arXiv subjects

Krishan Kumar

Publications and source records attributed to Krishan Kumar.

18 recordsLinked to original sources

arXiv2026

Coherent vibrational wave packet motion in ErCry4a proteins monitors the redox state of the flavin chromophore

Cryptochromes are blue-light-sensitive flavoproteins that play central roles in biological function. In European robin (Erithacus rubecula) ErCry4a proteins, optical excitation of their flavin chromophore forms a long-lived radical pair through a sequence of electron transfer steps across a tetradic chain of tryptophan residues, making them primary candidates for magnetoreception in night migratory songbirds. Recent quantum chemical calculations indicate that nonadiabatic couplings play a central role in the energy and charge transfer processes initiated by optical excitation. Here, we study these dynamics in ErCry4a using ultrafast transient absorption spectroscopy with 10-fs time resolution in the 450-nm spectral range. We uncover a rapid, sub-50 fs red shift in stimulated emission, quenched within 360 fs by electron transfer from a nearby tryptophan moiety. While high-frequency excited state vibrations are rapidly damped, coherent motion involving several low-frequency vibrations persists during both the initial energy relaxation and the subsequent electron transfer. This is evidenced by probing the coherent vibrational motion of the formed FAD$^{\bullet-}$ radical anion and is independently validated by blocking the electron transfer through site-selective tryptophan mutation. Our results only provide insight into the role of nonadiabatic couplings for the initial steps of cryptochrome photoactivation and suggest a general strategy for redox-state-specific monitoring of charge transfer dynamics by probing coherent vibrational motion.

physics.chem-ph↗

arXiv2026

Nonadiabatic couplings drive ultrafast, mode-selective intramolecular vibrational energy redistribution in flavins

Flavins are the chromophores in several blue-light-sensitive photoreceptor proteins and act as redox cofactors in many enzymes relevant for biological processes. Despite their biological relevance and numerous, detailed optical investigations of their photophysical properties, the ultrafast nonequilibrium dynamics of their elementary optical excitations are not yet fully known. Here, we use ultrafast coherent vibrational spectroscopy with 10-fs time resolution in the 450-nm spectral range to study their excited state coherent vibrational dynamics. We observe that coherent wavepacket motion along high-frequency C-C stretching modes with ~ 20-fs period is rapidly damped on a 20-fs timescale. In contrast, coherent motion along several low-frequency modes persists much longer. We attribute this to a mode-selective intramolecular vibrational energy redistribution driven by nonadiabatic couplings between the optical bright state and a close-lying dark electronic state, in accordance with model calculations. Our results may be of relevance for the formation of long-lived radical pair states in magnetic-field sensitive proteins.

physics.chem-ph↗

arXiv2025

Error analysis with exponential decay estimates for a fully discrete approximation of a class of strongly damped wave equations

This paper deals with the asymptotic behavior and FEM error analysis of a class of strongly damped wave equations using a semidiscrete finite element method in spatial directions combined with a finite difference scheme in the time variable. For the continuous problem under weakly and strongly damping parameters $\alpha$ and $\beta,$ respectively, a novel approach usually used for linear parabolic problems is employed to derive an exponential decay property with explicit rates, which depend on model parameters and the principal eigenvalue of the associated linear elliptic operator for the different cases of parameters such as $(i) \;\alpha, \beta >0$, $ (ii)\; \alpha>0, \beta \geq 0$ and $(iii)\;\alpha \geq 0, \beta >0$. Subsequently, for a semi-discrete finite element scheme keeping the temporal variable continuous, optimal error estimates are derived that preserve exponential decay behavior. Some generalizations that include forcing terms and spatially as well as time-varying damping parameters are discussed. Moreover, an abstract discrete problem is discussed, and as a consequence, uniform decay estimates for finite difference as well as spectral approximations to the damped system are briefly indicated. A complete discrete scheme is developed and analyzed after applying a finite difference scheme in time, which again preserves the exponential decay property. The given proofs involve several energies with energy-based techniques to derive the consistency between continuous and discrete decay rates, in which the constants involved do not blow up as $\alpha\to 0$ and $\beta\to 0$. Finally, several numerical experiments are conducted whose results support the theoretical findings, illustrate uniform decay rates, and explore the effects of parameters on stability and accuracy.

math.NA↗

arXiv2025

INTHOP: A Second-Order Globally Convergent Method for Nonconvex Optimization

Second-order Newton-type algorithms that leverage the exact Hessian or its approximation are central to solve nonlinear optimization problems. However, their applications in solving large-scale nonconvex problems are hindered by three primary challenges: (1) the high computational cost associated with Hessian evaluations, (2) its inversion, and (3) ensuring descent direction at points where the Hessian becomes indefinite. We propose INTHOP, an interval Hessian-based optimization algorithm for nonconvex problems to deal with these primary challenges. The proposed search direction is based on approximating the original Hessian matrix by a positive definite matrix. The novelty of the proposed method is that the proposed search direction is guaranteed to be descent and requires approximation of Hessian and its inversion only at specific iterations. We prove that the difference between the calculated approximate and exact Hessian is bounded within an interval. Accordingly, the approximate Hessian matrix is reused if the iterates are in that chosen interval while computing the gradients at each iteration. We develop various algorithm variants based on the interval size updating methods and minimum eigenvalue computation methods. We also prove the global convergence of the proposed algorithm. Further, we apply the algorithm to an extensive set of test problems and compare its performance with the existing methods such as steepest descent, quasi-Newton, and Newton method. We show empirically that the proposed method solves more problems in fewer function and gradient evaluations than steepest descent and the quasi-Newton method. While in the comparison to the Newton method, we illustrate that for nonconvex optimization problems, we require substantially less $O(n^3)$ operations.

math.OC↗

arXiv2025

Modeling of Experimentally Observed Two-Dimensional Precursor Solitons in a Dusty Plasma by the forced Kadomtsev-Petviashvili Equation

We compare model solutions of a forced Kadomtsev-Petviashvili (fKP) equation with experimental observations of dust acoustic precursor solitons excited by a supersonically moving charged cylindrical object in a dusty plasma medium. The fKP equation is derived from a three-fluid-Poisson model of the dusty plasma using the reductive perturbation technique and numerically solved for parameters close to the experimental investigations of cylindrical precursor solitons. The fKP model solutions show excellent agreement with the experimental results in reproducing the prominent geometric features of the two-dimensional solitons and closely matching the quantitative values of their velocities, amplitudes, and temporal evolutions. Our findings suggest that the fKP equation can serve as a very realistic model to investigate the dynamics of precursor solitons and can be usefully employed in practical applications such as space debris detection and tracking techniques that are based on observing/predicting nonlinear plasma excitations induced by the debris in the ionosphere.

physics.plasm-ph↗

arXiv2024

Error Estimates for Discontinuous Galerkin Approximations to the Vlasov-Unsteady Stokes System

In the first part of this paper, uniqueness of strong solution is established for the Vlasov-unsteady Stokes problem in 3D. The second part deals with a semi discrete scheme, which is based on the coupling of discontinuous Galerkin approximations for the Vlasov and the Stokes equations for the 2D problem. The proposed method is both mass and momentum conservative. Based on a special projection and also the Stokes projection, optimal error estimates in the case of smooth compactly supported initial data are derived. Moreover, the generalization of error estimates to 3D problem is also indicated. Finally, based on time splitting algorithm, some numerical experiments are conducted whose results confirm our theoretical findings.

math.NA↗

arXiv2023

Discontinuous Galerkin methods for the Vlasov-Stokes' system

This paper develops and analyses semi-discrete numerical method for two dimensional Vlasov-Stokes' system with periodic boundary condition. The method is based on coupling of semi-discrete discontinuous Galerkin method for the Vlasov equation with discontinuous Galerkin scheme for the stationary incompressible Stokes' equation. The proposed method is both mass and momentum conservative. Since it is difficult to establish non-negativity of the discrete local density, the generalized discrete Stokes' operator become non-coercive and indefinite and under smallness condition on the discretization parameter, optimal error estimates are established with help of a modified the Stokes' projection to deal with Stokes' part and with the help of a special projection to tackle the Vlasov part. Finally, numerical experiments based on the dG method combined with a splitting algorithm are performed.

math.NA↗

arXiv2023

Periodic Vlasov-Stokes' system: Existence and Uniqueness of strong solutions

This paper deals with the Vlasov-Stokes' system in three dimensions with periodic boundary conditions in the spatial variable. We prove the existence of a unique strong solution to this two-phase model under the assumption that initial velocity moments of certain order are bounded. We use a fixed point argument to arrive at a global-in-time solution.

math.AP↗

arXiv2023

Discontinuous Galerkin Methods with Generalized Numerical Fluxes for the Vlasov-Viscous Burgers' System

In this paper, semi-discrete numerical scheme for the approximation of the periodic Vlasov-viscous Burgers' system is developed and analyzed. The scheme is based on the coupling of discontinuous Galerkin approximations for the Vlasov equation and local discontinuous Galerkin approximations for the viscous Burgers' equation. Both these methods use generalized numerical fluxes. The proposed scheme is both mass and momentum conservative. Based on generalized Gauss-Radau projections, optimal rates of convergence in the case of smooth compactly supported initial data are derived. Finally, computational results confirm our theoretical findings.

math.NA↗

arXiv2022

Square lattice formation in a monodisperse complex plasma

We present the first observations of a square lattice formation in a monodisperse complex plasma system - a configurational transition phenomenon that has long been an experimental challenge in the field. The experiments are conducted in a tabletop L-shaped Dusty Plasma Experimental (DPEx-II) device in a DC glow discharge Argon plasma environment. By a careful control of the vertical potential confining the charged particles as well as the strength of the ion wake charge interactions with the dust particles we are able to steer the system towards a crystalline phase that exhibits a square lattice configuration. The transition occurs when the vertical confinement strength is slightly reduced below a critical value leading to a buckling of the monodisperse hexagonal 2D dust crystal to form a narrowly separated bilayer state (a quasi-2D state). Some theoretical insights into the transition process are provided through Molecular Dynamics (MD) simulations carried out for the parameters relevant to our experiment.

physics.plasm-ph↗

arXiv2021

Reflection of a dust acoustic solitary wave in a dusty plasma

We report the first experimental observations of the reflection of a dust acoustic solitary wave from a potential barrier in a dusty plasma medium. The experiments have been carried out in an inverted $\Pi$-shaped Dusty Plasma Experimental (DPEx) device in a DC glow discharge plasma environment. The dust acoustic solitary wave is excited by modulating the plasma with a short negative Gaussian pulse that is superimposed over the discharge voltage. The solitary wave structure is seen to move towards a potential barrier, created by the sheath around a biased wire, and turn back after reflecting off the barrier. The amplitude, width, and velocity of the soliton are recorded as a function of time. The experiment is repeated for different strengths of the potential barrier and for different initial amplitudes of the solitary wave. It is found that the distance of the closest approach of the solitary wave to the centre of the barrier increases with the increase of the strength of the potential barrier and with the decrease of the initial wave amplitude. An emissive probe is used to measure the sheath potential and its thickness by measuring the plasma potential profile in the axial direction over a range of resistances connected to the biased wire. A modified Korteweg de Vries equation is derived and numerically solved to qualitatively understand the experimental findings.

physics.plasm-ph↗

arXiv2021

Weak sharp minima for interval-valued functions and its primal-dual characterizations using generalized Hukuhara subdifferentiability

This article introduces the concept of weak sharp minima (WSM) for convex interval-valued functions (IVFs). To identify a set of WSM of a convex IVF, we provide its primal and dual characterizations. The primal characterization is given in terms of $gH$-directional derivatives. On the other hand, to derive dual characterizations, we propose the notions of the support function of a subset of $I(\mathbb{R})^{n}$ and $gH$-subdifferentiability for convex IVFs. Further, we develop the required $gH$-subdifferential calculus for convex IVFs. Thereafter, by using the proposed $gH$-subdifferential calculus, we provide dual characterizations for the set of WSM of convex IVFs.

math.OC↗

arXiv2021

Substrate Effects on the Bandwidth of CdSe Quantum Dot Photodetectors

We investigate the time-resolved photocurrent response of CdSe quantum dot (QD) thin films sensitized with zinc beta-tetraaminophthalocyanine (Zn4APc) on three different substrates, namely silicon with 230 nm SiO2 dielectric, glass as well as polyimide. While Si/SiO2(230 nm) is not suitable for any transient photocurrent characterization due to an interfering photocurrent response of the buried silicon, we find that polyimide substrates invoke the larger optical bandwidth with 85 kHz vs. 67 kHz for the same quantum dot thin film on glass. Upon evaluation of the transient photocurrent, we find that the photoresponse of the CdSe quantum dot films can be described as a combination of carrier recombination and fast trapping within 2.7 ns, followed by slower multiple trapping events. The latter are less pronounced on polyimide, which leads to the higher bandwidth. We show that all devices are RC-time limited and that improvements of the photoresistance are the key to further increasing the bandwidth.

cond-mat.mes-hall↗

arXiv2021

Music Generation using Three-layered LSTM

This paper explores the idea of utilising Long Short-Term Memory neural networks (LSTMNN) for the generation of musical sequences in ABC notation. The proposed approach takes ABC notations from the Nottingham dataset and encodes it to be fed as input for the neural networks. The primary objective is to input the neural networks with an arbitrary note, let the network process and augment a sequence based on the note until a good piece of music is produced. Multiple calibrations have been done to amend the parameters of the network for optimal generation. The output is assessed on the basis of rhythm, harmony, and grammar accuracy.

cs.SD↗

arXiv2020

Periodic Fluorescence Variations of CdSe Quantum Dots Coupled to Aryleneethynylenes with Aggregation Induced Emission

CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anti-correlated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. Energy transfer also affects the electric transport properties of the hybrid material under optical excitation. This work highlights that combining semiconductor nanocrystals with molecular aggregates, which exhibit aggregation induced emission, can result in unprecedented emerging optical properties.

physics.app-ph↗

arXiv2019

A Dye-Sensitized CdSe Nanocrystal Optical Transistor with High ON/OFF Ratio in the First Telecom Window with 74 ns Rise Time

We report an optically gated transistor composed of CdSe nanocrystals (NCs), sensitized with the dye Zinc beta-tetraaminophthalocyanine for operation in the first telecom window. This device shows a high ON/OFF ratio of six orders of magnitude in the red spectral region and an unprecedented 4.5 orders of magnitude at 847 nm. By transient absorption spectroscopy, we reveal that this unexpected infrared sensitivity is due to electron transfer from the dye to the CdSe NCs within 5 ps. We show by time-resolved photocurrent measurements that this enables fast rise times during near-infrared optical gating of 74 ns. Electronic coupling and accelerated non-radiative recombination of charge carriers at the interface between the dye and the CdSe NCs are further corroborated by steady-state and time-resolved photoluminescence measurements. Field-effect transistor measurements indicate that the increase in photocurrent upon laser illumination is mainly due to the increase in carrier concentration while the mobility remains unchanged. Our results illustrate that organic dyes as ligands for NCs invoke new optoelectronic functionalities, such as fast optical gating at sub-bandgap optical excitation energies.

cond-mat.mtrl-sci↗

arXiv2012

Mobile Ad Hoc Networks: A Comparative Study of QoS Routing Protocols

This Article presents a thorough overview of QoS routing metrics, resources and factors affecting performance of QoS routing protocols. The relative strength, weakness, and applicability of existing QoS routing protocols are also studied and compared. QoS routing protocols are classified according to the QoS metrics used type of QoS guarantee assured.

cs.NI↗

arXiv2011

Spin properties of a two dimensional electron system: valley degeneracy and finite thickness effects

The spin susceptibility of a two-dimensional electron system is calculated by determining the spin-polarization dependence of the ground-state energy within the self-consistent mean-field theory of Singwi et al. (STLS). Results are presented for three different devices, viz. the Si (100) inversion layer, the AlAs quantum well, and the GaAs heterojunction-insulated gate field-effect transistor. We find a fairly good agreement with experiments for the Si (100) system, on most of the experimental density range, whereas the agreement for the AlAs and GaAs systems is less satisfactory; in all cases, however, it is vital to include the characteristic device parameters like the valley degeneracy, the finite transverse thickness, etc. Further, the STLS theory predicts an abrupt spin-polarization transition at a sufficiently low electron density irrespective of the valley degeneracy and/or the finite thickness, with the partially spin-polarized states remaining unstable. Moreover, in the Si (100) inversion layer, the spin-polarization transition is preceded by the simultaneous valley- and spin- polarization; for its zero thickness model, these transitions however grossly disagree with the recent quantum Monte Carlo simulations. This drawback of the STLS theory is traced to its inaccuracy in treating electron correlations, which in turn become more and more important as the number of independent components (spin and valley) increases.

cond-mat.str-el↗