AlAsSb avalanche photodiodes with a sub-mV/K temperature coefficient of breakdown voltage

The temperature dependence of dark current and avalanche gain were measured on AlAsSb p-i-n diodes with avalanche region widths of 80 and 230 nm. Measurements at temperatures ranging from 77 to 295 K showed that the dark current decreases rapidly with reducing temperature while avalanche gain exhibits a weak temperature dependence. No measurable band to band tunneling current was observed in the thinner diodes at an electric field of 1.07 MV/cm, corresponding to a bias of 95% of the breakdown voltage. Temperature coefficients of breakdown voltage of 0.95 and 1.47 mV/K were obtained from 80 and 230 nm diodes, respectively. These are significantly lower than a range of semiconductor materials with similar avalanche region widths. Our results demonstrated the potential of using thin AlAsSb avalanche regions to achieve low temperature coefficient of breakdown voltage without suffering from high band to band tunneling current

Transport coefficients of graphene: Interplay of impurity scattering, Coulomb interaction, and optical phonons

We study the electric and thermal transport of the Dirac carriers in monolayer graphene using the Boltzmann-equation approach. Motivated by recent thermopower measurements [F. Ghahari, H.-Y.~Xie, T. Taniguchi, K. Watanabe, M.~S.~Foster, and P.~Kim, Phys.\ Rev.\ Lett.\ {\bf 116}, 136802 (2016)], we consider the effects of quenched disorder, Coulomb interactions, and electron--optical-phonon scattering. Via an unbiased numerical solution to the Boltzmann equation we calculate the electrical conductivity, thermopower, and electronic component of the thermal conductivity, and discuss the validity of Mott's formula and of the Wiedemann-Franz law. An analytical solution for the disorder-only case shows that screened Coulomb impurity scattering, although elastic, violates the Wiedemann-Franz law even at low temperature. For the combination of carrier-carrier Coulomb and short-ranged impurity scattering, we observe the crossover from the interaction-limited (hydrodynamic) regime to the disorder-limited (Fermi-liquid) regime. In the former, the thermopower and the thermal conductivity follow the results anticipated by the relativistic hydrodynamic theory. On the other hand, we find that optical phonons become nonnegligible at relatively low temperatures and that the induced electron thermopower violates Mott's formula. Combining all of these scattering mechanisms, we obtain the thermopower that quantitatively coincides with the experimental data.Comment: 20 pages, 9 figure

Mathematical Modeling of Product Rating: Sufficiency, Misbehavior and Aggregation Rules

Many web services like eBay, Tripadvisor, Epinions, etc, provide historical product ratings so that users can evaluate the quality of products. Product ratings are important since they affect how well a product will be adopted by the market. The challenge is that we only have {\em "partial information"} on these ratings: Each user provides ratings to only a "{\em small subset of products}". Under this partial information setting, we explore a number of fundamental questions: What is the "{\em minimum number of ratings}" a product needs so one can make a reliable evaluation of its quality? How users' {\em misbehavior} (such as {\em cheating}) in product rating may affect the evaluation result? To answer these questions, we present a formal mathematical model of product evaluation based on partial information. We derive theoretical bounds on the minimum number of ratings needed to produce a reliable indicator of a product's quality. We also extend our model to accommodate users' misbehavior in product rating. We carry out experiments using both synthetic and real-world data (from TripAdvisor, Amazon and eBay) to validate our model, and also show that using the "majority rating rule" to aggregate product ratings, it produces more reliable and robust product evaluation results than the "average rating rule".Comment: 33 page

Acting Globally while Thinking Locally: Is the Global Environment Protected by Transport Emission Control Programs?

Locally motivated air quality programs in Santiago and Mexico City have only minor collateral benefits for the global climate. If agencies with global and local agendas did business together, then individuals and firms and even cities would act globally when thinking locally, and one would see greater synergy. Eskeland and Xie find that locally motivated air quality programs for urban transport have limited collateral benefits in terms of protecting the global climate. This could puzzle some, since these two public goods one global, one local seem to be jointly produced. However, air quality in Mexico City, Santiago, and elsewhere is predominantly pursued by technical improvements (making cars and fuels cleaner), and not by reducing demand for polluting goods and services (though in Europe high fuel taxes help reduce demand). Control programs developed under joint stimulus to protect the global and local environment have not yet been seen, and they may surprise us when they come. However, they will likely rely more on reducing demand, using instruments such as corrective (Pigovian) taxes on fuels. The authors show how, if locally and globally charged agencies can do business together, consumers, producers, and cities will act globally when thinking locally. Only then will we know the extent to which local and global benefits are produced jointly.