After finishing PhD he started to work on the problem of a particle interacting with a quantum field. This problem is directly related to polaron theory, the effect of impurities on the energy spectrum of superfluids, and other problems in condensed matter physics. He was involved in the development of operator form of perturbation theory, approximate second quantization, adiabatic approximation for systems with translational invariance, and other theoretical physics methods which play an important role in the theory of many-particle systems.
