Controlling and Protecting Quasiparticles in 2-D Quantum Materials

In this presentation, we explore how quasiparticles can be controlled and stabilized within 2-D solids by creating carefully designed atomic hetero structures. Quasiparticles give raise to phenomena such as excitons (bound states of electrons and holes), superconducting states, polaritons (hybrids of light and matter), as well as more exotic systems like Tomonaga-Luttinger liquids. Each of these quasiparticles emerges due to the unique symmetries of the crystal structure.

By engineering precise atomic-scale patterns (heterostructures) within 2-D materials, we can not only protect and manipulate quasiparticles for potential applications, but potentially find new ones. For instance, heterostructures that confine matter in zero, one, or two dimensions allow us to control these emergent properties with unprecedented precision.

In the first part of my talk, I will focus on excitons - quasiparticles formed by electron-hole pairs. We have investigated excitons in stacks of the 2-D materials WS? and WSe?, which are promising candidates for next-generation quantum technologies. These stacks potentially host Bose-Einstein Condensates, a state where excitons behave like a collective whole. By coupling these excitons to plasmonic cavities, we have been able to study how they emit light, particularly “dark excitons,” which don’t normally emit photons. Additionally, we provide evidence of excitons traveling coherently over distances when strongly coupled to plasmons, forming a new hybrid quasiparticle called a plexciton.

In the second part of the talk, I will explore defects in 2-D materials that act as quantum emitters, which are critical for applications like quantum sensing. Using photo Scanning Tunneling Microscopy (photo-STM), we examine how tiny imperfections in the crystal structure of MoSe? and WS? - such as missing atoms - create unique energy states within the material’s band structure. These defects can emit single photons, which is a key requirement for quantum technologies. We have also shown how replacing individual atoms in the structure with elements like carbon or cobalt creates well-defined systems similar to color centers in diamonds, opening the door to new sensing and computational devices.

Finally, I will discuss how certain 1-D defects in 2-D materials - mirror twin boundaries - act as atomically thin conductors, which display a remarkable transition into a quantum liquid state at low temperatures - a Tomonaga-Luttinger liquid (TLl). We show that the formation of these TLls requires a combination out of a 2-D and 1 D heterostructure between graphene and 2-D WS2 and how they can give us new insights into highly correlated electron states.

Speaker:Alex Weber Bargioni, Lawrence Berkeley National Laboratory