Bulk diffusive relaxation mechanisms in optically excited topological insulators

Original Paper: A. Sterzi et al., Phys. Rev. B 95, 115431 (2017); DOI: 10.1103/PhysRevB.95.115431

Topological Insulators (TIs) represent a new quantum state of matter that is currently attracting great interest thanks to unique electrical transport properties. In order to provide a comprehensive description of their out-of-equilibrium electronic properties we investigated, by means of time- and angle-resolved photoemission spectroscopy, a wide set of differently doped topological compounds. Specifically, our study aimed at revealing the role of three-dimensional (bulk) and two-dimensional states (surface states) in influencing the microscopic scattering mechanisms.

The realization of new generation of spintronics devices requires novel materials of special electronic properties. Topological Insulators (TIs), because of their unique electronic band structure, are considered the best candidates for such applications. Differently from conventional insulators, these materials exhibit simultaneously insulating bulk and surface conductive states (SS). The most fascinating aspect of these systems arises from the linear dispersing metallic surface states, which connect the bulk valence band (VB) to the conduction band (CB) across the material bandgap. A number of interesting phenomena have been proposed to arise from the coupling of ultrashort laser pulses to TIs. In particular, the possibility to optically inject spin currents has stimulated intense studies of their out-of-equilibrium electronic properties. However, a comprehensive description of the electronic relaxation dynamics has been elusive, so far. Moreover, the specific role of bulk and surface state in determining the microscopic scattering mechanisms is still unclear. For this purpose, we investigated at the T-Rex laboratory a wide set of TIs, by means of time- and angle-resolved photoemission spectroscopy (TR-ARPES).

Figure 1: Band structure of the six TIs, as measured by ARPES before a)-d) and after e), f) optical excitation along the Γ-K high symmetry direction and at liquid nitrogen temperature (110K). The yellow dash line indicates the position of the Fermi energy, the name of each material is reported on top of each panel. Copyright (2017) by The American Physical Society

The materials under study, which can be grouped according to their different surface doping, are: GeBi₄Te₇, Bi₂Te₃, Sb₂Te₃, Sb₆Te₃, and GeBi₂Te₄. Fig. 1 summarizes the electronic states mapped in k-space and before (panels a-d) and after (panels e,f) the optical excitation. We clearly resolve the Dirac cone for each TI, showing how its energy position varies with respect to the E_F. Moreover, the ARPES spectra reported in Fig.1 clearly show the bottom of the CB and the top of the VB, located above and below the SS respectively. Our study focused on understanding the correlation between TIs’ electron dynamics and their energy position with respect to E_F. For this purpose, we selectively extracted and analyzed the temporal evolution of the band structures shown in Fig.1. As a result we obtained the temporal evolution of the hot electronic temperature (T_e). This fundamental parameter allows to directly monitor the energy flow after the optical perturbation from the electrons towards other degrees of freedom. In Fig. 2 we report the temporal evolution of T_e for each TI. The experimental curves show how compounds with different doping dissipate the energy provided by the pump. For all the samples under study a typical exponential behavior with similar decay times is observed, despite the case of intrinsic TIs. Unexpectedly, the T_e curves in Fig. 2c and Fig. 2d do not relax back to the equilibrium value within the investigated temporal window. In particular, in Bi₂Te₃ T_e forms a plateau which lasts more than 60 ps. To explain such effects we propose that different physical processes are at play when 3D (bulk) or 2D (surface) states cross the E_F, as schematized in figs. 2(g-h). In the n- and p-doped case (Fig. 2g and Fig. 2i), the CB and VB provide an efficient 3D channel to evacuate hot carries along the z-direction. On the contrary, in the intrinsic case (Fig. 2h), excited electrons are confined in the x-y plane due to a 2D diffusion mechanism. In conclusion, we ascribe the different relaxation dynamics to the dimensionality of states which cross the Fermi level. Our results highlight the different contributions of 3D bulk states and 2D SS to the energy relaxation mechanisms, clarifying the important role played by the surface doping in TIs.


























Figure 2: a-f) Temporal evolution of the electronic temperature T_e obtained by analyzing the electron dynamics as extracted from spectra shown in Fig. 1. For a), b) n-type and e-f) p-type TIs, Te recovers the equilibrium value with characteristic time τ of a few ps. On the contrary, in intrinsic TIs c), d) T_e forms a plateau, indicated by the brawn area. g), i) Schematization of the proposed diffusion mechanism in the bulk g), i) and at surface h). Copyright (2017) by The American Physical Society.