Showing papers by "Tanusri Saha-Dasgupta published in 1999"

Electronic structure and exchange interactions of the ladder vanadates CaV2O5 and MgV2O5

Abstract: We have performed ab-initio calculations of the electronic structure and exchange couplings in the layered vanadates CaV2O5 and MgV2O5. Based on our results we provide a possible explanation of the unusual magnetic properties of these materials, in particular the large difference in the spin gap between CaV2O5 and MgV2O5.

Developing the MTO Formalism

Abstract: The TB-LMTO-ASA method is reviewed and generalized to an accurate and robust TB-NMTO minimal-basis method, which solves Schrodinger's equation to Nth order in the energy expansion for an overlapping MT-potential, and which may include any degree of downfolding. For N = 1, the simple TB-LMTO-ASA formalism is preserved.For a discrete energy mesh, the NMTO basis set may be given as: /(N) (r) = gs n ϕ (∋n, r)L(N) n in terms of kinked partial waves, ϕ (∋, r), evaluated on the mesh, ∋0,..., ∋N. This basis solves Schrodinger's equation for the MT-potential to within an error α (∋. ∋0)... (∋. ∋N). The Lagrange matrix-coeficients, L(N)n, as well as the Hamiltonian and overlap matrices for the NMTO set, have simple expressions in terms of energy derivatives on the mesh of the Green matrix, defined as the inverse of the screened KKR matrix.The variationally determined single-electron energies have errors α (∋. ∋0)2... (∋. ∋N)2. A method for obtaining orthonormal NMTO sets is given and several applications are presented.

Electrical and magnetic properties of AuFe alloys

Abstract: We study the electronic and magnetic structure of AuFe alloys for varying Fe concentrations. The basis of our study is the augmented-space recursion in conjunction with the local spin-density approximation. We study magnetism from an itinerant-electron viewpoint and show that at low Fe concentrations the random phase arrangement is the more stable phase at T = 0 K as compared to the ferromagnetic, antiferromagnetic and paramagnetic arrangements. At higher concentrations the ferromagnetic arrangement becomes the most stable but the average moment decreases with increasing Au concentration.

Developing the MTO Formalism

Abstract: We review the simple linear muffin-tin orbital method in the atomic-spheres approximation and a tight-binding representation (TB-LMTO-ASA method), and show how it can be generalized to an accurate and robust Nth order muffin-tin orbital (NMTO) method without increasing the size of the basis set and without complicating the formalism. On the contrary, downfolding is now more efficient and the formalism is simpler and closer to that of screened multiple-scattering theory. The NMTO method allows one to solve the single-electron Schroedinger equation for a MT-potential -in which the MT-wells may overlap- using basis sets which are arbitrarily minimal. The substantial increase in accuracy over the LMTO-ASA method is achieved by substitution of the energy-dependent partial waves by so-called kinked partial waves, which have tails attached to them, and by using these kinked partial waves at N+1 arbitrary energies to construct the set of NMTOs. For N=1 and the two energies chosen infinitesimally close, the NMTOs are simply the 3rd-generation LMTOs. Increasing N, widens the energy window, inside which accurate results are obtained, and increases the range of the orbitals, but it does not increase the size of the basis set and therefore does not change the number of bands obtained. The price for reducing the size of the basis set through downfolding, is a reduction in the number of bands accounted for and -unless N is increased- a narrowing of the energy window inside which these bands are accurate. A method for obtaining orthonormal NMTO sets is given and several applications are presented.