arXiv subjects

Manoranjan P. Singh

Publications and source records attributed to Manoranjan P. Singh.

At least 19 recordsLinked to original sources

arXiv2026

Pair Production in time-dependent Electric field at Finite times

We investigate the finite-time behavior of pair production from the vacuum by a time-dependent Sauter pulsed electric field. By examining the temporal behavior of the single-particle distribution function, we observe oscillatory patterns in the longitudinal momentum spectrum of the particles at finite times. These oscillations arise due to quantum interference effects resulting from the various dynamical processes/channels leading to the creation of the (quasi-)particle of a given momentum. Furthermore, we derive an approximate and simplified analytical expression for the distribution function at finite times, allowing us to explain these oscillations' origin and behavior. The role of the vacuum polarization function and its counterterm are also discussed in this regard. The transverse momentum spectrum peaked at the nonzero value of the transverse momentum at finite times, which indicates the role of multiphoton transitions in the creation of quasiparticles.

hep-ph↗

arXiv2025

Electron-positron pair creation induced by multi-pulse train of electric fields: effect of randomness in time-delay

We investigate the creation of electron-positron pairs (EPPs) in a sequence of alternating-sign, time-dependent electric field pulse trains by solving the quantum Vlasov equations. Specifically, we focus on Sauter-like pulse trains with random time delays between successive pulses, drawn from a Gaussian distribution wherein the extent of fluctuations is controlled by the standard deviation $σ_T$ of the distribution. We find that increasing $σ_T$ leads to a dramatic transformation in the longitudinal momentum spectrum. The well-known fringe pattern, akin to that in the multi-slit interference, gets significantly modified. The averaged spectra exhibit a robust Gaussian-like envelope with residual oscillations, which are much more prominent in the central momentum region. Notably, we find that in certain cases, stochastic time delays lead to a pronounced enhancement in the central peak of the distribution function for pulse train containing $N$ pulses. For example, for $N=20$ pulses, $σ_T \approx 31$ $[m^{-1}]$(about $17\%$ of the mean time delay) yields nearly a tenfold increase in the central peak, which for $σ_T \approx 50$ $[m^{-1}]$ (about $27\%$ of the mean time delay), scales up to $10^3.$ This may open up new possibilities for optimizing multi-pulse field configurations and guide future experimental designs aimed at maximizing EPPs creation.

quant-ph↗

arXiv2025

Longitudinal Momentum Spectra of pair created in a pulsed field at finite times: Are Oscillations "Real"

We discuss the mechanism of production of electron-positron pairs from the vacuum in a time-varying, spatially uniform pulsed electric field. We analytically compute the probability of $(e^+ e^-) $pair production in momentum space using the exact solution of the one-particle time-dependent Dirac equation and compare the result with quantum kinetic theory (QKT). Both approaches allow us to study the particle momentum spectrum at any instant in time and can potentially unveil valuable information regarding quantum non-equilibrium physics. We analyze both approaches' momentum spectra of the created particles at any instant. We observe a multi-profile structure in the LMS. This multi-profile structure clearly illustrates the quantum interference effects associated with pair production. It is worth noting that both approaches exhibit quantum interference patterns at finite times, manifested as oscillations observed in the LMS. We highlight that this quantum signature is a universal behavior seen in the momentum spectra at finite times, where the electric field is nearly zero.

hep-ph↗

arXiv2025

Vacuum polarization current in presence of intense Sauter field

The quantum vacuum becomes unstable under an external field, leading to spontaneous particle-antiparticle pair creation. In canonical quantization, the time-dependent particle number, defined via Bogoliubov transformations lacks physical meaning until the external field vanishes. To address this, we explore dynamical quantities that remain well-defined at both asymptotic and intermediate times, focusing on the vacuum polarization current. Investigating this observable provides insights into the system's intermediate-time behavior. We consider pair creation in a spatially homogeneous, time-dependent, intense Sauter field. Specifically, we analyze the real and imaginary parts of the correlation function, linking them to vacuum polarization effects. The vacuum polarization current in an intense laser pulse is computed numerically, revealing that it correlates with the real part of the correlation function. Initially, the current changes sign and gradually decreases, but unlike the particle number, it does not reach a constant asymptotic value. Instead, for large times, it exhibits nearly undamped oscillations, a distinctive feature of scalar particles, oscillating strongly around zero. Additionally, we explore the uniqueness of the vacuum polarization current in the adiabatic basis, comparing different reference mode function choices. Notably, we find that the current remains independent of the basis choice.

hep-ph↗

arXiv2025

Dynamical Scaling in Pair Production for Scalar QED

We report on the dynamical scaling of momentum spectra for particle-antiparticle pairs at finite times within the framework of scalar Quantum Electrodynamics (QED). The analysis focuses on the momentum spectra in two different choices of adiabatic mode functions, which are related by a Wronskian normalization condition. Oscillations in the momentum spectra are attributed to quantum interference effects in the adiabatic number basis. A novel dynamical scaling behavior emerges when examining the oscillatory momentum spectra of pairs created by a Sauter pulsed field at intermediate times. While the oscillatory spectra are observed at distinct times in the two different choices, they overlap when time is rescaled by the point marking the initiation of the first occurrence of the Residual ParticleAntiparticle Plasma (RPAP) stage (or end of the transient stage) for the central momentum case. This scaling identifies the approximate time at which real particle-antiparticle pair formation becomes possible, shifting the focus from asymptotic times to finite-time dynamics. Additionally, in the multi-photon regime, we find that the momentum spectra exhibit a multi-modal profile structure at finite times, consistent across both choices and also follow the dynamical scaling in this case as well.

hep-ph↗

arXiv2025

Does the oscillatory behavior of the Momentum Spectrum depend on the basis in the Post-Transient Stage?

Pair creation by a spatially homogeneous, time-dependent electric field is studied within the framework of scalar quantum electrodynamics. We employ the standard Bogoliubov transformation approach to compute the single-particle distribution function in an adiabatic basis. We analyzed the distribution function of created particles in two different adiabatic bases (related by a unitary transformation). A novel dynamical scaling is observed while analyzing the oscillatory momentum spectrum of the pairs created by the Sauter pulsed field at intermediate times, calculated using the two adiabatic bases. In these bases, the same oscillatory momentum spectra are observed but at different times. However, when we scale the time by the point marking the end of the transient stage of dynamical evolution for each case of central momentum, the respective momentum spectra overlap. Furthermore, we study the time evolution of the momentum spectrum in the multi-photon regime and find that the spectra show a multi-modal profile structure at finite times for both choices of basis.

quant-ph↗

arXiv2023

Pair Production in time-dependent Electric field at Finite times

We investigate the finite time behavior of pair production from the vacuum by time-dependent Sauter pulsed electric fields in spinor quantum electrodynamics (QED). Using the exact analytic solution of the mode function, we find the one-particle distribution function in momentum space. The longitudinal momentum spectrum of particles shows oscillatory behavior at a finite time in a small window of longitudinal momentum where the electric field diminishes to around one-hundredth of its maximum magnitude and its oscillation time is close to Compton time. This oscillation is asymmetric, i.e., the amplitude of oscillation is maximum for negative longitudinal momentum compared to positive rate. The change in the longitudinal momentum spectrum can occur due to the quantum interference effect, and this quantum interference effect comes from the result of dynamical tunneling. The transverse momentum spectrum shows the Gaussian structure with a peak at zero transverse momentum when $t = 0$. After $t \approx τ/2$, the smooth Gaussian design becomes distorted, and we see inconstancy in spectrum structure, either a dip at the origin with an off-axis maximum or a peak at zero transverse momentum with small mountains up to $t \approx 2τ$ observed. After that, the spectrum shows a maximum height at zero transverse momentum with weakly pronounced peaks.

hep-ph↗

arXiv2022

Imprint of temporal envelope of ultrashort laser pulses on longitudinal momentum spectrum of $e^+e^-$ pairs

The effect of the temporal pulse shape of intense pulses on the momentum distribution of $e^+e^-$ pairs is studied using the quantum kinetic equation. Two closely resembling temporal envelopes namely, Gaussian and Sauter, keeping all the other pulse parameters the same, are considered to this end. Contrary to the common perception which can be gauged from the interchangeable use of these temporal profiles, the longitudinal momentum spectrum of the pairs created by the two pulses is found to differ significantly in all the temporal regimes. For the pulses having a few cycles of oscillations, the temporal profile of the pulse is revealed in the oscillatory interference pattern riding over the otherwise smooth longitudinal momentum spectrum at asymptotic times. The onset of the oscillation due to the quantum interference of reflection amplitudes from the scattering potential due to the pulses having a temporal structure of multiple barriers takes place for few-cycle oscillations for the Gaussian pulse than that for the Sauter pulse. Furthermore, the oscillation amplitude for the same number of oscillations within the pulse duration is larger for the Gaussian pulse. The presence of the carrier-envelope phase and the frequency chirping is found to magnify these differences. In the absence of any appreciable interference effect for the pulses having less than five oscillations, the longitudinal momentum spectrum has a higher peak value for the Sauter pulse at asymptotic times. On the hand, before the transient stage of evolution, the peak of the spectrum shows the opposite trend.

physics.plasm-ph↗

arXiv2018

Monte Carlo simulation of charge transport in disordered organic systems using buffer lattice at boundary

In this article, we present an alternative method for simulating charge transport in disordered organic materials by using a buffer lattice at the boundary. This method does not require careful tracking of carrier's hopping pattern across boundaries. Suitability of this method is established by reproducing the field dependence of mobility, carrier relaxation and carrier diffusion in disordered organic systems obtained by simulating the charge transport for the full length of the systems along the field direction without and boundary condition. The significance of the buffer lattice is emphasized by simulating field dependence of mobility without using a buffer lattice, which results in negative field dependence of mobiltiy (NFDM) at low field regime due to the extra bias the carrier gains from the neglected hops and boundaries along field direction.

cond-mat.dis-nn↗

arXiv2018

Transient and pre-transient stages in field induced phase transition of vacuum state

Evolution of modulus and phase of complex order parameter associated with field induced phase transition (FIPT) of the vacuum state interacting with time dependent Sauter pulse is studied to analyse different evolution stages of the order parameter e.g., quasi electron positron plasma (QEPP), transient, and residual electron-positron plasma (REPP) stages. By revisiting FIPT in presence of single-sheeted and multi-sheeted pulses, we attribute the transient stage to the nonlinear coupling in the differential equations governing the dynamics of the phase and the modulus of the order parameter. The appearance of the rapid oscillations in the modulus is shown to be associated with the abrupt change in the phase of the order parameter in the transient stage. FIPT is also studied for multi-sheeted Sauter pulse with linear and quadratic frequency chirp. QEEP stage is found to show complex dynamical behaviour with fast and irregular oscillations due to the frequency chirp. The formation of pre-transient region due to the quadratic frequency chirping is observed in the accelerating part of the QEPP stage before the electric field attains the maximum value. As the quadratic chirp is increased the pre-transient and transient stages move closer to the electric field maximum which leads to a decrease in temporal separation between the two stages. The early appearance of the transient stage and hence of the following REPP stage results in the enhancement of pair production rate.

hep-ph↗

arXiv2018

Phase control of Schwinger pair production by colliding laser pulses

We study the Schwinger electron-positron pair production by a strong electromagnetic field of two colliding e-polarized laser pulses with a relative phase shift $Ψ$. The spatio-temporal distribution of created pairs is very sensitive to this phase shift and to polarization of the pulses. We study this dependence in detail and demonstrate how it can be explained in terms of the underlying invariant field structure of the counterpropagating focused pulses.

hep-th↗

arXiv2016

Effect of polarization on the structure of electromagnetic field and spatiotemporal distribution of $e^+e^-$ pairs by colliding laser pulses

Electron-positron pair production by means of vacuum polarization in the presence of strong electromagnetic (EM) field of two counterpropagating laser pulses is studied. A 3-dimensional model of the focused laser pulses based on the solution of the Maxwell's equations proposed by Narozhny and Fofanov is used to find the structure of EM field of the circularly polarized counterpropagating pulses. Analytical calculations show that the electric and magnetic fields are almost parallel to each other in the focal region when pulses are completely transverse either in electric (e-wave) or magnetic (h-wave) field. On the other hand the electric and magnetic fields are almost orthogonal when the counterpropagating pulses are made up of equal mixture of e- and h- polarized waves. It is found that while the latter configuration of the colliding pulses has much larger threshold for pair production it can provide much shorter electron/positron pulses compared to the former case. The dependence of pair production and its spatiotemporal distribution on polarization of the laser pulses is analyzed using the structure of the EM field.

physics.plasm-ph↗

arXiv2013

Monte Carlo Simulation of Carrier Diffusion in Organic Thin Films with Morphological Inhomogeneity

Monte Carlo simulation was carried out to understand the influence of morphological inhomogeneity on carrier diffusion in organic thin films. The morphological inhomogeneity was considered in the simulation by incorporating the regions of low energetic disorder in a host lattice of high energetic disorder which decreases the overall energetic disorder of the system. For the homogeneous films, the carrier diffusion was found to decrease upon decreasing the energetic disorder. In contrast to this, in the case of inhomogeneous films the carrier diffusion enhanced upon decreasing the overall energetic disorder, up to an optimum value and beyond which the carrier diffusion decreased. Through our simulation, we observed that the behavior of carrier diffusion in the inhomogeneous case is due to the morphology dependent carrier spreading, which acts in addition to the thermal and non-thermal field assisted diffusion mechanisms. This morphological dependence of carrier spreading arises due to the generation of packets of carriers with different jump rates, which is after effect of slow relaxation of the carriers generated in the less disordered regions of inhomogeneous system. Our simulation of morphology dependent carrier spreading and its influence on the basic diffusion process provide deeper insight into the charge transport mechanisms in organic thin films.

cond-mat.mtrl-sci↗

arXiv2013

Influence of morphological inhomogeneity induced carrier diffusion on transient photocurrent pulse shape in organic thin films

The influence of film morphology induced carrier diffusion on the broadening of the time-of-flight transient photo-current pulse was investigated using Monte Carlo simulation in organic thin films. Assuming the Gaussian Disorder Model for the charge transport the simulation of the time-of-flight photo-current pulse shape was carried out for homogeneous and inhomogeneous films by varying the overall energetic disorder of the system. In the case of homogeneous system, the value of the tail broadening parameter (W) of the photocurrent pulse is found to decrease upon decreasing the energetic disorder. The observed behavior is explained by using the temporal evolution of carrier diffusion coefficient. In case of the inhomogeneous system, upon decreasing the overall energetic disorder of the system the value of W initially attained a maximum before it started to decrease. This is attributed to the morphology dependent carrier diffusion in the latter case. This study elicits the importance of the influence of the film morphology induced carrier diffusion on the experimentally measured shape of the time-of-flight transient photo-current pulses, which is found to be generally ignored.

cond-mat.mtrl-sci↗

arXiv2009

Electromagnetically induced transparency in cold 85Rb atoms trapped in the ground hyperfine F = 2 state

We report electromagnetically induced transparency (EIT) in cold 85Rb atoms, trapped in the lower hyperfine level F = 2, of the ground state 5$^{2}S_{1/2}$ (Tiwari V B \textit{et al} 2008 {\it Phys. Rev.} A {\bf 78} 063421). Two steady state $Λ$-type systems of hyperfine energy levels are investigated using probe transitions into the levels F$^{\prime}$ = 2 and F$^{\prime}$ = 3 of the excited state 5$^{2}P_{3/2}$ in the presence of coupling transitions F = 3 $\to$ F$^{\prime}$ = 2 and F = 3 $\to$ F$^{\prime}$ = 3, respectively. The effects of uncoupled magnetic sublevel transitions and coupling field's Rabi frequency on the EIT signal from these systems are studied using a simple theoretical model.

physics.atom-ph↗

arXiv2008

Negative electric field dependence of mobility in TPD doped Polystyrene

A total negative field dependence of hole mobility down to low temperature was observed in N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'diamine (TPD) doped in Polystyrene. The observed field dependence of mobility is explained on the basis of low values of energetic and positional disorder present in the sample. The low value of disorder is attributed to different morphology of the sample due to aggregation/crystallization of TPD. Monte Carlo simulations were also performed to understand the influence of aggregates on charge transport in disordered medium with correlated site energies. The simulation supports our experimental observations and justification on the basis of low values of disorder parameters.

cond-mat.dis-nn↗

arXiv2007

Charge transport in disordered organic solids: A Monte Carlo simulation study on the effects of film morphology

The influence of ordered regions (micro crystallites and aggregates) in the other wise disordered polymer host matrix on field and temperature dependence of mobility (μ) has been simulated. Increase in concentration of ordered regions leads to increase in magnitude of mobility and in high field regime the saturation of the mobility occurs at lower electric field strength. The influence of different mean and standard deviation of Gaussian density of states (DOS) of ordered regions on the field dependence of mobility was studied and found to be significant only at higher concentrations. Weak influence of these parameters at low concentrations are attributed to the strong interface effects due to the difference in the standard deviation of DOS of two regions (host and ordered region) and shallow trapping effect by ordered regions. For all the parameters of ordered regions under investigation the temperature dependence of mobility (logμ) and the slope of logμ Vs E^{1/2} plot show 1/T^2 dependence.

cond-mat.dis-nn↗

arXiv2005

Properties of Trapped Bose gas with vortices in large-gas-parameter regime

We study the properties of the vortex state of a trapped Bose gas in the large-gas-parameter regime. To test validity of the Gross-Pitaevskii theory in this regime for the vortex states we compare the results of the Gross-Pitaevskii and the modified Gross-Pitaevskii calculations for the total energy, the chemical potential, the density profile and the frequency shift of the quadrupole modes of the collective oscillations of the condensate. We find that in the large-gas-parameter regime two calculations give substantially different results for all the properties mentioned above

cond-mat.other↗