Current circulation near additional energy degeneracy points in quadratic Fermionic networks

Abstract We study heat and particle current circulation (CC) in quadratic Fermionic systems analysed using a general dissipative Lindbladian master equation. It was observed in an earlier study (Upadhyay et al 2023 Phys. Rev. E 107 034120) that CC occurs near the additional energy degeneracy point (AEDP) in Fermionic systems which have some form of asymmetry. We find general analytical expression to support this observation for quadratic Fermionic networks. We then apply these ideas to the Su–Schrieffer–Heeger (SSH) model with periodic boundary conditions and a tight binding model with unequal hopping strengths in the upper and lower branches. In both these cases, we find the specific conditions required for observing CC and study the behavior of these currents with various system parameters. We find that having unequal number of Fermionic sites in the upper and lower branches is enough for generating CC in the SSH model. However, this asymmetry is not adequate for the tight-binding model and we require unequal hopping strengths in the upper and lower branches to induce CC in this model. We also compare our results with the exact results obtained via the Non-equilibrium Green function formalism, and observe that the relationship between AEDP and CC also holds for the exact results. Finally, we observe that for certain system parameters, the onset point of particle and heat CC are not the same. Based on all these observations, we describe how carefully examining the energy spectrum of the system gives a great deal of information about the possibility and behavior of CC in Fermionic systems with asymmetries.