title: Direct observation of electron propagation and dielectric screening on the atomic length scale.
abstract: The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations--periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology (1 as = 10(-18) seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10(-15) seconds) extreme-ultraviolet light pulses to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few ångströms. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer--constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.
authors.affiliation: 1] Physik-Department, Technische Universität München, 85747 Garching, Germany [2] Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Neppl
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Ernstorfer
authors.affiliation: 1] Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany [2] Fakultät für Mathematik, Informatik und Naturwissenschaften, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany [3] Center for Free-Electron Laser Science (CFEL), Luruper Chaussee 149, 22761 Hamburg, Germany.
authors.forename: A L
authors.initials: AL
authors.lastname: Cavalieri
authors.affiliation: Institute for Theoretical Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/E136, A-1040 Vienna, Austria.
authors.forename: C
authors.initials: C
authors.lastname: Lemell
authors.affiliation: Institute for Theoretical Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/E136, A-1040 Vienna, Austria.
authors.forename: G
authors.initials: G
authors.lastname: Wachter
authors.affiliation: Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany.
authors.forename: E
authors.initials: E
authors.lastname: Magerl
authors.affiliation: Fakultät für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany.
authors.forename: E M
authors.initials: EM
authors.lastname: Bothschafter
authors.affiliation: 1] Physik-Department, Technische Universität München, 85747 Garching, Germany [2] Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Jobst
authors.affiliation: 1] Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany [2] Fakultät für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Hofstetter
authors.affiliation: 1] Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany [2] Fakultät für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Kleineberg
authors.affiliation: Physik-Department, Technische Universität München, 85747 Garching, Germany.
authors.forename: J V
authors.initials: JV
authors.lastname: Barth
authors.affiliation: 1] Physik-Department, Technische Universität München, 85747 Garching, Germany [2] Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Menzel
authors.affiliation: 1] Institute for Theoretical Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/E136, A-1040 Vienna, Austria [2] Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), 4001 Debrecen, Hungary.
authors.forename: J
authors.initials: J
authors.lastname: Burgdörfer
authors.affiliation: Physik-Department, Technische Universität München, 85747 Garching, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Feulner
authors.affiliation: 1] Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany [2] Fakultät für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Krausz
authors.affiliation: 1] Physik-Department, Technische Universität München, 85747 Garching, Germany [2] Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Kienberger