S. Auluck

Ab initio calculation of molecular field interactions in rare‐earth transition‐metal intermetallics (invited)

The interaction, KRM, between the rare‐earth 4f moment and the transition‐metal 3d moments in rare‐earth transition‐metal intermetallics is shown to depend upon the R‐5d moment, which is due to 3d–5d hybridization, and local 4f–5d exchange integrals. Both the R‐5d moment and KRM may be calculated ab initio from the local spin‐density approximation to density functional theory in self‐consistent energy‐band calculations with the localized 4f‐moments fixed at their Russel–Saunders values. Detailed examples are given for the RFe2 (R=Gd−Yb) series. The exchange integrals are similar to those entering into the density functional version of Stoner theory and their energy dependence must be treated carefully. The calculated local exchange integrals are shown to be related to the molecular fields derived from spin Hamiltonians, hence to the spin‐wave spectra. Reasonable agreement with values of the molecular fields extracted from inelastic neutron scattering and high field susceptibility measurements is obtained.

Optical properties of A1N

Abstract We have performed calculations of the anisotropic frequency dependent dielectric function of AlN, using the linear muffin-tin orbital method within the atomic sphere approximation (LMTO-ASA). Our calculations show that the anisotropy is very small. The effect of introducing empty spheres is to decrease the energy gap by about 20% and to make the dielectric function slightly anisotropic at higher energies. Comparisons with experiments must await experimental results.

Effect of site-disorder on magnetism and magneto-structural coupling in gallium ferrite: A first-principles study

We report a first-principles study of the magnetic properties, cation site disorder effect on magnetism and magneto-structural coupling in multiferroic gallium ferrite (GFO) using the local spin density approximation (LSDA+U) of the density functional theory. The calculation of the ground state A-type antiferromagnetic structure predicts magnetic moments consistent with the experiments while consideration of spin-orbit coupling yields a net orbital moment also in good accordance with the experiment. We find that though cation site disorder is not spontaneous in the ground state, interchange between octahedrally coordinated Fe2 and Ga2 sites is most favored in the disordered state. The results show that ferrimagnetism in GFO is primarily due to Ga-Fe site disorder such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1 sites as well as between Fe spins at Ga2 and Fe2 sites. Thus we are able to explain the origin of f...We report a first-principles study of the magnetic properties, site disorder and magneto-structural coupling in multiferroic gallium ferrite (GFO) using local spin density approximation (LSDA+U) of density functional theory. The calculations of the ground state A-type antiferromagnetic structure predict magnetic moments consistent with the experiments whilst consideration of spin-orbit coupling yields a net orbital moment of ~ 0.025 Bohr magneton/Fe site also in good accordance with the experiments. We find that though site disorder is not spontaneous in the ground state, interchange between Fe2 and Ga2 sites is most favored in the disordered state. The results show that ferrimagnetism in GFO is due to Ga-Fe site disordering such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1 sites as well as between Fe spins at Ga2 and Fe2 sites. The effect of spin configuration on the structural distortion clearly indicates presence of magneto-structural coupling in GFO.

Band structure calculations of heavy fermion YbSbPd and YbSbNi

Dhar et al. [J. Phys. F 18, L41 (1988); and Proceedings of the International Conference on Strongly Correlated Electron Systems Sendai, Japan, 1992 (Plenum, New York, 1987)] have recently measured the low‐temperature specific heat of YbSbPd and YbSbNi. Their measurements yield large values for the specific heat coefficient (1384 states/Ry cell for YbSbPd and 865 states/Ry cell for YbSbNi), and suggest a magnetic transition at low temperature. With a view to understand the ground state of these compounds, we have performed self‐consistent scalar relativistic linear muffin‐tin orbital band structure calculations. Our paramagnetic calculations for YbSbPd and YbSbNi give the density of the states at Fermi level to be 82.94 and 27.96 states/Ry cell, respectively. These give enhancement factors of 16.7 and 30.1 for YbSbPd and YbSbNi. We present results of our calculations of the band structure as well as the Stoner I parameter for these compounds.

Engineering polarization rotation in ferroelectric bismuth titanate

Here, we report a combined experimental-theoretical study showing that collective application of rare earth doping on A-site and epitaxial strain to ferroelectric bismuth titanate does not lead to a very large c-axis polarization as reported previously. Further first principles calculations based on the examination of polarization tensor suggest that simultaneous Bi and Ti site doping could result in moderate polarization along c-axis of bismuth titanate which is typically a preferential axis of film growth and thus enabling c axis oriented films to have appreciable polarization. This approach could also be applicable to other ferroic oxides where one can correlate the doping, epitaxial strain, and polarization to design materials compositions resulting in epitaxial films grown along desired directions yielding substantial polarization.

Density functional theory of molecular fields in R-M systems

Abstract In the local spin density approximation to the density functional theory the interaction, K RM , between the rare-earth 4f moment and local 4f-5d exchange integrals. Detailed examples are given for the RFe 2 (R=Gd-Yb) series. The calculated local exchange integrals are shown to be energy dependent and are then related to the molecular fields.

Calculated electronic and optical properties of a graphite intercalation compound

We have used the linear muffin-tin orbital method, without geometrical approximations, to calculate the electronic structure of . Using our self-consistent solution, we have calculated the anisotropic frequency-dependent dielectric function and the reflectivity spectrum. The calculated reflectivity spectrum is in good agreement with the experimental data.

Theory of the Curie temperatures of the rare earth metals

Abstract The curie temperatures of the rare earth metals have been calculated ab initio using the local pin density approximation. The exchange splitting of the 5d-states depends upon local 4f-5d exchange integrals, κ 415d , which are calculated ab initio in the local spin density approximation. The Curie temperatures may then be obtained without use of adjustable parameters but are calculated to be too high by a factors of two to three if normal itinerant electron mean field theory is used. We have found that it is necessary allow for disordered local 5d moments above T c and to calculate the local susceptibility. When the local fluctuation contribution to the Landau Parameter, A , is included the Curie temperatures are actually reduced to below those measured.

Electronic and optical properties of free-standing and supported vanadium nanowires

We have investigated theoretically the electronic and optical properties of free-standing and substrate-supported ultrathin nanowires (NWs) of the transition metal vanadium. Ground state of the structures studied, except free-standing zigzag geometry, is found to be magnetic in nature. We show that for some structures, study of the antiferromagnetic state necessitates considering various possible configurations. All the structures, except dimerized, show metallic behavior. Structure with helical geometry possesses decent value of magnetic moment and is exceptionally stable as well as most stiff of all the structures studied. The plasma frequency and dielectric function nicely exhibit the anisotropy due to one-dimensional nature of the nanowires. The latter is structure-dependent and markedly different from that of bulk. More realistic case of linear chains supported on a substrate shows fair impact of the substrate in comparison with free-standing case. There is substantial charge redistribution on relaxi...

Electronic and optical properties of InP

Abstract We have calculated the band structure and dielectric function of InP by means of an accurate first-principles method using the full potential linear muffin-tin orbital method (FPLMTO). Our calculated dielectric functions is compared with the recent data of Herzinger et al. [ J. Appl. Phys. , 77 , 1995, 1715]. The calculated dielectric function is in good agreement with the recent data. The calculated band gap is smaller by about 0.7 eV in comparison with experimental gap.