Sudhanshu S Jha
Articles written in Pramana – Journal of Physics
Volume 1 Issue 1 July 1973 pp 37-47 Solids
Within the framework of quasi equilibrium approximation, Josephson’s expression for current in a weakly coupled superconducting system is derived without the use of any specific microscopic model. It is based only on the existence of the off-diagonal long-range order in the two-particle reduced density matrix. Allowing for deviations from the quasi equilibrium approximation, generalised Josephson equations are obtained which include ohmic terms. The effect of relaxation and thermal fluctuations is examined in detail to emphasise the physical origin of various terms in the expression for current.
Volume 1 Issue 2 August 1973 pp 88-97 Plasmas
The problem of heating of a solid target to generate a nonequilibrium plasma by subnanosecond laser pulses is considered. For an appreciable absorption of energy from a Nd-glass laser, the critical density of the electrons in the plasma turns out to be 1021 cm−3. These electrons can be heated up to 107 K or more by using pulses of about 10 picosecond duration and absorbed energy flux of the order of 1021 erg cm−2 sec−1. Starting from neutral atoms in a solid with a high atomic number, e.g., Z=26, for times in the picosecond regime the relevant rate equations are solved analytically to predict densities of the atoms at different ionization levels. It is shown that during such a short time the population density of the ions isoelectronic to neon builds up to a very large amount. This in turn leads to the population inversion in the 4s → 3p soft x-ray laser transition, via the electron-impact excitation of the 4s level of the isoelectronic neon ion. For the effective pumping times of the order of 5 picoseconds, a gain of the order of 102 db cm−1 is predicted for the laser transition in Fe XVII, Co XVIII or Cu XX.
Volume 2 Issue 3 March 1974 pp 116-125 Solids
Simple approximation schemes are developed to calculate induced optical fields and local field corrections to the linear optical dielectric function in metals like aluminium and in insulators like germanium. In these calculations, the unperturbed electronic states in Ge are described within the framework of the bonding orbital approach, whereas the nearly-free-electron approximation is used for Al. As expected, explicit numerical calculations show that the contribution to secondary longitudinal induced fields is more appreciable in Ge. The second order susceptibility describing the non-linear mixing of an optical frequency with an x-ray frequency, which depends upon the magnitude of the microscopic induced optical charge density, is also calculated for these solids. For most relevant wavevectors of secondary optical fields, it is found to be of the order of 10−12 esu in Ge and 10−14 esu in Al.
Volume 6 Issue 3 March 1976 pp 161-182 Solids
The problem of superconductivity in a metal-semiconductor system has been studied, using the dielectric formulation of superconductivity. The charge redistribution due to the quantum penetration of the metallic electrons to the semi-conductor side is approximated by a simple exponential function. The interface exciton modes are obtained within the framework of classical electrostatics, and their effect in modifying the effective electron-electron interaction near the interface is investigated. It is found that the strength of the excitonic term is small, and by itself, insufficient to lead to superconductivity. Nevertheless, it can alter the superconducting transition temperature of a metal, if it is already superconducting due to some other mechanism. This has been studied as a function of the various parameters entering in the problem.
Volume 6 Issue 6 June 1976 pp 349-362 Solids
Analytical methods to investigate the interaction of magnetic monopoles with known magnetic media have been developed. Trapping energies of monopoles inside ferro-magnetic or super onducting materials of size greater than about 10−6 cm are found to be of the order of several kiloelectron volts. These are two to three orders of magnitude higher than in paramagnetic materials. Thus if stable magnetic monopoles exist at all in the universe, they are perhaps trapped in these magnetic materials. The effect of the finite size of the magnetic bodies is taken into account explicitly in our calculations of the trapping energy.
Volume 10 Issue 2 February 1978 pp 179-200 Solids
An approximate method is developed for investigating the nature of interface exciton modes in a composite spatially dispersive medium. The method is general enough to be applicable to any composite system, in which each component is described by an arbitrary bulk dielectric function
Volume 10 Issue 4 April 1978 pp 429-446 Plasma Physics
Approximate analytic solutions to the self similar equations of gas dynamics for a plasma, treated as an ideal gas with specific heat ratio
Volume 11 Issue 3 September 1978 pp 313-322 Optics
An initial value problem is set up to describe propagation of a low-frequency wave-field interacting with two almost transparent wave-fields in a dispersive medium. With no linear loss, perfect phase-matching, and equal group velocities for the two high-frequency wave-packets, it is shown how the solution of the above problem can evolve to well-known soliton solutions of the sine-Gordon equation. Other attempts for solving the more general problem in which all the group velocities are different are also discussed.
Volume 13 Issue 2 August 1979 pp 131-143 Solids
The superconducting transition temperature
Volume 22 Issue 3-4 March 1984 pp 173-182 Statistical Physics
A general nonlinear response theory for the case of linear coupling of physical systems to arbitrary external fields is formulated for applications in different branches of physics. This is done within the framework of non-relativistic density matrix approach of quantum mechanics. Some simple properties of response functions and other related functions, which are introduced here for convenience, are studied to obtain suitable representations of the nonlinear response functions, including important sum-rules. As an example, the sum rule for the second-order response function is applied to electronic dipole nonlinearity at optical frequencies which includes both the Raman nonlinearity arising from perturbation to the electronic motion from external ionic displacement field and the usual optical sum, difference and harmonic generations. This immediately allows us to visualize a rigorous connection between these two types of non-linearities.
Volume 22 Issue 5 May 1984 pp 431-437 Solid State Physics
An explicit expression for the superconducting transition temperature
Volume 29 Issue 6 December 1987 pp 1- Rapid Communications
A general mathematical formulation is developed for calculating the effective electron-electron interaction in layered crystals like YBa2Cu3O7−δ, and for finding the resulting superconducting transition temperature
Volume 34 Issue 4 April 1990 pp 259-277
New oxide superconductors with layered structure are expected to have anisotropic energy gap in the generalized BCS pairing theory. The gap parameter $$2\Delta (\hat k)$$ can be quite different for $$\hat k$$ perpendicular to the plane of the layers as compared to
Volume 39 Issue 6 December 1992 pp 615-631
A theoretical framework for treating the effects of magnetic field
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