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Nat Commun ; 9(1): 3720, 2018 09 13.
Artigo em Inglês | MEDLINE | ID: mdl-30213976


We present time-integrated four-wave mixing measurements on monolayer MoSe2 in magnetic fields up to 25 T. The experimental data together with time-dependent density function theory calculations provide interesting insights into the biexciton formation and dynamics. In the presence of magnetic fields the coherence at negative and positive time delays is dominated by intervalley biexcitons. We demonstrate that magnetic fields can serve as a control to enhance the biexciton formation and help search for more exotic states of matter, including the creation of multiple exciton complexes and excitonic condensates.

Sci Rep ; 6: 19762, 2016 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-26818899


Transition-metal oxides often exhibit complex magnetic behavior due to the strong interplay between atomic-structure, electronic and magnetic degrees of freedom. Cobaltates, especially, exhibit complex behavior because of cobalt's ability to adopt various valence and spin state configurations. The case of the oxygen-deficient perovskite Sr3YCo4O10+x (SYCO) has attracted considerable attention because of persisting uncertainties about its structure and the origin of the observed room temperature ferromagnetism. Here we report a combined investigation of SYCO using aberration-corrected scanning transmission electron microscopy and density functional theory calculations. Guided by theoretical results on Co-O distances projected on different planes, the atomic-scale images of several different orientations, especially of the fully oxygenated planes, allow the unambiguous extraction of the underlying structure. The calculated magnetic properties of the new structure are in excellent agreement with the experimental data.

Microsc Microanal ; 20(3): 784-97, 2014 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-24685384


We present a theoretical framework for calculating probe-position-dependent electron energy-loss near-edge structure for the scanning transmission electron microscope by combining density functional theory with dynamical scattering theory. We show how simpler approaches to calculating near-edge structure fail to include the fundamental physics needed to understand the evolution of near-edge structure as a function of probe position and investigate the dependence of near-edge structure on probe size. It is within this framework that density functional theory should be presented, in order to ensure that variations of near-edge structure are truly due to local electronic structure and how much from the diffraction and focusing of the electron beam.

Processamento de Imagem Assistida por Computador/métodos , Microscopia Eletrônica de Transmissão e Varredura/métodos , Espectroscopia de Perda de Energia de Elétrons , Modelos Teóricos , Óxidos/análise , Estrôncio/análise , Titânio/análise
Phys Rev Lett ; 109(20): 206803, 2012 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-23215517


Using a combination of Z-contrast imaging and atomically resolved electron energy-loss spectroscopy on a scanning transmission electron microscope, we show that the chemical bonding of individual impurity atoms can be deduced experimentally. We find that when a Si atom is bonded with four atoms at a double-vacancy site in graphene, Si 3d orbitals contribute significantly to the bonding, resulting in a planar sp(2) d-like hybridization, whereas threefold coordinated Si in graphene adopts the preferred sp(3) hybridization. The conclusions are confirmed by first-principles calculations and demonstrate that chemical bonding of two-dimensional materials can now be explored at the single impurity level.