, one can find an engine-to-refrigeration transition beyond a certain barrier height related to the energy difference between the energy barrier and the stable basins. Finally, we use the concavity-induced gain in information harnessing to device a BIE in the presence of a multistable potential that can harvest even more energy than monostable confinement.

cond-mat.soft↗

arXiv2024

Measurement of the Hoyle State Radiative Transition Width

The radiative decay of the Hoyle state is the doorway to the production of heavier elements in stellar environment. Here we report, an exclusive measurement of electric quadruple (E$_2$) transitions of the Hoyle state to the ground state of $^{12}$C through the $^{12}$C(p, p$^\prime$$γ$$γ$)$^{12}$C reaction. Triple coincidence measurement yields a value of radiative branching ratio $Γ_{rad}$/$Γ$ = 4.01 (30) $\times$ 10$^{-4}$. The result has been corroborated by an independent experiment based on the complete kinematical measurement $via.$ $^{12}$C(p, p$^\prime$)$^{12}$C reaction ($Γ_{rad}$/$Γ$ = 4.04 (30) $\times$ 10$^{-4}$). Using our results together with the currently adopted values of $Γ_π$(E$_0$)/$Γ$ and $Γ_π$($E_0$), the radiative width of the Hoyle state is found to be 3.75 (40) $\times$ 10$^{-3}$ eV. We emphasize here that our result is not in agreement with 34 $\%$ increase in the radiative decay width of the Hoyle state measured recently but consistent with the currently adopted value.

nucl-ex↗

arXiv2024

Chaotic Propellers of Barred Galaxies and Central Explosions

The central theme of this thesis work is to explore the possibilities of spiral arm formations from instabilities formed inside the central region of disc galaxies. These instabilities originate from the central baryonic feedback and have many prospects regarding the evolution of disc galaxies. They can trigger the gravitational collapse inside the dense molecular clouds that lead to the formation of stars under suitable astrophysical circumstances. In the present work, the role of parameters like the molecular cloud's magnetic field, rotation, etc., has been investigated behind this explosion-triggered star formation process with the help of Jeans instability analysis. From this study, our essential observation is that the formation of star clusters is favoured by a strong magnetic field ($\sim 10 \; μ$G), and the effect is enhanced at a more considerable distance from the centre. Again, this instability also contributes to the formation of stellar bars. This dense rotating component may drive out the chaotic stellar orbits from the disc through its two ends like a propeller. This process has been modelled in the thesis work from the viewpoint of chaotic scattering in open Hamiltonian systems. This analysis concludes that this bar-driven chaotic motion (or simply escaping motion) may lead to the forming of spiral arms or inner disc rings, depending on the bar strength. Our study also found that, compared to NFW dark haloes, the oblate dark haloes offer a more cohesive evolutionary framework for generating bar-driven escape structures in giant spiral and dwarf galaxies. Moreover, the formation of spiral arms via bar-driven escaping motion is only encouraged in galaxies with NFW dark haloes if they have highly energetic centres, like active galaxies.

astro-ph.GA↗

arXiv2023

Search for the origin of wobbling motion in the $ A \approx 130 $ region: The case of $^{131}$Xe

In-beam $ γ$-ray spectroscopy of $^{131}$Xe has been carried out to study the structure of the intruder $ νh_{11/2} $ band. Excited states were populated via an $ α$-induced fusion-evaporation reaction at E$ _α = 38 $ MeV. Inspection of $ γγ$-coincidence data resulted in the identification of a new rotational sequence. Based on the systematics of excitation energy, assigned spin-parity, decay pattern, and the electromagnetic character of the inter-band $ ΔI = 1 $ $ γ$-transitions, this sequence is proposed as the unfavoured signature partner of the $ νh_{11/2} $ band. The structure of this band is further illuminated in the light of the triaxial particle rotor model (TPRM). The possibility of wobbling excitation in $ N = 77 $ Xe-Ba-Ce isotones has been explored in a systematic manner.

nucl-ex↗

arXiv2023

Rate enhancement of gated drift-diffusion process by optimal resetting

`Gating' is a widely observed phenomenon in biochemistry that describes the transition between the activated (or open) and deactivated (or closed) states of an ion-channel, which makes transport through that channel highly selective. In general, gating is a mechanism that imposes an additional restriction on a transport, as the process ends only when the `gate' is open and continues otherwise. When diffusion occurs in presence of a constant bias to a {\it gated} target, i.e., to a target that switches between an open and a closed state, the dynamics essentially slows down compared to {\it ungated} drift-diffusion, resulting in an increase in the mean completion time. In this work, we utilize stochastic resetting as an external protocol to counterbalance the delay due to gating. We consider a particle that undergoes drift-diffusion in the presence of a stochastically gated target and is moreover subjected to a rate-limiting resetting dynamics. Calculating the minimal mean completion time rendered by an optimal resetting for this exactly-solvable system, we construct a phase diagram that owns three distinct phases: (i) where resetting can make gated drift-diffusion faster even compared to the original ungated process, (ii) where resetting still expedites gated drift-diffusion, but not beyond the original ungated process, and (iii) where resetting fails to expedite gated drift-diffusion. Gated drift-diffusion aptly models various stochastic processes such as chemical reactions that exclusively take place for certain activated state of the reactants. Our work predicts the conditions where stochastic resetting can act as a useful strategy to enhance the rate of such processes without compromising on their selectivity.

cond-mat.stat-mech↗

arXiv2023

Large electro-opto-mechanical coupling in VO2 neuristors

Biological neurons are electro-mechanical systems, where the generation and propagation of an action potential is coupled to generation and transmission of an acoustic wave. Neuristors, such as VO2, characterized by insulator-metal transition (IMT) and negative differential resistance, can be engineered as self-oscillators, which are good approximations of biological neurons in the domain of electrical signals. In this study, we show that these self-oscillators are coupled electro-opto-mechanical systems, with better energy conversion coefficients than the conventional electromechanical or electrooptical materials. This is due to the significant contrast in the material's resistance, optical refractive index and density across the induced temperature range in a Joule heating driven IMT. We carried out laser interferometry to measure the opto-mechanical response while simultaneously driving the devices electrically into self-oscillations of different kinds. We analyzed films of various thicknesses, engineered device geometry and performed analytical modelling to decouple the effects of refractive index change vis-a-vis mechanical strain in the interferometry signal. We show that the effective piezoelectric coefficient (d13*) for our neuristor devices is 660 pm/V, making them viable alternatives to Pb-based piezoelectrics for MEMS applications. Furthermore, we show that the effective electro-optic coefficient (r13*) is ~22 nm/V, which is much larger than that in thin-film and bulk Pockels materials.

physics.app-ph↗

arXiv2023

Modulation-Doping a Correlated Electron Insulator

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2 - especially without inducing structural changes - has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard x-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5x10^21 cm^(-3) without inducing any measurable structural changes. We find that the MIT temperature (T_MIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.

cond-mat.mtrl-sci↗

arXiv2022

Geometric Brownian Information Engine: Essentials for the best performance

We investigate a Geometric Brownian Information Engine (GBIE) in the presence of an error-free feedback controller that transforms the information gathered on the state of Brownian particles entrapped in monolobal geometric confinement into extractable work. Outcomes of the information engine depend on the reference measurement distance $x_m$, feedback site $x_f$ and the transverse force $G$. We determine the benchmarks for utilizing the available information in an output work and the optimum operating requisites for best work extraction. Transverse bias force ($G$) tunes the entropic contribution in the effective potential and hence the standard deviation ($σ$) of the equilibrium marginal probability distribution. We recognize that the amount of extracted work reaches a global maximum when $x_f = 2x_m$ with $x_m \sim 0.6σ$, irrespective of the extent of the entropic limitation. Because of the higher loss of information during the relaxation process, the best achievable work of a GBIE is lower in an entropic system. The feedback regulation also bears the unidirectional passage of particles. The average displacement increases with growing entropic control and is maximum when $x_m \sim 0.81σ$. Finally, we explore the efficacy of the information engine, a quantity that regulates the efficiency in utilizing the information acquired. With $x_f=2x_m$, the maximum efficacy reduces with increasing entropic control and shows a cross over from $2$ to $11/9$. We discover that the condition for the best efficacy depends only on the confinement length scale along the feedback direction. The broader marginal probability distribution accredits the increased average displacement in a cycle and the lower efficacy in an entropy-dominated system.

cond-mat.stat-mech↗

arXiv2022

Effect of dark matter halos on the orbital and escape dynamics of barred galaxies

This study examines the effect of dark matter halos on the orbital and escape dynamics of stars in the central region of barred galaxies. A three-dimensional gravitational model with a central bulge, bar, disc, and dark matter halo (or simply dark halo) has been set up and analyzed from the viewpoint of escape in open Hamiltonian systems for this purpose. Additionally, this model has been examined separately for the dark halo profiles: oblate and NFW. In both circumstances, an escape mechanism has been identified near the saddle points of the phase space, which correspond to the bar ends. The escaping motion of stars is seen using orbital maps and Poincaré surface section maps generated in various phase planes. Finally, the relationship between chaos and dark halo parameters such as mass, size, circular velocity, and nature has been studied. Our findings suggest that oblate dark halos are preferred over NFW dark halos for justifying the formation of full-fledged spiral arms and extended distribution of dark halos in giant spiral galaxies with supermassive black holes (SMBHs) at their centers. Again, the oblate dark halos well justify the emergence of less prominent or poor spiral arms and the core-dominated distribution of dark halos in dwarf and LSB galaxies in the absence of central SMBHs. On the other hand, extreme central baryonic feedback is required for the NFW halos to generate spiral patterns, and such dark halos should be preferred for galaxies with extremely energetic centers.

astro-ph.GA↗

arXiv2021

Escape kinetics of self-propelled particles from a circular cavity

We numerically investigate the mean exit time of an inertial active Brownian particle from a circular cavity with single or multiple exit windows. Our simulation results witness distinct escape mechanisms depending upon the relative amplitudes of the thermal length and self-propulsion length compared to the cavity and pore sizes. For exceedingly large self-propulsion lengths, overdamped active particles diffuse on the cavity surface, and rotational dynamics solely governs the exit process. On the other hand, the escape kinetics of a very weakly damped active particle is largely dictated by bouncing effects on the cavity walls irrespective of the amplitude of self-propulsion persistence lengths. We show that the exit rate can be maximized for an optimal self-propulsion persistence length, which depends on the damping strength, self-propulsion velocity, and cavity size. However, the optimal persistence length is insensitive to the opening windows' size, number, and arrangement. Numerical results have been interpreted analytically based on qualitative arguments. The present analysis aims to understand the transport controlling mechanism of active matter in confined structures.

cond-mat.stat-mech↗

arXiv2021

Role of galactic bars in the formation of spiral arms: A study through orbital and escape dynamics -- I

In the present work we have developed a three-dimensional gravitational model of barred galaxies, in order to study orbital and escape dynamics of the stars inside their central barred region. Our gravitational model is composed of four components, central nucleus, bar, disc and dark matter halo. Furthermore we have analysed the model for two different types of bar potentials. The study has been carried out for a Hamiltonian system and thorough numerical studies have been done in order to categorize regular and chaotic motions of stars. We have seen that escape mechanism has only seen near saddle points ($L_2$, $L_4$ and $L_2^{'}$, $L_4^{'}$) of the Hamiltonian system. Orbital structures in $x$ - $y$ plane indicate that this escaping motion corresponds to the two ends of the bar. Classifications of orbits are found by calculating maximal Lyapunov exponent of the stellar trajectories corresponding to a specific initial condition vector. Poincaré surface section maps are studied in both $x$ - $y$ and $x$ - $p_x$ ($p_x$ is the momentum along $x$ - direction) plane to get a complete view of the escape properties of the system in the phase space. Also we studied in detail how the chaotic dynamics varies with mass, length and nature of the bar. We found that under suitable physical conditions the chaos plays a pivotal role behind the formation of grand design or poor spiral pattern for stronger bars and ring structures for weaker bars.

astro-ph.GA↗

arXiv2020

Excitation energy and angular momentum dependence of the nuclear level density parameter around A$\approx $110

Neutron kinetic energy spectra in coincidence with low-energy $γ$-ray multiplicities have been measured around $A\approx $ 110 in the $^{16}$O, $^{20}$Ne + $^{93}$Nb reactions in a compound nuclear excitation energy range of $\approx $ 90 - 140 MeV. The excitation energy (temperature) and angular momentum (spin) dependence of the inverse level density parameter $k$ has been investigated by comparing the experimental data with statistical Hauser-Feshbach calculation. In contrast to the available systematic in this mass region, the inverse level density parameter showed an appreciable increase as a function of the excitation energy. The extracted $k$-values at different angular momentum regions, corresponding to different $γ$-multiplicities also showed an overall increase with the average nuclear spins. The experimental results have been compared with a microscopic statistical-model calculation and found to be in reasonable agreement with the data. The results provide useful information to understand the variation of nuclear level density at high temperature and spins.

nucl-ex↗