The structure of Green functions in quantum field theory with a general state

In quantum field theory, the Green function is usually calculated as the expectation value of the time-ordered product of fields over the vacuum. In some cases, especially in degenerate systems, expectation values over general states are required. The corresponding Green functions are essentially more complex than in the vacuum, because they cannot be written in terms of standard Feynman diagrams. Here, a method is proposed to determine the structure of these Green functions and to derive nonperturbative equations for them. The main idea is to transform the cumulants describing correlations into interaction terms.Comment: 13 pages, 6 figure

a-C/Me (Me=Pt, Au) Nanocomposite Films for Electrochemical Gas Sensors

Thin film vacuum deposition technology has been considered with respect to the production of electrodes for electrochemical gas sensors. Electrodes, employing Pt and Au as catalytic agents, are produced by Magnetron Sputtering (MS) co-deposition of graphite/noble metal. Once inserted in the sensor as working electrodes, the resultant amorphous carbon (a-C)–noble metal nanocomposite layers are shown to meet such requirements as sensitivity to a range of gases, linear response, reasonable (≈30 s) response time and good signal-to-noise ratio (inferred from the relatively weak temperature dependence of the background current). The time stability of the new (Pt based) electrodes was checked by letting the electrode work inside a sensor over a period of 3 months. No morphological or compositional change was observed for such “aged” electrodes asì compared to “as-deposited” one