arXiv subjects

Surajit Das

Publications and source records attributed to Surajit Das.

28 records · Page 2Linked to original sources

arXiv2024

Near-horizon chaos beyond Einstein gravity

We investigate chaos in the dynamics of massless particles near the horizon of static spherically symmetric black holes in two well-motivated models of $f(R)$ gravity. In both these models, we probe chaos in the particle trajectories (under suitable harmonic confinement) in the vicinity of the black hole horizons, for a set of initial conditions. The particle trajectories, associated Poincaré sections, and Lyapunov exponents clearly illustrate the role played by the black hole horizon in the growth of chaos. We find that with increasing energy, the particle trajectories explore regions closer to the black hole horizon, with reduced overlap between two initially close trajectories. We demonstrate how this energy range is controlled by the parameters of the modified gravity theory under consideration. The growth of chaos in such a classical setting is known to respect a surface gravity bound arising from universal aspects of particle dynamics close to the black hole horizon [K. Hashimoto and N. Tanahashi, Phys. Rev. D 95, 024007 (2017)], analogous to the quantum Maldacena, Shenker, and Stanford bound [J. Maldacena et al., J. High Energy Phys. 08 (2016) 106]. Interestingly, both models studied in our work respect the bound, in contrast to some of the other models of $f(R)$ gravity in the existing literature. The work serves as a motivation to use chaos as an additional tool to probe Einstein gravity in the strong gravity regime in the vicinity of black hole horizons.

gr-qc↗

arXiv2024

Influence of Pseudo-Jahn-Teller Activity on the Singlet-Triplet Gap of Azaphenalenes

We analyze the possibility of symmetry-lowering induced by pseudo-Jahn--Teller interactions in six previously studied azaphenalenes that are known to have their first excited singlet state (S$_1$) lower in energy than the triplet state (T$_1$). The primary aim of this study is to explore whether Hund's rule violation is observed in these molecules when their structures are distorted from $C_{\rm 2v}$ or $D_{\rm 3h}$ point group symmetries by vibronic coupling. Along two interatomic distances connecting these point groups to their subgroups $C_{\rm s}$ or $C_{\rm 3h}$, we relaxed the other internal degrees of freedom and calculated two-dimensional potential energy subsurfaces. The many-body perturbation theory (MP2) suggests that the high-symmetry structures are the energy minima for all six systems. However, single-point energy calculations using the coupled-cluster method (CCSD(T)) indicate symmetry lowering in four cases. The singlet-triplet energy gap plotted on the potential energy surface also shows variations when deviating from high-symmetry structures. A full geometry optimization at the CCSD(T) level with the cc-pVTZ basis set reveals that the $D_{\rm 3h}$ structure of cyclazine (1AP) is a saddle point, connecting two equivalent minima of $C_{\rm 3h}$ symmetry undergoing rapid automerization. The combined effects of symmetry lowering and high-level corrections result in a nearly zero singlet-triplet gap for the $C_{\rm 3h}$ structure of cyclazine. Azaphenalenes containing nitrogen atoms at electron-deficient sites -- 2AP, 3AP, and 4AP -- exhibit more pronounced in-plane structural distortion; the effect is captured by the long-range exchange-interaction corrected DFT method, $ω$B97XD. Excited state calculations of these systems indicate that in their low-symmetry energy minima, T$_1$ is indeed lower in energy than S$_1$, upholding the validity of Hund's rule.

physics.chem-ph↗

arXiv2024

$f(Q,T)$ gravity: From early to late-time cosmic acceleration

In this article, we explore the comprehensive narrative of cosmic evolution within a cosmological framework by utilizing a novel form of gravity known as generalized symmetric teleparallel gravity, denoted as $f(Q,T)$ gravity. Here, $Q$ represents the non-metricity scalar, while $T$ denotes the trace of the energy-momentum tensor. We present and analyze two distinct $f(Q,T)$ cosmological models, each characterized by its unique Lagrangian. Our investigation delves into the cosmological parameters of these models, scrutinizing various energy conditions, examining the inflationary dynamics of the early universe through scalar field formulations, and probing the mysterious nature of dark energy using statefinder diagnostics and $(ω-ω')$ phase space analysis. Ultimately, our findings offer a comprehensive account of cosmic evolution, spanning from the early universe to its late-time evolution.

physics.gen-ph↗

arXiv2024

Chemical Space-Informed Machine Learning Models for Rapid Predictions of X-ray Photoelectron Spectra of Organic Molecules

We present machine learning models based on kernel-ridge regression for predicting X-ray photoelectron spectra of organic molecules originating from the $K$-shell ionization energies of carbon (C), nitrogen (N), oxygen (O), and fluorine (F) atoms. We constructed the training dataset through high-throughput calculations of $K$-shell core-electron binding energies (CEBEs) for 12,880 small organic molecules in the bigQM7$ω$ dataset, employing the $Δ$-SCF formalism coupled with meta-GGA-DFT and a variationally converged basis set. The models are cost-effective, as they require the atomic coordinates of a molecule generated using universal force fields while estimating the target-level CEBEs corresponding to DFT-level equilibrium geometry. We explore transfer learning by utilizing the atomic environment feature vectors learned using a graph neural network framework in kernel-ridge regression. Additionally, we enhance accuracy within the $Δ$-machine learning framework by leveraging inexpensive baseline spectra derived from Kohn--Sham eigenvalues. When applied to 208 combinatorially substituted uracil molecules larger than those in the training set, our analyses suggest that the models may not provide quantitatively accurate predictions of CEBEs but offer a strong linear correlation relevant for virtual high-throughput screening. We present the dataset and models as the Python module, ${\tt cebeconf}$, to facilitate further explorations.

physics.chem-ph↗

arXiv2024

Cosmological effects on $f(\bar{R},\bar{T})$ gravity through a non-standard theory

This study aims to investigate the impact of dark energy in cosmological scenarios by exploiting $f(\bar{R},\bar{T})$ gravity within the framework of a {\it non-standard} theory, called {\it {\bf K-}essence} theory, where $\bar{R}$ represents the Ricci scalar and $\bar{T}$ denotes the trace of the energy-momentum tensor associated with the {\bf K-}essence geometry. The Dirac-Born-Infeld (DBI) non-standard Lagrangian has been employed to generate the emergent gravity metric $(\bar{G}_{μν})$ associated with the {\bf K-}essence. This metric is distinct from the usual gravitational metric $(g_{μν})$. It has been shown that under a flat FLRW background gravitational metric, the modified field equations and the Friedmann equations of the $f(\bar{R},\bar{T})$ gravity are distinct from the usual ones. In order to get the equation of state (EOS) parameter $ω$, we have solved the Friedmann equations by taking into account the function $f(\bar{R},\bar{T})\equiv f(\bar{R})+λ\bar{T}$, where $λ$ represents a parameter within the model. We have found a relationship between $ω$ and time for different kinds of $f(\bar{R})$ by treating the kinetic energy of the {\bf K-}essence scalar field ($\dotϕ^{2}$) as the dark energy density which fluctuates with time. Surprisingly, this result meets the condition of the restriction on $\dotϕ^{2}$. By presenting graphical representations of the EOS parameter with time, we show that our model is consistent with the data of $SNIa$+$BAO$+$H(z)$ within a certain temporal interval.

gr-qc↗

arXiv2024

Switchable Photovoltaic Effect in Ferroelectric CsPbBr3 Nanocrystals

Ferroelectric all-inorganic halide perovskites nanocrystals with both spontaneous polarizations and visible light absorption are promising candidates for designing functional ferroelectric photovoltaic devices. Three dimensional halide perovskite nanocrystals have the potential of being ferroelectric, yet it remains a challenge to realize ferroelectric photovoltaic devices which can be operated in absence of an external electric field. Here we report that a popular all-inorganic halide perovskite nanocrystal, CsPbBr3, exhibits ferroelectricity driven photovoltaic effect under visible light in absence of an external electric field. The ferroelectricity in CsPbBr3 nanocrystals originates from the stereochemical activity in Pb (II) lone pair that promotes the distortion of PbBr6 octahedra. Furthermore, application of an external electric field allows the photovoltaic effect to be enhanced and the spontaneous polarization to be switched with the direction of the electric field. Robust fatigue performance, flexibility and prolonged photoresponse under continuous illumination are potentially realized in the zero-bias conditions. These finding establishes all-inorganic halide perovskites nanocrystals as potential candidates for designing novel photoferroelectric devices by coupling optical functionalities and ferroelectric responses.

physics.app-ph↗

arXiv2023

Reconstruction of $f(R,T)$ gravity model via the Raychaudhuri equation

In this work, we investigate for an analytical solution under modified gravity theory, specifically the $f(R,T)$ gravity for two different eras, i.e., matter and dark energy dominated accelerating universe from completely geometrical and mathematical point of view with the help of the Raychaudhuri equation. To construct $f(R,T)$ gravity model, we consider the functional form of $f(R,T)$ as the sum of two independent functions of the Ricci scalar $R$ and the trace of the energy-momentum tensor $T$, respectively. Under the consideration of this type of power law expansion of the Universe we have studied the viability, stability and all the energy conditions. We note that the strong energy condition is not satisfied in our model, which is obvious for the present scenario of the Universe.

gr-qc↗

arXiv2023

Unveilling Chaos in Particle Motion: Analyzing the Impact of Horizon in $f(R)$ Gravity

This article is devoted to investigate the effects of $f(R)$ theory in the dynamics of a massless particle near the horizon of a static spherically symmetric (SSS) black hole. Deriving the equations of motion within $f(R)$ gravitational theories, novel solutions for charged and neutral black holes are obtained, introducing a dimensional parameter $a$ in $f(R)=R-2a\sqrt{R}$. Departing from General Relativity, these solutions showcase unique properties reliant on the dynamics of Ricci scalar. Analysis shows that chaos manifests within a specific energy range, with $a$ playing a crucial role. The study underscores the general applicability of the spherically symmetric metric, revealing insights into particle dynamics near black hole horizons. Despite an initially integrable nature, the introduction of harmonic perturbation leads to chaos, aligning with the Kolmogorov-Arnold-Moser theory. This research contributes to a nuanced understanding of black hole dynamics, emphasizing the importance of alternative theories of gravity.

gr-qc↗

arXiv2023

Leading-order corrections to the thermodynamics of Rindler modified Schwarzschild black hole

In this work, we present a thermodynamical study of a Rindler modified Schwarzschild black hole under the consideration of small thermal fluctuations. In particular, we compute various stable macroscopic thermodynamic variables such as Hawking temperature, entropy, Helmholtz free energy, internal energy, enthalpy, and Gibbs free energy. To explore the effects of small statistical thermal fluctuations on stable thermodynamical parameters, we estimated the corrections to the various thermodynamical potentials of Rindler modified Schwarzschild black hole up to the first (leading) order and do a comparative study for the different values of correction parameter and Rindler acceleration parameter for fixed values of a cosmological constant. In this study, we examine the stability of black holes in the presence of thermal fluctuations. We find that when the correction parameter is positive, small-sized black holes remain stable, while large-sized ones become unstable. Conversely, when the correction parameter is negative, both small and large black holes exhibit instability. Additionally, we demonstrate that the first law of thermodynamics remains valid even in the presence of thermal fluctuations.

gr-qc↗

arXiv2023

Raychaudhuri equation in $k$-essence geometry: conditional singular and non-singular cosmological models

We investigate how the Raychaudhuri equation behaves in the $k$-essence geometry. As far as we are concerned, both the early and current epochs of the universe are relevant to the $k$-essence theory. Here, we have studied the $k$-essence geometry using the Dirac-Born-Infeld (DBI) variety of non-standard action. The corresponding $k$-essence emergent spacetime is not conformally equivalent to the usual gravitational metric. We assume that the background gravitational metric is of the Friedmann-Lemaitre-Robertson-Walker (FLRW) type in this case. We have found that both the conditional singular and non-singular cosmological models of the universe through the modified Raychaudhuri equation are possible where we have used the spacetime as the flat $k$-essence emergent FLRW-type. We have also addressed to the Focusing theorem and conditional caustic universe construction. These conditional effects are caused by the additional interactions that arise as a result of the coupling that exists between the gravity and the $k$-essence scalar field.

gr-qc↗