Electronic and magnetic properties of the Ti5_5O9_9 Magn\'eli phase

Structural, electronic and magnetic properties of Ti$_5$O$_9$ have been studied by \textit{ab initio} methods in low-, intermediate- and high-temperature phases. We have found the charge and orbital order in all three phases to be non-stable, and the formation of Ti$^{3+}$-Ti$^{3+}$ bipolaronic states less likely as compared to Ti$_4$O$_7$. Several quasidegenerate magnetic configurations were calculated to have different width of the band gap, suggesting that the reordering of the unpaired spins at Ti$^{3+}$ ions might at least partially be responsible for the changes in conductivity of this material.Comment: 8 pages, 5 figures. Submitted to Phys. Rev.

High-temperature multiferroicity and strong magnetocrystalline anisotropy in 3d-5d double perovskites

Using density functional calculations we explore the properties of as-yet-unsynthesized $3d - 5d$ ordered double perovskites ($A_2BB'$O$_6$) with highly polarizable Bi$^{3+}$ ions on the $A$ site. We find that the Bi$_2$NiReO$_6$ and Bi$_2$MnReO$_6$ compounds are insulating and exhibit a robust net magnetization that persists above room temperature. When the in-plane lattice vectors of the pseudocubic unit cell are constrained to be orthogonal (for example, by coherent heteroepitaxy), the ground states are ferroelectric with large polarization and a very large uniaxial magnetocrystalline anisotropy with easy axis along the ferroelectric polarization direction. Our results suggest a route to multiferroism and electrically controlled magnetization orientation at room temperature

Willingness to communicate

This thesis deals with willingness to communicate (WTC). To be more precise, it deals with how WTC is related to age and level of proficiency. Participants in the present study were native speakers of Croatian. They belonged to three age groups: elementary, high-school students, and English majors. They differed in their levels of proficiency accordingly. The instrument we used for collecting data was a questionnaire. The results of our study showed that language anxiety differed significantly across the three age groups. English majors expressed greater anxiety than high-school students. Another finding was that elementary-school students liked communicating with other peers in class more than high-school students and English majors. The study showed that there was no difference between the groups concerning motivation, which might be explained by the fact that English is nowadays a lingua franca and most of the people are aware that mastering English gives many opportunities. And finally, our findings showed that the higher the mark, the more willing to communicate our participants were

Ferromagnetic spin coupling of 2p-impurities in band insulators stabilized by intersite Coulomb interaction: Nitrogen-doped MgO

For a nitrogen dimer in insulating MgO, a ferromagnetic coupling between spin-polarized $2p$-holes is revealed by calculations based on the density functional theory amended by an on-site Coulomb interaction and corroborated by the Hubbard model. It is shown that the ferromagnetic coupling is facilitated by a T-shaped orbital arrangement of the $2p$-holes, which is in its turn controlled by an intersite Coulomb interaction due to the directionality of the $p$-orbitals. We thus conjecture that this interaction is an important ingredient of ferromagnetism in band insulators with $2p$ dopants.Comment: Accepted for publication in Physical Review Letter

Scanning tunneling microscopy of surfaces of half-metals: an ab-initio study on NiMnSb(001)

We present a first-principles study of the unreconstructed (001) surfaces of the half-metallic ferromagnet NiMnSb. Both terminations (MnSb and Ni) are considered. We find that half-metallicity is lost at the surfaces. After a discussion of the geometric relaxations and the spin-polarized surface band structure, we focus on topography images which are expected to be found with spin-polarized scanning tunneling microscopy. For the MnSb-terminated surface we find that only the Sb atoms are visible, reflecting a geometric buckling caused by relaxations. For the Ni-terminated surface we find a strong contrast between the images of forward and reverse tip-sample-bias of 0.5 eV, as well as a stripe-like image for reverse bias. We interpret these findings in terms of highly directional surface states which are formed in the spin-down gap region.Comment: Submitted to J. Phys. D: Appl. Phy

Engineering quantum anomalous Hall phases with orbital and spin degrees of freedom

Combining tight-binding models and first principles calculations, we investigate the quantum anomalous Hall (QAH) effect induced by intrinsic spin-orbit coupling (SOC) in buckled honeycomb lattice with sp orbitals in an external exchange field. Detailed analysis reveals that nontrivial topological properties can arise utilizing not only spin but also orbital degrees of freedom in the strong SOC limit, when the bands acquire non-zero Chern numbers upon undergoing the so-called orbital purification. As a prototype of a buckled honeycomb lattice with strong SOC we choose the Bi(111) bilayer, analyzing its topological properties in detail. In particular, we show the emergence of several QAH phases upon spin exchange of the Chern numbers as a function of SOC strength and magnitude of the exchange field. Interestingly, we observe that in one of such phases, namely, in the quantum spin Chern insulator phase, the quantized charge and spin Hall conductivities co-exist. We consider the possibility of tuning the SOC strength in Bi bilayer via alloying with isoelectronic Sb, and speculate that exotic properties could be expected in such an alloyed system owing to the competition of the topological properties of its constituents. Finally, we demonstrate that 3d dopants can be used to induce a sizeable exchange field in Bi(111) bilayer, resulting in non-trivial Chern insulator properties

Ferromagnetism in Nitrogen-doped MgO

The magnetic state of Nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be magnetic with a moment of 1 Bohr magneton per Nitrogen atom and to interact ferromagnetically via the double exchange mechanism. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The Curie temperature increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single Nitrogen atoms, also interstitial Nitrogen atoms, clusters of Nitrogen atoms and their structural relaxation on the magnetism are discussed. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase of the Curie temperature is difficult to achieve, requiring a particular attention to the choice of chemistry

Ab initio phonon structure of h-YMnO3 in low-symmetry ferroelectric phase

We present a systematic first-principles study of the phonon spectrum of hexagonal YMnO3 in ferroelectric (multiferroic) phase. We investigated in detail the low-energy phonon modes, their dispersion, symmetry, as well as the infrared optical properties of the crystal and determined the phonon density of states

Thermal collapse of spin-polarization in half-metallic ferromagnets

The temperature dependence of the magnetization and spin-polarization at the Fermi level is investigated for half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab-initio calculations. The thermal properties are studied by mapping ab-initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method

Quality of Service Management in UMTS Network

UMTS (eng. Universal Mobile Telecommunications System) je mobilna mreža treće generacije, dizajnirana da podrži široki spektar aplikacija s različitim profilima kvalitete usluge. 3G sustav ima sposobnost prijenosa multimedijskih usluga i tradicionalnih mobilnih usluga. Kako bi pružao bolju kvalitetu usluge takvim uslugama potrebno je pravilno upravljanje radio resursima. Velika popularnost bežičnih i mobilnih mreža izazvala je u potrošačima zahtjevnije gledište kvalitete usluge. Cilj ovog rada je fokusirati se na UMTS mrežu i analizirati promjenu kvalitete usluge pomoću različitih metoda. Mjerenje kvalitete usluge temelji se na mrežnim parametrima, stoga je potrebno zadovoljiti sve zahtjeve kako bi zadovoljavajuća kvaliteta usluge bila dostignuta.UMTS (eng. Universal Mobile Telecommunications System) is the third generation mobile network, designed to support a wider range of applications with different quality of service profiles. 3G system has capability of transporting multimedia services and traditional mobile services. To provide better quality for those types of services right management of radio resources is necessary. A large popularity of wireless and mobile networks has caused consumers to have a more demanding view of quality of service. The aim of this paper is to focus on UMTS network and analyze changes in quality of service using different methods. Measurement of quality of service is based on network parameters, so it is necessary to meet all the requirements in order to achieve satisfactory quality of service