Ultrafast Dynamics of Atomic Motion Viewed by the Electrons in Solids

Thursday, 22nd November 2018Publication highlights, Miscellaneous

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.

Nicholson

Artists view of the excitation and formation of chemical bonds along Indiumnanowires (red balls)  on a Silicon(111) surface during the ultrafast photo­induced phase transition between the 8×2 and 4×1 structures. This real space view of atoms and bonds is complemented 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

Watching the motions of atoms in the course of a chemical reaction is generally thought of as the Holy Grail for understanding chemical transformations or phase transitions in solids. While recordings of such “molecular movies” have been achieved in recent years, the atomic motion does not reveal the whole story of why specific bonds break and others form. This is dictated by the arrangement of the electrons as the atoms move along gradients on an energy landscape defined by the electrons. It is therefore necessary to observe the dynamics of the electronic structure, which means to record an “electron movie”, to obtain a complete picture of the mechanisms driving chemical reactions.

An experimental team at the Fritz-Haber-Institut in Berlin and computational scientists at the University of Paderborn now filmed the electrons during a light-induced reaction. They investigated a single layer of indium atoms on top of a silicon crystal. At low temperatures, the indium atoms form an insulating layer with the atoms arranged as hexagons. At room temperature, however, the indium atoms rearrange and form conducting atomic wires. This phase transition can not only be induced by changing the temperature but also by exciting the cold material with a very short flash of light. This light pulse puts energy in the electrons of the material faster than the atoms can move. Due to the extra energy, the electrons reorganize and change the energy landscape for the atoms: the atoms immediately start to move. In turn, the swift electrons react to the change in the atomic structure. This dynamic interplay between electrons and atoms has been recorded with time- and angle-resolved photoemission spectroscopy: a second ultrashort laser pulse is used to emit few of the electrons at different times after the phase transition was initiated by the first laser pulses. By repeating this process billions of time, a movie of the electronic structure during the phase transition of the indium nanowires was obtained. This information, combined with simulations of the electronic structure dynamics, made it possible to translate the electronic structure dynamics into a movie of the atomic energy landscape. This detailed reconstruction of the reaction pathway reveals not only the motion of atoms but also the formation and breaking of chemical bonds during the phase transition.

The approach demonstrated by Nicholson et al. is generally applicable to physical processes like structural phase transitions in solids as well as to chemical reactions, for instance of molecules. The theoretical framework for describing the electronic structure, however, differ significantly between these cases: while electrons in a crystal are described as bands in momentum space, electrons in molecules are depicted as bonds in real space. The work by Nicholson et al. provides a bridge between the languages of physics and chemistry for describing photo-induced reactions. 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.

“Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition”
Science, Vol. 362, Issue 6416, pp. 821-825
DOI: 10.1126/science.aar4183
http://science.sciencemag.org/content/362/6416/821


(Deutsch) Reaktiver Zucker nach mehr als 100 Jahren Suche nachgewiesen

Wednesday, 17th October 2018Publication highlights

Sorry, this entry is only available in German.


Azubipreis der Max-Planck-Gesellschaft für Tuan Anh Mario Nguyen

Thursday, 20th September 2018Preise und Auszeichnungen

Sorry, this entry is only available in German.


Watching the first steps of magnetic information transport

Thursday, 13th September 2018Miscellaneous

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 nonmagnetic 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 neighbouring 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 Germany, England 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 information 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 simulations of the spin dynamics at the Tohoku University in Sendai, Japan.

Press contacts:
Prof. Tobias Kampfrath, Freie Universität Berlin and Fritz Haber Institute of the Max Planck Society, Berlin, +49 30 8413-5222, tobias.kampfrath@fu-berlin.de

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)   

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.

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.


What happens when we heat the atomic lattice of a magnet all of a sudden?

Monday, 16th July 2018Publication highlights

Magnets have fascinated humans for several thousand years and enabled the age of digital data storage. They occur in various flavors. Ferrimagnets form the largest class of magnets and consist of two types of atoms. Similar to a compass needle, each atom exhibits a little magnetic moment, also called spin, which arises from the rotation of the atom’s electrons about their own axes. In a ferrimagnet, the magnetic moments point in opposite directions for the two types of atoms (see panel A). Thus, the total magnetization is the sum of all magnetic moments of type 1 (M1, blue arrows) and type 2 (M2, green arrows). Due to the opposite direction, the magnitude of the total magnetization is M1M2.

When an insulating ferrimagnet is heated, the heat is first deposited in the atomic lattice which causes the atoms to move randomly around their cold positions. Finally, part of the heat also causes random rotation (precession) of the spins around their cold direction. Thus, magnetic order gets lost; the total magnetization (M1M2) decreases and eventually vanishes if the temperature of the ferrimagnet exceeds a critical temperature, the so-called Curie temperature. Although this process is of fundamental importance, its dynamics are not well understood. Even for the ferrimagnet yttrium iron garnet (YIG), one of the most intensely researched ferrimagnets, it is unknown how long it takes until the heated atomic lattice and the cold magnetic spins reach equilibrium with each other. Previous estimates of this time scale differ from each other by a factor of up to one million.

A team of scientists from Berlin (Collaborative Research Center/Transregio 227, Fritz Haber Institute and Max Born Institute), Dresden (Helmholtz Center), Uppsala (Sweden), St. Petersburg (Russia), and Sendai (Japan) have now revealed the elementary steps of this process. “To instantaneously and exclusively heat up the atomic lattice of a YIG film, we use a very specific and novel kind of stimulus: ultrashort bursts of laser light at terahertz frequencies. With a subsequently arriving visible laser pulse, we can then step-by-step trace the evolution of the initially cold magnetic spins. Essentially, we record a stop-motion movie of how the magnetization evolves.” says Sebastian Maehrlein, who conducted the experiments at the Fritz Haber Institute of the Max Planck Society. His colleague Ilie Radu from summarizes: “Our observations are striking. We found that sudden heating of the atomic lattice reduces the magnetic order of the ferrimagnet on two distinct time scales: an incredibly fast scale of only 1 ps and a 100,000 times slower scale of 100 ns.”

These two time scales can be understood in analogy to water in a closed pot that is put into a hot oven. The hot air of the oven corresponds to the hot atomic lattice whereas the magnetic spins correspond to the water inside the pot (see panel A). Once the atomic lattice is heated by the terahertz laser burst, the enhanced random oscillations of the atoms lead to a transfer of magnetic order from spin type 1 to spin type 2. Therefore, both the magnetic moments M1 (blue arrows in panel B) and M2 (green arrows) are reduced by exactly the same amount (red arrows). This process evolves on the fast time scale, and the atomic spins are forced to heat up while leaving the total magnetization M1M2 unchanged, just like water in a closed pot that has to keep its volume.

We know, however, that a heated ferrimagnet not only aims at reducing M1 and M2, but also its total magnetization M1M2. To do so, part of the spin must be released to the atomic lattice. This situation is again completely analogous to the hot water in a closed pot: the pressure inside the pot increases but is slowly released to the outside through little leaks in the lid (see panel C). This leakage of angular momentum to the atomic lattice is exactly what happens in the ferrimagnet through weak couplings between spins and lattice.

“We now have a clear picture of how the hot atomic lattice and the cold magnetic spins of a ferrimagnetic insulator equilibrate with each other.” says Ilie Radu. The international team of researchers discovered that energy transfer proceeds very quickly and leads to a novel state of matter in which the spins are hot but have not yet reduced their total magnetic moment. This “spin overpressure” is released through much slower processes that permit leakage of angular momentum to the lattice. “Our results are also relevant for applications in data storage.” Sebastian Maehrlein adds. “The reason is simple. Whenever we want to switch the value of a bit between 0 to 1 in a magnetic storage medium, angular momentum and energy have to finally be transferred between atomic lattice and spins.”

Press contacts:
Prof. Tobias Kampfrath, tobias.kampfrath@fu-berlin.de, +49 30 8413–5222; FHI PC Department Office: +49 30 8413–5112
Dr. Ilie Radu, radu@mbi-berlin.de, +49 30 6392 1357; Max Born Institute Berlin, Germany

Original Publication:
S. F. Maehrlein, I. Radu, P. Maldonado, A. Paarmann, M. Gensch, A. M. Kalashnikova, R. V. Pisarev, M. Wolf, P. M. Oppeneer, J. Barker, T. Kampfrath, Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation. Sci. Adv. 4, eaar5164 (2018).

 

Tobi

Heating a magnet without changing its magnetization. (A) A ferrimagnet consists of two spin sorts of opposite orientation (green and blue arrows). In the experiment, the atomic lattice of the ferrimagnet is heated by an extremely short terahertz laser pulse. This situation is analogous to heating the air (=atomic lattice) inside an oven that contains a pot with water (=spins). (B) Heat is transferred into the spin system and decreases the magnetization of each spin type by exactly the same amount. This process arises because spin is transferred from the blue to the green spin sort. Thus, the magnet is heated without changing its total magnetization! In the pot analogy, heat is transferred from the air outside the pot to the water inside. While the amount of water in the pot has not changed, an overpressure has built up. (C) Finally, the hot spins release their overpressure to the atomic lattice, thereby reducing the total magnetization. In the analogy, water overpressure is released through little leaks in the pot lid.


John B. Fenn Award for a Distinguished Contribution in Mass Spectrometry for Gert von Helden

Wednesday, 2nd May 2018Miscellaneous
Gert von Helden receives, together with Martin Jarrold and David Clemmer (both Indiana University, USA), the 2018 “John B. Fenn Award for a Distinguished Contribution in Mass Spectrometry” by the American Society for Mass Spectrometry (ASMS). The Award recognizes a focused or singular achievement in fundamental or applied mass spectrometry and is given for pioneering contributions to the development of ion mobility spectrometry (IMS).
See:  https://www.asms.org/about-asms-awards/distinguished-contribution

 


A comprehensive volume on chemical warfare entitled “One Hundred Years of Chemical Warfare: Research, Deployment, Consequences” has been published under the auspices of the Max Planck Society

Wednesday, 6th December 2017Publication highlights, Miscellaneous

ProductFlyer-9783319516639On April 22, 1915, the German military released 150 tons of chlorine gas at Ypres, Belgium. Carried by a long-awaited wind, the chlorine cloud passed within a few minutes through the British and French trenches, leaving behind at least 1,000 dead and 4,000 injured. This chemical attack, which amounted to the first use of a weapon of mass destruction, marks a turning point in world history. The preparation as well as the execution of the gas attack was orchestrated by Fritz Haber, the director of the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry in Berlin-Dahlem. During World War I, Haber transformed his research institute into a center for the development of chemical weapons (and of the means of protection against them).

Bretislav Friedrich and Martin Wolf (Fritz Haber Institute of the Max Planck Society, the successor institution of Haber’s institute) together with Dieter Hoffmann, Jürgen Renn, and Florian Schmaltz (Max Planck Institute for the History of Science) organized an international symposium to commemorate the centenary of the infamous chemical attack. The symposium examined crucial facets of chemical warfare from the first research on and deployment of chemical weapons in WWI to the development and use of chemical warfare during the century hence. The focus was on scientific, ethical, legal, and political issues of chemical weapons research and deployment — including the issue of dual use — as well as the ongoing effort to control the possession of chemical weapons and to ultimately achieve their elimination.

The volume consists of papers presented at the symposium and supplemented by additional articles that together cover key aspects of chemical warfare from 22 April 1915 until the summer of 2015.

The book was presented at a symposium on November 30, 2017 to the delegates of the 22nd Conference of State Parties of the Chemical Weapons Convention at the Organization for the Prohibition of Chemical Weapons in The Hague. Introduced by Paul Walker (Green Cross) and presented and moderated by Bretislav Friedrich (FHI), the symposium entitled “One Hundred Years since Ypres and Counting: Glimpses of the Past and the Present” explained the involvement of the Max Planck Society and provided a sampling of the book’s chapters by Edward Spiers (University of Leeds), Ulf Schmidt (University of Kent), Karin Mlodoch (Haukari), and Ralf Trapp (Chessenaz). Among the attendees were four survivors of the 1988 Halabja chemical attack.

Website: http://www.springer.com/de/book/9783319516639
eBook available at https://link.springer.com/book/10.1007/978-3-319-51664-6


This year’s ENI Award goes to Professor Robert Schlögl

Tuesday, 10th October 2017Preise und Auszeichnungen

Professor Schlögl received this year’s ENI Award, also known as “Nobel Prize for Energy”, in the Energy Transition category. We congratulate him on this prestigious distinction. Read more here:

The award and the awardees 2017: https://www.eni.com/en_IT/innovation/eni-award.page

Robert Schlögl: https://www.eni.com/en_IT/innovation/eni-award/2017-schlogl-energy-transition.page

ENI Press Release: https://www.eni.com/en_IT/media/2017/10/10th-eni-award-2017-prizes-awarded-for-scientific-research-in-the-field-of-the-energy-and-the-environment

ENI_Robert Schloegl

Source: ENI


(Deutsch) Azubipreis für Robert Hippmann Pena

Tuesday, 5th September 2017Miscellaneous

Sorry, this entry is only available in German.


Gert von Helden appointed as professor at Radboud University Nijmegen

Wednesday, 28th June 2017Preise und Auszeichnungen

The Radboud University has launched a press release concerning the appointment of Gert von Helden as professor of IR spectroscopy of biomacromolecules. Read the full press release.