1,758 research outputs found
Quantum spin liquids and the metal-insulator transition in doped semiconductors
We describe a new possible route to the metal-insulator transition in doped
semiconductors such as Si:P or Si:B. We explore the possibility that the loss
of metallic transport occurs through Mott localization of electrons into a
quantum spin liquid state with diffusive charge neutral "spinon" excitations.
Such a quantum spin liquid state can appear as an intermediate phase between
the metal and the Anderson-Mott insulator. An immediate testable consequence is
the presence of metallic thermal conductivity at low temperature in the
electrical insulator near the metal-insulator transition. Further we show that
though the transition is second order the zero temperature residual electrical
conductivity will jump as the transition is approached from the metallic side.
However the electrical conductivity will have a non-monotonic temperature
dependence that may complicate the extrapolation to zero temperature.
Signatures in other experiments and some comparisons with existing data are
made.Comment: 4 pages text + 3 pages Appendices, 3 Figures; v2 - References Adde
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Boreal forest CO2 exchange and evapotranspiration predicted by nine ecosystem process models: Intermodel comparisons and relationships to field measurements
Nine ecosystem process models were used to predict CO2 and water vapor exchanges by a 150-year-old black spruce forest in central Canada during 1994–1996 to evaluate and improve the models. Three models had hourly time steps, five had daily time steps, and one had monthly time steps. Model input included site ecosystem characteristics and meteorology. Model predictions were compared to eddy covariance (EC) measurements of whole-ecosystem CO2exchange and evapotranspiration, to chamber measurements of nighttime moss-surface CO2release, and to ground-based estimates of annual gross primary production, net primary production, net ecosystem production (NEP), plant respiration, and decomposition. Model-model differences were apparent for all variables. Model-measurement agreement was good in some cases but poor in others. Modeled annual NEP ranged from −11 g C m−2 (weak CO2source) to 85 g C m−2 (moderate CO2 sink). The models generally predicted greater annual CO2sink activity than measured by EC, a discrepancy consistent with the fact that model parameterizations represented the more productive fraction of the EC tower “footprint.” At hourly to monthly timescales, predictions bracketed EC measurements so median predictions were similar to measurements, but there were quantitatively important model-measurement discrepancies found for all models at subannual timescales. For these models and input data, hourly time steps (and greater complexity) compared to daily time steps tended to improve model-measurement agreement for daily scale CO2 exchange and evapotranspiration (as judged by root-mean-squared error). Model time step and complexity played only small roles in monthly to annual predictions
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