Electronic structure of copper intercalated transition metal dichalcogenides: First-principles calculations

We report first principles calculations, within density functional theory, of copper intercalated titanium diselenides, CuxTiSe2, for values of x ranging from 0 to 0.11. The effect of intercalation on the energy bands and densities of states of the host material is studied in order to better understand the cause of the superconductivity that was recently observed in these structures. We find that charge transfer from the copper atoms to the metal dichalcogenide host layers causes a gradual reduction in the number of holes in the otherwise semi-metallic pristine TiSe2, thus suppressing the charge density wave transition at low temperatures, and a corresponding increase in the density of states at the Fermi level. These effects are probably what drive the superconducting transition in the intercalated systems.Comment: 8 pages, 6 figure

Theoretical investigation of magnetic order in ReOFeAs, Re = Ce, Pr

Density functional theory (DFT) calculations are carried out on ReOFeAs, Re = Ce, Pr, the parent compounds of the high-T$_c$ superconductors ReO$_{1-x}$F$_{x}$FeAs, in order to determine the magnetic order of the ground state. It is found that the magnetic moments on the Fe sites adopt a collinear antiferromagnetic order, similar to the case of LaOFeAs. Within the generalized gradient approximation along with Coulomb onsite repulsion (GGA+U), we show that the Re magnetic moments also adopt an antiferromagnetic order for which, within the ReO layer, same spin Re sites lie along a zigzag line perpendicular to the Fe spin stripes. While within GGA the Re 4f band crosses the Fermi level, upon inclusion of onsite Coulomb interaction the 4f band splits and moves away from the Fermi level, making ReOFeAs a Mott insulator.Comment: 5 pages, 4 figure

Effect of hybridization on structural and magnetic properties of iron-based superconductors

We show that the strong hybridization between the iron 3d and the arsenic 4p orbitals, in the newly discovered iron-based high-T$_{c}$ superconductors, leads to an explanation of certain experimental observations that are presently not well understood. The existence of a lattice distortion, the smallness of the Fe magnetic moment in the undoped systems, and the suppression of both the lattice distortion and the magnetic order upon doping with fluorine, are all shown to result from this hybridization.Comment: 4 pages, 2 figure

Temperature-dependent striped antiferromagnetism of LaFeAsO in a Green's function approach

We use a Green's function method to study the temperature-dependent average moment and magnetic phase-transition temperature of the striped antiferromagnetism of LaFeAsO, and other similar compounds, as the parents of FeAs-based superconductors. We consider the nearest and the next-nearest couplings in the FeAs layer, and the nearest coupling for inter-layer spin interaction. The dependence of the transition temperature TN and the zero-temperature average spin on the interaction constants is investigated. We obtain an analytical expression for TN and determine our temperature-dependent average spin from zero temperature to TN in terms of unified self-consistent equations. For LaFeAsO, we obtain a reasonable estimation of the coupling interactions with the experimental transition temperature TN = 138 K. Our results also show that a non-zero antiferromagnetic (AFM) inter-layer coupling is essential for the existence of a non-zero TN, and the many-body AFM fluctuations reduce substantially the low-temperature magnetic moment per Fe towards the experimental value. Our Green's function approach can be used for other FeAs-based parent compounds and these results should be useful to understand the physical properties of FeAs-based superconductors.Comment: 12 page

The electronic structure of LiFeAs and NaFeAs probed by resonant inelastic x-ray scattering spectra

Results of resonant inelastic X-ray scattering (RIXS) measurements at Fe L-edges and electronic structure calculations of LiFeAs and NaFeAs are presented. Both experiment and theory show that in the vicinity of the Fermi energy, the density of states is dominated by contributions from Fe 3d-states. The comparison of Fe L2,3 non-resonant and resonant (excited at L2-threshold) X-ray emission spectra with spectra of LaOFeAs and CaFe2As2 show a great similarity in energy and I(L2)/I(L3) intensity ratio. The I(L2)/I(L3) intensity ratio of all FeAs-based superconductors is found to be more similar to metallic Fe than to correlated FeO. Basing on these measurements we conclude that iron-based superconductors are weakly or moderately correlated systems.Comment: 11 pages, 6 figure

Theoretical Investigation of Two-Dimensional Superconductivity in Intercalated Graphene Layers

First-principles calculations of the electronic structure and vibrational modes, in a system of graphene bilayers and trilayers intercalated with alkaline earth atoms, are presented. It is found that, in similarity to the case of superconducting graphite intercalation compounds, the Fermi level is crossed by an s-band derived from the intercalant states, as well as graphitic π-bands. The electron-phonon coupling parameter λ is found to be 0.60 and 0.80, respectively, in calcium intercalated graphene bilayers and trilayers. In superconducting CaC6 graphite intercalation compound, the calculated value for λ is 0.83. It is concluded that two-dimensional superconductivity is possible in a system of a few graphene layers intercalated with calcium