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Apr. 2019: Professor Jens Biegert joins us as a Friedrich Wilhelm Bessel Research Awardee.
The Alexander von Humboldt Foundation has announced that Prof. Dr. Jens Biegert, a researcher at the Institute of Photonic Sciences (ICFO), Barcelona, has been granted the Friedrich Wilhelm Bessel Research Award. Professor Biegert is an international leader in the fields of ultrafast laser physics and attosecond science, and noted as a pioneer atomic- and attosecond-resolution molecular imaging, and key advances towards element-selective attosecond x-ray spectroscopy of carrier dynamics in solid state systems.   During his stay in Berlin at the Physical Chemistry Department of the Fritz Haber Institute of the Max Planck Gesellschaft he will explore the connection between electrons and the lattice within electronic and structural phase transitions and for super- conductivity.
Contact persons: Prof. Dr. Martin Wolf  
Feb. 2019:  Mapping uncharted territory of electrons.

Physical properties often boil down to the arrangement of electrons within a solid, which means that mapping the electronic structure is a powerful way of classifying and understanding a wide range of materials. By applying ultrashort laser pulses with 22 eV photon energy from a novel 500 kHz extreme-ultraviolet light source to chains of indium atoms on a silicon surface, we are able to produce a detailed map of electronic states not normally accessible to traditional methods for measuring the electronic structure. The same technique also enables us to track the energy flow within the material as the highly excited electrons exchange energy with each other and the crystal lattice. Charting such previously inaccessible states and their dynamics allows us to better test the limits of current theories for understanding and predicting physical phenomena, and may in future lead to new designer materials.
Rev. Sci. Instrum. 90, 023104 (2019) (Editor's Pick); doi: 10.1063/1.5081938
Submitted: Phys. Rev. B (2019); preview at arXiv
Contact persons: Prof. Dr. Martin Wolf
Dr. Ralph Ernstorfer
Dr. Laurenz Rettig
Nov. 2018: Plasmon-Assisted Resonant Electron Tunneling in a Scanning Tunneling Microscope Junction.
This publication by Shuyi Liu, Martin Wolf, and Takashi Kumagai in Physical Review Letters reports about plasmon-assisted resonant electron tunneling from a silver or gold tip to field emission resonances (FERs) of a Ag(111) surface induced by continuous wave (cw) laser excitation of a scanning tunneling microscope (STM) junction at visible wavelengths. As a hallmark of the plasmon-assisted resonant tunneling, a downshift of the first peak in the FER spectra by a fixed amount equal to the incident photon energy is observed. STM-induced luminescence measurement for the silver and gold tip reveals the clear correlation between the laser-induced change in the FER spectra and the plasmonic properties of the junction. These results clarify a novel resonant electron transfer mechanism in a plasmonic nanocavity.

Contact person: Dr. Takashi Kumagai
Artists view of the excitation and formation of chemical bonds along Indium nanowires (red balls) on a Silicon(111) surface during the ultrafast photo-induced phase transition between the 8x2 and 4x1 structures. This real space view of atoms and bonds is com- plemented by detailed measurememets of the electronic structure of electrons in their “momentum space” exhibiting the evolution of the band stuctrue providing a complete picture of the phase transition.
© A.Lücke, Univ. Paderborn
  Nov. 2018:  Ultrafast Dynamics of Atomic Motion Viewed by the Electrons in Solids.

Capturing the motions of atoms in a so-called “molecular movie” is generally thought of as the Holy Grail for understanding chemical transformations or structural phase transitions in solids. However, atomic motion is not the whole story, as the forces driving these motions arise from details of the electronic structure and a gradient across a free energy landscape. Therefore, to obtain a complete picture of the processes driving structural changes, it is necessary to observe the dynamics of the electronic structure and track the temporal evolution of electronic states and their populations. By using femtosecond lasers to perform time- and angle-resolved photoemission spectroscopy, the changes of the electronic structure during the phase transition in indium nanowires on a silicon surface could be closely monitored, allowing a detailed reaction pathway to be extracted. This information combined with simulations of the electronic structure dynamics, made it possible to translate the electronic structure dynamics into a potential energy landscape and therefore extract not only the motion of atoms, but also the formation and breaking of chemical bonds during the phase transition. This provides a bridge between the languages of physics and chemistry for describing structural changes in both real and momentum space. Understanding how the transient electronic structure results in bond dynamics may in future allow the tailoring of chemical reactions and phase transitions via engineered light pulses. (more . . . )
Press contacts:   Prof. Martin Wolf, Fritz Haber Institute of the Max Planck Society, Berlin,
phone: +49 30 8413-5111; email:   
The Original publication:   C. W. Nicholson, A. Lücke, W. G. Schmidt, M. Puppin, L. Rettig, R. Ernstorfer, M. Wolf,
Beyond the molecular movie: Dynamics of bands and bonds during a photo- induced phase transition,
Science, Vol. 362, Issue 6416, pp. 821-825 (2018)
  Nov. 2018:  Christopher Nicholson receives Carl Ramsauer Award for his Thesis.

The Physikalische Gesellschaft zu Berlin has announced that Christopher Nicholson who had prepared his Thesis “Electronic Structure and Dynamics of Quasi-One Dimensional Materials” at the Dynamics of Correlated Materials group has been awarded the Carl Ramsauer Award 2018 for his Thesis.

The thesis of Christopher Nicholson (who is meanwhile at the Université de Fribourg) explores the electronic structure and ultrafast dynamics of quasi-one dimensional materials by means of high resolution angle-resolved photoemission spectroscopy (ARPES) and of femtosecond time-resolved ARPES (trARPES). Observing how confining electrons to quasi-one dimensional environments induces a range of broken symmetry ground states, and emergent properties that result from the increased inter-particle couplings and reduced phase space that such a confinement enforces, the work furthermore studies the interaction of such quasi-one dimensional phases with a higher dimensional environment.
A number of model quasi-one-dimensional systems were analysed: the bulk one-dimensional compound NbSe3 (see left image); the possibly one-dimensional system Ag/Si(557); the atomic nanowire system In/Si(111) that is known to undergo concomitant structural and metal-to-insulator transitions; and the spin density wave phase transition in thin films of Cr driven by photoexcitation.
Contact persons: Prof. Dr. Martin Wolf   
Jul. 2018:  Watching the first steps of magnetic information transport.
In conventional electronics, information is encoded in bits (0 or 1) by the presence or absence of electron charges. A promising new approach—spintronics—aims to use the electron ‘spin’ as an information carrier. This method takes advantage of the orientation (up or down) of the electron spin to encode information. The speed at which electronics operate continues to increase and is expected to work at terahertz speeds in the future. To be competitive and compatible with charge- based electronics, spintronic operations must, therefore, also work at these high frequencies.

An elementary but vital spintronic operation is the transport of spin-based information from a magnetic metal layer into an attached nonmagnetic metal layer (see figure). It was discovered only a few years ago that this transfer can happen simply by heating the magnet and metal to different temperatures. When heating the magnetic layer, hot  electrons  move  into  the  colder  nonmag- netic metal,  thereby carrying magnetic information
  across the interface of the two layers.

What is remarkable is that this transfer still occurs when the magnetic layer is an electrical insulator—meaning electron currents cannot move across the interface. The spin transfer happens instead from the torque exerted by the immobile spins of the magnetic layer onto the spins of the neigh- bouring mobile electrons in the metal layer. This phenomenon is called the spin Seebeck effect.

In the framework of the CRC/TRR 227 at the Freie Universität Berlin, a team of scientists from Ger- many, Great Britain and Japan aimed to discover just how quickly the spin transfer can happen. “Answering this question is not only interesting for potential applications in future high-speed infor- mation technology. It is also relevant to understand the elementary steps that lead to the emergence of the spin current”, says physicist Dr. Tom Seifert, who conducted the experiments at the Fritz Haber Institute of the Max Planck Society in Berlin.
  In their experiment, the researchers used a pulse from a femtosecond laser to heat up a metal film on top of a magnetic insulator in less than one millionth of a millionth of a second (see figure). The metal itself then emitted an electromagnetic pulse caused by the spin current flowing into it— behaving like an ultrafast spin-amperemeter. Using the emitted pulse, the researchers observed the formation of the spin current caused by the spin Seebeck effect. Once heated, the electrons in the metal hit the metal-insulator interface and are reflected back. During this scattering event, the magnet exerts torque on the incident electron’s spin, aligning it a little more parallel to the magnetization M of the insulator. Thus, spin information of the magnetic insulator is transported into the metal (see figure at time 0 femtoseconds).

The researchers made a surprising observation —the spin transport does not begin immediately, taking about 200 femtoseconds to peak. The reason is that the laser pulse excites relatively few electrons, but they receive a lot of energy and collide with ‘cold’ electrons, redistributing the energy. This avalanche-like process heats up a large number of electrons which also hit the interface, becoming a part of the spin transport (see figure at time 100 femtoseconds). “The photoexcited electrons need to multiply their numbers to generate sizeable spin transport”, says theorist Dr. Joseph Barker, who conducted simu- lations of the spin dynamics at the Tohoku Uni- versity in Sendai, Japan.

Finally, the electrons cool down by transferring heat to the atomic lattice of the metal, and after 1000 femtoseconds, the spin transport finishes (see figure). In effect, the instantaneous spin current is also a measure of the effective temperature of the electrons in the metal. “Our ultrafast amperemeter also acts like an ultrafast thermometer. This is very useful for studying spin and electron dynamics in a broad range of materials which hold a great potential for applications in spintronics and terahertz photonics”, notes Dr. Tom Seifert.

Figure: Watching the ultrafast spin Seebeck effect. At time 0 femtoseconds, a bilayer made of a magnetic insulator (with magnetization M ) and a nonmagnetic metallic layer is heated by an extremely short ultrafast laser pulse. The resulting transport of spin from the magnet to the metal leads to the emission of a terahertz pulse whose measurement allows the researchers to monitor the dynamics of the excited metal electrons with an extremely fine time resolution of 20 femtoseconds. Their work reveals that the strength of the spin transfer is directly proportional to the number of heated metal electrons, which is maximal at about 100 femtoseconds after laser excitation. After about 1000 femtoseconds, the excited metal electrons have cooled down and transferred their excess energy to the atomic lattice.
Press contactts: Prof. Tobias Kampfrath, Freie Universität Berlin and Fritz Haber Institute of the Max Planck Society, Berlin, +49 30 8413-5222,
The Original publication: T. Seifert, S. Jaiswal, J. Barker, S.T. Weber, I. Razdolski, J. Cramer, O. Gueckstock, S. Maehrlein, L. Nadvornik, S. Watanabe, C. Ciccarelli, A. Melnikov, G. Jakob, M. Münzenberg, S.T.B. Goennenwein, G. Woltersdorf, B. Rethfeld, P.W. Brouwer, M. Wolf, M. Kläui, T. Kampfrath,
Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy,
Nature Communications 9, Article number: 2899 (2018)
  Jul. 2018:  hot-electrons-induced atomic disorder in Au nanoclusters.

Researchers of the Structural and Electronic Surface Dynamics group of the FHI Department of Physical Chemistry, Berlin, in cooperation with colleagues at the Nanoscale Physics Research Laboratory of the University of Birmingham and the College of Engineering of the University of Swansea, have published a joint article in ACS Nano, observing various forms of atomic motion, such as thermal vibrations, thermal expansion, and lattice disordering on size-selected Au nanoclusters on thin-film substrates as distinct and reciprocal-space manifestations. Thermal equilibration proceeds through intrinsic heat flow between electrons and lattice, and extrinsic heat flow between the nanoclusters and the substrate. The two-temperature model was extended to 0D/2D heterostructures so as to describe energy flow among various subsystems, to quantify interfacial coupling contents and to elucidate the role of optical and thermal substrate properties. At lattice heating of the nanoclusters dominated by intrinsic heat flow, a reversible disordering of atomic positions occurs, which is absent when heat is injected as hot substrate phonons. The analysis suggests that hot electrons can distort the lattice of nanoclusters, even at lattice temperatures below the equilibrium threshold for surface premelting, and this is interpreted as activation of surface diffusion due to modifications of the potential energy surface at high electronic temperatures.

  Jul. 2018:  Daniela Zahn receives the Physics-studies Prize.

The Physikalische Gesellschaft zu Berlin has announced that Daniela Zahn of the Structural and Electronic Surface Dynamics group has been awarded the Physics-studies Prize 2018. The Physics-studies Prize is awarded by the Physikalische Gesellschaft zu Berlin to outstanding radiates with diploma or masters degree.
Contact person: Dr. Ralph Ernstorfer
  Jul. 2018:  Impact of crystal-lattice vibrations on magnetic properties.

Heating a ferromagnet beyond some critical temperature leads to loss of magnetic properties. The precise mechanism and time-scale of the process remained undescribed. A team of scientists from Berlin (Fritz Haber Inst., FU Physics Dept., TU Inst. for Optics & Atomic Physics, Max Born Inst.), Dresden (Helmholtz Center), Uppsala (Dept. of Physics and Astronomy), St. Petersburg (Ioffe Inst.), and Sendai, Japan (inst. of Materials Research) have now revealed the elementary steps of this process. 
The researchers have directly probed the flow of energy and angular momentum in the model insulating ferrimagnet yttrium iron garnet. After ultrafast resonant lattice excitation, one could observe that magnetic order reduces on distinct time scales of 1 ps and 100 ns. Temperature-dependent measurements, a spin-coupling analysis, and simulations show that the two dynamics directly reflect wo stages of spin-
lattice equilibration. On the 1-ps scale, spins and phonons reach quasi-equilibrium in terms of energy through phonon-induced modulation of the exchange interaction. This mechanism leads to identical demagnetization of the ferrimagnet’s two spin sublattices and to a ferrimagnetic state of increased temperature yet unchanged total magnetization. Finally, on the much slower, 100-ns scale, These findings are relevant for all insulating ferrimagnets and indicate that spin manipulation by phonons, including the spin Seebeck effect, can be extended to anti- ferromagnets and into the terahertz frequency range.
This has been published in  Science Advances.
See further details in the Fritz Haber Institute public information site,
  Nov. 2017:  an ERC Consolidator Grant for Kramer R. Campen.

The Grant is awarded to individual principal investigators based on the quality of the research proposal and the scientific track record of the applicant.
Kramer R. Campen, leader of the Interfacial Molecular Spectroscopy group, has received the Consolidator Grant of the European Research Council (ERC) for the research proposal “Electron Transfer Across Solid/Liquid Interfaces: Elucidating Elementary Processes from Femtoseconds to Seconds”,

Contact person: Dr. Kramer R. Campen
Jul. 2017: Laurenz Rettig receives a grant of the Emmy Noether Programme.
The Emmy Noether Programme of the DFG (Deutsche Forschungsgemeinschaft)) has accorded Laurenz Rettig, leader of the research group “Dynamics of Correlated Materials ” its support for his research project “Beyond time constants: Quantifying interactions in correlated materials by complementary ultrafast time-domain approaches”
Project Description:
Correlated materials are characterised by the variety of interactions between the elementary degrees of freedom, leading to novel ground states with broken symmetries and often intriguing properties. The quantitative determination of those couplings and their relevance for the formation of broken-symmetry ground states and phase transitions is a major challenge in solid state physics. In particular, in thermal equilibrium the various interactions are present simultaneously in a system, making it difficult to separate them due to their similar energy scale. Studies of those interactions in the time domain in a non-equilibrium system created after ultrafast optical excitation promise a way to separate such contributions by their intrinsically
  different dynamics. Such an approach, however, is often hindered by the unspecific nature of the employed probes and thus frequently limited to a qualitative discussion of time constants. The goal of this research project is to combine complementary, quantitative time-resolved spectroscopies, which directly address the dynamics of specific degrees of freedom, in order to determine the couplings between the different degrees of freedom and their relevance for a phase transition from their temporal evolution. Combining time- and angle-resolved photoemission spectroscopy, time-resolved resonant and non-resonant x-ray diffraction and electron diffraction techniques provides direct and quantitative access to electronic, magnetic and structural ordering phenomena. This approach will be employed for different selected classes of phase transitions, such as charge-density waves, antiferromagnetically ordered system and complex charge- and orbitally ordered systems, in order to determine and quantify the couplings between the different degrees of freedom responsible for the formation of those broken-symmetry states.
  Jul. 2017:  Nikolai Paßler receives the Physics-studies Prize.

The Physikalische Gesellschaft zu Berlin has announced that Nikolai Paßler of the Lattice Dynamics group has been awarded the Physics-studies Prize 2017. The Physics-studies Prize is awarded by the Physikalische Gesellschaft zu Berlin to outstanding radiates with diploma or masters degree.
Contact person: Dr. Alex Paarmann

  Jul. 2017:  photoexcited carriers interact with lattice vibrations in thin films of the layered WSe2.

A team of researchers from the Structural and Electronic Surface Dynamics group of the FHI Department of Physical Chemistry, Berlin, and the MPI for Structure and Dynamics of Matter, Hamburg, have published an outstanding article in the Physical Review Letters, investigating the interactions of photoexcited carriers with lattice vibrations in thin films of the layered transition metal dichalcogenide WSe2. Employing femtosecond electron diffraction with monocrystalline samples and first-principles density functional theory calculations, a momentum-resolved picture of the energy transfer from excited electrons to phonons is obtained. The measured momentum-dependent phonon population dynamics are compared to first-principles calculations of the phonon linewidth and rationalized in terms of electronic phase-space arguments. The momentum-dependent electron-phonon coupling leads to a nonthermal phonon distribution, which relaxes to a thermal distribution via electron-phonon and phonon-phonon collisions. The results provide a basis for monitoring and predicting out of equilibrium electrical and thermal transport properties for nanoscale applications of transition metal dichalcogenides.

Contact person: Dr. Ralph Ernstorfer
  Feb. 2017:  simultaneous probe of electron-lattice interactions in photoexcited antimony.

Researchers of the Structural and Electronic Surface Dynamics group of the FHI Department of Physical Chemistry, Berlin, in cooperation with colleagues at Theoretical Physics II & Center for Interdisciplinary Nanostructure Science and Technology of the University of Kassel, have published a joint article in the Physical Review B, demonstrating in the example of photoexcited atimony, that the entire electron-lattice interaction, particularly coherent and incoherent electron-phonon coupling, can be probed simultaneously. Femtosecond electron diffraction (FED) with high temporal resolution allowed to simultaneously observe coherent excitation of the fully symmetric A1g optical phonon mode as well as energy transfer from electrons to phonons via incoherent electron-lattice scattering.

Contact person: Dr. Ralph Ernstorfer

Oct. 2016: Takashi Kumagai receives PRESTO promotion.
The PRESTO (Precursory Research for Embryonic Science and Technology) managed by JST (Japan Science & Technology Agency) has announced that Takashi Kumagai, leader of the research group “Nanoscale Surface Chemistry” will be supported in his research topic “Eluciation of microscopic mechanism of catalytic effects via localized plasminon excitation”.   on plasmonic metal nano-structures can enhance chemical reactions. However, the microscopic mechanism of the process induced via near-field excitation remains poorly understood. This research project investigates near-field induced chemical reactions using a low-temperature scanning probe microscope and aims at obtaining the detailed reaction mechanism at the single-molecule level. The application for methane activation via near-field excitation will also be examined.

Sep. 2016: Julia Stähler wins the Edith Flanigen Award 2016.

The Collaborative Research Centre 1109 (CRC 1109) has announced that the winner of the Edith Flanigen Award 2016 is Dr. A Julia Stähler, leader of the Max Planck Research Group “Electron Dynamiχ”. The award is conferred annually by the CRC 1109 to an exceptional female scientist at an early stage of her career for outstanding results on metal oxide water systems.
The announcement mentions that in her research, the awardee investigates ultrafast non-equilibrium dynamics after optical excitation in a wide range of fields: Electron- and exciton-transfer processes at inorganic-organic hybrid systems, photo- and electron-induced chemical reactions at interfaces, elementary excitations at surfaces, and photoinduced phase transitions.
  Sep. 2016:  Lutz Waldecker receives Carl Ramsauer Award for his Thesis.

The Physikalische Gesellschaft zu Berlin has announced that Lutz Waldecker of the Structural & Electronic Surface Dynamics group has been awarded the Carl Ramsauer Award 2016 for his Thesis “Electron-Lattice Interactions and Ultrafast Structural Dynamics of Solids”.

Lutz Waldecker’s thesis studies the interactions between the atomic cores of solids and their outer electrons. These interactions determine fundamental material properties such as electric and thermal conductivity, and are responsible for how a material behaves in extreme conditions. To investigate these interactions, materials were placed in non-equilibrium states by very short laser pulses. The subsequent relaxation proceeds within an extremely short period of a few hundred femtoseconds (1 Femtosekunde = 10-15). Complementary femtosecond optical- and diffraction techniques based on the pump-probe principle were developed, allowing to observe these processes on their fundamental time scale. Applying these methods it became possible to elucidate new details of the electron-lattice interactions in simple metals as well as in two-dimensional semiconductors. Additional experiments with strongly excitated phase-change materials gave new insights into the pathway of the transition between their two crystallographic states and on what role the electron-lattice interactions play in this process.
Contact persons: Dr. Ralph Ernstorfer
  Apr. 2016:  Observing basic interactions in solids.

Interactions between electrons and vibrations of atomic ions, of which all condensed matter is composed, and which determine such fundamental features like electron and thermal conductivity of materials, energy dissipation in electronic devices, and emergence of quantum phenomena like superconductivity, are a central subject in solid-state physics.

Lutz Waldecker and Roman Bertoni of the Max Planck Research Group headed by Ralph Ernstorfer, in coopertion with Jan Vorberger of the Max-Planck-Institut für Physik komplexer Systeme, Dresden, have now visualized and quantified electron-lattice interactions after a sudden very short external disturbance (a 50 femtosecond infrared laser pulse) in aluminum, a proto-typical metal. Due to these interactions, the excited electrons proceed to equilibrate with the atomic vibrations, and successive snapshots of these latter provide a “movie” of the relaxation process. Comparing the experimental findings with state-of-the-art numerical calculations of atomic mean squared displacements made it possible to revise the existing model of such interactions.

The report of these results has now been published in Physical Review X vol. 6.
  Feb. 2016:  ERC double feature at the PC department.

Tobias Kampfrath and Ralph Ernstorfer have been successful in the 2015 Call for Consolidator Grants of the European Research Council (ERC). ERC Consolidator Grants are awarded to single principal investigators based on the quality of the research proposal and the scientific track record of the applicant. Tobias Kampfrath’s and Ralph Ernstorfer’s projects each have a duration of 5 years and are funded with 2.0 and 2.6 million Euro, respectively.

In his TERAMAG project, Tobias Kampfrath will implement spintronic operations, i.e. operations utilizing the spin of electrons, at extremely high frequencies in the elusive terahertz range. This research is not only relevant for future processing of magnetically stored information, but will also result in novel and highly efficient emitters of terahertz electromagnetic radiation.

Ralph Ernstorfer will investigate the interaction between electrons, their spin and the vibrations of atoms in semiconductors with layered crystalline structure. The project FLATLAND aims at obtaining a microscopic understanding of relaxation and dissipation effects in two-dimensional semiconductors and stacked heterostructures comprised thereof.
Further information on the grantee’s research and the employed methodology can be found on the websites of the Terahertz Physics group and the Max Planck Research Group Structural & Electronic Surface Dynamics.
  Contact persons:  Dr. Tobias Kampfrath &
Dr. Ralph Ernstorfer
Jan. 2016:  Ultrafast dynamics of elementary processes and many-body effects
at surfaces and in solids.

We are happy to announce that the Deutsche Vakuumgesellschaft (DVG) awarded the Gaede-Preis 2016 to Dr. Julia Stähler in recognition of her outstanding work on ultrafast dynamics of elementary processes and many-body effects at surfaces and in solids. The prize will be handed over on March 8, 2016 in a Special Ceremonial Session at the 80th Annual Meeting of the DPG in Regensburg.
Contact person: Dr. Julia Stähler

Order versus disorder: Electrons are diffracted differently in crystalline structured and amorphous Ge2Sb2Te5 (GST), leading to distinctly different diffraction images of the crystal (left) and of the amorphous material (right).
 © Fritz Haber Institute of the MPG
Jul. 2015:  Light switches on a DVD.

A germanium, antimony and tellurium compound, Ge2Sb2Te5, also known by its acronym GST, has long been used as data storage material, in that its structure is rearranged by laser pulses, switching it between transparent and non-transparent states A researcher team at the Physical-Chemistry Department of the Fritz Haber Institute (FHI) in Berlin, in collaboration with colleagues from the Institut de Ciències Fotòniques (ICFO) in Barcelona, have discovered that the material’s optical properties change much faster than the structure, which opens new perspectives for its use in photonics components. Hence, while DVDs may soon become obsolete, the implemented GST may find applications other than as storage material.

These findings, together with a video, have been published in Nature Materials.
MPG research news: “Light switches on a DVD”  (doi: 10.1038/nmat4359)

Contact person: Dr. Ralph Ersntorfer
Apr. 2015:  Solvation of electrons in water.

The behavior in water of naked electrons, i.e. of such which are not attached to atoms or molecules, plays a significant role in the progress of chemical reactions when water is involved. As a first step, the negatively charged electrons become solvated, they surround themselves with water molecules by attracting the positive electrical poles of the latter.

A research team of the Department of Physical Chemistry has now obtained detailed information about the solvation process, determining the energy with which a naked electron is bound directly when immersed in water, when its charge is not yet buffered off by the water molecules, and has also measured the time it takes until the naked electron begins to gather water molecules around it. The findings on the immersion process of an electron were made using time-resolved two-photon photoelectron spectroscopy.

The findings have been published as cover story in Journal of the American Chemical Socierty.
MPG research news: “Diving electrons” Contact person: Dr. Julia Stähler
Jun. 2014: “Karl Scheel Prize 2014” is awarded to Dr. Tobias Kampfrath.

The Physikalische Gesellschaft zu Berlin (PGzB) has decided to award the Karl Scheel Prize for 2014 to Tobias Kampfrath in recognition of his outstanding research on “manipulation of spins and light at terahertz frequencies”. The prize will be handed over on June 27, 2014, at a meeting of the PGzB in the Magnus House in Berlin, at which occasion the awardee will present a talk on that subject.
Dec. 2013:  Every single atom counts.

The environment of every single molecule is important in any chemical process as it defines the potential energy landscape in a reaction. A research team at the Fritz-Haber-Institute Berlin (in collaboration with theoreticians from Liverpool and chemists from Warszaw) could for the first time control a chemical process by single atoms in the vicinity of the molecule. Porphycene molecules were studied on a copper surface and an intramolecular proton transfer, a process that is important in nature, was induced by the tip of a scanning tunneling microscope. The researchers then placed very precisely individual copper atoms close to a porphycene molecule and found a dramatic influence of the atom location on the proton transfer rate in the molecule. It is thus possible to tune the transfer rate up and down, depending on the atom position.
This surprising effect is caused by the modified potential energy landscape of the molecule in the presence of an atom. It could be extended to rows of porphycene molecules on the surface. In such an arrangement it was found that even the position of the protons in the centre of a molecule can modifiy the proton transfer in the neighbour molecule. Hence, positive and negative cooperativity could be directly imaged in real space. These results show on the one hand the importance of controlling the environment of molecules with atomic precision and demonstrate on the other hand the potential to regulate processes that occur in a single molecule.  
These findings have been published in Nature Chemistry.

Nov. 2013: “Carl Ramsauer Award of the German Physical Society Berlin” Ceremony at the Technical University Berlin.

The 2013 awards for outstanding PhD theses will be conferred today, November 13, at the Technical University Berlin. Amongst the awardees are, besides Laurenz Rettig (see below), also Marc Herzog who defended his thesis “Structural Dynamics of Photoexcited Nanolayered Perovskites Studied by Ultrafast X-ray Diffraction” at the University of Potsdam, but presently is in the Electron Dynamics group of the FHI Physical Chemistry Department.
Aug. 2013: “Carl Ramsauer Award of the German Physical Society Berlin” for Dr. Laurenz Rettig.

The German Physical Society in Berlin will award the Carl-Ramsauer-Award 2013 to Laurenz Rettig for his PhD thesis “Ultrafast dynamics of correlated electrons”. The Carl-Ramsauer-Award recognizes one outstanding PhD thesis in physics at each of the 3 universities in Berlin and at the University of Potsdam. The award is named after Carl Wilhelm Ramsauer, who discovered in 1920 the high transmission of slow electrons through gases demonstrating non-classical behavior of electrons (Ramsauer-Townsend-Effect).

Schematic representation of the experiment. Left: An extremely short ultraviolet pulse creates hot excited electrons in the semiconductor titanium dioxide. This changes the spatial distribution of the electrons within the lattice, resulting in a shift of the potentials for the atomic cores, i.e., their rest position (center). The subsequent cooling of the electrons which takes about 20 femtoseconds further amplifies this effect (right). The combined effect of electron excitation and cooling leads to a force on the oxygen atomic cores, resulting in a coherent oscillation within the crystal structure.
  Feb. 2013:  Cooling of electrons drives ultrafast lattice dynamics.

At the atomic scale, solids are made of atomic cores, i.e., nuclei with tightly bound electrons, and weakly bound valence electrons. The valence electrons strongly interact with each other and thereby act as a kind of glue that holds the atomic cores together within the crystal. The fundamental properties of a material, e.g., its electrical conductivity, optical properties, or crystal structure, are the result of the continuous interplay between the positions of the atomic cores and the valence electrons. The investigation these correlations, in particular for complex materials, manifests one of the central topics of modern solid state physics. One experimental approach uses extremely short light pulses to excite the material and observes the response of the crystal lattice to this perturbation. These processes occur on time scales of femtoseconds (1 fs = 10–15 s).

The correlations between electronic and atomic structures are particularly strong for transition metal oxides. For some of them it is even possible to induce a structural phase transition with an optical excitation. A team of researchers from the Fritz Haber Institute, in collaboration with colleagues from the Max Planck Institute for Quantum Optics, the Technical University of Munich, and the University of Kassel have now shown that even a small redistribution of electrons can produce a significant force on the atomic cores in the crystal lattice. They excited the semiconductor titanium dioxide with extremely short ultraviolet light pulses and measured the subsequent changes of the crystals reflectivity. The excitation initially generates a small number of very hot electrons which thereby also redistribute spatially within the crystal: the electron concentration is reduced around the oxygen cores while it is increased around the titanium cores. In consequence, the potential energy surface for the atomic cores, which is due to the valence electron distribution, changes and the rest position of the oxygen cores is shifted relative to the position of the titanium cores. Since these changes occur faster than motion of the atom cores in the crystal, each oxygen core experiences the same force, and all of them start oscillating in phase.

This effect is best understood imagining a ball (oxygen atom core) in a bowl (potential surface of the crystal). In the ground state, the ball is in the center at the bottom of the bowl. The excitation of the electrons causes a sudden shift of the bowl, and the ball starts oscillating.

Detailed analysis of the phase of these lattice oscillations and extensive theoretical calculations revealed a surprising effect: it is not only the initial excitation of the electrons that is important for the new rest position of the atomic cores, but also the subsequent cooling of the electrons. The initially hot electrons cool down from several thousand Kelvins to room temperature within about 20 femtoseconds. While the crystals warms up only slightly on those time scales, a significant change of the spatial redistribution of valence electrons and, in consequence, the rest positions of the atomic cores is observed. Such dependence of the crystal structure on the electronic temperature has been long predicted, and could now be shown experimentally for the first time. The results show how the equilibrium state of a crystal can be extremely sensitive to small changes in the electronic structure. The work is another step towards understanding the complex interactions in transition metal oxides and opens up new ways of designing materials for specific applications.
The findings are published in Physical Review Letters. (doi: 10.1103/PhysRevLett.110.067402)

Contact person: Dr. Ralph Ersntorfer
 Oct. 2012: “2011 Foresight Institute Feynman Prize” awarded to Dr. Leonhard Grill.
Foresight  Institute  Feynman  Prize 
  The Foresight Institute at Palo Alto (CA), which awards annual Feynman Prizes for Nanotechnology Theory and Experiment in the amount of $5,000 each, announced on Oct. 16, 2012, that the winner of the 2011 Feynman Prize for Experimental work is Leonhard Grill of the Department of Physical Chemistry of the FHI: “in recognition of his pioneering and continuing work on manipulating and structuring functional matter at the atomic scale. He has used scanning tunneling microscopy to characterize the electronic and mechanical properties of single molecules; constructed atomically precise covalent molecular nanostructures from single molecules; and used an STM tip to roll a 0.8 nanometer molecular wheel on a surface.”

Experiment scheme: a single graphen strip is drawn off a gold surface and it’s conductance is measured.
Oct. 2012:  Single graphene stripes as molecular wires.

The transport of electrons and thus electrical current is not only of central importance in modern society, but also for scientists in fundamental research where it is of interest for biological processes and for potential applications for future molecular electronics (with single molecules as devices). A research team at the Fritz-Haber-Institute Berlin (in collaboration with theoreticians from Toulouse and Singapore) could measure for the first time the electrical current through single molecules at different electrode voltages, thus characterizing various charge transport regimes. Graphene stripes were chosen, due to their interesting electronic properties, and assembled directly on the surface by in-situ polymerization. The central challenge for such measurements is to measure the current thourgh an object at the atomic scale with macroscopic electrodes, ensuring for a well-defined arrangement.
In this work, a scanning tunneling microscope was used to pick up single graphene ribbons from a surface and thus realizing the desired geometry. In this way, the decay of the electric current with the molecular length, the key property for charge transport efficiency, can be measured in real time. It is shown that the conductance properties of a single molecule can be correlated with its electronic states. Comparison with calculation reveals that the conductance depends on the precise atomic structure and bending of the molecule in the junction. 
These findings have been published in Nature Nanotechnology.

Light induced changes in crystal vibrations enable researchers to optically probe changes in atomic structure.
  Mar. 2012:  Measuring ultrafast phase transitions with light.

Short, femtosecond (10-15 s) pulses of intense laser light offer the fastest ways to manipulate material properties. One active area of research is to investigate how such pulses can be used to trigger ultrafast phase transitions, which results in colossal changes in a material’s properties on very short timescales. During these phase transitions the structure of the material can evolve extremely rapidly so that conventional probes of the crystal structure, such as X-ray and electron diffraction, produce blurred images, and the transition pathway cannot be resolved. Although optical pulses have the required temporal resolution, their wavelength is too long to measure the atomic positions directly. To overcome this limitation, researchers at the Fritz Haber Institute instead used an optical technique to measure the forces that dictate the atomic positions, thus enabling an ultrafast optical probe of the crystal structure. 
The technique was used to measure the structural component of the photoinduced insulator-metal phase transition in VO2, using samples grown and characterized at the Vanderbilt University. When the laser strikes the material, vibrations, which are characteristic of the insulating phase, are created. By using a high power laser, these vibrations could be completely eliminated, providing insights into the nature of the timescale and driving mechanism of the phase transition. These findings have been published in Nature Communications — see elaborate information in German at the MPG news site.
(doi: 10.1038/ncomms1719)

Contact person: Dr. Simon Wall
Feb. 2012:  Modifying the light spectrum with a spectral lens.

Left: schematic of a spectral lens. A pulse of light modifies the light spectrum of the lens, so that its initially broad frequency range (shown red) is compressed to a narrower, higher-frequency spectrum (blue).
Right: experimental realization. When a probe pulse is sent through a light conductor, the low frequencies infiltrate a wider area of the membrane while the high frequencies remain concentrated in the middle. An intensive pump pulse then mainly affects the lower frequencies, while the higher frequencies are protected by a gold stripe.
  A conventional optical lens can be used to magnify an image, or to reduce it. Quite different is the effect of a so-called spectral lens developed by researchers of the Department of Physical Chemistry of the FHI, the Amsterdam FOM Institute for Atomic and Molecular Physics (AMOLF), and the School of Physics and Astronomy at the University of St. Andrews (Scotland, UK). It narrows or widens the spectral range of a light beam.

Its construction, and mode of functioning, is of course quite different from that of an optical lens. To narrow the spectral range of a light beam, the frequencies are shifted to higher frequencies such, that the frequency increase is dependent of the initial frequency. The lower the initial frequency, the greater the upwards shift. This is achieved in a so-called photonic crystal, that is a solid with a periodic structural modulation, for example a regularly repeating pattern of tiny holes. This influences the index of refraction of the solid, and hence the dispersal of light in it.
The photonic crystal of the researchers around Tobias Kampfrath of the FHI PC Department consists of a 220 nanometer (a nanometer is a millionth of a millimeter) thin silicon foil with regularly distributed little holes, each a few hundred nanometer in size. In the middle there is a very thin strip without holes, that serves as light conductor. When a pulse of light is directed through this strip, the higher (blue) frequencies remain concentrated within it, while the lower (red) frequencies penetrate far into the perforated part. As a result, only the higher frequencies are influenced by the perpendicularly incident controlling pump pulse. The results have been published in Physical Review Letters — see also the elaborate information in German at the MPG news site, and in the FHI archives. 
Jan. 2012:  Hierarchical linking of individual molecules into complex structures
Researchers at the Fritz-Haber-Institute demonstrate the controlled assembly of nanoscopic building blocks on the path towards novel materials and molecular electronics
  A central challenge of nanotechnology, which is crucial to visions such as molecular machines, novel materials and molecular electronics, lies in the “bottom-up” connection of molecular building blocks in a well-defined assembly. One can think of it as similar to LEGO, however at the nanometer scale. Due to their exceptionally low size, such structures hold the promise of low energy consumption, high operating speeds and low production costs. However, to date, only simple structures could be formed, because the assembly process was limited to a single step. A research group at the Fritz Haber Institute, Berlin, could now, in collaboration with scientists from the Humboldt University, Berlin, and the Laboratorio TASC ,Trieste, for the first time demonstrate hierarchical growth in several steps by the suitable provision of distinct side groups. This was performed by sequentially supplying reactive sites on molecular building blocks, which allowed the formation of considerably more sophisticated structures. With this method the researchers could affect the formation of networks made of two different molecular building blocks with high selectivity with regard to the connection sites, which would not have been possible with a non-hierarchical process. These results have been reported in the journal Nature Chemistry.
(doi: 10.1038/nchem.1242)

Contact person: Dr. Leonhard Grill

Drawing an individual polymer from a surface and applying an electric tension (potential difference) opens the way to an atomic scale study of charge transport through molecular wires.
Jul. 2011:  Can molecular wires become the basis for new computers?

In their pursuit of ever faster and more efficient processors, the producers of computer chips are approaching the physical limits of maniaturisation. The tiniest transistors in modern microprocessors, measuring just a few nanometers (1 nanometer is one billionth of a meter), can no longer be made smaller by conventional techniques of so-called top-down procedures. For this reason, the semiconductor industry is compelled to place more transistors on a chip, and to operate them at higher clock frequencies — both having the consequence of rising energy consumption and higher waste heat. A major European project, the Atomic Scale and Single Molecule Logic Gate Technologies (AtMol), in which researchers of the Fritz Haber Istitute are taking part, aims for solutions of the problem. 
In this, the research group headed by Leonhard Grill at the Department of Physical Chemistry is to play an important role, in that they will measure the electric conductivity of individual molecular wires, i.e. of polymers (molecule chains). In cooperation with Stefan Hecht of the Humboldt University, Grill will implement a jointly developed method to produce the polymers directly on surfaces so as to assemble the first logical circuit elements with custom-built molecular modules.
(more info, in German)

Contact person: Dr. Leonhard Grill
  Jun. 2011: “Professor Bernhard Heß Lectureship”, Regensburg.
  The Bernhard Heß Foundation of the Regensburg University awards an annual “Professor Bernhard Heß Lectureship” prize of € 2,000. The award is designed to give external upcoming researchers the opportunity to hold guest lectures at the Faculty of Physics in Regensburg, so as in this way to widen the range of courses of the University. The presentation of the award will be in February, on the Physics Day at the Regensburg University. The awardee for 2011, Dr. Tobias Kampfrath, is preparing a lecture series on the subject “spectroscopy and photonics with a femtosecond laser”, that will magnificently augment the courses offered at the University.
(more info, in German)

Contact person: Dr. Tobias Kampfrath

The basic principle: Electrons (blue spheres) carry a spin that can be seen as a persistent rotation of the electron about its own axis, resulting in a magnetic moment (blue arrows). In the experiment, spins are kicked by an intense terahertz magnetic transient (orange waveform), leading to a precession about their equilibrium directions, similar to a spinning top.
Jan. 2011:  Coherent terahertz control of antiferromagnetic spin waves.

Researchers of our Department — with colleagues from the University of Konstanz and the Hemlholtz Institute in Bonn — have succeeded in controlling the electron spin direction, which had heretofore been inaccessible. The experiment employs electromagnetic radiation in the terahertz frequency range (1 THz = 1012 Hz), which is currently attracting attention because of its use in body scanners in airports. Like many other applications, these scanners exploit the electric component of the terahertz wave. The present work shows that the magnetic component of terahertz radiation has important further potential applications. Their findings were reported in the journal Nature Photonics.

Turing patterns in a complex network.
  Jul. 2010:  Turing patterns in network-organized activator–inhibitor systems.

Researchers of our Department and the Department of Physics of the Kyoto University have made an important step towards understanding self-organization phenomena, extending theoretical analysis to complex networks. Their attention was focused on patterns under conditons of instblility first proposed in 1952 by the British mathematician Alan Turing. However, the treatment of Turing instability in networks has so far only been considered in the example of regular lattices and small networks. The present work studies Turing patterns in large random networks, which reveal striking differences from the classical behaviour. Their findings were reported in the journal Nature Physics.
Nov. 2009:  A new book by Gerhard Ertl: “Reactions at Solid Surfaces”

A new book entitled “Reactions at Solid Surfaces” by Nobel Prize winner Gerhard Ertl has appeared, based on Ertl‘s Baker Lectures at Cornell University in 2007. The book describes how surface chemistry works and how to probe and understand the dynamics of reactions at surfaces. The book is a thoughtful and compact introduction to surface reactions for graduate students and professional scientists alike.
The book was published by Wiley.

isbn: 978-0-470-26101-9
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