Robust nonequilibrium edge currents with and without band topology

TL;DR

This paper investigates nonequilibrium edge currents in 2D lattice systems, revealing robust chiral currents that persist due to topological and dissipative symmetries, even without band topology.

Contribution

It demonstrates the existence of robust nonequilibrium edge currents in both bosonic and fermionic systems, independent of topological band effects, under dissipative conditions.

Findings

Chiral edge currents are robust against reservoir coupling and defects.

Edge currents can flow against the temperature gradient without external work.

Fermionic systems exhibit topologically protected boundary currents that do not circulate around all edges.

Abstract

We study two-dimensional bosonic and fermionic lattice systems under nonequilibrium conditions corresponding to a sharp gradient of temperature imposed by two thermal baths. In particular, we consider a lattice model with broken time-reversal symmetry that exhibits both topologically trivial and nontrivial phases. Using a nonperturbative Green function approach, we characterize the nonequilibrium current distribution in different parameter regimes. For both bosonic and fermionic systems, we find chiral edge currents that are robust against coupling to reservoirs and to the presence of defects on the boundary or in the bulk. This robustness not only originates from topological effects at zero temperature but, remarkably, also persists as a result of dissipative symmetries in regimes where band topology plays no role. Chirality of the edge currents implies that energy locally flows against the temperature gradient without any external work input. In the fermionic case, there is also a regime with topologically protected boundary currents, which nonetheless do not circulate around all system edges.