Quantum control of spins in nanostructures of monolayer transition metal dichalcogenides

Abstract

Single spins localized in semiconductor nanostructures have been considered as carriers of qubits in quantum computing and quantum spintronics. Recently, monolayer transition metal dichalcogenides (TMDs) have attracted vast interest as a new class of two-dimensional (2D) semiconductors with extraordinary properties including the direct band gap, the strong spin-orbit coupling, and the additional valley degree of freedom for localized electrons (holes). In this thesis, we investigated the possibility of realizing optically controllable spin-valley qubit carried by single localized electrons in monolayer TMDs nanostructures, including small quantum dots and impurity systems. Optical quantum control schemes are proposed for two scenarios: (a) in presence of valley hybridization caused by the confinement; (b) in the absence of valley hybridization. Since lattice nuclear spins are the ultimate environment for spin qubit at low temperature, the hyperfine interplay with nuclear spins is also investigated. The forms of hyperfine interaction between the electron and hole with the lattice nuclear spins are derived in the envelope function approximation. The symmetry properties of the hyperfine interaction between electron (hole) and nuclear spins in monolayer TMDs nanostructures are analyzed and numerical estimation of the hyperfine interaction strength is made using band edge Bloch functions obtained from: (a) the Rothaan-Hatree-Fock atomic orbitals; (b) first principle Abinit all electron wavefunction calculation. A quantum control scheme to realize the intervalley spin flip is proposed and analyzed, utilizing the intervalley terms of the hyperfine interaction. The feedback control schemes of the nuclear field for the spin-valley qubit in monolayer TMDs quantum dots are also studied. A negative feedback control process of the nuclear field is found to suppress the statistical fluctuations of the nuclear configurations. The positive feedback scheme is also discussed and we give the critical conditions for these two feedback scenarios. In the last part of this thesis, the generation of a pure flow of spin and valley pseudospin based on the trigonal warping of Fermi pockets in monolayer TMDs is studied.