RESUMEN
The Berry phase has found applications in building topological order parameters for certain condensed matter systems. The question whether some geometric phase for mixed states can serve the same purpose has been raised, and proposals are on the table. We analyse the intricate behaviour of Uhlmann's geometric phase in the Kitaev chain at finite temperature, and then argue that it captures quite different physics from that intended. We also analyse the behaviour of a geometric phase introduced in the context of interferometry. For the Kitaev chain, this phase closely mirrors that of the Berry phase, and we argue that it merits further investigation.
RESUMEN
Conventional passive sampling devices for monitoring pollution input often prove to be cost-prohibitive when assessing large spatial and temporal scales. The Kenai River, a major salmon-producing river in Alaska (USA), served as the perfect laboratory to test the utility of polyethylene membrane devices for assessing chronic nonpoint-source inputs to a large riverine watershed. Comparison of the relative levels of polycyclic aromatic hydrocarbons (PAHs) at a large number of locations over time allowed us to assess the significance and potential source of these compounds in the river. Concentrations of PAH were greatest near urban areas and peaked during the late winter, when streams flows and dilution were low. Vessel activity and PAH levels peaked in July and were heaviest in the lower 16 km of the river, where fishing activity was concentrated. Nearly one-third of the vessels observed on the river were powered by two-stroke engines, which release a higher proportion of unburned fuel into the water than the cleaner burning four-stroke engines. The low concentrations of hydrocarbons upriver of the boat traffic suggest very little remote delivery of these contaminants to the watershed. Polyethylene strips proved to be an excellent, low-cost tool for determining the PAH patterns in a large watershed.