Ab initio prediction of magnetically dead layers in freestanding γ\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.Comment: 6 pages, 3 figures, 1 tabl

Electronic structure and electric-field gradients analysis in CeIn3CeIn_3

Electric field gradients (EFG's) were calculated for the $CeIn_3$ compound at both $^{115}In$ and $^{140}Ce$ sites. The calculations were performed within the density functional theory (DFT) using the augmented plane waves plus local orbital (APW+lo) method employing the so-called LDA+U scheme. The $CeIn_3$ compound were treated as nonmagnetic, ferromagnetic, and antiferromagnetic cases. Our result shows that the calculated EFG's are dominated at the $^{140}Ce$ site by the Ce-4f states. An approximately linear relation is intuited between the main component of the EFG's and total density of states (DOS) at Fermi level. The EFG's from our LDA+U calculations are in better agreement with experiment than previous EFG results, where appropriate correlations had not been taken into account among 4f-electrons. Our result indicates that correlations among 4f-electrons play an important role in this compound and must be taken into account

Tunable relativistic quasiparticle electronic and excitonic behavior of the FAPb(I1−xBrx)3alloy

We study the structural, electronic, and excitonic properties of mixed FAPb(I1−xBrx)30 ≤x≤ 1 alloys by first-principles density functional theory as well as quasiparticle GW and Bethe Salpeter equation (BSE) approaches with the inclusion of relativistic effects through spin orbit coupling. Our results show that the system volume decreases with increasing Br content. The quasiparticle band gaps vary from 1.47 eV for pure α-FAPbI3to 2.20 eV for Br-rich α-FAPbBr3and show stronger correlation with the structural changes. The optical property analysis reveals that the overall excitonic peaks are blue shifted with the Br fraction. Our results further reveal strong Br concentration dependence of the variation in the exciton binding energy (from 74 to 112 meV) and the carrier effective masses as well as the high frequency dielectric constants. These findings provide a way to tune the carrier transport properties of the material by doping, which could be utilized to improve the short circuit currents and power conversion efficiencies in multijunction solar cell devices

Ab inito calculations of Hubbard parameters for NiO and Gd crystals

In this research the Hubbard parameters have been calculated for NiO and Gd crystals, as two strongly correlated systems with partially full 3d and 4f levels, respectively. The calculations were performed within the density functional theory (DFT) using the augmented plane waves plus the local orbitat (APW+lo) method. We constructed a suitable supercell and found that the Hubbard parameters for the NiO and Gd compounds are equal to 5.9 eV and 5.7 eV, respectively. Our results are in good agreement with experimental data and results of other computational methods. Then we used the obtained parameters to study the structural properties of NiO and Gd by means of LDA+U approximation. Our results calculated by the LDA+U method which are in better agreement with the experiment show a significant improvement compared to the GGA approximation. The result shows that our method for calculating U parameter can be considered as a satisfactory method to study a strongly correlated system

C atom endohedral doping effect on the bond lengths in the crystal structure of fcc-C60

Single and double equilibrium bond lengths of the fcc-C60 crystal were calculated in the absence and presence of the endohedral C atom as an impurity doped into each C60 cluster, i.e., fcc-C@C60, by means of fully-relaxed self-consistent calculations within the density functtional theory (DFT) employing the full potential-augmented plane waves plus local orbital (FP-APW+lo) method. The result shows that the single and double bond lengths were decreased for the doped case of fcc-C@C60 when compared with the pure fcc-C60. The reduction in the bond lengths by the carbon impurity doping is attributed to the bond alternation effect and reduction of the symmetry in the C60 molecule. The result shows that the impurity injection gives rise to change in the electron charge distribution and as a result to change in electronic properties

Electronic properties of antiferromagnetic UBi2 metal by exact exchange for correlated electrons method

This study investigated the electronic properties of antiferromagnetic UBi2 metal by using ab initio calculations based on the density functional theory (DFT), employing the augmented plane waves plus local orbital method. We used the exact exchange for correlated electrons (EECE) method to calculate the exchange-correlation energy under a variety of hybrid functionals. Electric field gradients (EFGs) at the uranium site in UBi2 compound were calculated and compared with the experiment. The EFGs were predicted experimentally at the U site to be very small in this compound. The EFG calculated by the EECE functional are in agreement with the experiment. The densities of states (DOSs) show that 5f U orbital is hybrided with the other orbitals. The plotted Fermi surfaces show that there are two kinds of charges on Fermi surface of this compound