Spectroscopy and dynamics of unoccupied electronic states of the topological insulators Sb2Te3 and Sb2Te2S

Time- and angle-resolved two-photon photoemission (2PPE) was used to study the electronic structure and ultrafast electron dynamics of the p-doped topological insulator Sb2Te3 and its derivative Sb2Te2S. Our 2PPE experiments directly reveal that the massless Dirac-cone like energy dispersion of topological surface states is realized above the Fermi energy in both materials. The observed bulk conduction bands of Sb2Te2S are found to be shifted to higher energies as compared to Sb2Te3. This shift has, however, surprisingly almost no influence on the electron dynamics in the topological surface state, which proceed on a picosecond time scale.We acknowledge funding by the Deutsche Forschungsgemeinschaft through SPP1666.Peer Reviewe

Buildup and dephasing of Floquet-Bloch bands on subcycle time scales

Strong light fields have created spectacular opportunities to tailor novel functionalities of solids. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle time scales, lightwaves can simultaneously drive intraband currents and interband transitions, which enable high-harmonic generation (HHG) and pave the way towards ultrafast electronics. Yet, the interplay of intra- and interband excitations as well as their relation with Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this pivotal link by pioneering the ultrafast buildup of Floquet-Bloch bands with time- and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator with mid-infrared fields - strong enough for HHG - and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum nonequilibrium calculations explain the simultaneous occurrence of Floquet states with intra- and interband dynamics. Our joint experiment-theory study opens up a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.Comment: 45 pages, 4 figures, 10 extended data figure

Response theory for time-resolved second-harmonic generation and two-photon photoemission

A unified response theory for the time-resolved nonlinear light generation and two-photon photoemission (2PPE) from metal surfaces is presented. The theory allows to describe the dependence of the nonlinear optical response and the photoelectron yield, respectively, on the time dependence of the exciting light field. Quantum-mechanical interference effects affect the results significantly. Contributions to 2PPE due to the optical nonlinearity of the surface region are derived and shown to be relevant close to a plasmon resonance. The interplay between pulse shape, relaxation times of excited electrons, and band structure is analyzed directly in the time domain. While our theory works for arbitrary pulse shapes, we mainly focus on the case of two pulses of the same mean frequency. Difficulties in extracting relaxation rates from pump-probe experiments are discussed, for example due to the effect of detuning of intermediate states on the interference. The theory also allows to determine the range of validity of the optical Bloch equations and of semiclassical rate equations, respectively. Finally, we discuss how collective plasma excitations affect the nonlinear optical response and 2PPE.Comment: 27 pages, including 11 figures, version as publishe

Temperature dependence of the dynamics of the first image-potential state on Ag(111)

The temperature dependence of the dynamics of electrons in the n=1 image potential state on the Ag(111) surface has been investigated by means of time-resolved two-photon photoemission spectroscopy and many-body calculations. We show that the decay rate of electrons in this state grows linearly with temperature. The thermal shortening of the lifetime is caused by the increase of the electron-electron scattering rate, due to deeper penetration of the image state wave function into the bulk metal at higher temperature. The electron-phonon scattering in this state is found to be small. © 2012 American Physical Society.We acknowledge funding by the Deutsche Forschungsgemeinschaft through Grants No. GRK 790 and No. GU 495/2, the Ikerbasque Foundation, the University of the Basque Country (Project No. GV-UPV/EHU, Grant No. IT-366-07), and Ministerio de Ciencia e Inovación (Grant No. FIS2010-19609-C02-00).Peer Reviewe