16 research outputs found
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
Spectral and spatial distribution of polarization at the LaAlO3/SrTiO3 interface
The polarization induced at the LaAlO3/SrTiO3 interface has been explored by optical second harmonic generation (SHG) spectroscopy. Our data reveal that an orbital reconstruction of the SrTiO3 surface occurs when three epitaxial monolayers of LaAlO3 are deposited on top of it. The reconstruction manifests as a spectral modification of a Ti4+(3d) state at the LaAlO3/SrTiO3 interface which is accompanied by a displacement of the Ti4+ ions in adjacent SrTiO3 monolayers in order to compensate for the polarization discontinuity induced at the interface. According to the SHG experiment, the carriers transferred to the interface are trapped because in spite of the orbital reconstruction, the interface remains insulating. The onset of conductivity with the formation of a two-dimensional electron liquid appears to be a subordinate effect related to a minority of carriers and occurs
upon further increase of the LaAlO3 coverage
Gallium gradients in Cu In,Ga Se2 thin film solar cells
The gallium gradient in Cu(In,Ga)Se2 (CIGS) layers, which forms during the two industrially relevant deposition
routes, the sequential and co-evaporation processes, plays a key role in the device performance of CIGS thin-film
modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium
gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization
process by in situ X-ray measurements. In addition, the gallium grading of a CIGS layer grown with an in-line coevaporation
process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real
solar cell served as input data for device simulations. Depth-dependent occurrence of lateral inhomogeneities on the
μm scale in CIGS deposited by the co-evaporation process was investigated by highly spatially resolved luminescence
measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi-Fermi level
splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS
cross-sections point to a difference in gallium content in the near surface region of neighboring grains. Migration
barriers for a copper-vacancy-mediated indium and gallium diffusion in CuInSe2 and CuGaSe2 were calculated using
density functional theory. The migration barrier for the InCu antisite in CuGaSe2 is significantly lower compared with
the GaCu antisite in CuInSe2, which is in accordance with the experimentally observed Ga gradients in CIGS layers
grown by co-evaporation and selenization processes
Anatomy and systematics of Anodontites Elongatus (Swainson) from Amazon and Parana Basins, Brazil (Mollusca, Bivalvia, Unionoida, Mycetopodidae)
The anatomy of Anodontiies elongatus (Swainson, 1823), a rare species restricted to the Amazon and Parana Basins, is described by first time, showing a group of conchological and anatomical characters exclusive of this species that may be analyzed to identify it. Diagnosis of A. elongatus: shell long antero-posteriorly, umbones prominent, periostracum opaque and smooth, two posterior radial striae; middle fold of mantle edge veiy tall; gill long antero-posteriorly and short dorso-ventrally, extending about a half of it total length beyond visceral mass; palps proportionally small, several furrows in its outer surface; stomach without esophageal transversal ridjp, dorsal hood and gastric shield poorly developed, major typhlosole entering in ddd , posterior pouch of sa³ very-long; style sac reduced, without crystalline style; distal region of intestine and rectum with a well developed typhlosole, "T" in section, other intestinal regions without folds; gonad gonochoristic