Fano resonance through Higgs bound states in tunneling of Nambu-Goldstone modes

TL;DR

This paper investigates how Higgs bound states in superfluid Bose gases near a Mott transition can cause Fano resonances in tunneling of Nambu-Goldstone modes, revealing universal phenomena in systems with particle-hole symmetry.

Contribution

It demonstrates the existence of Higgs bound states localized around potential barriers and their role in mediating Fano resonance with NG modes, a novel insight into collective excitations.

Findings

Higgs bound states are localized around certain barriers.

Fano resonance occurs due to coupling between Higgs bound states and NG modes.

Universal presence of Higgs bound states in systems with particle-hole symmetry.

Abstract

We study collective modes of superfluid Bose gases in optical lattices combined with potential barriers. We assume that the system is in the vicinity of the quantum phase transition to a Mott insulator at a commensurate filling, where emergent particle-hole symmetry gives rise to two types of collective mode, namely a gapless Nambu-Goldstone (NG) phase mode and a gapful Higgs amplitude mode. We consider two kinds of potential barrier: One does not break the particle-hole symmetry while the other does. In the presence of the former barrier, we find Higgs bound states that have binding energies lower than the bulk Higgs gap and are localized around the barrier. We analyze tunneling properties of the NG mode incident to both barriers to show that the latter barrier couples the Higgs bound states with the NG mode, leading to Fano resonance mediated by the bound states. Thanks to the universality of the underlying field theory, it is expected that Higgs bound states may be present also in other condensed matter systems with a particle-hole symmetry and spontaneous breaking of a continuous symmetry, such as quantum dimer antiferromagnets, superconductors, and charge-density-wave materials.