Evolution of the interfacial structure of LaAlO3 on SrTiO3

The evolution of the atomic structure of LaAlO3 grown on SrTiO3 was investigated using surface x-ray diffraction in conjunction with model-independent, phase-retrieval algorithms between two and five monolayers film thickness. A depolarizing buckling is observed between cation and oxygen positions in response to the electric field of polar LaAlO3, which decreases with increasing film thickness. We explain this in terms of competition between elastic strain energy, electrostatic energy, and electronic reconstructions. The findings are qualitatively reproduced by density-functional theory calculations. Significant cationic intermixing across the interface extends approximately three monolayers for all film thicknesses. The interfaces of films thinner than four monolayers therefore extend to the surface, which might affect conductivity

Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO3_3/SrTiO3_3 interfaces

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.Comment: 4 figures in main text; 4 figures in the supplemen

DLTS investigations on CIGS solar cells from an inline co-evaporation system with RbF post-deposition treatment

In this study, Deep Level Transient Spectroscopy (DLTS) measurements have been performed on Cu(In,Ga)Se$_{2}$ (CIGS) solar cells from an inline co-evaporation system. The focus of this investigation is directed on the effect of rubidium-fluoride (RbF)-post-deposition treatment (PDT) on the defects in the CIGS absorber layer. Different traps can be identified and their properties are calculated. Herein, different methods of evaluations have been used to verify the results. Specifically, one minority trap around 400 meV was found to show a significant reduction of the trap density due to the alkali treatment. In contrast, a majority trap at approximately 600 meV is unaffected

Diodes with Breakdown Voltages Enhanced by the Metal-Insulator Transition of LaAlO3_3-SrTiO3_3 Interfaces

Using the metal-insulator transition that takes place as a function of carrier density at the LaAlO$_3$-SrTiO$_3$ interface, oxide diodes have been fabricated with room-temperature breakdown voltages of up to 200 V. With applied voltage, the capacitance of the diodes changes by a factor of 150. The diodes are robust and operate at temperatures up to 270 C

Two-dimensional electron liquid state at LaAlO3-SrTiO3 interfaces

Using tunneling spectroscopy we have measured the spectral density of states of the mobile, two-dimensional electron system generated at the LaAlO3-SrTiO3 interface. As shown by the density of states the interface electron system differs qualitatively, first, from the electron systems of the materials defining the interface and, second, from the two-dimensional electron gases formed at interfaces between conventional semiconductors

LaAlO3 stoichiometry found key to electron liquid formation at LaAlO3/SrTiO3 interfaces

Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators LaAlO3 and SrTiO3 distinguish this rich system from conventional two-dimensional electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated with focus on the role of defects in the SrTiO3 while the LaAlO3 has been assumed perfect. Our experiments and first principles calculations show that the cation stoichiometry of the nominal LaAlO3 layer is key to 2-DEL formation: only Al-rich LaAlO3 results in a 2-DEL. While extrinsic defects including oxygen deficiency are known to render LaAlO3/SrTiO3 samples conducting, our results show that in the absence of such extrinsic defects, an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides a roadmap for identifying other interfaces where emergent behaviors await discovery

Three new species of Scissurellidae (Gastropoda, Prosobranchia) from the coast of Brazil

A consultation of the original descriptions of the genera of Scissurellidae led the author to conclude that the valid names for the genera of this family are: Scissurella d'Orbigny, 1823 (redescription by Sowerby, 1824), Anatoma Woodward, 1859, Incisura Hedley, 1904, Scissurona Iredale, 1924, Sinezona Finlay, 1927. Scissurella and Anatoma are cosmopolites, the first generally living in shallow waters, associated to sea-weeds, while the second is found in deep waters. The three last genera are restricted to Australasia. Three new species are described from the Brazilian coast: Scissurella alexandrei, Scissurella electilis and Scissurella morretesi. The internal anatomy of S. alexandrei is described. These three new species have been found in littoral shallow waters, the two first on the northeast Brazilian coast, the third on the coast of the State of São Paulo. The fourth known species of Scissurellidae from Brazil, Anatoma aedonia (Watson, 1886), was dredged by H.M.S, "Challenger", in 1873, off Pernambuco, 350 fm (640 m).Pela leitura das descrições originais dos gêneros de Scissurellidae , conclue-se que os nomes válidos destes gêneros são: Scissurella d'Orbigny, 1823 (redescrição por Sowerby, 1824), Anatoma Woodward, 1859, Incisura Hedley,1904, Scissurona Iredale, 1924 e Sinezona Finlay, 1927. Scissurella e Anatoma são cosmopolitas; o primeiro encontra-se geralmente em águas rasas e associado a algas; o segundo habita águas profundas. Os três últimos gêneros são restritos à Australásia. Três novas espécies são descritas para a costa do Brasil: Scissurella alexandrei, Scissurella electilis e Scissurella morretesi. Da primeira foi estudada a anatomia interna. Estas três espécies novas foram encontradas em águas rasas; S. alexandrei e S.electilis provém de amostras de algas do mesolitoral inferior. Conhece-se uma quarta espécie de Scissurellidae para o Brasil: Anatoma aedonia (Watson, 1886), dragada em 1873 pelo 'Challenger', ao largo da costa de Pernambuco, em profundidade de 640 m