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Sayantan Ghosh

Publications and source records attributed to Sayantan Ghosh.

At least 37 records · Page 2Linked to original sources
arXiv2024

Unmasking noise transients masquerading as intermediate-mass black hole binaries

In the first three observation runs, ground-based gravitational wave (GW) detectors have observed close to 100 compact binary coalescence (CBC) events. The GW detection rates for CBCs are expected to increase with improvements in the sensitivity of the International Gravitational-Wave Observatory Network (IGWN). However, with improved sensitivity, non-Gaussian instrumental transients or ``glitches'' are expected to adversely affect GW searches and characterisation algorithms. The most detrimental effect is due to short-duration glitches, which mimic the morphology of short-duration GW transients, in particular Intermediate-mass black hole (IMBH) binaries. They can be easily misidentified as astrophysical signals by current searches, and if included in astrophysical analyses, glitches mislabelled as IMBH binaries can affect IMBH population studies. In this work, we introduce a new similarity metric that quantifies the consistency of astrophysical parameters across the detector network and helps to distinguish between IMBH binaries and short-duration, loud glitches which mimic such binaries. We develop this method using a simulated set of IMBH binary signals and a collection of noise transients identified during the third observing run of the Advanced LIGO and Advanced Virgo detectors.

gr-qc↗
arXiv2024

Study of charged gravastar model in $f(\mathcal{Q})$ gravity

In recent days gravastar has been a very lucrative alternative to black holes, as it does not suffer from the singularity problem as well and it is based on sound physical grounds. Modified Symmetric teleparallel equivalent of gravity also has seen quite a few successes in recent years both in cosmology as well as in astrophysical objects like black holes and wormholes. In this paper, we have considered the charged gravastar in $f(\mathcal{Q})$ formulation and have solved it fully analytically and found various physical characteristics like energy density, entropy and EoS for the gravastar. We have used the Israel junction condition to make some phenomenological predictions regarding the potential of the thin shell around the gravastar. We have also studied the deflection of the angle caused by the gravastar. Finally, we conclude by noting how future radio telescopes could detect the gravastar shadow and how can one distinguish it from the black hole event horizon.

gr-qc↗
arXiv2024

Exploring the Macroscopic Properties and Nonradial Oscillations of Proto-Neutron Stars: Effects of Temperature, Entropy, and Lepton Fraction

Neutron stars (NSs) have traditionally been viewed as cold, zero-temperature entities. However, recent progress in computational methods and theoretical modelling has opened up the exploration of finite temperature effects, marking a novel research frontier. This study examines Proto-Neutron Stars (PNSs) using the BigApple parameter set to investigate their macroscopic properties. Two approaches are employed: one with constant temperatures (10-50 MeV) and the other fixing entropy per baryon (S) at predefined levels (S = 1 and S = 2). Notably, S remains constant with increasing baryon density due to electron-positron pair formation at finite temperatures. Analysis of PNS mass-radius profiles, considering neutrino trapping and temperature effects, reveals flattened curves and expanded radii with increasing temperature, resulting in slightly higher masses compared to zero temperature. The influence of lepton fraction ($Y_l$) on maximum PNS mass is explored, indicating that higher $Y_l$ values lead to a softer Equation of State (EoS), reducing maximum mass and increasing the canonical radius ($R_{1.4}$). Further investigation of a constant entropy EoS demonstrates that higher entropy is associated with increased maximum PNS masses and flatter mass-radius curves. Central temperature versus maximum mass relationships suggest a correlation between NS mass and temperature. Lastly, we investigate the behaviour of $f$-mode frequencies in PNS. It reveals that the frequency of these modes decreases with increasing entropy and temperature, reflecting complex thermodynamic interactions within the stars.

nucl-th↗
arXiv2024

Exploring Radial Oscillations in Slow Stable and Hybrid Neutron Stars

In the era of gravitational wave astronomy, radial oscillations hold significant potential for not only uncovering the microphysics behind the internal structure but also investigating the stability of neutron stars (NSs). We start by constructing families of static NSs following nucleonic, quarkyonic, and hybrid equations of state and then subject them to radial perturbations in order to explore the stability of these stars. Unlike other literature where the fluid elements are assumed to be in chemical equilibrium, we consider the out-of-equilibrium effects on the chemical composition of fluid elements for the calculation of radial modes. Taking these considerations into account, we observe that the sound speed ($c^2_s$) and adiabatic index ($γ$) avoid singularities and discontinuities over the equilibrium case. We elucidate the response of the fundamental radial modes by examining the out-of-equilibrium matter distribution scenario, offering insights into its dynamic variations. We also demonstrate that this approach extends the stable branches of stellar models, enabling stars to sustain stable higher-order mass doublets, shedding some light on observation and existence of PSR J0740+6620.

nucl-th↗
arXiv2024

The Impact of Anisotropy on Neutron Star Properties: Insights from I-f-C Universal Relations

This study presents a universal relation for anisotropic neutron stars, called the $I-f-C$ relation, which accounts for the local anisotropic pressure using the Quasi-Local (QL) Model proposed by Horvat et al. \cite{QL_Model} to describe the anisotropy inside the neutron star. This study analyzes approximately 60 unified tabulated EoS-ensembles, spanning from relativistic to non-relativistic mean-field models, that comply with multimessenger constraints and cover a broad range of stiffness. The results indicate that the relationship between the parameters becomes more robust with positive anisotropy, while it weakens with negative anisotropy. With the help of the GW170817 \& GW190814 tidal deformability limit, a theoretical limit for the canonical $f$-mode frequency for both isotropic and anisotropic stars is established. For isotropic case the canonical $f$-mode frequency for event GW170817 \& GW190814 is $f_{1.4} = 2.605^{+0.487} _ {-0.459}\ \mathrm{kHz}$ and $ f_{1.4} = 2.093^{+0.150} _ {-0.125} \ \mathrm{kHz}$ respectively. These established relationships have the potential to serve as a reliable tool to limit the equation of state of nuclear matter when measurements of relevant observables are obtained.

nucl-th↗
arXiv2023

Probing the impact of Delta-Baryons on Nuclear Matter and Non-Radial Oscillations in Neutron Stars

The presence of heavy baryons, such as $Δ$-baryons and hyperons can significantly impact various properties of Neutron Stars (NSs), like oscillation frequencies, dimensionless tidal deformability, mass, and radii. We explored these effects within the Density-Dependent Relativistic Mean Field formalism. Our analysis considered $Δ$-admixed NS matter in both hypernuclear and hyperon-free scenarios, providing insights into particle compositions and their effects on NS properties. Our study of non-radial $f$-mode oscillations revealed a distinct increase in frequency due to the additional baryons. The degree of increase was significantly influenced by the meson-baryon coupling strengths. Notably, the coupling between $Δ$-resonances and $σ$-mesons played a highly influential role. In some cases, it led to an approximately 20\% increase in the $f$-mode oscillation frequency of canonical NSs. These couplings also affect other bulk properties of NSs, including mass, radii, and dimensionless tidal deformability ($Λ$). Comparing our results with available observational data from pulsars (NICER) and gravitational waves (LIGO-VIRGO collaboration), we found strong agreement, particularly concerning $Λ$.

nucl-th↗
arXiv2023

Investigating Dark Matter-Admixed Neutron Stars with NITR Equation of State in Light of PSR J0952-0607

The fastest and heaviest pulsar, PSR J0952-0607, with a mass of $M=2.35\pm0.17 \ M_\odot$, has recently been discovered in the disk of the Milky Way Galaxy. In response to this discovery, a new RMF model, `NITR' has been developed. The NITR model's naturalness has been confirmed by assessing its validity for various finite nuclei and nuclear matter properties, including incompressibility, symmetry energy, and slope parameter values of 225.11, 31.69, and 43.86 MeV, respectively. These values satisfy the empirical/experimental limits currently available. The maximum mass and canonical radius of a neutron star (NS) calculated using the NITR model parameters are 2.355 $M_\odot$ and 13.13 km, respectively, which fall within the range of PSR J0952-0607 and the latest NICER limit. This study aims to test the consistency of the NITR model by applying it to various systems. As a result, its validity is extensively calibrated, and all the nuclear matter and NS properties of the NITR model are compared with two established models such as IOPB-I and FSUGarnet. In addition, the NITR model equation of state (EOS) is employed to obtain the properties of a dark matter admixed NS (DMANS) using two approaches (I) single-fluid and (II) two-fluid approaches. In both cases, the EOS becomes softer due to DM interactions, which reduces various macroscopic properties such as maximum mass, radius, tidal deformability, etc. The various observational data such as NICER and HESS are used to constrain the amount of DM in both cases. Moreover, we discuss the impact of dark matter (DM) on the nonradial $f$-mode frequency of the NS in a single fluid case only and try to constrain the amount of DM using different theoretical limits available in the literature.

nucl-th↗
arXiv2023

Dynamical system analysis of scalar field cosmology in coincident $f(Q)$ gravity

In this article, we investigate scalar field cosmology in the coincident $f(Q)$ gravity formalism. We calculate the motion equations of $f(Q)$ gravity under the flat Friedmann-Lemaître-Robertson-Walker background in the presence of a scalar field. We consider a non-linear $f(Q)$ model, particularly $f(Q)=-Q+αQ^n$, which is nothing but a polynomial correction to the STEGR case. Further, we assumed two well-known specific forms of the potential function, specifically the exponential from $V(ϕ)= V_0 e^{-βϕ}$ and the power-law form $V(ϕ)= V_0ϕ^{-k}$. We employ some phase-space variables and transform the cosmological field equations into an autonomous system. We calculate the critical points of the corresponding autonomous systems and examine their stability behaviors. We discuss the physical significance corresponding to the exponential case for parameter values $n=2$ and $n=-1$ with $β=1$, and $n=-1$ with $β=\sqrt{3}$. Moreover, we discuss the same corresponding to the power-law case for the parameter value $n=-2$ and $k=0.16$. We also analyze the behavior of corresponding cosmological parameters such as scalar field and dark energy density, deceleration, and the effective equation of state parameter. Corresponding to the exponential case, we find that the results obtained for the parameter constraints in Case III is better among all three cases, and that represents the evolution of the universe from a decelerated stiff era to an accelerated de-Sitter era via matter-dominated epoch. Further, in the power-law case, we find that all trajectories exhibit identical behavior, representing the evolution of the universe from a decelerated stiff era to an accelerated de-Sitter era. Lastly, we conclude that the exponential case shows better evolution as compared to the power-law case.

gr-qc↗
arXiv2023

GUP Corrected Casimir Wormholes in $f(Q)$ Gravity

We have presented systematically the effect of the Generalized Uncertainty Principle (GUP) in Casimir wormholes space-time in the recently proposed modified gravity, the so-called symmetric teleparallel gravity or $f(Q)$ gravity. Here $Q$ is the non-metricity scalar that drives the gravitation interaction. We consider two famous GUP relations, such as the Kempf, Mangano, and Mann (KMM) model and the Detournay, Gabriel, and Spindel (DGS) model, in our study. Besides this, we investigate with three different redshift functions under anisotropic fluid located at the throat. Further, we analyzed the obtained wormhole solutions with energy conditions, especially null energy conditions (NEC) at the throat of the wormhole, and encountered that some arbitrary quantity disrespects the classical energy conditions at the wormhole throat of radius $r_0$. Later, the ADM mass and the volume integral quantifier are also discussed to calculate the amount of exotic matter required near the wormhole throat. Additionally, we show the behavior of the equation of state parameters under the effect of GUP.

gr-qc↗
arXiv2023

Exploration of strongly correlated states in SmB6 through a comparison of its two-coil pick-up response to that of Bi2Se3

Earlier studies on the Kondo insulator SmB6 reveal the presence of a bulk Kondo insulating gap between 30 - 50 K, and the emergence of a conducting surface state only below 4 K. Here, we compare the two-coil mutual inductance pick-up response of SmB6 single crystal with that of a conventional topological insulator (TI), Bi2Se3 single crystal. From these studies we identify three distinct temperature regimes for SmB6, viz., (i) T >= T*(~ 66 K), (ii) (40 K~) T_g <= T < T*, and (iii) T < T_g. At T* in SmB6, we observe a peak in the temperature-dependent AC pickup signal which corresponds to the peak in the broad hump feature in the bulk DC susceptibility measurements and features in the resistivity measurements. A dip in the pickup signal at T_g in SmB6 correlates with the evidence for the opening of a bulk Kondo gap in transport measurements. Our study of the pickup signal in SmB6 suggests the presence of a thin (submicron order thickness) high conducting surface layer from a temperature just below T_g. In this T regime in SmB6, the pickup signal shows a distinct square root frequency (f) dependence compared to the linear f dependence found in Bi2Se3. Across all the different T regimes, distinct AC frequency dependence and scaling properties are observed. Our results suggest that above T*, weak exchange interactions cause electrons to scatter from random ion sites. Electronic correlations gradually strengthen with the onset of Kondo like hybridization, setting in from below T*, and at T_g, a strongly correlated Kondo gap opens up in the bulk of the material. The appearance of the thin high conducting surface layer is nearly coincident with the onset of bulk Kondo insulating state below T_g in SmB6.

cond-mat.str-el↗
arXiv2023

Onset of glassiness in two-dimensional ring polymers: interplay of stiffness and crowding

The effect of ring stiffness and pressure on the glassy dynamics of a thermal assembly of two-dimensional ring polymers is investigated using extensive coarse-grained molecular dynamics simulations. In all cases, dynamical slowing down is observed with increasing pressure and thereby a phase space for equilibrium dynamics is identified in the plane of obtained monomer density and ring stiffness. When the rings are highly flexible, i.e. low ring stiffness, glassiness sets in via crowding of crumpled polymers which take a globular form. In contrast, at large ring stiffness, when the rings tend to have large asphericity under compaction, we observe the emergence of local domains having orientational ordering, at high pressures. Thus, our simulations highlight how varying the deformability of rings leads to contrasting mechanisms in driving the system towards the glassy regime.

cond-mat.soft↗
arXiv2023

Scalar field evolution at background and perturbation levels for a broad class of potentials

In this paper, we investigate a non-interacting scalar field cosmology with an arbitrary potential using the $f$-deviser method that relies on the differentiability properties of the potential. Using this alternative mathematical approach, we present a unified dynamical system analysis at a scalar field's background and perturbation levels with arbitrary potentials. For illustration, we consider a monomial and double exponential potential. These two classes of potentials comprise the asymptotic behaviour of several classes of scalar field potentials, and, therefore, they provide the skeleton for the typical behaviour of arbitrary potentials. Moreover, we analyse the linear cosmological perturbations in the matterless case by considering three scalar perturbations: the evolution of the Bardeen potentials, the comoving curvature perturbation, the so-called Sasaki-Mukhanov variable, or the scalar field perturbation in uniform curvature gauge. Finally, an exhaustive dynamical system analysis for each scalar perturbation is presented, including the evolution of Bardeen potentials in the presence of matter.

gr-qc↗
arXiv2023

Non-exotic static spherically symmetric thin-shell wormhole solution in $f(Q,T)$ gravity

In this study, we have conducted an analysis of traversable wormhole solutions within the framework of linear $f(Q, T) = αQ + βT$ gravity, ensuring that all the energy conditions hold for the entire spacetime. The solutions presented in this study were derived through a comprehensive analytical examination of the parameter space associated with the wormhole model. This involved considering the exponents governing the redshift and shape functions, as well as the radius of the wormhole throat ($r_0$), the redshift function value at the throat ($ϕ_0$), and the model parameters ($α$ and $β$). Also, we have established bounds on these free parameters that guarantee the satisfaction of the energy conditions throughout spacetime and have also provided two solutions. Further, we have used the Israel junction condition to see the stability of a thin-shell around the wormhole. We have also calculated the NEC criteria and potential for such a thin-shell and how it varies with the chosen shape function.

gr-qc↗
arXiv2022

Casimir Wormholes in Modified Symmetric Teleparallel Gravity

In recent years there has been a growing interest in the field of Casimir wormhole. In classical general relativity (GR), it is known that the null energy condition (NEC) has to be violated to have a wormhole to be stable. The Casimir effect is an experimentally verified effect that is caused due to the vacuum field fluctuations in quantum field theory. Since the Casimir effect provides the negative energy density, thus this act as an ideal candidate for the exotic matter needed for the stability of the wormhole. In this paper, we study the Casimir effect on the wormhole geometry in modified symmetric teleparallel gravity or $f(Q)$ gravity, where the non-metricity scalar $Q$ drives the gravitation interaction. We consider three systems of the Casimir effect such as (i) two parallel plates, (ii) two parallel cylindrical plates, and (iii) two-sphere separated by a large distance to make it more experimentally feasible. Further, we studied the obtained wormhole solutions for each case with energy conditions at the wormhole throat with radius $r_0$ and found that some arbitrary quantity violates the classical energy conditions at the wormhole throat. Furthermore, the behavior of the equation of state (EoS) is also analyzed for each case. Finally, we investigate the stability of the obtained Casimir effect wormhole solutions with the generalized Tolman-Oppenheimer-Volkoff (TOV) equation.

gr-qc↗
arXiv2021

Wavelength-Controlled Photocurrent Polarity Switching in BP-MoS$_2$ Heterostructure

Layered two-dimensional van der Waals (vdW) semiconductors and their heterostructures have been shown to exhibit positive photoconductance (PPC) in many studies. A few recent reports have demonstrated negative photoconductance (NPC) as well that can enable broadband photodetection besides multi-level optoelectronic logic and memory. Controllable and reversible switching between PPC and NPC is a key requirement for these applications. This report demonstrates visible-to-near infrared wavelength-driven NPC and PPC, along with reversible switching between the two, in an air stable, high mobility, broadband black phosphorus (BP) field effect transistor (FET) covered with a few layer MoS$_2$ flake. The crossover switching wavelength can be tuned by varying the MoS$_2$ bandgap through its flake thickness and the NPC and PPC photoresponsivities can be modulated using electrostatic gating as well as laser power. Recombination-driven NPC and PPC allows for reversible switching at reasonable time scales of a few seconds. Further, gate voltage-dependent negative persistent photoconductance enables synaptic behavior that is well-suited for optosynaptic applications.

physics.app-ph↗
arXiv2020

Enhanced gas sensing performance and all-electrical room temperature operation enabled by a WSe2/MoS2 heterojunction

Gas sensors built using two-dimensional (2D) MoS2 have conventionally relied on a change in field-effect-transistor (FET) channel resistance or a change in Schottky contact/pn homojunction barrier. This report demonstrates, for the first time, an NO2 gas sensor that leverages a gate tunable type II WSe2 (p)/MoS2 (n) heterojunction to realize a 4x enhancement in sensitivity, 8x lower limit of detection and improved dynamic response when compared to an MoS2 FET sensor on the same flake. Comprehensive sensing measurements over a range of analyte concentrations, gate biases and MoS2 flake thicknesses indicate a novel two-fold electrical response to NO2 exposure underlying the enhanced sensitivity of the heterojunction- (i) a series resistance change that leads to an exponential change in thermionic current at high bias, and, (ii) a carrier concentration change that leads to a linear change in interlayer recombination current near zero bias. The heterojunction diode also exhibits fast and tunable recovery under negative gate biasing. All-electrical (gate controlled) sensing and recovery operation at room temperature makes this a simple, low-overhead sensor. The ability to sense tri-nitro toluene (TNT) molecules down to a concentration of 80PPB highlights its potential as a comprehensive chemical sensing platform.

physics.app-ph↗
arXiv2020

Imaging the topological current carrying state and the surface to bulk transformation, in Bi2Se3 single crystal and thin film

Magneto-optics based current imaging technique compares the nature of topological current distribution in a single crystal and thin film of topological insulator material, Bi2Se3. The single crystal, at low temperatures, has uniform topological surface current sheets which are about 3.6 nm thick. With increasing temperature, the current partially diverts into the crystal bulk and concomitantly, the sheet break up into a patchy network of high and low current density regions. The temperature dependence of the high current density areas shows that the surface to bulk transformation in the crystal has features of classical phase transition phenomena. The surface area fraction with topological high current density behaves like an order parameter. This phase transition is driven by disorder. In Bi2Se3 thin film we show the presence of quasi one-dimensional topological edge currents which are suppressed with a weak applied magnetic field. The edge current transforms into a uniform bulk current in the film.

cond-mat.mtrl-sci↗
arXiv2020

Coupling-decoupling of conducting topological surface states in thick Bi$_2$Se$_3$ single crystals

Sensitive ac susceptibility measurements of a topological insulator, Bi$_2$Se$_3$ single crystal, using mutual two coil inductance technique (Ref. 32) shows coupling and decoupling of high conducting surface states. The coupling of the surface states exists upto thickness of 70 microns, which is much larger than the direct coupling limit of thickness approximately 5 to 10 nanometers found in thin films. The high conducting topological surface states are coupled through the crystal via high electrically conducting channels, generated by Selenium vacancies. These conducting channels through the bulk disintegrate beyond 70 micron thickness and at high temperatures, thereby leading to decoupling of the topological surface states. We show the decoupled surface states persist upto room temperature in the topological insulator. Analysis of Nyquist plot of ac-susceptibility response of the TI using a resistor (R) Inductor (L) model shows an inductive nature of the coupling between surface states found in these Bi$_2$Se$_3$ crystals.

physics.app-ph↗