Tales in 2D crystals

DIPC Seminars

Juanjo Palacios, Universidad Autónoma de Madrid
Donostia International Physics Center
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Tales in 2D crystals In this talk I will present three stories related to three different two- dimensional crystals recently studied in my group. [1] GaSe: We have predicted that long-lived excitons with very large binding energies can also exist in a single or few layers of monochalcogenides such as GaSe. Our theoretical study shows that excitons confined by a local strain field are unable to recombine despite of electrons and holes co- existing in space. The localized single-particle states and excitons formed thereof are calculated in the envelope function approximation based on a three-band k · p Hamiltonian obtained from DFT calculations. The interplay between the localized strain and the caldera-type valence band, characteristic of few- layered monochalcogenides, creates localized electron and hole states with very different quantum numbers which hinders the recombination even for singlet excitons. [2] Antimonene: Despite intensive efforts, a practical use of the topological protection of surface states in 3D topological insulators (TIs) remains a challenge due to the intrinsic difficulties in obtaining defect-free bulk and pristine surfaces. Antimonene or few-layer antimony is lately receiving considerable attention since its recent addition to the experimentally accessible family of two-dimensional crystals. The presence of topologically protected surface states may place antimonene closer to TIs rather than common semi-metals. However, the absence of a bulk gap seems a handicap when confronted against the potential of topological insulators. Understanding whether the trivial or nontrivial character dominates under different circumstances is thus of utmost importance for antimonene’s future role in the family of topological materials. I will present our combined numerical effort between finite-size Kubo formalism and realistic DFT modeling to understand the conductivity of this system. [3] 1T’ MoS2: The quantum-spin-Hall (QSH) phase in topological insulating 2D crystals is attracting significant attention. In particular, large bulk gaps with vanishing edge gaps in atomically thin layers have been reported recently, while observations of the QSH phases by resistance measurements are comparatively few. Here, I will present the first observation of the QSH phase at room temperature in the few-layer 1T’-phase of MoS2 patterned by low- power and short-time laser beam irradiation on the 2H semiconducting phase along with theoretical results supporting the observation. [1] J. J. Esteve-Paredes, Sahar Pakdel, and J. J. Palacios, Phys. Rev. Lett. 123, 077402 (2019). [2] Sahar Pakdel and J. J. Palacios, submitted to 2D Materials. [3] H. Mine et al., Phys. Rev. Lett. (in press). Host: Fernando de Juan