Session W40: Exotic Exciton Transport and Polaritons in Nanostructures

Exciton localization and transport in van der Waals heterostructuresXiaoqin Li

Transition metal dichalcogenide heterostructures provide a versatile platform to explore electronic and excitonic phases. Depending on the exciton density, several distinct phases of localized excitons, mobile exciton gases, and ionized hot plasma can be observed in heterostructures. At low excitation density, interlayer excitons can be localized by moire potential in heterostructures with a small twist angle and relatively large supercells. As the excitation density exceeds the critical Mott density, interlayer excitons are ionized into an electron-hole plasma phase. At the excitation density well exceeding the Mott density, we find a surprisingly rapid initial expansion of hot plasma to a few microns away from the excitation source within 0.2 ps. Microscopic calculations reveal that this rapid expansion is mainly driven by Fermi pressure and Coulomb repulsion, while the hot carrier effect has only a minor effect in the plasma phase. I will also discuss a completely different type of moire superlattices that can be used to localize excitons without reducing quantum efficiency in the strong confinement regime with small supercells.   

Counterflow Conductivity and Quantum Geometry in Excitonic Condensates

Excitons are bound electron-hole pairs that Bose condense to form an excitonic condensate with macroscopic phase coherence. Motivated by recent experiments in TMDs, we focus on inter-layer excitons and investigate the effects of particle-hole asymmetry and quantum geometry on the condensate within a mean-field framework. We make connections between phase stiffness, Coulomb drag and counterflow conductivity, and highlight how they are enhanced by the quantum geometry of Bloch wavefunctions. We then add deviations from particle-hole symmetry in excitons and uncover an effective magnetic field that aids in pairing but quenches stiffness. Combining quantum geometry, particle-hole asymmetry with different pairing symmetries of the excitons, we comment on various regimes that optimize the BKT temperature. Our mean-field results have direct implications on the ongoing search for excitonic condensates in Moire materials.   

Contact Electrification of Two-Dimensional Materials

Contact electrification is a ubiquitous phenomenon that is still poorly understood. It has been proposed as a clean energy source for the Internet of Things in triboelectric nanogenerators (TENGs). Recently, we proposed a microscopic model in one-dimension in order to study the dynamics of electrons between atomic chains [M. Willatzen, L. C. Lew Yan Voon, and Z.-L. Wang, Adv. Func. Mat. 1910461 (2020)]. In this talk, we present an extension of the work to include charge transfer between normal and/or topological materials in one and two dimensions in order to assess the relevance of using two-dimensional materials in the design of TENGs.

    

Exciton Transport on a Polaritonic Quantum Wire

Polaritonic materials have been shown to possess enhanced transport properties. This observation is attributed to the strong interaction between light and matter promoted within these materials. Theoretical investigations on these systems traditionally employ a Tavis-Cumming effective picture that attempts to provide a general qualitative view of the phenomena. However, the effect of disorder and a multimode description of light on kinetic properties remains unclear. In this work, we provide extensive numerical simulations of wave packet evolution on polaritonic wires. We investigate the convergence of our results with respect to the size of the system and the number of cavity modes describing the radiation field. Moreover, transport and localization properties are evaluated for a range of parameters including disorder strength, detuning, and effective coupling strength. We discuss the implication of our results for theoretical models as well as the connection to recent experimental findings.   

Impact of free carriers on exciton and trion diffusion in monolayer WSe2

Propagation of light-emitting exciton quasiparticles is of central importance in the fields of condensed matter physics and nanomaterials science. Monolayer transition metal dichalcogenides are particularly suitable to study this phenomenon due to strong Coulomb interaction and efficient light-matter coupling. In these systems the excitons are tightly bound, but also mobile at both room and cryogenic temperatures, as well as interact strongly with free charge carriers. The impact of the exciton-electron interaction in the context of exciton propagation, however, remains unclear, while there is increasingly mounting evidence of mobile composite exciton-carrier states, known as trions and Fermi polarons. Here we address this question in a controlled experimental scenario and demonstrate diffusion of excitons in the presence of a continuously tunable Fermi sea. Studying hBN-encapsulated WSe₂ monolayers by ultrafast microscopy we reveal a non-monotonic dependence of the exciton diffusion coefficient on the charge carrier density in both electron and hole doped regimes. We identify distinct regimes of elastic scattering and quasiparticle formation  determining exciton diffusion and highlight the importance of treating exciton-electron scattering in the presence of additional energy and momentum dissipation via phonons. We further emphasize that both excitons and trions remain mobile down to 5 K, with an effective trion mobility reaching up to 3000 cm²/(Vs).

    

Exciton ground state bleaching in (6,5) single walled carbon nanotubes

We model the optical absorption of single walled carbon nanotubes using the quantum Boltzmann equation.

We explore a wide range of initial conditions and determine the dependency of the absorption spectrum as a function of the exciton and phonon populations.

Specifically, we investigate the behaviour of the dominant excitonic peak and the phonon side-bands arising from exciton-phonon coupling.

We observe two distinct behaviours depending on the excitonic statistics.

At low occupancy (i.e. bosonic excitons) there is little to no change in the absorption spectrum.

On the other hand, at high occupancy (i.e. fermionic excitons) we observe an important weakening of both the main excitonic peak and the phonon side-bands.

Most importantly, we observe a "negative" absorption corresponding to the emission of light caused by an excitonic population-inversion effect.

This effect is cause by a Pauli blocking mechanism which is responsible for the bleaching effect observed in many pump-probe experiments.   

Temperature-dependent photoresponse and transport properties of silicon MoS2 Van der Waals heterostructure

Defects present in two-dimensional materials (2D) have a strong influence on the electrical and optical properties of devices based on 2D materials. Here, the temperature-dependent electronic transport and photoresponse of a silicon-MoS₂ p-n junction heterostructure are studied to understand the effect of defects, and a method of improving the photoresponse is demonstrated. The temperature-dependent I-V characteristics suggest the presence of a space charge limited transport with exponentially distributed trap states. The temperature dependence of ideality factor and intensity-dependent photoresponse also elucidate the nature of defects. The defects present in the heterostructure can cause recombinations diminishing the photoresponse and severely degrading the optoelectronic properties. A significant enhancement in photoresponse by reducing the recombination centers can be obtained by altering the dielectric environment. The study shows that for a particular dielectric, the enhancement is more prominent towards low temperatures. The low-frequency noise is observed to decrease with an increase in temperature and the dielectric medium suppressed the noise levels in these devices. Insights from this study would help in designing and improving the optoelectronic properties of low-dimensional optoelectronic devices.

    

Ballistic transverse magnetic focusing in monolayer graphene on WSe2

It has been found that a large spin-orbit coupling (SOC) is induced in monolayer graphene by proximity with transition metal dichalcogenides (TMDC), as compared to the very weak one in pristine graphene. Theoretical calculation showed that the SOC splits the monolayer band and there exist two types: the valley-Zeeman term which couples the out-of-plane spin and valley degrees of freedom, and the Rashba term which couples the in-plane spin and sublattice degrees of freedom. Here we employ the transverse magnetic focusing technique to probe the effects of the SOC on the Fermi surface profile and electron dynamics in monolayer graphene-TMDC heterostructure. We clearly observed the spin-split band in graphene on WSe₂, and quantitively estimated the sizes of both SOC terms by analyzing the TMF peaks. Interestingly, the second TMF peak didn’t show the splitting and theoretical analysis further shows that this is closely related to the effect of scattering at the sample edges and the spin conservation.   

Extreme doping of graphene

Extreme doping of two-dimensional (2d) van der Waals materials can lead to emergent phases and next generation electronic devices and photonics. This also requires clean and local charge control. Here, we demonstrate that the large work function narrow-band Mott insulator RuCl3 enables modulation doping of exfoliated graphene. We characterized the doping level in trilayer structure RuCl3/graphene/RuCl3 using Raman. The G and 2D peak of graphene showed significant upshifts, indicating that graphene is heavily doped. By performing Raman measurements at different excitation energies, we show that, with excitation wavelength longer than 633nm, the 2D peak can be suppressed by Pauli blocking as laser energy below the doping level.

    

Mobile interlayer excitons at the Mott transition in Moiré-free heterostructures

Vertically stacked heterostructures of transition metal dichalcogenides (TMDCs) present an exciting platform to study electronic many-body phenomena. The type-II band alignments, commonly encountered in TMDC heterobilayers and the presence of strong Coulomb interactions results in the formation of tightly bound and mobile interlayer excitons. What remains barely explored, however, is the high-density regime between excitons and dense plasma in the context of exciton propagation. Moreover, the heterostructures can exhibit substantial complexity due to formation of Moiré-type superlattices. It motivates investigation of high-density exciton transport phenomena in the absence of such potentials, to disentangle the effects of dipolar excitons from those stemming from Moiré effects. This is the main topic of our study, taking advantage of hBN-encapsulated WSe2/MoSe2 heterostructures studied in the Moiré-free limit of large, atomically reconstructed domains. Using ultrafast microscopy, we show that the interlayer excitons propagate freely even at cryogenic temperatures and low densities. At elevated exciton densities, we demonstrate that in addition to broadly assumed exciton-exciton repulsion, the non-linear increase of the diffusion coefficient also originates from efficient exciton-exciton annihilation. Remarkably, at the exciton ionization threshold of the Mott transition and beyond, we reveal a highly unusual regime of negative effective diffusion that persist for many 100's of ps after the excitation. This observation presents a particularly interesting case of non-equilibrium phenomena in composite many-particle systems, highlighting the rich physics of optical excitations in van der Waals heterostructures.   

Strong plasmon-exciton coupling probed through electroluminescence and polarization control

The realization and manipulation of polaritons, coupled quantum excitations, is of fundamental importance in the prospect of realizing novel photonic devices. Here, we investigate the formation of plasmon-exciton polaritons in hybrid structures consisting of a two-dimensional transition-metal dichalcogenide (TMDC) transferred onto a plasmonic metal tunnel junction. These excitations are driven using plasmon-based electroluminescence for incoherent optical excitation. We determine the polaritonic spectrum from the electroluminescence and show that the polarization dependence of the emission is sensitive to the plasmon-exciton coupling for local junction plasmon modes, finding with that Rabi splittings exceeding 100 meV can readily be achieved. We thus believe that this incoherent, electrically driven plexciton coupling provides potential applications for engineering compact photonic devices with tunable optical and electrical properties.   

Electron Optics and Valley Hall Effect in Undulated Graphene

Electron optics is the systematic use of electro-magnetic (EM) fields to control electron motions and forms the basis of many technological applications such as particle accelerators, tokamak reactors, or electron microscopes. The same principles have been applied to 2D materials to create Hall effects or Veselago-lens. However, the small length-scale of nanomaterials have made it challenging to create more complex “optical” designs to truly achieve 2D electron optics. Graphene offers a unique opportunity as its strain gradients produce pseudo-electromagnetic fields to guide its electron motion. Based on this concept, we demonstrate the use of substrate topography to impart desirable strain patterns on graphene to induce useful pseudo-EM fields. We derive the quasi-classical equation of motion for Dirac Fermions in a pseudo-EM field in graphene. Based on the trajectory analysis, we design sculpted substrates to realize various “optical devices” such as a converging lens or a collimator, and further propose a setup to achieve valley Hall effect solely through substrate patterning, without any external fields, to be used in valleytronics applications. Finally, we discuss how the predicted strain/pseudo-EM field patterns can be experimentally sustained by typical substrates and generalized to other 2D materials.