DEPARTMENT OF
PHYSICAL CHEMISTRY
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Phase-Change materials such as the Ge3Sb2Te6 compound (GST) can be reversibly switched between the amorphous and crystalline crystal phases by ultrashort laser pulses. At infrared wavelengths, the two phases of GST exhibit a large permittivity contrast, which can be used to control the dielectric background sensed by guided optical modes that propagate in close proximity.

In their work, the authors demonstrate experimentally and theoretically that a thin GST film on a polaritonic substrate (SiC) serves as an actively tunable material system for the infrared spectral range. The system not only supports index-shifted p-polarized surface phonon polaritons (SPhPs), but additionally also s-polarized waveguide modes. Intriguingly, the waveguide modes replicate the properties of the SPhP that define its suitability for nanophotonic applications, such as the enhancement of local electric fields and subwavelength confinement.

The authors show experimentally that both modes can be actively tuned by switching the GST phase, with an exceptionally high tuning figure of merit of up to 7.7. With this unique tunability of its omnipolarized guided modes, the material system provides a promising platform for nanophotonic applications such as in-plane metasurfaces or polariton lenses.

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Atomic scale heterostructures offer unique possibilities for new material properties that emerge from the chemical bonding at the material interfaces rather than from the bulk. Often this idea is brought forward using two-dimensional van-der-Waals bonded crystals. In contrast here, researches show that the same concept can also be applied to traditional semiconductors with atomic-level control over the individual few-monolayer-thick sheets using epitaxial methods. The enhanced interlayer bonding strength leads to tremendous modifications of the material properties. The heterostructure behaves as a new crystal hybrid material dominated by the hetero-interfaces, with properties which cannot be derived from the individual constituents.
Using the Nitride semiconductors GaN and AlN, the authors demonstrate this concept experimentally for the first time, focusing on the optical phonons. In the crystal hybrid, new optical phonon modes emerge and others get modified significantly. These phonon modes, in turn, determine the infrared dielectric function and thereby the characteristics of phonon polaritons supported by the new material. Apart from shifted and newly emerging surface polariton bands, the strong anisotropy of the layered structure additionally introduces extreme birefringence, with several pronounced hyperbolic bands emerging. Thus, the new material provides rich physics which is absent its constituents, and which can be controlled via the layer thicknesses. This first demonstration of the crystal hybrid concept paves the way for a new era of material design for infrared-to-terahertz photonics.

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Nanotechnologies utilize physical and chemical properties which arise by shrinking a material in size to the nanometer length scale. The structural stability of nanoscale building blocks is a central aspect in nanoscience as it governs the long-term stability of a device. Function, however, is only achieved once a device is energized, which is why nanotechnologies require materials which are structurally stable under operation conditions.

On the atomic level, the structure is prone to ultrafast motions that range from atomic vibrations and diffusion to translations and rotations of entire nanostructures. These motions, for instance, governs the sintering of nanoparticles to larger complexes which occurs at elevated temperatures. To study the motion of entire nanoparticles, we establish femtosecond electron diffraction as goniometer of ultrafast nanocrystal rotations. We have investigated nanoclusters containing 923 gold atoms, which have been deposited on a graphene sheet, with ultrashort pulses of electrons. We observe motions of the atoms in the cluster as well as constrained rotations, so-called librations, of the entire nanoclusters. This motion is induced by suddenly energizing the material, which is realized by illumination with an ultrashort laser pulse.

These experiments are compared to molecular dynamics and electron diffraction simulations. Through the combination of the different approaches, the mechanism for the conversion of laser energy to rotational motions could be revealed. This study adds a new aspect to the understanding of nanoscopic heterostructures in out-of-equilibrium conditions and to the mechanisms of heat- and mass-transport in nanoscale materials.

This research results from an international collaboration between the research group Structural & Electronic Surface Dynamics at the Fritz Haber Institute in Berlin headed by Ralph Ernstorfer, the group of Richard Palmer at Swansea University in the UK, and the group of Vlasios Mavrantzas located at the University of Patras (Greece) and the ETH Zurich. This project was supported from the European Union through the ERC grant FLATLAND.

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The researchers of the Department report that plasmonic properties of a scanning tunneling microscope (STM) junction can be manipulated by nanofabrication of Au tips using focused ion beam (FIB) milling. An exemplary Fabry–Pérot like resonator of surface plasmons is demonstrated by producing a single groove on a smoothed tip shaft. STM-induced luminescence spectra of these tips exhibit spectral modulation originating from the interference between localized and propagating surface plasmon modes. In addition, the researchers show that the quality factor of the plasmonic Fabry–Pérot interference can be improved by optimizing the overall tip shape. This approach paves the way for near-field imaging and spectroscopy with a high degree of accuracy.

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

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 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&nbsp 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 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 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 simu- lations of the spin dynamics at the Tohoku University 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.

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)

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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 two 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.

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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.

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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.

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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”.