THEORY DEPARTMENT
Fritz Haber Institute of the Max Planck Society

All FHI Seminars

Layer-by-layer growth of TiO2 - anatase: evolution 2D/3D of the electron states
Speaker:Alessandro Troglia
 Università degli Studi di Milano
Time:Tuesday, 23rd April 2019, 10:00 AM
Location: PC Seminar Room G2.06, Building G
Organized: PC
Abstract: Titanium dioxide (TiO2) is mainly present in nature as three different polymorphs: rutile, brookite and anatase. In particular, the latter is largely studied due to its promising future applications in several devices like memristors and solar cells, as well as implementations in spintronics and transparent conductive oxides. In this framework, the most important physical quantity is certainly conductivity: it is thus fundamental to analyze and control the electronic properties of anatase with a particular attention to the surface, which plays a remarkable role in the previous applications. Being thermodinamically favoured at the nanometric scale, a controlled and functionalized growth of anatase thin films is achieved through the extremely versatile and accurate technique of Pulsed Laser Deposition (PLD), while in-situ synchrotron radiation photo-emission spectroscopy measurements allow the investigation of the surface electronic properties. Anatase TiO2 is an insulator, however, recent studies report the presence of a dispersive electronic state at Fermi level and a non-dispersive in-gap state: despite the tremendous research activity, the nature of these states, especially the dispersive one, is still debated. In particular, for the latter it is contradictorily reported both a three-dimensional and a purely two-dimensional nature. Thus, to try to clarify this discrepancy, the evolution of the dispersive state has been analyzed as a function of thin film thickness, trying to find a possible correlation with the out-of-plane direction (namely, perpendicular to the surface). A preliminary investigation of the state evolution as a function of external solicitations like epitaxial strain induced by the substrate and thermal annealing has also been perforemed, with the purpose of providing more information regarding its physical properties. Anatase thin films deposition by means of PLD required an intense materials science activity and the optimization of a suitable growth protocol. Nevertheless, the samples turned out to be of excellent quality, both regarding the optimal structural and superficial properties: the films possess an unique crystal phase, a well long range order and a very low surface roughness. Finally, the use of synchrotron radiation allowed an accurate investigation of the electronic properties: in particular, the dispersive state at Fermi level presents a quasi-two-dimensional nature, because its filling with electrons is correlated with the film thickness, while its response as a function of epitaxial strain and thermal annealing suggests a potential tunability behaviour controllable by appropriate external solicitations.

Realtime tracking of the electron dynamics in molecules
Speaker:Prof. Francesca Calegari
 Attosecond Science Group, Center for Free-Electron Laser Science (CFEL), and Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany
Time:Friday, 26th April 2019, 9:30 AM
Location: Haber Villa
Organized: MP
Abstract: Observing electron dynamics in matter on its natural time scale requires attosecond technology. I will show how isolated attosecond pulses can be used in combination with phase-stable infrared/ ultraviolet pulses to track ultrafast charge dynamics in bio-relevant molecules such as aromatic amino acids. These schemes can be also used to investigate the ultrafast mechanisms behind the photo stability of our own DNA. Our results open new important perspectives for a future understanding of the role of the electronic motion in the photochemistry and photobiology of complex molecules.

Pulsed infrared FELs for resonant time-resolved and nonlinear studies of solid matter
Speaker:Sergey Pavlov
 DLR - German Aerospace Center, Berlin
Time:Monday, 29th April 2019, 11:00 AM
Location: PC Seminar Room G2.06, Building G
Organized: PC
Abstract: Infrared picosecond pulsed FELs open up unique opportunities to study fast phenomena in solids, in particular when electronic resonances are excited selectively with sufficient intensity. Time-resolved single-color experi-ments provide important insights about the lifetimes of excited states of major impurity centers and electron-phonon interactions in elemental semiconductors doped by hydrogen-like and multiple-charge impurities [1]. Furthermore, the unique spectral brightness of IR FEL´s have been utilized for saturation intensity experiments [2], the demonstration of THz range stimulated emission [3] and resonant stimulated Raman emission at long wavelengths, up to 40 microns [4], from doped silicon. Also, simultaneous operation of inversion population-based and inversion-less lasing has been observed in a simple three atomic-like level system [5]. Another research topic of interest is the study of THz range multi-photon absorption spectra of atomic-like optical transitions in doped elemental semiconductors [6]. Finally, control of the repetition rate of the ultra-short FEL pulses can be utilized to investigate the dynamics of Raman stimulated emission towards its extreme: transient mode [7]. [1] Dynamics of nonequilibrium electrons on neutral center states of interstitial magnesium donor in silicon, S. G. Pavlov, N. Deßmann, A. Pohl, V. B. Shuman, L.М. Portsel, А.N. Lodygin, Yu.A. Astrov, S. Winnerl, H. Schneider, N. Stavrias, A. F. G. van der Meer, V. V. Tsyplenkov, K. A. Kovalesky, R. Kh. Zhukavin, V. N. Shastin, N. V. Abrosimov and H.-W. Hübers, Phys. Rev. B 94, 075208 (2016). Dynamics of infrared excitations in boron doped diamond, S. G. Pavlov, S. A. Tarelkin, V. S. Bormashov, N. Stavrias, K. Saeedi, A. F. G. van der Meer, N. A. Bekin, R.Kh. Zhukavin, V. N. Shastin, M. S. Kuznetsov, S. A. Terentiev, S. A. Nosukhin, D. D. Prikhodko, V. D. Blank, M. Wienold, H.-W. Hübers, Diamond and Related Materials 92, 259-265 (2019). [2] Short pulse effects in FIR optical saturation: shallow donors in silicon, K. K. Geerinck, J. E. Dijkstra, J.N. Hovenier, T.O. Klaassen, W.Th. Wenckebach, A. F. G. van der Meer, P.W. van Amersfoort, Nuclear Instr. and Methods in Phys. Research A. 341, 162-164 (1984). [3] Terahertz Stimulated Emission from Silicon Doped by Hydrogenlike Acceptors, S. G. Pavlov, N. Deßmann, V. N. Shastin, R. Kh. Zhukavin, B. Redlich, A. F. G. van der Meer, M. Mittendorff, S. Winnerl, N. V. Abrosimov, H. Riemann, and H.-W. Hübers, Phys. Rev. X 4, 021009 (2014). [4] Stimulated terahertz Stokes emission of silicon crystals doped with antimony donors, S. G. Pavlov, H.-W. Hübers, J. N. Hovenier, T. O. Klaassen, D. A. Carder, P. J. Phillips, B. Redlich, H. Riemann, R. Kh. Zhukavin, and V. N. Shastin, Phys. Rev. Lett. 96, 037404 (2006). [5] Competing Inversion-Based Lasing and Raman Lasing in Doped Silicon, S. G. Pavlov, N. Deßmann, B. Redlich, A. F. G. van der Meer, N. V. Abrosimov, H. Riemann, R. Kh. Zhukavin, V. N. Shastin, and H.-W. Hübers, Phys. Rev. X 8, 041003 (2018). [6] Giant multiphoton absorption for THz resonances in silicon hydrogenic donors, M. van Loon, N. Stavrias, N. Le, K. Litvinenko, P. Greenland, C. Pidgeon, K. Saeedi, B. Redlich, G. Aeppli, B. Murdin Nature Photonics 12, 179-184 (2018). [7] Transient THz Stimulated Emission from Silicon Pumped by FELs, S. G. Pavlov, N. Deßmann, A. Pohl, A. F. G. van der Meer, J.-M. Ortega, R. Prazeres, R. Kh. Zhukavin, N. V. Abrosimov, H.-W. Hübers, 42nd Int. Conf. on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 27.8.-1.9.2017, Cancun, Mexico, PID4694267 (2017). DOI: 10.1109/IRMMW-THz.2017.8067094

Helical structures for electron spin filtering
Speaker:Helmut Zacharias
 Westfälische Wilhelms-Universität Münster
Time:Monday, 6th May 2019, 11:00 AM
Location: PC Seminar Room G2.06, Building G
Organized: PC
Abstract: Helical molecules adsorbed in monolayers on various substrates preferentially transmit one longitu-dinal spin component of electrons emitted by the substrate. First shown for natural biomolecules, like oligo-DNA with a spin polarization up to 60%, bacteriorhodopsin, and oligo-peptides, which only appear in their natural enantiomeric form, this effect has recently also been observed for hepta-helicene molecules adsorbed on precious metal surfaces Cu(332), Ag(110), and Au(111). Using both pure enantiomers it could be shown that the sign of the electrons spin polarization is directly con-nected with the chiral sense of the helicene. Further, this conjugated organic molecule is conducting, different from the hopping conduction of the biomolecules. A recent extension of this effect to chiral inorganic CuO thin films opens applications in catalytic systems. Further, first realizations for spintronics devices are shown.

Photoionization dynamics of core excited atoms and molecules in the tender x-ray domain (2-10 keV) using Synchrotron Radiation.
Speaker: Prof. Marc Simon
 Sorbonne Université, CNRS, Paris, and Synchrotron SOLEIL
Time:Friday, 10th May 2019, 9:30 AM
Location: Haber-Villa, Faradayweg 8
Organized: MP
Abstract: Our research group is working on uphotoionization dynamics after absorption of a tender x-ray photon (2-10 keV) by an isolated atom or molecule. Characteristics of this energy domain are connected to the very short lifetime (1 fs or less), the creation of multiply charged ions by cascade Auger, emission of fast Auger or x-ray photon of high energy. We have developed experimental setups allowing to measure x-ray emission, photoemission as well as vectorial coincidences between the momenta of electrons and ions. I will show recent results related to the dynamics and spectroscopy of Double Core Hole states [1-3]. I will show ultrafast fragmentation exemples [4-5]. High photon energy photoionization leads to strong recoil effects [6-8]. We took advantage of the fragmentation of multiply charged ions to study the localization of inner shells [9]. Two examples of attosecond electronic relaxation will be presented [10, 11]. Références [1] R. Püttner et al., Physical Review Letters 114, 093001 (2015) [2] G. Goldsztejn et al. Physical Review Letters 117, 133001 (2016) [3] T. Marchenko et al., Physical Review Letters 119, 133001 (2017) [4] O. Travnikova et al., Physical Review Letters 116 213001 (2016) [5] O. Travnikova et al., Physical Review Letters 118, 213001 (2017) [6] M. Simon et al., Nature Communications 5 4069 (2014) [7] E. Kukk et al., Physical Review Letters 121 073002 (2018) [8] D. Céolin et al., PNAS 116 4877 (2019) [9] R. Guillemin et al., Nature Communications 6 6166 (2015) [10] T. Marchenko et al, Physical Review X 5 031021 (2015) [11] M. N. Piancastelli et al, Phys. Rev A 95 061402(R) (2017)

TBA
Speaker: Prof. Nicola Poli
 Department of Physics and Astronomy, University of Florence
Time:Friday, 17th May 2019, 9:30 AM
Location: Haber Villa, Faradayweg 8
Organized: MP

Anisotropic Hot-Electron Relaxation Lifetimes in Black Phosphorus Studied by TR-2PPE
Speaker:Can Wang
 Friedrich-Alexander-Universität Erlangen-Nürnberg
Time:Tuesday, 28th May 2019, 11:00 AM
Location: PC Seminar Room G2.06, Building G
Organized: PC
Abstract: Black phosphorus (BP), a new 2D semiconductor, has received broad interest for its promising mechanical, electric, optical, and thermoelectric properties. Here, we demonstrate applications of time-resolved two-photon photoemission (TR-2PPE) which are unique in that they allow for a direct observation of ultrafast hot-electron dynamics at the conduction band in single crystal BP. We report the strong anisotropy of hot-electron relaxation lifetimes in bulk BP using TR-2PPE. Electrons excited from the armchair (kx) direction (140 fs) exhibit a much longer relaxation time than the zigzag (ky) direction (60 fs) at the same intermediate-state energy (E – EF = 1:8 eV). Using photoemission spectroscopy (PES), we also study that charge transfer at organic/BP interface which plays an important role in determining the interfacial electronic properties, such as energy-level alignment and charge carrier dynamics. We make a systematical research on the electronic structure between organic semiconductor and BP interfaces. Besides, we will also introduce spin-resolved PES results on rubrene/Co(001) interface and metalation effect on porphyrins/CoO(111) interfaces briefly.

Optical Clocks: from frequency measurements to industrial devices
Speaker: Prof. Simon Stellmer
 Physikalisches Institut der Universität Bonn
Time:Friday, 7th June 2019, 9:30 AM
Location: Haber Villa
Organized: MP
Abstract: Optical Clocks are probably the most precise measurement devices mankind has ever built, the finest of them now reaching fractional uncertainties at the 19th digit. This stunning performance - an improvement by 10 orders of magnitude over the past 30 years - was made possible by landmark progress in the field ultracold atoms and optical cavities, and the invention of the frequency comb. Current trends towards future developments include the exploration of new approaches, such as the nuclear clock and the superradiant clock, as well as the quest of preparing these clocks for operation in an industrial environment. I will give a broad introduction that spans from the many variations of optical clocks to the most recent applications in relativistic geodesy.

TBA
Speaker: Prof. Wilson Ho
 University of California, Irvine, CA
Time:Friday, 21st June 2019, 11:00 AM
Location: Seminarraum Faradayweg 10
Organized: CP

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