Das Preiskomitee der SPG unter dem Vorsitz von Prof. Minh Quang Tran hat die Gewinner wieder aus zahlreichen Kandidaten ausgewählt. Die Gewinner präsentierten ihre Arbeiten aufgrund der besonderen Situation am 1. Juli 2020 in einer virtuellen Preisverleihung. Nachfolgend die Zusammenfassungen der jeweiligen Gewinner.*(Zur Zeit sind keine deutschen Übersetzungen verfügbar.)*

The SPS Award in General Physics is given to **Hiske Overweg** for her work on "*Electrostatically induced nanostructures in bilayer graphene*".

**Electrostatically induced nanostructures in bilayer graphene**

Soon after the discovery of graphene, the idea was put forward that this material could be an attractive host for spin qubits. Because of the low atomic weight of carbon, spin-orbit interactions in graphene are small. On top of that, the material mostly consists of nuclear spin-free carbon-12 isotope, which leads to a small hyperfine interaction. These facts should lead to a long coherence time of electron spins, which is an important criterion for a quantum computation platform.

A first step towards the creation of a spin qubit in graphene is the implementation of confinement of charge carriers on the nanoscale. Single layer graphene has a gapless band structure, which makes it hard to realize controlled confinement. The tunable band structure of bilayer graphene provides a solution. By applying a vertical electric field to this material, a band gap is opened. When the Fermi level is locally tuned into the band gap, confinement can be realized.

We investigated the confinement of charge carriers in bilayer graphene by electrostatic gating. We fabricated samples consisting of exfoliated bilayer graphene encapsulated in hexagonal boron nitride, a good insulator. This “sandwich” was placed on a doped silicon substrate, which functions as a back gate. After evaporating metallic gates on top, electric fields could be applied.

Despite the observation of various interesting physical phenomena in dual-gated devices, the maximal resistance in these devices usually stayed in the range of tens of kiloohms, which is insufficient for electrostatic definition of nanostructures. We demonstrated that the incorporation of a graphite back gate into the device structure reproducibly leads to induced resistances in the megaohm or even gigaohm regime, paving the path for electrostatically defined nanostructures.

With a graphite back gate in place, we measured quantized conductance in a device, thereby demonstrating the formation of a quantum point contact. We investigated the magnetic field dependence of the conductance through the constriction, which shows a fascinating pattern of level crossings [1].

The introduction of a graphite back gate in bilayer graphene devices has opened doors to explore various quantum dot arrangements in this material [2].

[1] Hiske Overweg, Angelika Knothe, Thomas Fabian, Lukas Linhart, Peter Rickhaus, Lucien Wernli, Kenji Watanabe, Takashi Taniguchi, David Sánchez, Joachim Burgdörfer, Florian Libisch, Vladimir I. Fal'ko, Klaus Ensslin and Thomas Ihn, *Phys. Rev. Lett.***121**, 257702 (2018)

[2] Annika Kurzmann, Hiske Overweg, Marius Eich, Alessia Pally, Peter Rickhaus, Riccardo Pisoni, Yongjin Lee, Kenji Watanabe, Takashi Taniguchi, Thomas Ihn and Klaus Ensslin, *Nano Lett.***19**, 8, 5216-5221 (2019)

**Shantanu Mishra **receives the SPS Award in Condensed Matter Physics for his work on "*Atomic-scale investigations of carbon magnetism*".

**Engineering intrinsic π-magnetism in carbon**

Magnetism is ordinarily thought of in terms of the d- and f-block elements of the periodic table, which form the basis for modern magnetic technologies. In this context, magnetism in light elements, in particular carbon, may hold several advantages over current inorganic materials. The low atomic mass of carbon, combined with the zero nuclear spin of the ^{12}C isotope implies weak spin-orbit and hyperfine couplings, which are the major sources of spin relaxation and decoherence. This makes carbon nanomaterials ideal for transport of spin polarized currents with high fidelity, or toward realization of fault-tolerant qubits for quantum computation. In addition, carbon nanomaterials offer the intriguing prospect of electric field control of spin transport, which remains difficult to achieve in inorganic materials.

The electronic structure of polycyclic aromatic hydrocarbons (or, nanographenes) critically depends on the topology of the underlying π-electron network, which provides a tunable platform to realize all-carbon magnetism at the nanoscale. Combining rational design principles with on-surface chemistry, we engineer elusive magnetic nanographenes, and probe their structural, electronic and magnetic properties at submolecular resolution with scanning tunneling microscopy. The simplest route toward inducing magnetism in nanographenes involves inducing a sublattice imbalance in the bipartite honeycomb lattice, which translates to a net spin imbalance. As an experimental verification of this concept, we synthesize π-extended triangulene, a nanographene consisting of ten benzenoid rings fused in a triangular fashion [1]. An inherent sublattice imbalance in π-extended triangulene leads to the appearance of non-bonding states in the electronic energy spectrum, which become spin polarized due to electron-electron interactions, leading to a spin-quartet ground state. Moreover, we show for the first time that nanographenes without a sublattice imbalance may also host non-bonding states. Herein, we demonstrate the experimental realization of an elusive spin-singlet nanographene known as Clar's goblet, after its first prediction almost fifty years ago [2]. The magnetic exchange coupling in Clar's goblet is found to exceed the thermodynamic threshold, which, in principle, enables room temperature spintronic applications.

[1] S. Mishra et al., *Synthesis and characterization of π-extended triangulenes*, J. Am. Chem. Soc. **141**, 10621 (2019).

[2] S. Mishra et al., *Topological frustration induces unconventional magnetism in a nanographene*, Nat. Nanotechnol. **15**, 22 (2020).

The SPS Award in Applied Physics is given to **Michael A. Becker** for his work on *"Exciton dynamics and light-matter interactions of colloidal semiconductor nanocrystals"*.

**Exciton dynamics and light-matter interactions of colloidal perovskite quantum dots**

Colloidal quantum dots are tiny semiconductor crystals, whose size is in the order of only few nanometers. At these length scales quantum effects within the nanocrystal become apparent that alter its optical and electronic properties. Colloidal quantum dots possess a broad range of applications ranging from bio-imaging applications to photovoltaic devices and displays. Recently, the synthesis of a novel type of colloidal semiconductor quantum dot with a perovskite crystal structure was developed. These cesium lead halide perovskite quantum dots exhibit outstanding optical properties compared to conventional quantum dots such as a high photoluminescence quantum yield, a fast photoluminescence decay and a broad tunability of the emission energy.

We investigated their optical properties at cryogenic temperature and demonstrated that these nanocrystals possess a bright triplet exciton state with a typical fine-structure splitting in the millielectronvolt range. The bright triplet exciton state is responsible for their ultrafast radiative decay, which is roughly 1000 times faster at cryogenic temperature compared to other conventional nanocrystals [1]. Using transient three-beam four-wave mixing we measured the characteristic dephasing time T_{2}, which is in the range of several tens of picoseconds. The low dephasing rate together with the ultrafast radiative decay enable coherent coupling phenomena among nanocrystals. By investigating perovskite quantum-dot superlattices, that are long-range-ordered three-dimensional lattices consisting of well-separated individual quantum dots, we demonstrated a collective emission effect known as superfluorescence. Using excitation-power dependent streak camera measurements, we observed all key signatures of superfluorescence, such as a shortening of the radiative decay time, a superlinear increase of the initial intensity in time-correlated decay measurements, a shortening of the so-called delay time and Rabi-type oscillations of the intensity [2].

[1] Becker, M. A.; Vaxenburg, R.; Nedelcu, G.; Sercel, P. C.; Shabaev, A.; Mehl, M. J.; Michopoulos, J. G., Lambrakos, S. G.; Bernstein, N.; Lyons, J. L.; Stöferle, T.; Mahrt, R. F.; Kovalenko, M. V.; Norris, D. J.; Rainò, G.; Efros, A. L., *Nature*, **553**, 187-193 (2018)

[2] Rainò, G.; Becker, M. A.; Bodnarchuk, M. I.; Mahrt, R. F.; Kovalenko, M. V.; Stöferle, T., *Nature*, **563**, 671-675, (2018)

**Katharina Schmeing** is honored with the SPS Award related to Metrology for her work on *"Integrated Gallium Phosphide Photonics"*.

**Integrated nanophotonics with gallium phosphide**

Gallium phosphide (GaP) is an intriguing but mostly unexplored material for integrated photonics. It possesses an attractive combination of a large refractive index (n_{0} > 3) and a large electronic bandgap (2.26 eV). These values offer the possibility of creating devices with strong light confinement and enhanced light-matter interaction, while simultaneously providing transparency into the visible as well as weak two-photon absorption at typical data communication wavelengths in the infrared. GaP also has large second- and third-order non-linear optical coefficients and is piezoelectrically active. This uncommon confluence of properties has led to numerous proposals utilizing GaP as a platform for solid-state cavity-quantum electrodynamics, optomechanics, solar cell technology, and even cold-atom physics, in addition to non-linear photonics.

The main challenge has been the development of methods for fabricating GaP structures on a low-refractive-index substrate. To address this problem, we employed a direct wafer-bonding approach for scalable integration of high quality, epitaxially-grown GaP onto silicon dioxide. Exploiting new techniques for patterning GaP into high-aspect-ratio structures with nanometer precision while maintaining good material quality, we realized a catalog of low-loss photonic devices, including free-standing photonic crystal cavities exhibiting optomechanical coupling in the resolved-sideband regime [1] and grating-coupled waveguide resonators with loaded quality factors of 3 × 10^{5} [2]. The photonic crystal cavities had optical quality factors as high as 1.1 × 10^{5} and were optimized to couple an optical mode at ~ 200 THz via radiation pressure to a co-localized mechanical mode with a frequency of 2.9 GHz. Notably, the large vacuum optomechanical coupling rate (400 kHz) permitted amplification of the mechanical mode into the so-called mechanical lasing regime with input power as low as ~ 20 µW. For non-linear optics, we used waveguide resonators pumped at 1550 nm to generate second- and third-harmonic light as well as Kerr frequency combs. Parametric threshold powers as low as 3 mW were realized, followed by broadband (> 100 nm) frequency combs with sub-THz spacing, frequency-doubled combs and, in a separate device, efficient Raman lasing.

Taken together, our results herald the emergence of GaP as a new platform for integrated nonlinear photonics.

[1] K. Schneider, Y. Baumgartner, S. Hönl, P. Welter, H. Hahn, D. J. Wilson, L. Czornomaz, and P. Seidler, “Optomechanics with one-dimensional gallium phosphide photonic crystal cavities,” *Optica***6**, 577–584 (2019).

[2] D. J. Wilson*, K. Schneider*, S. Hönl*, M. Anderson, T. J. Kippenberg, and P. Seidler, “Integrated gallium phosphide nonlinear photonics,” *Nature Photonics***14**, 57–62 (2020)

The SPS Award in Computational Physics is given to **Frank Schindler** for his work on "*Higher-order topological insulators*".

**Higher-order topological insulators**

The mathematical field of topology has become a framework in which to describe the low-energy electronic structure of crystalline solids. Typical of a bulk insulating three-dimensional topological crystal are conducting two-dimensional surface states. This constitutes the topological bulk–boundary correspondence. We extend the notion of three-dimensional topological insulators to systems that host no gapless surface states but exhibit topologically protected gapless hinge states [1]. We numerically predict tin telluride as a material candidate realizing this kind of higher-order topology. The hinge states discussed by us may be used for lossless electronic transport, spintronics, or — when proximitized with superconductivity — for topological quantum computation.

Furthermore, we establish that the electronic structure of bismuth, an element consistently described as bulk topologically trivial, is in fact topological and follows a generalized bulk–boundary correspondence of higher-order [2]. Its hinge states are protected against localization by time-reversal symmetry locally, and globally by the three-fold rotational symmetry and inversion symmetry of the bismuth crystal. In addition to extensive first-principles and tight-binding calculations, we provide supporting evidence from two complementary experimental techniques.

[1] Schindler, Frank, et al. "Higher-order topological insulators." *Science advances ***4**.6 (2018): eaat0346.

[2] Schindler, Frank, et al. "Higher-order topology in bismuth." *Nature physics ***14**.9 (2018): 918-924.