Topological orbital ladders

TL;DR

This paper introduces a novel topological phase in interacting fermions on a two-leg ladder with unequal orbitals, characterized by a simple $Z_2$ invariant, and explores its extension to two dimensions with potential for strongly correlated states.

Contribution

It reveals a new topological phase arising from $sp$-orbital tunneling without spin-orbit coupling, and shows how edge modes form a flat band in two dimensions, enabling strongly correlated states.

Findings

Identification of a $Z_2$ topological invariant from $sp$-orbital tunneling.

Edge modes form a parity-protected flat band in 2D.

Experimental signatures include density correlations and phase transitions.

Abstract

We unveil a topological phase of interacting fermions on a two-leg ladder of unequal parity orbitals, derived from the experimentally realized double-well lattices by dimension reduction. $Z_2$ topological invariant originates simply from the staggered phases of $sp$-orbital quantum tunneling, requiring none of the previously known mechanisms such as spin-orbit coupling or artificial gauge field. Another unique feature is that upon crossing over to two dimensions with coupled ladders, the edge modes from each ladder form a parity-protected flat band at zero energy, opening the route to strongly correlated states controlled by interactions. Experimental signatures are found in density correlations and phase transitions to trivial band and Mott insulators.