Theoretical study of $alpha$-U/W(110) thin films from density functional theory calculations: Structural, magnetic and electronic properties

Structural, electronic and magnetic properties were calculated for the optimized $alpha$-U/W(110) thin films (TFs) within the density functional theory. Our optimization for 1U/7W(110) shows that the U-W vertical interlayer spacing (ILS) is expanded by 14.0% compared to our calculated bulk W-W ILS. Spin and orbital magnetic moments (MMs) per U atom were found to be enhanced from zero for the bulk of $alpha$-U to 1.4 $mu_B$ and -0.4 $mu_B$ at the interface of the 1U/7W(110), respectively. Inversely, our result for 3U/7W(110) TFs shows that the surface U-U ILS is contracted by 15.7% compared to our obtained bulk U-U spacing. The enhanced spin and orbital MMs in the 1U/7W(110) were then found to be suppressed in 3U/7W(110) to their ignorable bulk values. The calculated density of states (DOS) corroborates the enhancement and suppression of the MMs and shows that the total DOS, in agreement with experiment, is dominated in the vicinity of Fermi level by the 5f U states. Proximity and mismatch effects of the nonmagnetic W(110) substrate were assessed and found to be important for this system.

Ab initio prediction of magnetically dead layers in freestanding $gamma$-Ce(111)

It is well known that the surface of nonmagnetic $alpha$-Ce is magnetically ordered, i.e., $gamma$-like. One then might conjecture, in agreement with previous theoretical predictions, that the $gamma$-Ce may also exhibit at its surfaces even more strongly enhanced $gamma$-like magnetic ordering. Nonetheless, our result shows that the (111)-surfaces of magnetic $gamma$-Ce are neither spin nor orbitally polarized, i.e., $alpha$-like. Therefore, we predict, in contrast to the nonmagnetic $alpha$-phase which tends to produce magnetically ordered $gamma$-like thin layers at its free surfaces, the magnetic $gamma$-phase has a tendency to form $alpha$-like dead layers. This study, which explains the suppressed (promoted) surface magnetic moments of $gamma$-Ce ($alpha$-Ce), shows that how nanoscale can reverse physical properties by going from bulk to the surface in isostructural $alpha$- and $gamma$-phases of cerium. We predict using our freestanding surface results that a typical unreactive and non-diffusive substrate can dramatically influence the magnetic surface of cerium thin films in contrast to most of the uncorrelated thin films and strongly correlated transition metals. Our result implies that magnetic surface moments of $alpha$-Ce(111) can be suddenly disappeared by increasing lattice mismatch at the interface of a typical unreactive and non-diffusive substrate with cerium overlayers.

Quantum Size Effects in Pb/Si(111) Thin Films from Density Functional Calculations

The Pb/Si(111) thin films were simulated within the density functional theory (DFT). The well-known Perdew-Burke-Ernzerhof (PBE) version of the generalized gradient approximation (GGA) and its recent nonempirical successor Wu-Cohen (WC) issue were used to estimate the exchange-correlation functional. Lattice parameters were calculated for Bulk of the Pb and Si compounds to obtain more reliable lattice mismatch at the interface to be consistent with our used full-potential method of calculations. The WC-GGA result predicts the lattice constants of the Pb and Si compounds better than the GGA when compared with experiment. We have found that the spin-orbit coupling (SOC) does not significantly influence the results. Our finding is in agreement with the recent observation of the Rashba-type spin-orbit splitting of quantum well states in ultrathin Pb/Si(111) films. Our result shows, in agreement with experiment, that the top site (T1) is the most stable phase. A combination of tight $sigma$ and feeble $pi$ bonds has been found at the interface, which results in a covalent Pb-Si bond. Our calculated electric field gradient (EFG) predicts quantum size effects (QSE) with respect to the number of deposited Pb layers on the Si substrate. The QSE prediction shows that the EFG dramatically drops on going from first to second layer. The EFG calculation shows that this system is not an ideal paradigm to freestanding films.