Dark states of electrons in a quantum system with two pairs of sublattices

TL;DR

This paper reports the discovery of condensed matter dark states in palladium diselenide, where certain valence bands are undetectable across the Brillouin zone due to interference effects from sublattice symmetries.

Contribution

It introduces a mechanism for dark states in systems with two pairs of sublattices, explaining their role in condensed matter phenomena and optoelectronic properties.

Findings

Valence bands are practically unobservable across the Brillouin zone.

Dark states arise from destructive interference due to sublattice symmetries.

The mechanism explains phenomena in cuprates, perovskites, and density wave systems.

Abstract

A quantum state of matter that is forbidden to interact with photons and is therefore undetectable by spectroscopic means is called a dark state. This basic concept can be applied to condensed matter where it suggests that a whole band of quantum states could be undetectable across a full Brillouin zone. Here we report the discovery of such condensed matter dark states in palladium diselenide as a model system that has two pairs of sublattices in the primitive cell. By using angle-resolved photoemission spectroscopy, we find valence bands that are practically unobservable over the whole Brillouin zone at any photon energy, polarisation, and scattering plane. Our model shows that two pairs of sublattices located at half-translation positions and related by multiple glide-mirror symmetries make their relative quantum phases polarised into only four kinds, three of which become dark due to double destructive interference. This mechanism is generic to other systems with two pairs of sublattices, and we show how the phenomena observed in cuprates, lead-halide perovskites, and density wave systems can be resolved by the mechanism of dark states. Our results suggest that the sublattice degree of freedom, which has been overlooked so far, should be considered in the study of correlated phenomena and optoelectronic characteristics.