Enhanced repulsively bound atom pairs in topological optical lattice ladders

Abstract

There is a growing interest in using cold-atom systems to explore the effects of strong interactions in topological band structures. Here we investigate interacting bosons in a Cruetz ladder, which is characterised by topological flat energy bands where it has been proposed that interactions can lead to the formation of bound atomic pairs giving rise to pair superfluidity. By investigating realistic experimental implementations, we understand how the lattice topology enhances the properties of bound pairs giving rise to relatively large effective pair-tunnelling in these systems which can lead to robust pair superfluidity, and we find lattice supersolid phases involving only pairs. We identify schemes for preparation of these phases via time-dependent parameter variation and look at ways to detect and characterise these systems in a lattice. This work provides a starting point for investigating the interplay between the effects of topology, interactions and pairing in more general systems, with potential future connections to quantum simulation of topological materials.

Pairs, trimers and BCS-BEC crossover near a flat band: the sawtooth lattice

“…4a we display the result obtained from Eq. (19) for the weakest interaction considered, U = −0.25 (dashed line). There is a wide region around the FB point, where the numerical result is far apart from the weak coupling expansion.…”

“…The physics of two-body bound states in the presence of a flat band has been recently explored theoretically in different contexts, including topological matter [17][18][19][20] and the link between the inverse effective mass of the bound state and the quantum metric of the singleparticle states [21][22][23]. This second direction is related to the more general question of understanding how transport and superconductivity can occur in system with quenched kinetic energy [24][25][26][27][28][29][30][31][32].…”

“…(9) by retaining only the contribution from the lower band. The dashed line corresponds to the weak coupling result E b = U 2 f , where f (t) is defined in Eq (19)…”

Superconducting pairing from repulsive interactions of fermions in a flat-band system

“…Systems with flat-band dispersions have recently attracted much theoretical and experimental attention [1][2][3][4][5][6][7][8][9][10][11][12][13], because when the kinetic energy of quantum particles is near zero at some or all momenta, interactions become completely decisive for the many-body ground state. In particular, a major line of work has been devoted to the enhancement of superconductivity in electronic systems with flat bands near the Fermi level [6][7][8][9][10][11][12][13][14].…”

“…For this purpose, 1D systems are ideal targets, as density-matrix renormalization group (DMRG) techniques [16][17][18][19] yield quasiexact results. It also allows us to study the effect of flat bands in 1D systems specifically, a burgeoning field in itself [4,14,[20][21][22]. In particular, flat-band systems might fall outside the widely used low-energy Luttinger liquid (LL) description of 1D fermions, as the necessary linearization of the dispersion breaks down [23].…”