Getting more from pushing less: Negative specific heat and conductivity in nonequilibrium steady states

For students familiar with equilibrium statistical mechanics, the notion of a positive specific heat, being intimately related to the idea of stability, is both intuitively reasonable and mathematically provable. However, for systems in nonequilibrium stationary states, coupled to more than one energy reservoir, a negative specific heat is entirely possible. We present a minimal system that displays this phenomenon. For a system in contact with two thermal baths at different temperatures, the (internal) energy may increase when a thermostat is turned down. In another context, a similar phenomenon is negative conductivity, where a current may increase by decreasing the drive (for example, an external electric field). The counter-intuitive behavior in both processes may be described as getting more from pushing less. The crucial ingredients for this phenomenon and the elements needed for a minimal system are also presented. (C) 2002 American Association of Physics. Teachers.