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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.

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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.

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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.

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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.