# What is 2D topological insulator?

## What is 2D topological insulator?

Two-dimensional topological insulators (2D TIs) are a remarkable class of atomically thin layered materials that exhibit unique symmetry-protected helical metallic edge states with an insulating interior. Recent years have seen a tremendous surge in research of this intriguing new state of quantum matter.

### What is a weak topological insulator?

Three-dimensional topological insulators are classified into “strong” (STI) and “weak” (WTI) according to the nature of their surface states. While the surface states of the STI are topologically protected from localization, this does not hold for the WTI.

**What is an excitonic insulator?**

Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs—excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects.

**How can we identify a topological insulator?**

A topological insulator is a material that behaves as an insulator in its interior but whose surface contains conducting states, meaning that electrons can only move along the surface of the material.

## Can a topological insulator be found in an exciton polariton?

Recently, topological insulators were suggested to be possible in exciton-polariton systems 16, 17, 18 organized as honeycomb (graphene-like) lattices, under the influence of a magnetic field. Exciton-polaritons are part-light, part-matter quasiparticles that emerge from strong coupling of quantum-well excitons and cavity photons 19.

### Can a topological insulator exist without a magnetic field?

Topological insulators were originally observed in the integer quantum Hall effect 2 (in which conductance is quantized in a strong magnetic field) and subsequently suggested 3, 4, 5 and observed 6 to exist without a magnetic field, by virtue of other effects such as strong spin–orbit interaction. These were systems of correlated electrons.

**What makes a chiral exciton different from other excitons?**

Unlike other known excitons composed of massive quasiparticles, chiral excitons are the bound states of surface massless electrons and surface massive holes, both subject to strong spin–orbit coupling which locks their spins and momenta into chiral textures.

**Why is the excitonic part of a polariton dissipative?**

While the excitonic fraction provides strong nonlinearity, the photonic part results in a low effective mass, allowing the formation of a Bose–Einstein condensate that is dissipative in nature and can be driven by a laser beam 20. Polaritons have thus been described as “quantum fluids of light” 21.