We work out the non-equilibrium steady state properties of a harmonic lattice
which is connected to heat reservoirs at different temperatures. The heat
reservoirs are themselves modeled as harmonic systems. Our approach is to write
quantum Langevin equations for the system and solve these to obtain steady
state properties such as currents and other second moments involving the
position and momentum operators. The resulting expressions will be seen to be
similar in form to results obtained for electronic transport using the
non-equilibrium Green's function formalism. As an application of the formalism
we discuss heat conduction in a harmonic chain connected to self-consistent
reservoirs. We obtain a temperature dependent thermal conductivity which, in
the high-temperature classical limit, reproduces the exact result on this model
obtained recently by Bonetto, Lebowitz and Lukkarinen.Comment: One misprint and one error have been corrected; 22 pages, 2 figure