Prolonged duration of nonequilibrated Dirac fermions in neutral topological insulators

Abstract

Topological insulators (TIs) possess spin-polarized Dirac fermions on their surface but their unique properties are often masked by residual carriers in the bulk. Recently, (Sb1−xBix)2Te3 was introduced as a non-metallic TI whose carrier type can be tuned from n to p across the charge neutrality point. By using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast carrier dynamics in the series of (Sb1−xBix)2Te3. The Dirac electronic recovery of ∼10 ps at most in the bulk-metallic regime elongated to >400 ps when the charge neutrality point was approached. The prolonged nonequilibration is attributed to the closeness of the Fermi level to the Dirac point and to the high insulation of the bulk. We also discuss the feasibility of observing excitonic instability of (Sb1−xBix)2Te3.

Magnetic-impurity-induced modifications to ultrafast carrier dynamics in the ferromagnetic topological insulators Sb2−xVxTe3

“…The Dirac points for x=0, 0.015 and 0.03 are located at 200, 210 and 220 meV, respectively. The locus of the Dirac point of the pristine sample is consistent with previous reports [48][49][50][51]. Around t ∼ 0 ps, we observe TARPES signals of IPSs with quantum numbers n=2 and 3 which are excited by the UV pulses and probed by the mid-IR pulses.…”

Role of matrix elements in the time-resolved photoemission signal

“…the Fermi momentum k F ) and velocity (v F ) extracted from an MDC fitting procedure are summarized for all delays in figure 2(e). Such an observation requires consideration of the possible impact of photoinduced surfacephotovoltage (SPV) effects, which have been reported widely in a variety of bulk-insulating TIs [36,37,[41][42][43][44]. We note that SPV would induce a rigid band shift, instead of the polarization dependent photoemission intensity dynamics reported below and in figure 4, and would thus not affect our analysis and conclusions (see supplementary material available online at stacks.iop.org/NJP/22/023031/mmedia).…”

Surface State Dynamics of Topological Insulators Investigated by Femtosecond Time- and Angle-Resolved Photoemission Spectroscopy

“…This occurs when the TSS is well isolated from bulk states at the FE and leads to a different transient chemical potential of electrons and holes which creates a barrier for relaxation. However, its relevance to the observed enhancement of the electronic lifetime in TSS is still debated [36].…”