Diagrammatic $λ$ series for extremely correlated Fermi liquids

The recently developed theory of extremely correlated Fermi liquids (ECFL), applicable to models involving the physics of Gutzwiller projected electrons, shows considerable promise in understanding the phenomena displayed by the $t$-$J$ model. Its formal equations for the Greens function are reformulated by a new procedure that is intuitively close to that used in the usual Feynman-Dyson theory. We provide a systematic procedure by which one can draw diagrams for the $\lambda$-expansion of the ECFL introduced in Ref. (9), where the parameter $\lambda \in (0,1)$ counts the order of the terms. In contrast to the Schwinger method originally used for this problem, we are able to write down the $n^{th}$ order diagrams ($O(\lambda^n)$) directly with the appropriate coefficients, without enumerating all the previous order terms. This is a considerable advantage since it thereby enables the possible implementation of Monte Carlo methods to evaluate the $\lambda$ series directly. The new procedure also provides a useful and intuitive alternative to the earlier methods.