Mechanism of charge transfer/disproportionation in LnCu3Fe4O12 (Ln: Lanthanides)

The Fe-Cu intersite charge transfer and Fe charge disproportionation are interesting phenomena observed in some LnCu3Fe4O12 (Ln: Lanthanides) compounds containing light and heavy Ln atoms, respectively. We show that a change in the spin state is responsible for the intersite charge transfer in the light Ln compounds. At the high spin state, such systems prefer an unusual Cu-d^8 configuration, whereas at the low spin state they retreat to the normal Cu-d^9 configuration through a charge transfer from Fe to Cu-3d_{xy} orbital. We find that the strength of the crystal field splitting and the relative energy ordering between Cu-3d_{xy} and Fe-3d states are the key parameters, determining the intersite charge transfer (charge disproportionation) in light (heavy) Ln compounds. It is further proposed that the size of Ln affects the onsite interaction strength of Cu-3d states, leading to a strong modification of the Cu-L_3 edge spectrum, as observed by the X-ray absorption spectroscopy.Comment: 6 pages, 5 figures, 1 table. To appear in PR

Topological protection of bound states against the hybridization

Topological invariants are conventionally known to be responsible for protection of extended states against disorder. A prominent example is the presence of topologically protected extended-states in two-dimensional (2D) quantum Hall systems as well as on the surface of three-dimensional (3D) topological insulators. Distinct from such cases, here we introduce a new concept, that is, the topological protection of bound states against hybridization. This situation is shown to be realizable in a 2D quantum Hall insulator put on a 3D trivial insulator. In such a configuration, there exist topologically protected bound states, localized along the normal direction of 2D plane, in spite of hybridization with the continuum of extended states. The one-dimensional edge states are also localized along the same direction as long as their energies are within the band gap. This finding demonstrates the dual role of topological invariants, as they can also protect bound states against hybridization in a continuum.Comment: 21 pages, 7 figure

Rich structural phase diagram and thermoelectric properties of layered tellurides Mo1-xNbxTe2

MoTe2 is a rare transition-metal ditelluride having two kinds of layered polytypes, hexagonal structure with trigonal prismatic Mo coordination and monoclinic structure with octahedral Mo coordination. The monoclinic distortion in the latter is caused by anisotropic metal-metal bonding. In this work, we have examined the Nb doping effect on both polytypes of MoTe2 and clarified a structural phase diagram for Mo1-xNbxTe2 containing four kinds of polytypes. A rhombohedral polytype crystallizing in polar space group has been newly identified as a high-temperature metastable phase at slightly Nb-rich composition. Considering the results of thermoelectric measurements and the first principles calculations, the Nb ion seemingly acts as a hole dopant in the rigid band scheme. On the other hand, the significant interlayer contraction upon the Nb doping, associated with the Te p-p hybridization, is confirmed especially for the monoclinic phase, which implies a shift of the p-band energy level. The origin of the metal-metal bonding in the monoclinic structure is discussed in terms of the d electron counting and the Te p-p hybridization.Comment: 16 pages, 6 figures, 1 table, to be published in APL Material

Spin-orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS2_2-superconductors

We develop the realistic minimal electronic model for recently discovered BiS$_2$ superconductors including the spin-orbit coupling based on a first-principles band structure calculations. Due to strong spin-orbit coupling, characteristic for the Bi-based systems, the tight-binding low-energy model necessarily includes $p_x$, $p_y$, and $p_z$ orbitals. We analyze a potential Cooper-pairing instability from purely repulsive interaction for the moderate electronic correlations using the so-called leading angular harmonics approximation (LAHA). For small and intermediate doping concentrations we find the dominant instabilities to be $d_{x^2-y^2}$-wave, and $s_{\pm}$-wave symmetries, respectively. At the same time, in the absence of the sizable spin fluctuations the intra and interband Coulomb repulsion are of the same strength, which yields the strongly anisotropic behaviour of the superconducting gaps on the Fermi surface in agreement with recent ARPES findings. In addition, we find that the Fermi surface topology for BiS$_2$ layered systems at large electron doping can resembles the doped iron-based pnictide superconductors with electron and hole Fermi surfaces with sufficient nesting between them. This could provide further boost to increase $T_c$ in these systems.Comment: 10 pages, 3 figure

Giant thermoelectric effect in graphene-based topological insulators with nanopores

Designing thermoelectric materials with high figure of merit $ZT=S^2 G T/\kappa$ requires fulfilling three often irreconcilable conditions, i.e., the high electrical conductance $G$, small thermal conductance $\kappa$ and high Seebeck coefficient $S$. Nanostructuring is one of the promising ways to achieve this goal as it can substantially suppress lattice contribution to $\kappa$. However, it may also unfavorably influence the electronic transport in an uncontrollable way. Here we theoretically demonstrate that this issue can be ideally solved by fabricating graphene nanoribbons with heavy adatoms and nanopores. These systems, acting as a two-dimensional topological insulator with robust helical edge states carrying electrical current, yield a highly optimized power factor $S^2G$ per helical conducting channel. Concurrently, their array of nanopores impedes the lattice thermal conduction through the bulk. Using quantum transport simulations coupled with first-principles electronic and phononic band structure calculations, the thermoelectric figure of merit is found to reach its maximum $ZT \simeq 3$ at $T \simeq 40$ K. This paves a way to design high-$ZT$ materials by exploiting the nontrivial topology of electronic states through nanostructuring.Comment: 7 pages, 4 figures; PDFLaTe

Inclusion of intercultural Education Criteria in Iranian Official English Textbooks

The present study, with the concern for inclusion of the intercultural dimensions in the process of language learning and teaching, looked at the content of the some official English textbooks designed and authored by Ministry of Education in Iran to find out whether the cultural dimensions of English speaking world have been included in official high school English textbooks. The study is a qualitative study in nature, based on content analysis. It examined the subject of each lesson and the content of different parts included in each units to find the representation of intercultural studies. The results of the study revealed the lack of intercultural perspective inclusion within official English textbooks in Iran. Keywords: Intercultural education, National curriculum of Islamic Republic of Iran. Official English textbook

Origin of giant bulk Rashba splitting: Application to BiTeI

We theoretically propose the necessary conditions for realization of giant Rashba splitting in bulk systems. In addition to (i) the large atomic spin-orbit interaction in an inversion-asymmetric system, the following two conditions are further required; (ii) a narrow band gap, and (iii) the presence of top valence and bottom conduction bands of symmetrically the same character. As a representative example, using the first principles calculations, the recently discovered giant bulk Rashba splitting system BiTeI is shown to fully fulfill all these three conditions. Of particular importance, by predicting the correct crystal structure of BiTeI, different from what has been believed thus far, the third criterion is demonstrated to be met by a negative crystal field splitting of the top valence bands.Comment: 3 figure

The Effect of Translation -Shifts Instruction on Translation Quality

Inspired by the notion of "Descriptive Translation Studies", the present study looked for adopting new translation teaching methods. It tried to introduce the concept of translation shifts to a number of translation students in translation workshops. The assumption was based on the idea that the study of translation shifts can lead to finding some generalizations happened in the process of translation, the study of these general rules can help the translation students create translated texts which enjoy higher quality in comparison with translated texts, performed earlier. In pursuit of this aim, some English translation students were chosen from among BA students in North-Branch, Tehran-Azad University. The homogeneity of the subjects was determined through a TOEFL examination. The students attended some translation workshops to receive some instruction and became familiar and conscious about the general rules of translation process. Through the treatment, students were required to recognize the shifts happened in the process of translation in several translation texts. Finally, the differences between translation quality of the subjects before and after treatment were examined through two translation production tests. The tests were corrected according to Farahzad's "evaluation model" (1992). The post test scores after treatment confirmed the positive effects of the teaching on translation quality.. Keywords: Descriptive Translation Studies (DTS), Translation Shifts