7 research outputs found
Theory of high energy optical conductivity and the role of oxygens in manganites
Recent experimental study reveals the optical conductivity of
LaCaMnO over a wide range of energy and the occurrence of
spectral weight transfer as the system transforms from paramagnetic insulating
to ferromagnetic metallic phase [Rusydi {\it et al.}, Phys. Rev. B {\bf 78},
125110 (2008)]. We propose a model and calculation within the Dynamical Mean
Field Theory to explain this phenomenon. We find the role of oxygens in
mediating the hopping of electrons between manganeses as the key that
determines the structures of the optical conductivity. In addition, by
parametrizing the hopping integrals through magnetization, our result suggests
a possible scenario that explains the occurrence of spectral weight transfer,
in which the ferromagnatic ordering increases the rate of electron transfer
from O orbitals to upper Mn orbitals while simultaneously
decreasing the rate of electron transfer from O orbitals to lower
Mnorbitals, as temperature is varied across the ferromagnetic
transition. With this scenario, our optical conductivity calculation shows very
good quantitative agreement with the experimental data.Comment: 10 pages, 6 figures (accepted
Surface characterization and resistance changes of silver-nanowire networks upon atmospheric plasma treatment
Highly conductive silver-nanowire (Ag-NW) networks are used in composite materials as conductive channels. Their resistance tuning can be accomplished by changing the Ag-NW concentration, and, therefore, changing the network structure. In this study, an alternative pathway to resistance engineering of conductive Ag-NW networks by local atmospheric plasma treatment is employed. The corresponding changes in nanowire network morphology and crystallinity as a function of plasma etching time are investigated by time-resolved grazing-incidence X-ray scattering, field-effect scanning electron microscopy, and X-ray photoelectron spectroscopy. Three characteristic etching phases are identified. The first two phases enable the controlled engineering of the electrical properties with different rates of resistance change, which results from changes in nanowire shape, network morphology, and different oxidation rates. Phase III is characterized by pronounced fragmentation and destruction of the Ag-NW networks. These results show the feasibility of atmospheric plasma treatments to tune the local electrical properties of conductive Ag-NW networks. Furthermore, we present a physical Monte Carlo model explaining the electrical network properties as a function of plasma etching time based on the network connectivity and a constant plasma etching rate of 570 ng s cm
Structural dynamics upon photoexcitation-induced charge transfer in a dicopper(I)-disulfide complex
The structural dynamics of charge-transfer states of nitrogen-ligated copper complexes has been extensively investigated in recent years following the development of pump-probe X-ray techniques. In this study we extend this approach towards copper complexes with sulfur coordination and investigate the influence of charge transfer states on the structure of a dicopper(I) complex with coordination by bridging disulfide ligands and additionally tetramethylguanidine units [CuI2(NSSN)(2)](2+). In order to directly observe and refine the photoinduced structural changes in the solvated complex we applied picosecond pump-probe X-ray absorption spectroscopy (XAS) and wide-angle X-ray scattering (WAXS). Additionally, the ultrafast evolution of the electronic excited states was monitored by femtosecond transient absorption spectroscopy in the UV-Vis probe range. DFT calculations were used to predict molecular geometries and electronic structures of the ground and metal-to-ligand charge transfer states with singlet and triplet spin multiplicities, i.e. S-0, (MLCT)-M-1 and (MLCT)-M-3, respectively. Combining these techniques we elucidate the electronic and structural dynamics of the solvated complex upon photoexcitation to the MLCT states. In particular, femtosecond optical transient spectroscopy reveals three distinct timescales of 650 fs, 10 ps and 4100 ps, which were assigned as internal conversion to the ground state (Sn -> S-0), intersystem crossing (MLCT)-M-1 -> (MLCT)-M-3, and subsequent relaxation of the triplet to the ground state, respectively. Experimental data collected using both X-ray techniques are in agreement with the DFT-predicted structure for the triplet state, where coordination bond lengths change and one of the S-S bridges is cleaved, causing the movement of two halves of the molecule relative to each other. Extended X-ray absorption fine structure spectroscopy resolves changes in Cu-ligand bond lengths with precision on the order of 0.01 angstrom, whereas WAXS is sensitive to changes in the global shape related to relative movement of parts of the molecule. The results presented herein widen the knowledge on the electronic and structural dynamics of photoexcited copper-sulfur complexes and demonstrate the potential of combining the pump-probe X-ray absorption and scattering for studies on photoinduced structural dynamics in copper-based coordination complexes