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

Hamoon Hedayat,Ettore Carpene,Claudia Dallera,Xingjiang Zhou,Davide Bugini,Hemian Yi,Chaoyu Chen,Giulio Cerullo

doi:10.1038/s41598-021-83848-z

Abstract

We use circular dichroism (CD) in time- and angle-resolved photoemission spectroscopy (trARPES) to measure the femtosecond charge dynamics in the topological insulator (TI) hbox {Bi}_{2}hbox {Te}_{3}. We detect clear CD signatures from topological surface states (TSS) and surface resonance (SR) states. In time-resolved measurements, independently from the pump polarization or intensity, the CD shows a dynamics which provides access to the unexplored electronic evolution in unoccupied states of hbox {Bi}_{2}hbox {Te}_{3}. In particular, we are able to disentangle the unpolarized electron dynamics in the bulk states from the spin-textured TSS and SR states on the femtosecond timescale. Our study demonstrates that photoexcitation mainly involves the bulk states and is followed by sub-picosecond transport to the surface. This provides essential details on intra- and interband scattering in the relaxation process of TSS and SR states. Our results reveal the significant role of SRs in the subtle ultrafast interaction between bulk and surface states of TIs.

Highlights

We use circular dichroism (CD) in time- and angle-resolved photoemission spectroscopy to measure the femtosecond charge dynamics in the topological insulator (TI) Bi2Te3
At 250 fs delay (Fig. 1b), two unoccupied bands (B1 and B2) can be seen: one is located 0.8 eV above the Fermi level, the other at lower energy has parabolic dispersion and is well-separated from the topological surface states (TSS). These unoccupied states have been widely investigated in TIs, and predicted theoretically[23,51]. Their nature is still under debate: while some studies considered them as bulk states[23,24], other investigations suggested that these bands are SRs20
We will demonstrate that the photoemission (PE) signal is detected from both surface resonance (SR) and bulk bands (BBs), B1 and B2 bands in Fig. 1b,c are the superposition of BBs and SRs

Summary

Introduction

We use circular dichroism (CD) in time- and angle-resolved photoemission spectroscopy (trARPES) to measure the femtosecond charge dynamics in the topological insulator (TI) Bi2Te3. Our study demonstrates that photoexcitation mainly involves the bulk states and is followed by sub-picosecond transport to the surface. This provides essential details on intra- and interband scattering in the relaxation process of TSS and SR states. The electronic dynamics can be significantly affected by the spin constraints This has motivated out-of-equilibrium experiments on TIs. In particular, time- and angle-resolved photoemission spectroscopy (trARPES) provides valuable information on electron interactions[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]. The same approach can be generalized to study the bulk and surface electron dynamics in a wide class of TIs and open a route towards their advanced engineering for innovative opto-electronic and spintronics applications

Methods

Results

Conclusion