K. C. Sharma

Mixed valence systems: Anomalous thermal expansion in the presence of crystalline electric field

Abstract Thermal expansion (Δα) of mixed valence systems in the presence of crystalline electric field (CEF) has been investigated in the framework of Anderson model as studied earlier by Coqblin and co-workers. The Δα has been analysed in two limiting cases of temperature. In the low temperature limit it is found to be proportional to T and in the high temperature limit the proportionality is T -2 . The contribution to Δα goes through a maximum. The effect of CEF is to increase the value of Δα which reveals a qualitative change in the thermal expansion. Qualitatively, Δα is estimated by choosing proper values of T and δ. For a typical case with T ≌ 100 K, δ ≌ 200 K; Δα is found to be an anomalously large value of the order of 8 × 10 -3 K -1 .

Partial swapping of Fano resonance and bound states in continuum in a correlated double-quantum-dot system

We study the partial swapping of the Fano resonance in a correlated double-quantum-dot (DQD) system connected to normal leads by employing the Keldysh non-equilibrium Green function technique in a Coulomb blockade regime. Two types of asymmetries have been considered which occurs when the DQD system undergoes transition from symmetric parallel to i) series and ii) T-shape geometries. It is found that both types of asymmetries exhibit swapping on varying the interdot Coulomb interaction (U12), however, mostly in the first one or first two pairs (out of four pairs) of Fano and Breit-Wigner peaks. Further, for particular values of U12 and dot-lead couplings, one, two or three Fano peaks collapse forming bound states in the continuum (BIC). This partial (non-magnetic) swapping and the formation of BIC is found to result from an interplay of correlation effects and asymmetries in dot-lead couplings.

Compressibility and collisional effects on thermal instability of a partially ionized medium

The thermal instability of a finitely conducting hydromagnetic composite and compressible medium is studied to include the frictional effects with neutrals. The effect of compressibility is found to be stabilizing. In contrast to the nonoscillatory modes for (Cp/g)β > 1 in the absence of a magnetic field;Cv, β andg being specific heat at constant pressure, uniform adverse temperature gradient and acceleration due to gravity respectively, the presence of magnetic field introduces oscillatory modes in the system. The overstable case is also discussed. The magnetic field is found to have a stabilizing effect on the system for (Cp/g)β > 1.

Anisotropic ferromagnet with biquadratic exchange

A formulation, using the double time Green’s function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of Ising part of the biquadratic exchange and an expression is given for the critical temperatureTc in terms of two parameters. One is associated with the uniaxial anisotropy (treated exactly) and the other one corresponds to the biquadratic interaction (treated in RPA). The bilinear Heisenberg exchange term is treated in Callen’s approximation emphasizing the role of intersite correlation for small parameters. The prediction thatTc increases with the biquadratic exchange is found to be consistent with the recent calculation based on high temperature series expansion.

Critical temperature of an anisotropic ferromagnet in the presence of biquadratic exchange

Abstract A formulation, using the double time Greens function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of biquadratic exchange and an expression is given for the critical temperature T c in terms of parameters associated with the uniaxial anisotropy (treated exactly) and the biquadratic interaction (only the longitudinal part, treated in RPA). The prediction that T c increases with the biquadratic exchange is found to be consistent with the recent calculation.

Composite Anderson-Newns model and density of states due to chemisorption: Quasi-chemical approximation

In this paper a variation in density of states (DOS) of the substrate due to chemisorption of hydrogen on transition metals using composite Anderson-Newns model has been investigated for different coverages in quasi-chemical approximation of Fowler and Guggenhiem, which in the limitz→∞ gives the Bragg-Williams approximation as a special case. Variation in density of states has been studied for one-dimensional periodic substrate with change in adatom interaction energy and coverage. With increase in coverage, the bonding and antibonding (B-AB) peaks are found to shift towards higher energies and at the same time relative height of the peaks also increases. The interesting feature to observe is that both approximations for a particular coverage, give split-off states with different height for both (B-AB) peaks. It particularly indicates change in B-AB states, representing amount of chemisorption, with the change in interaction energy between adatoms. At the same time bond strength is also found to decrease with interaction between adatoms.

Composite Anderson-Newns model and chemisorption characteristics of nickel-hydrogen system

The composite Anderson-Newns model and triangular weighted density of states are used to study the electron density of states, magnetic moment and charge transfer of adatoms for a chemisorbed system. The model has been applied to hydrogen chemisorbed on nickel. It has been observed that with the increase in coverage, number of B-AB states as well as bond strength increases, whereas the magnetic moment and adatom charge decrease with coverage. These results match with the experimental data.