Crystalline topological states at a topological insulator junction

We consider an interface between two strong time-reversal invariant topological insulators having surface states with opposite spin chirality, or equivalently, opposite mirror Chern number. We show that such an interface supports gapless modes that are protected by mirror symmetry. The interface states are investigated with a continuum model for the Bi2Se3 class of topological insulators that takes into account terms up to third order in the crystal momentum, which ensures that the model has the correct symmetry. The model parameters are obtained from ab initio calculations. Finally, we consider the effect of rotational mismatch at the interface, which breaks the mirror symmetry and opens a gap in the interface spectrum.Comment: 10 pages, 5 figure

Exceeding the Shockley-Queisser limit within the detailed balance framework

The Shockley-Queisser limit is one of the most fundamental results in the field of photovoltaics. Based on the principle of detailed balance, it defines an upper limit for a single junction solar cell that uses an absorber material with a specific band gap. Although methods exist that allow a solar cell to exceed the Shockley-Queisser limit, here we show that it is possible to exceed the Shockley-Queisser limit without considering any of these additions. Merely by introducing an absorptivity that does not assume that every photon with an energy above the band gap is absorbed, efficiencies above the Shockley-Queisser limit are obtained. This is related to the fact that assuming optimal absorption properties also maximizes the recombination current within the detailed balance approach. We conclude that considering a finite thickness for the absorber layer allows the efficiency to exceed the Shockley-Queisser limit, and that this is more likely to occur for materials with small band gaps.Comment: 6 pages, 3 figure

Cartografia de las Transiciones Electronicas en Cristales Ferromagneticos

El conocimiento de las transiciones electronicas posibles entre estados de unsistema, dentro del marco de la mecanica cuantica, es fundamental para podercomprender las propiedades del sistema y su respuesta a un estimulo exterior.En este trabajo estudiamos los 'mapas' de las transiciones de intercambioposibles entre estados electronicos en un modelo de cristal ferromagnetico,tomando en cuenta las transiciones de Stoner, con inversion del spinelectronico, y las transiciones sin inversion de spin. Estos mapas ponen enevidencia el origen de la estructura de la respuesta del material cuandosometido a un analisis de espectroscopia electronica

Charge localization, frustration relief, and spin-orbit coupling in U3_3O8_8

Research efforts on the low temperature magnetic order and electronic properties of U$_3$O$_8$ have been inconclusive so far. Reinterpreting neutron scattering results, we use group representation theory to show that the ground state presents collinear out-of-plane magnetic moments, with antiferromagnetic coupling both in-layer and between layers. Charge localization relieves the initial geometric frustration, generating a slightly distorted honeycomb sublattice with N\'eel order. We show, furthermore, that spin-orbit coupling has a giant effect on the conduction band states and band gap value. Our results allow a reinterpretation of recent optical absorption measurements.Comment: 12 pages, including supplemental materia

Positron surface state as a spectroscopic probe for characterizing surfaces of topological insulator materials

Topological insulators are attracting considerable interest due to their potential for technological applications and as platforms for exploring wide-ranging fundamental science questions. In order to exploit, fine-tune, control, and manipulate the topological surface states, spectroscopic tools which can effectively probe their properties are of key importance. Here, we demonstrate that positrons provide a sensitive probe for topological states and that the associated annihilation spectrum provides a technique for characterizing these states. Firm experimental evidence for the existence of a positron surface state near Bi2Te2Se with a binding energy of Eb=2.7±0.2eV is presented and is confirmed by first-principles calculations. Additionally, the simulations predict a significant signal originating from annihilation with the topological surface states and show the feasibility to detect their spin texture through the use of spin-polarized positron beams.Academy of Finland (Projects No. 285809)Academy of Finland (Projects No. 293932)United States. Department of Energy. Office of Basic Energy Sciences (Grant No. DE-FG02- 07ER4635)United States. Department of Energy (Grant No. DE-AC02-05CH11231)National Science Foundation (U.S.) (Grants No. DMR-MRI-1338130)National Science Foundation (U.S.) (Grants No. DMR-1508719)National Science Foundation (U.S.) (Grants No. DMR-1231319)National Science Foundation (U.S.) (DMR-1207469)United States. Office of Naval Research (Grant No. N00014-13-1-0301)Laboratoire de physique théorique. École Normale Supérieule. International Centre for Fundamental Physics (Grant No. ANR-10-LABX-0010/ANR- 10-IDEX-0001-02 PSL

Effect of Zinc Oxide Modification by Indium Oxide on Microstructure, Adsorbed Surface Species, and Sensitivity to CO

Additives in semiconductor metal oxides are commonly used to improve sensing behavior of gas sensors. Due to complicated effects of additives on the materials microstructure, adsorption sites and reactivity to target gases the sensing mechanism with modified metal oxides is a matter of thorough research. Herein, we establish the promoting effect of nanocrystalline zinc oxide modification by 1–7 at.% of indium on the sensitivity to CO gas due to improved nanostructure dispersion and concentration of active sites. The sensing materials were synthesized via an aqueous coprecipitation route. Materials composition, particle size and BET area were evaluated using X-ray diffraction, nitrogen adsorption isotherms, high-resolution electron microscopy techniques and EDX-mapping. Surface species of chemisorbed oxygen, OH-groups, and acid sites were characterized by probe molecule techniques and infrared spectroscopy. It was found that particle size of zinc oxide decreased and the BET area increased with the amount of indium oxide. The additive was observed as amorphous indium oxide segregated on agglomerated ZnO nanocrystals. The measured concentration of surface species was higher on In2O3-modified zinc oxide. With the increase of indium oxide content, the sensor response of ZnO/In2O3 to CO was improved. Using in situ infrared spectroscopy, it was shown that oxidation of CO molecules was enhanced on the modified zinc oxide surface. The effect of modifier was attributed to promotion of surface OH-groups and enhancement of CO oxidation on the segregated indium ions, as suggested by DFT in previous work