Research Sara Conti

Abstract

The study of excitons in two-dimensional materials is rapidly evolving with promising applications in various fields such as optoelectronics and quantum information. Of particular interest are the excitons in van der Waals bilayers where the electrons and holes are confined in different layers giving rise to dipolar interactions and long life times. Their properties can be tuned by applying pressure, electric fields or by inducing a moiré modulation by twisting the layers. When a two-dimensional material is embedded in an optical microcavity, it can give rise to the formation of hybrid light-matter quasi-particles, the so-called exciton-polaritons. In this project, we wish to explore theoretically the properties of such exciton-polaritons and exploit their tunability in order to give them desirable properties. In particular, a long standing goal in the field of polariton physics is the realisation of polariton-polariton interactions that are larger than their linewidth and allow to reach exotic phases of strongly correlated photons. A large part of this project will be devoted to the study of the interactions between the polaritons and the exploration of how to enhance them by controlling experimental parameters, in particular the moiré-induced potential. In the second part of the project, we will investigate the many body polariton phases that can be realised with this system, taking into account the particularities of the system unveiled in the first part of our study.

Researcher(s)

• Promoter: Wouters Michiel
• Co-promoter: Conti Sara
• Fellow: Van Hoorebeke Manon

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The supersolid, an intriguing counter-intuitive quantum state in which a rigid lattice of particles flows without resistance, has attracted long-time interest but has to date not been unambiguously realised. Alternative approaches have been proposed to Chester's original idea of a supersolid, where within the macroscopic quantum condensate the single particles are localized on each lattice site by strong repulsion. These include periodic density-modulated superfluids or clusters of condensates observed in cold atoms gases in optical lattices. Most recently, we have revealed a supersolid in double-layer heterostructures with interlayer excitons: electrons confined in a layer, coupled with holes, confined in a separated layer. This exciton supersolid is a Chester-type supersolid with one exciton per site and it shows over a wide range of layer separations, well within reach of the experimental capabilities but outside the focus of recent experiments. In this project, we aim to theoretically investigate how the existence and the stability of an interlayer exciton supersolid can be controlled and enhanced, by providing the phase diagram augmented by all supersolid phases. By controlling the layer separation (length of the exciton dipole) and the exciton density of the system, the exciton repulsions can be tuned to stabilize the supersolid with respect to the other excitonic phases and rich novel phenomena can be explored in the vicinity of the phase transitions.

Researcher(s)

• Promoter: Milosevic Milorad
• Co-promoter: Tempere Jacques
• Fellow: Conti Sara

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project

Abstract

Recent experimental results on superconductivity in twisted bilayer graphene indicates that flat bands are a key feature to explore strongly correlated phases. A straightforward access to quasi-flat bands has also been realised by means of twisting or periodic straining in van der Waals heterobilayers made of transition metal dichalcogenides. By controlling the doping in bilayer systems, it is possible to generate interlayer excitons: electrons are confined in a layer and couples with holes, confined in the opposite and separated layer. In this project, I propose to theoretically study the effects of tunable flat bands on interlayer excitons with the aim to investigate the emergence of excitonic strongly correlated phases. At low temperature, a number of competing phases have been predicted, including electron-hole superfluidity, exciton insulator, coupled Wigner crystallisation and charged density waves. The enhancement of the effective masses of the carriers increases of the excitonic binding energy and this makes the excitonic phases more robust. By tuning the flatness of the bands it will become possible to enhance the critical temperature for electron-hole superfluidity and to control the emergence of the competing strongly correlated phases that appear in the phase diagram.

Researcher(s)

• Promoter: Partoens Bart
• Co-promoter: Neilson David
• Fellow: Conti Sara

Research team(s)

• Condensed Matter Theory

Project type(s)

• Research Project