Efficient tight-binding approach for the study of strongly correlated systems

Abstract

In this work, we present results from self-consistent charge density functional based tight-binding ͑DFTB͒ calculational scheme, including local-density approximation +U ͑LDA+U͒ and simplified self-interactioncorrected-like potentials for the simulation of systems with localized strongly correlated electrons. This approach attempts to combine the efficiency of tight binding with the accuracy of more sophisticated ab initio methods and allows treatment of highly correlated electrons for very large systems. This is particularly interesting for the case of rare earths in GaN, where dilute amount of rare earth ions is used. In this work, we show the results of test calculations on bulk ErN and on the substitutional Er Ga in wurtzite GaN, which we choose as representatives of bulk and point defects in solids with strongly correlated electrons. We find that ErN is a half metal in the ferromagnetic phase and that the substitutional Er Ga in wurtzite GaN has C 3v symmetry. These examples show that the DFTB approach reproduces well the results of more demanding calculation schemes with a very low computational cost, making it suitable for the study of extended systems beyond the capabilities of density functional theory.

Magnetic properties of ErN films

“…However, another LSDAþ U calculation found [1] a half-metallic zero-gap in ErN. A half metallic ground state has also been obtained in a spin polarized charge-self-consistent density functional based tight binding (DFTB) approach [16]. Both the electronic structure and magnetic ordering have been investigated theoretically in ErAs and ErN [17].…”

Excited Multiplets of Eu in GaN

“…[6,10,20] have been used with a 4x4x3 wurtzite supercell with 4 3 Monkhorst-Pack k-points [21] and a Fermi distribution temperature of 300 K. Eu is placed on a Ga site, and the geometry and lattice constants fully relaxed using collinear spin-polarized DFTB. This structure is then used for the subsequent calculations.…”

“…In this work the atomic-like LDA+U correction [11] is included, this contribution pushes the on-site electronic states towards being either fully localized on the rare earth or fully empty. In this material system, this term acts to move state which would otherwise be partially filled rare-earth levels from the GaN band gap [10,12]. Following the discussion in Ref.…”

“…The Δq μ terms are fluctuation of the charge and magnetization Mulliken populations from the neutral spin-unpolarized reference state of the system for atomic shell μ. The coupling between these fluctations, W, is on-site in the case of magnetic interactions, but contains both long range coulombic and short range exchangecorrelation contributions for charge fluctuations [4,8,10]. The final term in the total energy is the short ranged pairwise repulsive interaction between the atoms in the system (containing most of the DFT double counting contribution for the reference system).…”

Theoretical study of rare earth point defects in GaN

“…The method has been extended to include LDA+U [7] and simplified self interaction cor-rected (SIC)-like potentials [8,11] for the simulation of systems with localized and strongly correlated electrons. This approach attempts to combine the efficiency of tightbinding with the accuracy of more sophisticated ab initio methods, allowing the treatment of highly correlated electrons for very large systems, like the supercells needed to simulate the dilute concentrations of lanthanide impurities in GaN [9,10].…”