Showing papers by "Viktor Zólyomi published in 2013"

Monolayer MoS 2 : Trigonal warping, the Γ valley, and spin-orbit coupling effects

Abstract: We use a combined ab initio calculations and k · p theory based approach to derive a low-energy effective Hamiltonian for monolayer MoS2 at the K point of the Brillouin zone. It captures the features which are present in first-principles calculations but not explained by the theory of Xiao et al. [Phys Rev Lett 108, 196802 (2012)], namely the trigonal warping of the valence and conduction bands, the electron-hole symmetry breaking, and the spin splitting of the conduction band. We also consider other points in the Brillouin zone which might be important for transport properties. Our findings lead to a more quantitative understanding of the properties of this material in the ballistic limit.

Band structure and optical transitions in atomic layers of hexagonal gallium chalcogenides

Abstract: We report density-functional-theory calculations of the electronic band structures and optical absorption spectra of two-dimensional crystals of Ga${}_{2}{X}_{2}$ ($X=$ S, Se, and Te). Our calculations show that all three two-dimensional materials are dynamically stable indirect-band-gap semiconductors with a sombrero dispersion of holes near the top of the valence band. We predict the existence of Lifshitz transitions---changes in the Fermi-surface topology of hole-doped Ga${}_{2}{X}_{2}$---at hole concentrations ${n}_{S}=7.96\ifmmode\times\else\texttimes\fi{}{10}^{13}$ cm${}^{\ensuremath{-}2}$, ${n}_{Se}=6.13\ifmmode\times\else\texttimes\fi{}{10}^{13}$ cm${}^{\ensuremath{-}2}$, and ${n}_{Te}=3.54\ifmmode\times\else\texttimes\fi{}{10}^{13}$ cm${}^{\ensuremath{-}2}$.

Spin-orbit coupling, quantum dots, and qubits in monolayer transition metal dichalcogenides

Abstract: We derive an effective Hamiltonian which describes the dynamics of electrons in the conduction band of transition metal dichalcogenides (TMDC) in the presence of perpendicular electric and magnetic fields. We discuss in detail both the intrinsic and the Bychkov-Rashba spin-orbit coupling (SOC) induced by an external electric field. We point out interesting differences in the spin-split conduction band between different TMDC compounds. An important consequence of the strong intrinsic SOC is an effective out-of-plane $g$-factor for the electrons which differs from the free-electron g-factor $g\simeq 2$. We identify a new term in the Hamiltonian of the Bychkov-Rashba SOC which does not exist in III-V semiconductors. Using first-principles calculations, we give estimates of the various parameters appearing in the theory. Finally, we consider quantum dots (QDs) formed in TMDC materials and derive an effective Hamiltonian which allows us to calculate the magnetic field dependence of the bound states in the QDs. We find that all states are both valley and spin split, which suggests that these QDs could be used as valley-spin filters. We explore the possibility of using spin and valley states in TMDCs as quantum bits, and conclude that, due to the relatively strong intrinsic spin-orbit splitting in the conduction band, the most realistic option appears to be a combined spin-valley (Kramers) qubit at low magnetic fields.