The `Higgs' Amplitude Mode at the Two-Dimensional Superfluid-Mott Insulator Transition

TL;DR

This paper reports the experimental observation of a Higgs amplitude mode in a two-dimensional superfluid near the Mott insulator transition, confirming theoretical predictions about its existence and behavior in low-dimensional quantum systems.

Contribution

First experimental identification of a Higgs mode in a 2D superfluid close to a quantum critical point, using ultracold atoms and spectral response measurements.

Findings

Higgs mode softens approaching the quantum critical point

Mode is clearly distinguishable from Nambu-Goldstone modes

System described by an effective relativistic field theory

Abstract

Spontaneous symmetry breaking plays a key role in our understanding of nature. In a relativistic field theory, a broken continuous symmetry leads to the emergence of two types of fundamental excitations: massless Nambu-Goldstone modes and a massive `Higgs' amplitude mode. An excitation of Higgs type is of crucial importance in the standard model of elementary particles and also appears as a fundamental collective mode in quantum many-body systems. Whether such a mode exists in low-dimensional systems as a resonance-like feature or becomes over-damped through coupling to Nambu-Goldstone modes has been a subject of theoretical debate. Here we reveal and study a Higgs mode in a two-dimensional neutral superfluid close to the transition to a Mott insulating phase. We unambiguously identify the mode by observing the expected softening of the onset of spectral response when approaching the quantum critical point. In this regime, our system is described by an effective relativistic field theory with a two-component quantum-field, constituting a minimal model for spontaneous breaking of a continuous symmetry. Additionally, all microscopic parameters of our system are known from first principles and the resolution of our measurement allows us to detect excited states of the many-body system at the level of individual quasiparticles. This allows for an in-depth study of Higgs excitations, which also addresses the consequences of reduced dimensionality and confinement of the system. Our work constitutes a first step in exploring emergent relativistic models with ultracold atomic gases.