Abstract
Electronic structure and electron dynamics in the ternary topological insulator $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{2}\mathrm{Se}$ are studied with time- and angle-resolved photoemission spectroscopy using optical pumping. An unoccupied surface resonance split off from the bulk conduction band previously indirectly observed in scanning tunneling measurements is spectroscopically identified. Furthermore, an unoccupied topological surface state (TSS) is found, which is serendipitously located at about 1.5 eV above the occupied TSS, thereby facilitating direct optical transitions between the two surface states at $\ensuremath{\hbar}\ensuremath{\omega}=1.5\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ in an $n$-type topological insulator. An appreciable nonequilibrium population of the bottom of the bulk conduction band is observed for longer than 15 ps after the pump pulse. This leads to a long recovery time of the lower TSS, which is constantly populated by the electrons coming from the bulk conduction band. Our results demonstrate $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{2}\mathrm{Se}$ to be an ideal platform for designing future optoelectronic devices based on topological insulators.
Highlights
Three-dimensional topological insulators (TIs) are an exotic quantum phase of matter with an insulating bulk and a conducting surface
The unoccupied surface resonance state (SRS) split off from the bulk conduction band was identified in a first-principles calculation in our previous work on Bi2Te2Se (BTS), where it was drawn on to explain the spin-selective scattering of the topological surface state (TSS) [17]
This is advantageous for the efficient generation of the spin-polarized photocurrent by the direct photoexcitation of the lower TSS, which is stationarily occupied in an n-type topological insulator; see Fig. 1(b)
Summary
Three-dimensional topological insulators (TIs) are an exotic quantum phase of matter with an insulating bulk and a conducting surface. The advantage stems from the topological character and the similar helical spin texture of the TSS, which is not the case in the photogalvanic effect in the spin-degenerate bulk conduction band, where the spin polarization of the electrons optically excited from the TSS is significantly reduced [20]. This makes the studies of the nonequilibrium surface electrons indispensable for the development of TI-based optical devices. We will discuss the ultrafast dynamics of the unoccupied surface resonance and of the upper TSS populated by the resonant optical transitions due to the 1.5-eV laser pulse
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