A quantum-statistical-mechanical theory of nonequilibrium stationary states (NESS) is presented. The average of an arbitrary dynamical operator is calculated. The theory is applied to correlation functions in a heat-conducting NESS of the Heisenberg ferromagnet. Significant changes (as compared with equilibrium) are found. Correlation functions that vanish in equilibrium due to symmetry are allowed and are very important for $\overrightarrow{\mathrm{k}}$ vectors in the hydrodynamic regime. In the absence of magnetic field, the change with respect to equilibrium is pronounced. A static correlation that depends on $k$ as $\frac{1}{k^7}$ is found. Such correlations are expected to be seen experimentally when magnon-phonon interactions can be neglected (i.e., $k>\mathrm{}{10}^6$ ${\mathrm{cm}}^{-1\mathrm{}}$). The influence on time correlation functions is examined and found to be well within experimental detectability.

• Received 5 December 1979

DOI:https://doi.org/10.1103/PhysRevA.22.1720