Sudhanshu S. Jha

Theory of second harmonic generation at a metal surface with surface plasmon excitation

Theory of second harmonic generation at a metal surface is developed, when surface plasmon-polaritons are resonantly excited by the incident electromagnetic field. For harmonic generation from small metallic spheres characterized by the dielectric function ϵ1(ω) + iϵ2(ω), the resonant enhancement of the second harmonic intensity is predicted to be of the order of (ϵ1ϵ2)4. The resonant enhancement is much smaller for the case of a metallic grating.

Energy distribution of thermal neutrons in a finite solid moderator assembly

The equilibrium neutron energy spectrum inside a finite Be assembly was calculated by solving the energydependent Boltzmann diffusion equation by numerical iteration. A pulsed source of neutrons at time t = 0 was assumed. A reasonable approximation to the values given by Bhandari for the transport cross- sections was used. For small assemblies the spectrum found to deviate appreciably from the Maxwellian, panticularly in the low energy region. The calculations also gave the values of the decay constant lambda of neutrons in different sizes of the assembly. (auth)

Generalized pairing theory of superconductivity in layered crystals

Abstract A generalized pairing theory of superconductivity in layered crystals is formulated in terms of an arbitrary dynamical interaction, V(r1r2; t1−t2), among the electrons. This has been done within the framework of the Gorkov mean field theory, with the introduction of an appropriate set of states to represent the single particle motion in the layered crystals. The layer representation is band-like in the plane of the layers and localized in the perpendicular direction. This enables us to derive the equation for the superconducting state which includes intra-and inter-layer polarization interactions involving exchange of all possible electronic and ionic excitations in the system within the strong coupling formalism. An equation for the critical temperature Tc of the system has been obtained by solving the linearized gap equation in terms of a suitably averaged dynamical interaction. We specify from first principles the various approximations required to obtain the simpler forms of the Tc-equation used in the literature for investigating its dependence on the structure and number of conducting layers in the new high-Tc systems. In general, we find that the dynamical interactions can involve a maximum of any four different layers with intra- and inter-layer pairings. In the extreme layer approximation, where the excitations are highly localized to within individual layers, the interaction connects only a maximum of any two layers. These considerations still imply that the required attractive interaction for intra-layer Cooper pairing in a conducting layer may arise from both intra- and inter-layer couplings and possible exchange of excitations in the neighboring layers whether conducting or insulating.

Critical temperature in high-Tc layered superconductors: Dependence on the number of interacting conducting layers per unit cell

Abstract The generalized BCS pairing thoery for layered superconductors is used to investigate the dependence of the transition temperature T c on the number N of interacting conducting Cu-O layers in any unit cell. The coupling constants, which describe intralayer and interlayer interactions in these layers where the Cooper pairing is assumed to take place, are explicity related to the microscopic polarization functions of all layers in the unit cell, including the insulating layers. With reasonable interlayer couplings it is possible to understand the experimental values of T c in many of the high- T c compounds discovered recently. One can indeed increase T c as a function of N , but we find T c (∞) to be less than 150K in polytypes of the [-(CuO 2 ) N -] class, as well as in the 1:2:3 class polytypes [-(CuO 2 )-(CuO-CuO 2 ) n -], N =2+1. This is based on the approximation involving only up to the nearest neighbour conducting layer couplings and the use of experimental results for T c in various thallium, bismuth and 1:2:3 compounds. A much higher T c (∞) is possible only of the single-layer T c (1) is high (>90 K) and interlayer couplings are significant.

Electronic plus phonon-exchange mechanism for high-temperature superconductivity in layered crystals

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 temperatureTc in such systems within the framework of the conventional BCS pairing arising from various possible excitations in the medium. This differs considerably from the usual case of an effective three-dimensional homogeneous system, and should be relevant in the calculation ofTc for the new class of high-Tc perovskites in which oxygen deficiencies in Cu-O layers and their distribution in the crystal play a crucial role. The explicit form of the effective interactionVjj(qt,ω) in a given layerj in the unit cell of the crystal is found to be determined not only by the true polarization functionπj(qt,ω) of that layer, but also of other layers. The exchange of electronic excitations of a nearby insulating layer by carriers in a conducting layer thus becomes possible to get highTc, with or without the usual phonon exchange.

On electronic Raman scattering from high-Tc layered superconducting materials

Abstract The nature of electronic Raman scattering from anisotropic layered high- T c materials has been considered. Approximate expressions for the electronic Raman scattering from both charge-density fluctuations (CDF) and spin-density fluctuations (SDF) in these materials with nonparabolic energy bands and anisotropic Fermi surfaces have been derived. It is shown that in the normal state the unscreened part of the single-particle electronic Raman scattering has a width of about 2ω cn in the collision-dominated regime in which the normal state collision frequency ω cn ⪢ q · v f , where ħ q is the momentum transfer in the scattering process and v F is the Fermi velocity. Although, the normal state Raman spectrum is not as flat as observed experimentally, the possibility that most of the spectrum may be arising from such collision-dominated single-particle excitations, cannot be ruled out completely. More careful experimentation is required to confirm unambiguously the existence of any novel type of electronic excitations in the system.

Pairing mechanisms and anisotropic superconductivity in layered crystals

Abstract A brief outline of the generalized BCS pairing theory is presented. The pairing of carriers can be due to the exchange of lattice-phonons or due to the exchange of electronic charge-density and spin-density excitations. It is argued that anisotropic physical properties in the normal as well as superconducting states in the new high-Tc materials are crucual in the development of any realistic theoretical approach, and in comparison of experimental results with correct BCS predictions involving superconductivity in layered crystals. The possibility of the break-down of the mean-field approximation is also discussed. As of now, the generalized BCS pairing approach is the only realistic microscopic theory available which may be applied to high-Tc superconductors.

Superconducting transition temperature of a paramagnetic material close to magnetic ordering

An explicit expression for the superconducting transition temperatureTc in a paramagnetic material is derived, when the transition occurs just before a possible magnetic ordering. As first noted by Uspenskii, such a transition may arise from electronic mechanism itself, without the necessary role played by the usual phonon-exchange mechanism. The result is discussed in terms of some recent experimental observations on the binary alloy Y9Co7.

Anomalous normal state electronic Raman scattering continuum in YBCO: Inadequacy of frequency-dependent collision-rate models

Abstract We calculate the intraband electronic Raman scattering cross section from charge density fluctuations of holes in YBCO and the real part of the transverse electromagnetic conductivity in the normal state, using phenomenological linear frequency-dependent collision rates ω c (ω, T ). Whereas there is a general agreement with the experimental data on infrared conductivity, we are unable to obtain the extent of flatness of the electronic Raman continuum as observed in several experiments.

Picosecond time evolution of photoexcited hot‐electron mobility in GaAs and the speed of photoresponse

The transient mobility of hot electrons photoexcited in undoped GaAs by subpicosecond laser pulses is calculated. For this, we solve the time‐dependent Boltzmann transport equation in the presence of a low‐frequency, weak electric field. The attention is focused mainly on the role of intracentral Γ valley scattering in determining the delay in the mobility rise on the picosecond time scale, and the hot‐electron energies are assumed to be below the thresholds for possible side‐valley transfers (Γ→L,X). We consider the mobility response under two separate conditions of excited carrier density, namely, (1) low‐density excitations for which the electron–longitudinal phonon (LO) Frohlich interaction initially dominates in the carrier relaxation and (2) high‐density excitations for which the electron‐electron interaction is faster than all other collisions. The mobility of hot electrons is very small (<1000 cm2/V s) just after photogeneration. It rises to its maximum value with a time constant decided by the va...