Evidence of reduced surface electron-phonon scattering in the conduction band of Bi2Se3by nonequilibrium ARPES

Abstract

The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electronphonon scattering at the surface. By comparing time-resolved ARPES measurements of the TI Bi2Se3 with different probing depths we show that the relaxation dynamics of the electronic temperature of the conduction band is much slower at the surface than in the bulk. This observation suggests that surface phonons are less effective in cooling the electron gas in the conduction band.The scientific and technological interest on topological insulators (TIs) stems from the unusual properties of their topologically protected metallic surface states, which exhibit a linear dispersion and a characteristic spin helicity [1][2][3][4][5][6][7]. For the Dirac quasiparticles elastic backscattering is forbidden by time-reversal symmetry, and transport is controlled by scattering events mediated by phonons. Attempts to measure the strength of the electron-phonon coupling in the representative TI Bi 2 Se 3 by angle-resolved photoelectron spectroscopy (ARPES) have produced somewhat conflicting results. The estimated values of the dimensionless coupling constant λ vary from small (λ ∼ 0.08) [8] to moderate (λ ∼ 0.25) [9]. Time-resolved ARPES (tr-ARPES) can tackle the problem in the time domain, complementary to the energy domain of conventional ARPES at equilibrium [10][11][12][13]. In pumpprobe tr-ARPES experiments, the electrons excited by a light pulse are described by an effective Fermi-Dirac (FD) distribution. The relaxation of the electronic temperature (T e ), as well as the variation of the chemical potential (µ) that reflects photo-doping of the conduction band, provide fundamental information on the de-excitation mechanisms, namely between the conduction band (CB) and the topologically protected surface state [12,13]. In a recent experiment on Bi 2 Se 3 , the contribution of various phonon modes to the electronic cooling has been addressed by comparing the relaxation dynamics of the FD distribution at various sample temperatures and for different charge densities [12].In this work we present a tr-ARPES investigation of the conduction band dynamics in Bi 2 Se 3 , where two different photon energies are exploited to vary the surface sensitivity. Standard tr-ARPES experiments, performed with laser-based sources at 6.2 eV photon energy, are rather bulk sensitive, due to the very low kinetic energy of the photo-electrons [14]. The comparison between more bulk sensitive (hν = 6.2 eV; UV) and more surface sensitive (hν = 17.5 eV; extreme UV, EUV) measurements reveals two different relaxation dynamics for T e in the conduction band. Namely, we observe a freezing of T e to an elevated value (∼ 600 K) at the surface but not in the bulk, suggesting a reduced efficiency of the phonons in the electronic cooling at the surface.A quantitative estimation of the photo-electron mean free path, l, as a function of the photo-electron kinetic energy is challenging. In particular, it is well established that at low kinetic energies ...

Effect of Mn doping on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3

“…The fits of the experimental data shown in figures 3 and 4 using equations (8) and (10) reproduce all the variations of τ D1 and τ R with n 0 and n t , thus proving the validity of the model. We note that if the electron-electron scattering rate in equation (9) switches to the 2D structure case ( ) τ − ñ R 1 [29], the experimental data can also be fitted quite well, thus indicating that Mn doping effect on electron-electron thermalization dynamics is not sensitive enough to distinguish between 2D and 3D states in TIs.…”

Time-resolved terahertz dynamics in thin films of the topological insulator Bi2Se3

“…This is consistent with the bulk relaxation times (∼2 ps) obtained in previous experiments. [14,[16][17][18][19] Thus, the simplest way to interpret the observed behavior is that photoinduced change ∆σ of 10 QL film is produced by hot surface carriers, while in thicker films the majority of photoexcited carriers live in the bulk of the material, and survive for only a few picoseconds after which surface carrier start to dominate the THz transport.…”

“…), they cannot reliably distinguish signatures of the surface carriers from that of the bulk carriers present in non-ideal TI. Optical pump-optical probe spectroscopy (OPOP) [14][15][16] and time-resolved angle resolved photoemission spectroscopy (tr-ARPES) [17][18][19] were used to reveal dominant role of electron-phonon scattering in the carrier relaxation process following photoexcitation of single Bi 2 Se 3 crystals. However, OPOP is most sensitive to interband processes, and tr-ARPES lacks high energy resolution near the chemical potential and is only sensitive to surface carriers.…”

Ramifications of optical pumping on the interpretation of time-resolved photoemission experiments on graphene

“…This time, energy and momentum resolving capability is a virtue precluded to time resolved all-optical techniques. It has opened unprecedented possibilities to study the quasiparticle dynamics in a wide range of condensed matter systems, including the ultrafast amplitude oscillations in charge density wave materials [2][3][4][5], the quasiparticle recombination rate near the d -wave node in cuprate superconductors [1,[6][7][8], the dynamics taking place on the Dirac cone in graphene [9][10][11][12][13] and on the surface of topological insulators [14][15][16][17][18][19].…”