Impact of ultrafast transport on the high-energy states of a photoexcited topological insulator

Abstract

Ultrafast dynamics in three-dimensional topological insulators (TIs) opens new routes for increasing the speed of information transport up to frequencies thousand times faster than in modern electronics. However, up to date, disentangling the exact contributions from bulk and surface transport to the subpicosecond dynamics of these materials remains a difficult challenge. Here, using timeand angle-resolved photoemission, we demonstrate that driving a TI from the bulk-conducting into the bulk-insulating transport regime allows to selectively switch on and off the emergent channels of ultrafast transport between the surface and the bulk. We thus establish that ultrafast transport is one of the main driving mechanisms responsible for the decay of excited electrons in prototypical TIs following laser excitation. We further show how ultrafast transport strongly affects the thermalization and scattering dynamics of the excited states up to high energies above the Fermi level. In particular, we observe how inhibiting the transport channels leads to a thermalization bottleneck that substantially slows down electron-hole recombination via electron-electron scatterings. Our results pave the way for exploiting ultrafast transport to control thermalization time scales in TI-based optoelectronic applications, and expand the capabilities of TIs as intrinsic solar cells.

Ultrafast evolution of bulk, surface and surface resonance states in photoexcited $$\hbox {Bi}_{2}\hbox {Te}_{3}$$

“…Our results demonstrate that in an intrinsic TI, Bi 2 Te 3 , the picosecond relaxation process is mainly due to surface (resonant) states with strongly limited scattering channels. The complete relaxation occurring at long delays is explained by the energy transfer from the bottom of the TSS to low energy phonons [56][57][58] and electron-hole recombination [30] at the surface (Fig. 4d).…”

“…This has motivated out-of-equilibrium experiments on TIs. In particular, time-and angle-resolved photoemission spectroscopy (trARPES) provides valuable information on electron in-teractions [23,[25][26][27][28][29][30] as it gives access to the time evolution of excited carriers in the reciprocal space [31][32][33]. However, detecting the spin degrees of freedom requires a more sophisticated method [18,34,35].…”

Effective mass enhancement and ultrafast electron dynamics of Au(111) surface state coupled to a quantum well

“…On one hand, this result highlights the impact of quantization effects on the efficiency of transversal electron transport near E F , which is the main reason for the increasing values of τ l with decreasing thickness. On the other hand, it also provides further evidence for the importance of electronelectron scattering which is, in any case, a slow channel of thermalization due to Auger-like secondary electron emission upon recombination of the hot electron with a hole [41].…”

“…There are several effects that can explain this observation. One of them is the influence of ultrafast carrier transport (ballistic or diffusive) [38][39][40][41] from the surface to the bulk of the Au film with increasing thickness. However, the efficiency of these channels should be weak as no Au bulk states are observed in our photoemission spectra.…”

Impurity band assisted carrier relaxation in Cr doped topological insulator Bi2Se3

“…The performance of these devices depends on the dynamics of the hot carriers in TIs, which can be observed directly by TR-ARPES [4][5][6] . So far, a number of works have been carried on illustrating the recovery process of the excited electrons in TIs due to various mechanisms: such as the phonons assisted, 7,8 the surface states assisted, 9 and the electron-electron scattering assisted. 10,11 Recently, numerous work have reported that the doping with transition metals can introduce novel physical phenomena in TIs, such as quantum anomalous Hall effect (QAHE) 12 , the giant magneto-optical Kerr effect 13 , and chiral Majorana fermions 14 .…”