Coherently Controlled Electrical Currents at Surfaces

Abstract

During the last decade, great progress has been made by investigating electron transport through single molecules, metallic point contacts, or chains of single atoms by using scanning tunneling microscopy or break junctions [1,2]. Many of these works have focused on the coherent regime where inelastic scattering of the electrons due to vibronic or electronic excitation within the junction can be neglected. In this regime, nanoscale wires show no ohmic behavior and a quantization of the conductance has been observed as can be described by the Landauer theory [3]. While these experiments observe electron transport on the atomic scale, information about the dynamics of electron transport on the timescale of the relevant scattering mechanisms is scarce. The microscopic understanding of the mechanisms of electron transport, however, is not only of great interest for the development of new small-scale electronic devices, the connection of the electric conductivity s of a material with the microscopic scattering processes of individual charge carriers is also a fundamental issue, central to solid state physics [4]. This question goes back to Paul Drude [5] who has connected the electric conductivity s of a metal to an empirical scattering time t of free electrons that are accelerated in an external electric field, resulting in the well known Drude formula s ¼ e 2 n e t=m e , where e, m e , and n e are the electron charge, mass and density, respectively. Even if this model is oversimplified as it does not consider the quantum nature and the many-body aspects of electronelectron interaction, it gives the correct order of magnitude for the timescale of electron scattering processes. For Cu at room temperature, for example, it gives a scattering time of 27 fs [4], which shows that usual conductivity measurements cannot resolve the individual scattering events because available electronic equipment can neither produce trigger signals nor detect transients that are shorter than tens of picoseconds.Recently, we have introduced a new experimental technique that is capable of accessing the dynamics of electrical currents on the femtosecond timescale [6]. This contact-free technique combines the pure optical generation of electric

Towards full surface Brillouin zone mapping by coherent multi-photon photoemission

“…Finally, we emphasize that the proposed methodology is applicable to probe coherent ultrafast phenomena in the full surface Brillouin zone. For example, interferometrically time-resolved multiphoton photoemission techniques [2,5,6,40,[47][48][49][50] are applicable to those electronic bands. Moreover, it opens a new approach for performing measurements where the excitation field can both drive dipole transitions and interband ballistic electron acceleration in solids [51][52][53].…”

“…Time-and angle-resolved two-photon photoemission spectroscopy (TR-2PP) enables mapping the energy and momentum (k||, k⊥)-resolved electronic structure and dynamics of the occupied and unoccupied electronic bands of solids [1][2][3]. Using excitation frequencies from the infrared (IR) to the ultraviolet (UV) range, TR-2PP has been applied to a wide range of condensed matter systems ranging from pristine metals to complex materials and interfaces [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In TR-2PP spectroscopy, the photon energies ℏ𝜔 of the pump and the probe laser pulses are chosen such that the pump excites the sample to a real or a virtual intermediate state and the probe induces further upward transition from the excited system to induce photoemission.…”

Ultrafast dynamics of photocurrents in surface states of 3D topological insulators

“…This includes a scheme that additionally allows for a coherent control of the size and direction of the photocurrent by tuning the relative phase of two phaselocked laser fields with photon energies ω and ω/2, which simultaneously drive a one-and two-photon transition, respectively. This method has been used to induce electric [13][14][15] and spin currents [16,17] in bulk direct-band-gap semiconductors and has been combined with time-and angle-resolved 2PPE to investigate the ultrafast dynamics of electric currents in image-potential states at a Cu(001) surface [18]. The generation of a dc current by this process can be understood as third-order nonlinear optical rectification which allows to generate photocurrents even in unbiased centrosymmetric materials [19].…”

“…Time-and angle-resolved 2PPE is even capable to investigate electron transport on an ultrafast time scale if the initial optical excitation by the pump pulses creates an asymmetry in the momentum distribution of the electrons parallel to the surface, i.e. a photocurrent [18]. The time-resolved observation of the redistribution and decay of the initially inhomogeneous momentum distribution provides microscopic information on the different scattering mechanisms of just those electrons that carry the photocurrent.…”

Local and non-local electron dynamics of Au/Fe/MgO(001) heterostructures analyzed by time-resolved two-photon photoemission spectroscopy

“…Early optical experiments used back side pump / front side probe schemes and analyzed the propagation dynamics through the bulk of thin films [4]. Time-and angle-resolved two-photon photoelectron spectroscopy exploited the sensitivity to electron energy and momentum and was key to develop a comprehensive understanding of the microscopic nature of the engaged elementary processes which hot electrons experience [5][6][7][8][9]. In heterostrutures such an analysis is challenging but highly desired given the widespread application of these materials.…”