Broken Symmetries in Low-Dimensional Materials

Lenferink, Erik

doi:https://doi.org/10.21985/n2-s3jt-bz73

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Broken Symmetries in Low-Dimensional Materials

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In monolayer materials with a honeycomb-like lattice an emergent degree of freedom arises for charge carriers corresponding to their valley index. Due to spatial inversion asymmetry and the presence of strong spin-orbit coupling, this valley degree of freedom strongly affects the optoelectronic properties of carriers in monolayer transition metal dichalcogenides (TMDs). While the valley-dependent optical properties of carriers have been well studied, their electronic properties have not. Motivated by the potential of the valley degree of freedom for information processing, this dissertation primarily focuses on the valley-dependent electronic effects originating in coupling between charge and valley currents. The phenomena of current-induced valley polarization and valley Hall effect are studied utilizing a transport model similar to that applied to the spin Hall effect and their dependencies on valley lifetimes determined. The valley Hall effect measurement is then reproduced in $n$-type monolayer MoS$_2$ devices and a nontrivial temperature dependence obtained. With the goal of extending valley Hall effect measurements to $p$-type WSe$_2$ where valley lifetimes extend into the microsecond range, a bottom-gated $p$-$n$ diode device architecture is developed using hexagonal BN for stable low temperature conduction. With this, current-induced valley polarization is measured with spatially-resolved Kerr rotation and shown to be distinct from a spurious background. In the latter part of this dissertation, focus is shifted to excitonic states localized to defects in monolayer TMDs. Confined laterally as well as vertically, these states mimic conventional quantum dots and exhibit single photon emission. We incorporate recent methods developed to deterministically create these states with strain engineering with our $p$-$n$ diode device architecture. By performing photon antibunching measurements of the electroluminescence, we demonstrate electrically-pumped single photon emission. Finally we illustrate how the chiral optical selection rules of monolayer TMDs could be implemented in a integrated photonic device to realize coherent optical non-reciprocity.

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