Yasser Saleem, Hamburg University

Leuchs-Russell-Auditorium, A.1.500, Staudtstr. 2

Location details

Abstract:

There is currently great interest in semiconductor quantum dots (QDs) as building blocks of quantum technology. This includes lateral gated QDs confining spins of electrons or holes for quantum computation, self-assembled QDs confining carriers for emitters, detectors, lasers, displays, and single and entangled photon pair sources. The gated lateral QDs allow for high tunability of their electronic properties but are generally limited to confining either electrons or holes, while self-assembled dots confine both electrons and holes but are difficult to tune. Three main topics will be discussed here.

First, we present a theory of electronic and optical properties of a few-electrons, an exciton, and a trion confined in a bilayer graphene (BLG) QD [1-3]. We utilize the atomistic tight-binding approach to compute the single-particle energies and wave functions of quasiparticles confined in a BLG QD consisting of ~1.6 million atoms, followed by the microscopic calculation of Coulomb and dipole matrix elements. This allows us to formulate a Hamiltonian of interacting quasiparticles accounting for the BLG valleys, trigonal warping, and details of the displacement and lateral confinement potentials on equal footing. We fill the BLG QD with N (N=2 to 6) electrons and calculate the spin and valley polarization of their ground state as a function of the strength of Coulomb interactions relative to the single-particle energy (tuned, e.g., by changing the QD size). We find that as the interactions grow stronger, the system undergoes a series of phase transitions increasing both their spin and valley polarization and forms highly correlated states for the largest interaction strength. We will then investigate the origin of the exchange coupling in coupled spin-1 triangulene chains. A triangulene molecules is a 22 carbon atom triangular graphene quantum dot with zigzag edges. A single triangulene molecule has a spin-1 ground state with the spin-1 quasiparticle localized at the edges of the triangulene. Recently, in 2021 Mishra et al [4] observed signatures of fractional edge states, and gapped spin excitations characteristic of an antiferromangetic (AFM) spin-1 chain by connecting triangulene molecules together. Using a combination of tight-binding, Hartree-Fock and configuration interaction, we compute the single-particle and many-body states of coupled triangulene chains with a focus on $N_s=2,4$ sites. We show that neighboring spins are coupled antiferromagnetically through an intermediate state resembling that of the superexchange mechanism found in transition metal oxides.

Finally, the last topic of the talk outlines the continuum model, frequently used in both twisted transition metal dichalcogenides and twisted bilayer graphene. Focusing on the example of twisted MoTe2 bilayer, the details of the electronic structure calculation will be presented. In the next step the topological properties of twisted MoTe2 will be presented with the analysis of the Berry curvature and the Chern number for the bands near the Fermi level.

[1] M. Korkusinski, Y. Saleem, A Dusko, D. Miravet & P. Hawrylak, Spontaneous Spin and Valley Symmetry-Broken States of Interacting Massive Dirac Fermions in a Bilayer Graphene Quantum Dot, Nano Lett. 23, 7546 (2023).
[2] Y. Saleem, K. Sadecka, M. Korkusinski, D. Miravet, A. Dusko & P. Hawrylak, Theory of Excitons in Gated Bilayer Graphene Quantum Dots, Nano Lett. 23, 2998 (2023).
[3] K. Sadecka, Y. Saleem, D. Miravet, M. Albert, M. Korkusinski, G Bester, and P. Hawrylak, Electrically Tunable Fine Structure of Negatively Charged Excitons in Gated Bilayer Graphene Quantum Dots. arXiv preprint arXiv:2312.06264 (2023).
[4] S. Mishra, G. Catarina, F. Wu, R. Ortiz, D. Jacob, K. Eimre, J. Ma et al. Observation of fractional edge excitations in nanographene spin chains. Nature 598, no. 7880 (2021): 287-292.