Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect

Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe valence and the degree of B-site order were probed by 57Fe Mossbauer spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms occupying wrong sites, i.e. antisite disorder. It had very little effect on the Fe valence: a small but visible increase in the isomer shift was seen for the mixed-valent FeII/III atoms occupying the right site indicating a slight movement towards divalency of these atoms, which was more than counterbalanced by the increase in the fraction of antisite Fe atoms with III valence state. It is therefore argued that the bulk of the electron doping is received by antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the lower IV/V valence state. Thus under Nd substitution, the charge-neutrality requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.Comment: 15 pages, 6 figures, to appear in Solid State Commu

Frequency dependence of signal power and spatial reach of the local field potential

The first recording of electrical potential from brain activity was reported already in 1875, but still the interpretation of the signal is debated. To take full advantage of the new generation of microelectrodes with hundreds or even thousands of electrode contacts, an accurate quantitative link between what is measured and the underlying neural circuit activity is needed. Here we address the question of how the observed frequency dependence of recorded local field potentials (LFPs) should be interpreted. By use of a well-established biophysical modeling scheme, combined with detailed reconstructed neuronal morphologies, we find that correlations in the synaptic inputs onto a population of pyramidal cells may significantly boost the low-frequency components of the generated LFP. We further find that these low-frequency components may be less `local' than the high-frequency LFP components in the sense that (1) the size of signal-generation region of the LFP recorded at an electrode is larger and (2) that the LFP generated by a synaptically activated population spreads further outside the population edge due to volume conduction

Valence-state mixing and separation in SmBaFe2O5+w

A mixed-valence state, formally denoted as Fe2.5+, is observed in the 300 K Mössbauer spectra of the most reduced samples of SmBaFe2O5+w. Upon cooling below the Verwey-type transition temperature (TV≈200K), the component assigned to Fe2.5+ separates into a high-spin Fe3+ state and an Fe2+ state with an unusually low internal field. The separation of the mixed-valence state at TV is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w.Peer reviewe

Electroactive Polyhydroquinone Coatings for Marine Fouling Prevention—A Rejected Dynamic pH Hypothesis and a Deceiving Artifact in Electrochemical Antifouling Testing

Nanometer-thin coatings of polyhydroquinone (PHQ), which release and absorb protons upon oxidation and reduction, respectively, were tested for electrochemically induced anti-biofouling activity under the hypothesis that a dynamic pH environment would discourage fouling. Antifouling tests in artificial seawater using the marine, biofilm-forming bacterium Vibrio alginolyticus proved the coatings to be ineffective in fouling prevention but revealed a deceiving artifact from the reactive species generated at the counter electrode (CE), even for electrochemical bias potentials as low as |400| mV versus Ag|AgCl. These findings provide valuable information on the preparation of nanothin PHQ coatings and their electrochemical behavior in artificial seawater. The results further demonstrate that it is critical to isolate the CE in electrochemical anti-biofouling testing

Valence-state mixing and separation in SmBaFe2O5+w

A mixed-valence state, formally denoted as Fe2.5+, is observed in the 300 K Mössbauer spectra of the most reduced samples of SmBaFe2O5+w. Upon cooling below the Verwey-type transition temperature (TV≈200K), the component assigned to Fe2.5+ separates into a high-spin Fe3+ state and an Fe2+ state with an unusually low internal field. The separation of the mixed-valence state at TV is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w.Peer reviewe

Decay times in turnover statistics of single enzymes

The first passage times for enzymatic turnovers in non-equilibrium steady state display a statistical symmetry property related to non-equilibrium fluctuation theorems, that makes it possible to extract the chemical driving force from single molecule trajectories in non-equilibrium steady state. Below, we show that this system violates the general expectation that the number of decay constants needed to fit a first passage time distribution reflects the number of states in the escape problem. In fact, the structure of the kinetic mechanism makes half of the decay times vanish identically from the turnover time distribution. The terms that cancel out correspond to the eigenvalues of a certain sub-matrix of the master equation matrix for the first exit time problem. We discuss how these results make modeling and data analysis easier for such systems, and how the turnovers can be measured.Comment: 4 pages, 1 figure v2: Published version, minor corrections in response to referee comment