Title: Seminar: Excitons in 2D Materials
Location: TEK Ellehammer - SDU, Campusvej 55, 5230 Odense M
Online participation: is possible. Sign up and you will receive a Zoom link.
Date: 23 January 2024
Time: 12.00-17.00 - lunch is included
Program
"Elementary theoretical methods for computing the excitonic properties of 2D materials" by Nuno Peres, University of Minho/SDU POLIMA
Abstract: In this seminar Nuno Peres will introduce several theoretical methods of determining the exciton wave functions and the corresponding eigenenergies. The methods covered are either analytical, semianalytical, or numeric. All the details associated with the different methods are made explicit time permitting, thus allowing newcomers to do research on their own, without experiencing a steep learning curve. In the seminar Nuno Peres will start with the variational method and end with a simple semi-analytical approach to solve the Bethe–Salpeter equation (BSE) in gapped 2D materials. We address single layer of hexagonal boron nitride (hBN) and of transition metal dichalcogenide (TMD), as these are exemplary materials in the field of 2D excitons. For explaining the Bethe–Salpeter method, the biased bilayer graphene is chosen. The system has the right amount of complexity (without being excessive). This allows the presentation of the solution in a context that can be easily generalized to more complex systems or to apply it to simpler models.
Abstract: The appearance of the moiré period resulting from stacking atomic thin layers with rotational misalignment and/or lattice mismatch creates moiré superlattices, which have emerged as a unique platform for exploring novel physics, e.g., superconductor in the magic-angle superlattice, correlated insulator, and exciton localization. The formation of periodic energy potential traps in such a system strongly modulates existing excitonic states and their transport properties even at room temperature.
In this talk, Sanshui Xiao will mainly discuss recent experimental projects: (1) localization and interaction of interlayer excitons in MoSe2/WSe2 heterobilayers: addressing different trap mechanisms, the emergence of biexcitons, and the impact of potential traps on biexciton formation. (2) observation of room-temperature moiré exciton in the MoS2/WSe2 system, the impact of moiré potential on excitonic light emission, and potential application for lasing at optical fiber communication O-band (1260-1360 nm).
"Theoretical calculations of excitons and trions in two dimensional materials" by André Chaves, Instituto Tecnológico de Aeronáutica (ITA)
Abstract: André Chaves will divide this seminary in three topics i) Dielectric engineering of interlayer and intralayer excitons in double layers of transition metal dichalcogenides (TMDs), using a formalism based on the expansion of Chebyshev polynomials to solve the Wannier equation. We show how the binding energies and wave functions of inter-and intralayer excitons depend on on the spacer width and the dielectric constant. ii) Excitons in biased twisted bilayer graphene (TBG) under pressure, using a tight-binding model and semiconductor Bloch equations. We show that biased TBG under pressure opens a bandgap and hosts highly hybridized and anisotropic excitons. iii) Using the Faddeev Formalism, we solve the three body problem of the trion in momentum space using two different regularization methods for the Rytova-Keldysh potential. We obtain that both methods converge to the same value for the trion binding energy.
"Manipulation of excitonic dynamics in two-dimensional materials" by Sergii Morozov, SDU POLIMA
Valley polarization is a key concept in quantum computing, enabling the encoding and processing of quantum information. However, valley depolarization due to temperature-dependent intervalley scattering has posed a significant challenge for practical applications of TMD monolayers at room temperature. In our experiments, we utilized electron-doping techniques, pushing electron densities beyond 10^13 cm⁻², which resulted in remarkable valley contrasts of 61% in tungsten diselenide (WSe2) and 37% in molybdenum diselenide (MoSe2) monolayers at room temperature. This breakthrough suggests that charged excitons in TMD monolayers can serve as quantum units for the development of practical valleytronic devices operating at 300 K.
The precise manipulation of excitonic dynamics in two-dimensional materials holds immense promise for advancing quantum technology and photonics. In our study, we also explore the effects of strain engineering in transition metal dichalcogenide (TMD) monolayers to achieve efficient exciton funneling for localization of TMD-based qubits. By employing strain-induced perturbations, we establish control over exciton dynamics, resulting in the funneling of excitons with enhanced precision and efficiency, while also providing the control over valley dynamics. This transformative approach not only sheds light on the fundamental mechanisms governing excitonic behavior in TMD monolayers but also paves the way for the development of novel quantum devices, ultra-sensitive optical sensors, and advanced photonic systems. Our findings offer a promising avenue for harnessing strain engineering as a powerful tool in the pursuit of tailored quantum states and optimized photonic functionalities in two-dimensional materials.