Faculty

Research Overview

We are a theory group with interest in the physics of excitons (bound electron-hole pairs), spin-polarized electrons, and spin/valley interactions in semiconductors. Our focus since 2013 is on atomically thin monolayer semiconductors. Below, we provide five synopses of our research results since 2020 .

Roles of exciton-electron and trion-electron interactions in WSe₂ monolayers 

We have presented photoluminescence measurements in monolayer WSe2, performed by our collaborators in Toulouse, which point to the importance of the interaction between charged particles and excitonic complexes. Our theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer WSe2 on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their interaction with electrons or holes, our analysis sheds light on several key features that are commonly seen in the photoluminescence of this monolayer semiconductor. We can understand why the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole doped rather than charge neutral or electron doped. Similarly, we can understand the reason for the dramatic increase of the photoluminescence intensity of negatively charged excitons (trions) as soon as electrons are added to the monolayer. To self-consistently explain the findings, we have further studied the photoluminescence spectra at different excitation energies and analyze the behavior of the elusive indirect exciton. 

Ref: Phys. Rev. B. 105, 085302 (2022)

Hexciton and Oxcitons in monolayer transition-metal dichalcogenides 

In the archetypal monolayer semiconductor WSe2, the distinct ordering of spin-polarized valleys (low-energy pockets) in the conduction band allows for studies of not only simple neutral excitons and charged excitons (i.e., trions), but also more complex many-body states that are predicted at higher electron densities. We have analyzed magneto-optical measurements of electron-rich WSe₂ monolayers, performed by our collaborators, and interpret the spectral lines that emerge at high electron doping as optical transitions of six-body exciton states (“hexcitons”) and eight-body exciton states (“oxcitons”). These many-body states emerge when a photoexcited electron-hole pair interacts simultaneously with multiple Fermi seas, each having distinguishable spin and valley quantum numbers. In addition, we have explained the relations between dark trions and satellite optical transitions of hexcitons in the photoluminescence spectrum.

Ref: Phys. Rev. Lett. 129, 076801 (2022)

Composite excitonic states in doped semiconductors 

We have presented a theoretical model of composite excitonic states in doped semiconductors. Many-body interactions between a photoexcited electron-hole pair and the electron gas are integrated into a computationally tractable few-body problem, solved by the variational method. We focused on electron-doped monolayer (ML)-MoSe₂ and ML-WSe₂ due to the contrasting character of their conduction bands. In both cases, the core of the composite is a tightly bound trion (two electrons and a valence-band hole) surrounded by a region depleted of electrons. The composite in ML-WSe₂ further includes a satellite electron with different quantum numbers. The

theory is general and can be applied to semiconductors with various energy-band properties, allowing one to calculate their excitonic states and to quantify the interaction with the Fermi sea. 

Ref: Phys. Rev. B. 106 L081301 (2022)

Measurement of conduction and valence bands g-factors in a TMD Monolayer

Together with our experimental collaborators in Toulouse, we have developed an original technique to measure the g-factors of electrons and holes in WSe₂.  The method exploits the optical selection rules of exciton complexes, in particular the ones involving intervalley phonons, avoiding strong renormalization effects that compromise single particle g-factor determination in transport experiments.

Ref: Phys. Rev. Lett. 126, 067403 (2021)

Zone edge phonons and exciton complexes in WSe₂ monolayers

Together with our experimental collaborators in Seattle, we have provided the physical origin for a myriad of optical transitions in WSe₂ monolayers. The identification relies on the the coupling between spin, charge, and lattice degrees of freedom plays. We have reported and analyzed the observation of multiple valley phonons – phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone – and the resulting exciton complexes in the WSe₂ monolayers. We found that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation, leading to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncover an intervalley exciton near charge neutrality. Our work not only identifies several previously unknown 2D excitonic species, but also shows that monolayer WSe₂ is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.

Ref: Nature communications 11, 618 (2022).