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Register a new userMultiband tight-binding model of local magnetism in GaMnAs
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We present the spin and orbitally resolved local density of states (LDOS) for a single Mn impurity and for two nearby Mn impurities in GaAs. The GaAs host is described by a sp^3 tight-binding Hamiltonian, and the Mn impurity is described by a local p-d hybridization and on-site potential. Local spin-polarized resonances within the valence bands significantly enhance the LDOS near the band edge. For two nearby parallel Mn moments the acceptor states hybridize and split in energy. Thus scanning tunneling spectroscopy can directly measure the Mn-Mn interaction as a function of distance.
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We consider the electronic properties of ferromagnetic bulk GaMnAs at zero temperature using two realistic tight-binding models, one due to Tang and Flatte and one due to Masek. In particular, we study the density of states, the Fermi energy, the inverse participation ratio, and the optical conductivity with varying impurity concentration x=0.01-0.15. The results are very sensitive to the assumptions made for the on-site and hopping matrix elements of the Mn impurities. For low concentrations, x<0.02, Maseks model shows only small deviations from the case of p-doped GaAs with increased number of holes while within Tang and Flattes model an impurity-band forms. For higher concentrations x, Maseks model shows minor quantitative changes in the properties we studied while the results of the Tang and Flatte model exhibit qualitative changes including strong localization of eigenstates with energies close to the band edge. These differences between the two approaches are in particular visible in the optical conductivity, where Maseks model shows a Drude peak at zero frequency while no such peak is observed in Tang and Flattes model. Interestingly, although the two models differ qualitatively the calculated effective optical masses of both models are similar within the range of 0.4-1.0 of the free electron mass.
A method for incorporating electromagnetic fields into empirical tight-binding theory is derived from the principle of local gauge symmetry. Gauge invariance is shown to be incompatible with empirical tight-binding theory unless a representation exists in which the coordinate operator is diagonal. The present approach takes this basis as fundamental and uses group theory to construct symmetrized linear combinations of discrete coordinate eigenkets. This produces orthogonal atomic-like orbitals that may be used as a tight-binding basis. The coordinate matrix in the latter basis includes intra-atomic matrix elements between different orbitals on the same atom. Lattice gauge theory is then used to define discrete electromagnetic fields and their interaction with electrons. Local gauge symmetry is shown to impose strong restrictions limiting the range of the Hamiltonian in the coordinate basis. The theory is applied to the semiconductors Ge and Si, for which it is shown that a basis of 15 orbitals per atom provides a satisfactory description of the valence bands and the lowest conduction bands. Calculations of the dielectric function demonstrate that this model yields an accurate joint density of states, but underestimates the oscillator strength by about 20% in comparison to a nonlocal empirical pseudopotential calculation.
We consider the mapping of tight-binding electronic structure theory to a local spin Hamiltonian, based on the adiabatic approximation for spin degrees of freedom in itinerant-electron systems. Local spin Hamiltonians are introduced in order to describe the energy landscape of small magnetic fluctuations, locally around a given spin configuration. They are designed for linear response near a given magnetic state and in general insufficient to capture arbitrarily strong deviations of spin configurations from the equilibrium. In order to achieve this mapping, we include a linear term in the local spin Hamiltonian that, together with the usual bilinear exchange tensor, produces an improved accuracy of effective magnetic Weiss fields for non-collinear states. We also provide examples from tight-binding electronic structure theory, where our implementation of the calculation of exchange constants is based on constraining fields that stabilize an out-of-equilibrium spin configuration. We check our formalism by means of numerical calculations for iron dimers and chains.
We extend a tight-binding method to include the effects of spin-orbit coupling, and apply it to the study of the electronic properties of the actinide elements Th, U, and Pu. These tight-binding parameters are determined for the fcc crystal structure using the equivalent equilibrium volumes. In terms of the single particle energies and the electronic density of states, the overall quality of the tight-binding representation is excellent and of the same quality as without spin-orbit coupling. The values of the optimized tight-binding spin-orbit coupling parameters are comparable to those determined from purely atomic calculations.
A dynamics of the precession of coupled atomic moments in the tight-binding (TB) approximation is presented. By implementing an angular penalty functional in the energy that captures the magnetic effective fields self-consistently, the motion of the orientation of the local magnetic moments is observed faster than the variation of their magnitudes. This allows the computation of the effective atomic magnetic fields that are found consistent with the Heisenbergs exchange interaction, by comparison with classical atomistic spin dynamics on Fe, Co and Ni magnetic clusters.
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