Research David Neilson

Abstract

Recent observations of possible signatures of Bose-Einstein condensation and superfluidity of excitons have drawn a lot of attention to excitonic bilayer systems. An exciton bilayer is a two-dimensional device where there are two conducting layers, one doped with electrons and one with holes, separated by few nanometers. In the last decade there has been a huge search effort to find superfluid phases in exciton bilayers, and there are experimental indications of a superfluid phase but to date the evidence is not clear. The aim of this project is to investigate three definitive fingerprints of exciton superfluidity: identification-markers. 1) We propose to employ the Josephson effect in exciton bilayers taken for the first time in combination with Coulomb drag measurements to definitively identify superfluidity. 2) Mapping out the collective modes in the various phases of the exciton bilayer system at different temperatures and densities. Characterization of the excitation spectra (i) in the exciton superfluid, (ii) exciton normal-fluid and (iii) decoupled normal-fluid phases. 3) Examination of the pseudogap region as a function of temperature and density. This is a vital high-temperature precursor of the superfluid transition. This understanding will provide a new theoretical basis for the experiments that aim to map out the various phases in the exciton bilayer system.

Researcher(s)

• Promoter: Milosevic Milorad
• Co-promoter: Neilson David
• Fellow: Pascucci Filippo

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