SUNIL KUMAR
Articles written in Pramana – Journal of Physics
Volume 10 Issue 1 January 1978 pp 75-81 Quantum Mechanics
Extremum uncertainty product and sum states
We consider the states with extremum products and sums of the uncertainties in non-commuting observables. These are illustrated by two specific examples of harmonic oscillator and the angular momentum states. It shows that the coherent states of the harmonic oscillator are characterized by the minimum uncertainty sum 〈(Δ
Volume 15 Issue 1 July 1980 pp 1-32 Optics
We give the theory of the free induction decay of two-level saturation resonances in the time-resolved laser saturation spectroscopy. The saturating and probe fields may be either co- or counter-propagating. The change-signal line-shape is analysed in detail in two cases of practical importance, namely when the saturating field is either strong or weak. The time evolution of the change-signal exhibits many interesting features which include Ramsey-type fringes, oscillatory decay, narrow resonances, line-narrowing etc. It is shown that by analysing the decay of the change-signal in time one can distinguish (i) various laser interaction processes such as population effects, coherence effects, three-photon Raman-type process, dynamic Stark splitting, power-broadening etc., and (ii) various relaxation mechanisms such as phase-changing collisions, etc.
Volume 23 Issue 5 November 1984 pp 541-546 Mathematical Physics
Superposition of special non-abelian potentials
The superposition of the non-abelian potentials of the form
Volume 25 Issue 3 September 1985 pp 291-309 Atomic Physics
Theory of coherent, degenerate two-photon absorption and emission
The theory of the coherent, two-photon resonant interaction of a monochromatic field with N atoms is given. It is seen that the dynamics of the atom-field system can be completely determined when the field is “strong”. Two specific examples are given: (i) two-photon absorption by atoms in ground state, and (ii) stimulated two-photon emission by fully excited atoms, assuming a coherent field in both cases. In case (ii), the field shows photon-antibunching after the decay of half of the atoms. The merits of our approach are shown by comparing with other treatments. Our results can also be applied to certain degenerate four-wave mixing processes which are described by a similar Hamiltonian.
Volume 76 Issue 2 February 2011 pp 223-239 Invited Papers
Inﬂuence of near-edge processes in the elemental analysis using X-ray emission-based techniques
Gurjeet Singh Sunil Kumar N Singh J Goswamy D Mehta
The near-edge processes, such as X-ray absorption ﬁne structure (XAFS) andresonant Raman scattering (RRS), are not incorporated in the available theoretical attenuation coefﬁcients, which are known to be reliable at energies away from the shell/subshell ionization thresholds of the attenuator element. Theoretical coefﬁcients are generally used to estimate matrix corrections in routine quantitative elemental analysis based on various X-ray emission techniques. A tabulation of characteristic X-ray energies across the periodic table is provided where those X-rays are expected to alter the attenuation coefﬁcients due to XAFS from a particular shell/subshell of the attenuator element. The inﬂuence of XAFS to the attenuation coefﬁcient depends upon the atomic environment and the photoelectron wave vector, i.e., difference in energies of incident X-ray and the shell/subshell ionization threshold of the attenuator element. Further, the XAFS at a shell/subshell will signiﬁcantly alter the total attenuation coefﬁcient if the jump ratio at that shell/subshell is large, e.g., the K shell, L_{3} subshell and M$_5$ subshell. The tabulations can be considered as guidelines so as to know what can be expected due to XAFS in typical photon-induced X-ray emission spectrometry.
Volume 79 Issue 3 September 2012 pp 471-481
Sunil Kumar M Khorasaninejad M M Adachi K S Karim S S Saini A K Sood
We investigate the relaxation dynamics of photogenerated carriers in silicon nanowires consisting of a crystalline core and a surrounding amorphous shell, using femtosecond time resolved differential reflectivity and transmission spectroscopy at 3.15 eV and 1.57 eV photon energies. The complex behaviour of the differential transmission and reflectivity transients is the mixed contributions from the crystalline core and the amorphous silicon on the nanowire surface and the substrate where competing effects of state-filling and photoinduced absorption govern the carrier dynamics. Faster relaxation rates are observed on increasing the photogenerated carrier density. Independent experimental results on crystalline silicon-on-sapphire (SOS) help us in separating the contributions from the carrier dynamics in crystalline core and the amorphous regions in the nanowire samples. Further, single-beam z-scan nonlinear transmission experiments at 1.57 eV in both open- and close-aperture configurations yield two-photon absorption coefficient 𝛽 (∼3 cm/GW) and nonlinear refraction coefficient 𝛾 ($−2.5 × 10^{−4}$ cm^{2} /GW).
Volume 84 Issue 2 February 2015 pp 317-325
We apply random matrix theory (RMT) to investigate the structure of cross-correlation in 20 global financial time series after the global financial crisis of 2008. We find that the largest eigenvalue deviates from the RMT prediction and is sensitive to the financial crisis. We find that the components of eigenvectors corresponding to the second largest eigenvalue changes sign in response to the crisis. We show that 20 global financial indices exhibit multifractality. We find that the origin of multifractality is due to the long-range correlations as well as broad probability function in the financial indices, with the exception of the index of Taiwan, as in all other indices the multifractal degree for shuffled and surrogate series is weaker than the original series. We fit the binomial multifractal model to the global financial indices.
Volume 86 Issue 1 January 2016 pp 97-108
B J Roy A Parmar T Nandi Biraja Mohanty M Oswal Sunil Kumar A Jhingan V Jha D C Biswas
Cross-sections for one- and multinucleon transfer reactions, namely, ^{58}Ni(^{12}C, ^{13}C), ^{58}Ni(^{12}C, ^{11}C), ^{58}Ni(^{12}C, ^{11}B), ^{58}Ni(^{12}C, ^{10}B), ^{58}Ni(^{12}C, ^{10}Be), ^{58}Ni(^{12}C, ^{9}Be), ^{58}Ni(^{12}C, ^{8}Be_{𝑔.s.}), ^{58}Ni(^{12}C, ^{7}Be), ^{58}Ni(^{12}C, ^{7}Li) and ^{58}Ni(^{12}C, ^{6}Li) have been measured at an incident energy of 60 MeV. The reaction cross-section for the corresponding transfer channels in the system ^{12}C+^{56}Fe have also been measured under the same kinematical conditions. Angular distribution of the elastic scattering cross-section is measured at 60 MeV. The measured elastic scattering angular distributions for these two systems have been analysed using the optical model search code SFRESCO and the potential parameters are extracted. The multinucleon transfer data are analysed to obtain cross-section dependence on the number of nucleons transferred and on the ground state 𝑄-values. The transfer probabilities for multinucleon stripping are extracted. A detailed comparison in the multiparticle stripping and elastic scattering cross-sections between these two systems are made to understand the mechanism of multinucleon transfer and possible role of two extra protons in ^{58}Ni target nucleus as compared to the ^{56}Fe core.
Volume 95 All articles Published: 6 May 2021 Article ID 0075 Reserach Article
THz pulses from optically excited Fe-, Pt- and Ta-based spintronic heterostructures
SANDEEP KUMAR ANAND NIVEDAN ARVIND SINGH SUNIL KUMAR
Spintronic heterostructures are considered to be the new generation terahertz (THz) sources because of their capability of producing high power and broadband THz radiation. Here, we provide a brief review on the state-of-the-art in this field. The optically excited bi- and tri-layer combinations of ferromagnetic and non-magnetic thin films have become increasingly popular. Towards optimising the THz conversion efficiency and broadband gapless spectrum from these THz emitters, various control parameters need to be taken into consideration. The inverse spin Hall effect in the heavy metal layer of the heterostructure is primarily responsible for the generation of THz pulses. A few new results on iron-, platinum- and tantalum-based heterostructures have also been reported here. It is observed that the Ta(2 nm)/Fe(2 nm)/Pt(2 nm) tri-layer heterostructure generates ~40(250)% stronger THz signal than the counterpart Fe(2 nm)/Pt(2 nm) (Fe(3 nm)/Ta(2 nm)) bi-layer heterostructure.
Volume 96, 2022
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