Manoranjan P. Singh

Negative electric field dependence of mobility in TPD doped Polystyrene

Abstract 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 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.

Retrieval Properties of a Hopfield Model with Random Asymmetric Interactions

The process of pattern retrieval in a Hopfield model in which a random antisymmetric component is added to the otherwise symmetric synaptic matrix is studied by computer simulations. The introduction of the antisymmetric component is found to increase the fraction of random inputs that converge to the memory states. However, the size of the basin of attraction of a memory state does not show any significant change when asymmetry is introduced in the synaptic matrix. We show that this is due to the fact that the spurious fixed points, which are destabilized by the introduction of asymmetry, have very small basins of attraction. The convergence time to spurious fixed-point attractors increases faster than that for the memory states as the asymmetry parameter is increased. The possibility of convergence to spurious fixed points is greatly reduced if a suitable upper limit is set for the convergence time. This prescription works better if the synaptic matrix has an antisymmetric component.

Measurements on impulsive force-induced dynamics of a cold 85Rb atom cloud in a magneto-optical trap

We report on the study of impulsive force-induced dynamics of a cold 85Rb atom cloud in a magneto-optical trap. The impulse, imparted by a laser beam of variable pulsewidth, forces the atom cloud away from the centre of the trap. Depending on the initial drift velocity of the atom cloud, we observe either oscillation or escape of the cloud. We investigate these processes as a function of the trap parameters, especially around the onset of the multiple scattering regime.

Effect of polarization on the structure of electromagnetic field and spatiotemporal distribution of e + e – pairs generated by colliding laser pulses

We have studied the production of electron–positron pairs due to polarization of vacuum in the presence of the strong electromagnetic field of two counterpropagating laser pulses. The structure of the electromagnetic field with the circular polarization has been determined using the 3D model of focused laser pulses, which was proposed by Narozhny and Fofanov. Analytic calculations have shown that the electric and magnetic fields are almost parallel to each other in the focal region when the laser pulses are completely transverse in the electric (E-wave) or magnetic (H-wave) field. On the other hand, the electric and magnetic fields are almost orthogonal when laser pulses consist of a mixture of E- and H-waves of the same amplitude. It has been found that although the latter configuration of colliding laser pulses has a much higher pair production threshold, it can generate much shorter electron–positron pulses as compared to the former configuration. The dependence of the production efficiency of pairs and their spatiotemporal distribution on the polarization of laser pulses has been analyzed using the structure of the electromagnetic field in the focal plane.

Elementary excitations of a trapped Bose gas in the large-gas-parameter regime

We study the effect of going beyond the Gross-Pitaevskii theory on the frequencies of collective oscillations of a trapped Bose gas in the large gas parameter regime. We go beyond the Gross-Pitaevskii regime by including a higher-order term in the interatomic correlation energy. To calculate the frequencies we employ the sum-rule approach of many-body response theory coupled with a variational method for the determination of ground-state properties. We show that going beyond the Gross-Pitaevskii approximation introduces significant corrections to the collective frequencies of the compressional mode.

Ground state and vortex states of bosons in an anisotropic trap: A variational approach

Abstract:We propose a simple variational form of the wave function to describe the ground state and vortex states of a weakly interacting Bose gas in an anisotropic trap. The proposed wave function is valid for a wide range of the particle numbers in the trap. It also works well in the case of attractive interaction between the atoms. Further, it provides an easy and fast method to calculate the physical quantities of interest. The results compare very well with those obtained by purely numerical techniques. Using our wave function we have been able to verify, for the first time, the predicted behaviour of the aspect ratio.

Analysis of the performance of a thermal defocusing optical limiter in the transient regime

A theoretical analysis of the performance of a thermal defocusing optical limiter in the transient regime-i.e., when the laser pulse duration is comparable to the acoustic transit time which governs the limiter response-is presented. First, an expression is obtained for the time- and space-dependent refractive index changes induced by a laser pulse in an absorbing liquid. This is used to calculate the nonlinear phase change experienced by the laser pulse while traversing the absorbing medium and then the transmission of the optical limiter. Results on the dependence of the optical limiters performance on laser pulse duration are presented in a general form and also by considering a specific example.

Electron–positron pair creation by counterpropagating laser pulses: role of carrier envelope phase

Abstract The effect of carrier envelope phase (CEP) on the spatio-temporal distribution of electron–positron pairs created by ultra-intense counterpropagating femtosecond laser pulses is studied. When the laser pulses are linearly polarized, the temporal distribution of the pairs is found to be sensitive with CEP. Same analysis is also done for the counterpropagating circularly e-polarized laser pulses. It is seen that when the counterpropagating laser pulses are both right (left) circularly polarized, the effect of the CEP is insignificant. On the other hand when the superimposed fields are in the combination of right and left circular polarizations, the CEP dependence comes in the invariant electric and magnetic fields structure and hence it reflects in the particle–antiparticle temporal distribution. However, the average number of total pairs is not greatly influenced by CEP for both the polarizations. Graphical abstract

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

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

Monte Carlo simulation of charge transport in disordered organic thin films: Applicability of Meyer-Neldel rule for extracting energetic disorder

Multi-carrier Monte Carlo simulation of charge transport is employed to test the suitability of Meyer-Neldel rule (MNR) in extracting energetic disorder from homogeneous organic thin films in diode geometry. Studies validate the use of MN rule for extracting energetic disorder from homogeneous organic thin films.