Application of the modified Dubinin-Astakhov equation for a better understanding of high-pressure hydrogen adsorption on activated carbons

International audienc

Application of the modified Dubinin-Astakhov equation for a better understanding of high-pressure hydrogen adsorption on activated carbons

International audienc

Study of coupled heat and water transfer in proton exchange membrane fuel cells by the way of internal measurements

Abstract. Measurements of electrode temperatures within a proton exchange membrane fuel cell were performed using platinum wires. A temperature difference of 7°C between the electrodes and the bipolar plates was observed for a cell operating at a current density of 1.5 A.cm -2 . These measurements show a strong non-uniformity of the temperature profile through membrane electrode assembly (MEA) that future phenomenological models must take into account. In addition, the simultaneous measurements of heat and water flux through the MEA leads to the conclusion that produced water crosses the diffusion layer in vapor phase. A very simple heat transfer model is proposed

Hall Effect of La2/3(Ca,Pb)1/3MnO3 Single Crystals near the Critical Temperature

The Hall resistivity rho_{xy} of a La_{2/3}(Ca,Pb)_{1/3}MnO_3 single crystal has been measured as a function of temperature and field. The overall behavior is similar to that observed previously in thin-films. At 5 K, rho_{xy} is positive and linear in field, indicating that the anomalous contribution $R_S$ is negligible. However, the effective carrier density in a free electron model is n_{eff}=2.4 holes/Mn, even larger than the 0.85-1.9 holes/Mn reported for thin-films and far larger than the 0.33 holes/Mn expected from the doping level. As temperature increases, a strong, negative contribution to rho_{xy} appears, that we ascribe to R_S. Using detailed magnetization data, we separate the ordinary (\propto B) and anomalous (\propto M) contributions. Below T_C, R_S \propto rho_{xx}, indicating that magnetic skew scattering is the dominant mechanism in the metallic ferromagnetic regime. At and above the resistivity-peak temperature, we find that rho_{xy}/rho_{xx}M is a constant, independent of temperature and field. This implies that the anomalous Hall coefficient is proportional to the magnetoresistance. A different explanation based on two fluid model is also presented.Comment: revtex, 11 pages, 4 figure

Reaction of dialkyl carbonates with alcohols:Defining a scale of the best leaving and entering groups

A series of dialkyl and methyl alkyl carbonates has been synthesized and their reactivity investigated. The behavior of preferential leaving and entering groups for the newly synthesized carbonates has been accurately investigated. Both experimental and computational studies agreed that the scale of leaving groups follows the trend: PhCH2O–, MeO– ≥ EtO–, CH3(CH2)2O–, CH3(CH2)7O– > (CH3)2CHO– > (CH3)3CO–. Accordingly, the scale of the entering group has the same trend, with t-butoxide being the worst entering group. A preliminary attempt to rationalize the nucleofugality trends, limited to the (CH3)3CO– and CH3O– groups, has indicated that a likely origin of the observed trends lies in the different entropic contributions and solvation effects

Chirality driven anomalous Hall effect in weak coupling regime

Anomalous Hall effect arising from non-trivial spin configuration (chirality) is studied based on the $s$-$d$ model. Considering a weak coupling case, the interaction is treated perturbatively. Scattering by normal impurities is included. Chirality is shown to drive locally Hall current and leads to overall Hall effect if there is a finite uniform chirality. This contribution is independent of the conventional spin-orbit contribution and shows distinct low temperature behavior. In mesoscopic spin glasses, chirality-induced anomalous Hall effect is expected below the spin-glass transition temperature. Measurement of Hall coefficient would be useful in experimentally confirming the chirality ordering

Electronic transport in EuB6_6

EuB$_6$ is a magnetic semiconductor in which defects introduce charge carriers into the conduction band with the Fermi energy varying with temperature and magnetic field. We present experimental and theoretical work on the electronic magnetotransport in single-crystalline EuB$_6$. Magnetization, magnetoresistance and Hall effect data were recorded at temperatures between 2 and 300 K and in magnetic fields up to 5.5 T. The negative magnetoresistance is well reproduced by a model in which the spin disorder scattering is reduced by the applied magnetic field. The Hall effect can be separated into an ordinary and an anomalous part. At 20 K the latter accounts for half of the observed Hall voltage, and its importance decreases rapidly with increasing temperature. As for Gd and its compounds, where the rare-earth ion adopts the same Hund's rule ground state as Eu$^{2+}$ in EuB$_{6}$, the standard antisymmetric scattering mechanisms underestimate the $size$ of this contribution by several orders of magnitude, while reproducing its $shape$ almost perfectly. Well below the bulk ferromagnetic ordering at $T_C$ = 12.5 K, a two-band model successfully describes the magnetotransport. Our description is consistent with published de Haas van Alphen, optical reflectivity, angular-resolved photoemission, and soft X-ray emission as well as absorption data, but requires a new interpretation for the gap feature deduced from the latter two experiments.Comment: 35 pages, 12 figures, submitted to PR

Charge Transport in Manganites: Hopping Conduction, the Anomalous Hall Effect and Universal Scaling

The low-temperature Hall resistivity \rho_{xy} of La_{2/3}A_{1/3}MnO_3 single crystals (where A stands for Ca, Pb and Ca, or Sr) can be separated into Ordinary and Anomalous contributions, giving rise to Ordinary and Anomalous Hall effects, respectively. However, no such decomposition is possible near the Curie temperature which, in these systems, is close to metal-to-insulator transition. Rather, for all of these compounds and to a good approximation, the \rho_{xy} data at various temperatures and magnetic fields collapse (up to an overall scale), on to a single function of the reduced magnetization m=M/M_{sat}, the extremum of this function lying at m~0.4. A new mechanism for the Anomalous Hall Effect in the inelastic hopping regime, which reproduces these scaling curves, is identified. This mechanism, which is an extension of Holstein's model for the Ordinary Hall effect in the hopping regime, arises from the combined effects of the double-exchange-induced quantal phase in triads of Mn ions and spin-orbit interactions. We identify processes that lead to the Anomalous Hall Effect for localized carriers and, along the way, analyze issues of quantum interference in the presence of phonon-assisted hopping. Our results suggest that, near the ferromagnet-to-paramagnet transition, it is appropriate to describe transport in manganites in terms of carrier hopping between states that are localized due to combined effect of magnetic and non-magnetic disorder. We attribute the qualitative variations in resistivity characteristics across manganite compounds to the differing strengths of their carrier self-trapping, and conclude that both disorder-induced localization and self-trapping effects are important for transport.Comment: 29 pages, 20 figure

Large Anomalous Hall effect in a silicon-based magnetic semiconductor

Magnetic semiconductors are attracting high interest because of their potential use for spintronics, a new technology which merges electronics and manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently emerged as the most popular materials for this new technology. While Curie temperatures are rising towards room temperature, these materials can only be fabricated in thin film form, are heavily defective, and are not obviously compatible with Si. We show here that it is productive to consider transition metal monosilicides as potential alternatives. In particular, we report the discovery that the bulk metallic magnets derived from doping the narrow gap insulator FeSi with Co share the very high anomalous Hall conductance of (GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens up a new arena for spintronics, involving a bulk material based only on transition metals and Si, and which we have proven to display a variety of large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure