In the early 1950s, Kubo transformed research into the linear response properties of near-equilibrium condensed-matter systems, in particular the understanding of electron transport and conductivity, through the Kubo formalism, a Green's function approach to linear response theory for quantum systems. In 1977 Ryogo Kubo was awarded the Boltzmann Medalfor his contributions to the theory of non-equilibrium statistical mechanics, and to the theory of fluctuation phenomena. He is cited particularly for his work in the establishment of the basic relations between transport coefficients and equilibrium time correlation functions: relations with which his name is generally associated.
"Statistical-mechanical theory of irreversible processes. 1. general theory and simple applications to magnetic and conduction problems". J. Phys. Soc. Jpn.12: 570. 1957. doi:10.1143/JPSJ.12.570. – cited more than 4300 times on Web of Science.