Raza A. Tahir-Kheli

Antiferromagnetic-spin-flop critical field in MnxZn1-xF2

The antiferromagnetic resonance in MnxZn1-xF2, measured recently by Wiltshire (1977), gives information about the antiferromagnetic-spin-flop critical field Hc(x). The authors calculate Hc(x) on the basis of known exchange and anisotropy parameters of the MnF2 system. The concentration dependence of the anisotropy is determined through a separation between short-range and long-range parts of the dipolar forces while the Mn-Mn exchange interactions are assumed to be unaffected by the zinc dilution. The theoretical treatment is based on an extension of the Izyumov type of perturbation theory (1966) to non-magnetic impurity problems. Coherent potential approximation (CPA) analyses are also carried out. Results are in reasonable agreement with experiment.

Simultaneous occurrence of magnetic and spatial long-range order in binary alloys

The authors study an ordering binary alloy with magnetic and spatial interactions. Solutions for a two sub-lattice model are obtained within a combination of Bragg-Williams and the molecular-field approximations. The presence of magnetic interactions can have far-reaching effects on the spatial ordering characteristics of the alloy. In particular, the spatial long-range order (LRO) parameter can be substantially renormalized and the location of the order-disorder transition temperature can be appreciably shifted. Similarly, the presence of spatial LRO, in general, causes the occurrence of more than one type of magnetization for each magnetic species. Moreover, it can also seriously renormalize the temperature dependence of the magnetizations as well as shift the location of the magnetic transition temperature.

Direct and exchange‐correlation carrier interaction effects in a resonant tunnel diode

Results are presented of a density functional calculation of the effects due to both electron‐electron direct and exchange‐correlation interactions on the effective, single‐electron potential, the conduction electron density profile, the device transmission coefficient, and the current‐voltage characteristics of a GaAs‐AlGaAs quantum‐well tunnel diode. The calculation proceeds by self‐consistently solving the Kohn–Sham and Poisson equations, accounting for the strong, electric‐field‐induced nonequilibrium effects by employing a spatially varying chemical potential which is essential for maintaining net charge neutrality. The exchange potential used is the Dirac local‐density approximation and the correlation potential is determined from the Wigner interpolation formula. The results show a number of interesting effects: it is found that the direct, electrostatic repulsion due to the buildup of charge within the spacer layers extends far beyond the device, well into the contact layers. Furthermore, the depr...

Zero-frequency behavior of thermodynamic green's functions☆

Abstract In thermodynamic systems (as contrasted with zero-temperature quantum systems) all correlation functions must decouple in the long-time limit: 〈 A ( t ) B (0)〉 → 〈 A 〉 〈 B 〉 as t → ∞. We propose this property to resolve the zero-frequency problem discussed by Stevens and Toombs and by Fernandez and Gersch.

CPA theory for jump diffusion of particles in periodic lattices

The incoherent dynamical structure factor and the diffusion coefficient of the nearest neighbour (NN) jump diffusion model is studied. Diffusion of a specific particle in a static but random arrangement of background particles is considered. The virtual crystal approximation and coherent potential approximation procedures are used to find solutions. First five exact frequency moments of the spectral density are calculated and compared with those given by these theories. In simple cubic and body-centred cubic lattices four first moments are exactly reproduced by the CPA solution.

Flop-para critical field in diluted antiferromagnet

Abstract The behaviour of the critical field, H c , separating the spin-flop and the paramagnetic phases of a randomly diluted, anisotropic antiferromagnet is analyzed within a simple CPA scheme. The rate of decrease of H c with dilution is found to be lower than that predicted by virtual crystal approximation (VCA).

Plaquette percolation on frustrated lattices in two dimensions

Abstract We study the ±J quenched-random-bond Ising model in two dimensions. Critical concentrations for percolation of frustrated and normal plaquettes are calculated for the triangular and square lattices by computer simulation. Connection with T = 0 statistical properties is discussed.

Amorphous Ferro- and Antiferromagnetism Within an RPA-CPA Theory

We consider the system to be governed by a Heisenberg spin Hamiltonian with vanishing external field, i.e. u→0

Excitations in randomly diluted ferromagnet

H = A\sum\limits_{f,p} {J(f,p)} {S_f}.{S_p} - \mu \sum\limits_I {S_f^z}