Charge-Neutral Electronic Excitations in Quantum Insulators

TL;DR

This paper reviews recent advances in detecting and understanding charge-neutral electronic excitations in quantum insulators, crucial for exploring exotic quantum phases and strongly correlated materials.

Contribution

It provides a comprehensive overview of theoretical and experimental progress in probing neutral excitations in unconventional insulators and discusses future opportunities in quantum materials research.

Findings

Progress in detecting neutral fermionic, bosonic, and anyonic excitations

Identification of candidate quantum spin liquids and excitonic insulators

Challenges and prospects in utilizing quantum insulators for quantum technologies

Abstract

Experiments on quantum materials have uncovered many interesting quantum phases ranging from superconductivity to a variety of topological quantum matter including the recently observed fractional quantum anomalous Hall insulators. The findings have come in parallel with the development of approaches to probe the rich excitations inherent in such systems. In contrast to observing electrically charged excitations, the detection of charge-neutral electronic excitations in condensed matter remains difficult, though they are essential to understanding a large class of strongly correlated phases. Low-energy neutral excitations are especially important in characterizing unconventional phases featuring electron fractionalization, such as quantum spin liquids, spin ices, and insulators with neutral Fermi surfaces. In this perspective, we discuss searches for neutral fermionic, bosonic, or anyonic excitations in unconventional insulators, highlighting theoretical and experimental progress in probing excitonic insulators, new quantum spin liquid candidates and emergent correlated insulators based on two-dimensional layered crystals and moir\'e materials. We outline the promises and challenges in probing and utilizing quantum insulators, and discuss exciting new opportunities for future advancements offered by ideas rooted in next-generation quantum materials, devices, and experimental schemes.