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[en] The structural and electronic properties of the monolayer and bilayer stanene structures have been studied using first-principles calculations. For the monolayer, the buckled structure is more stable than the flat one, with an opening of the band gap when spin-orbit coupling is taken into account, as mentioned in recent studies. For the bilayer, three types of stacking are considered: parallel layers, anti-parallel layers, and parallel layers where the first layer is shifted from the second one. These three configurations are named AA1, AA2, and AB, respectively. The two layers are separated by the distance d. The interactions between two layers of stanene are strong for a short distance, while the van der Waals bonding appears for a longer distance. Furthermore, stanene was fabricated experimentally on a substrate; thus, we proposed another study of electronic properties of stanene deposited on Ge(111) to reveal other behavior as a topological insulator and show the existence of the quantum spin Hall effect.
[en] Full Potential Linearized Augmented Plane Wave (FP-LAPW) based on density functional theory (DFT) is carried out to study the density of states, optical properties and XMCD properties of bulk spinel ferrite CdFe_2O_4. The exchange correlation potential was solved by Generalized Gradient Approximation (GGA) that underestimates the band gap value, therefore, the later needed to be corrected using two approaches: Tran–Blaha modified Becke–Johnson exchange potential approximation (TB-mBJ) and GGA plus the multi-orbital mean-field Hubbard potential (GGA+U). These methods provide accurate results in good agreement for the optical properties and band gap with experimental data. CdFe_2O_4 has a coefficient of absorption more than 10"4/cm in the visible range. The first critical point known as optical absorption edge calculated with GGA+U and TB-mBJ appears at 1.56 eV and 1.88 eV, respectively. The XMCD spectra show a double pic in L_3 edge indicating the presence of both Fe"2"+ and Fe"3"+ ions, occupying the octahedral sites. However, using GGA+U the charge of the atoms is well localized and subsequently the double peak does not reproduce. The crystal field creates additional levels that provide radiation in the visible, having these properties; this compound promotes its use for device applications operating over a wide energy range based on CdFe_2O_4 in optoelectronics. - Highlights: • TB-mBJ approximation improve the band gap of CdFe_2O_4. • TB-mBJ approximation provide accurate optical properties of CdFe_2O_4. • GGA+U approximation provide accurate optical properties of CdFe_2O_4
[en] The electronic structure, magnetic, and optical properties in cubic crystalline phase of Zr1−xTMxO2 (TM = V, Mn, Fe, and Co) at x = 6.25% are studied using density functional theory with the Generalized Gradient Approximation and the modified Becke-Johnson of the exchange-correlation energy and potential. In our calculations, the zirconia is a p-type semiconductor and has a large band gap. We evaluated the possibility of long-range magnetic order for transition metal ions substituting Zr. Our results show that ferromagnetism is the ground state in V, Mn, and Fe-doped ZrO2 and have a high value of energy in Mn-doped ZrO2. However, in Co-doped ZrO2, antiferromagnetic ordering is more stable than the ferromagnetic one. The exchange interaction mechanism has been discussed to explain the responsible of this stability. Moreover, it has been found that the V, Mn, and Fe transition metals provide half-metallic properties considered to be the leading cause, responsible for ferromagnetism. Furthermore, the optical absorption spectra in the TM -doped cubic ZrO2 are investigated
[en] Recently, a reversible and a giant rotating magnetocaloric effect has been pointed out in the multiferroic TbMnO single crystal, opening the way for new designs of low-temperature magnetic cooling. In this paper, we report a preliminary theoretical work with the aim of enlarging our understanding on the electronic, magnetic and accordingly magnetocaloric features of the TbMnO compound. Particularly, the TbMnO magnetic anisotropy is analyzed in terms of X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) spectra.
[en] Highlights: • Experimental and theoretical investigation of Nd doped ZnO. • Oxygen vacancies play a critical role in the activation of ferromagnetism in Nd doped ZnO. • Nd positions play a critical role in the activation of ferromagnetism in Nd doped ZnO. - Abstract: Nd-doped ZnO (ZnO:Nd), have been synthesized by coprecipitation method, x-ray diffraction (XRD), and Magnetic Properties Measurement System (MPMS) techniques were used to characterize crystal structure and magnetic properties. Wurtzite structure of single phase is obtained according to XRD results. Magnetic measurement indicates that the sample is paramagnetic at room temperature. However, a large increase in the magnetization is observed below 50 K. By density functional calculations, we have investigated the electronic structures and magnetic properties of Nd doped ZnO with and without oxygen vacancies (VO) and for different Nd positions. The results suggest that the VO and disorder effect of Nd positions play a critical role in the activation of ferromagnetism in Nd doped ZnO.