Seminars

Abstract:
Light induced phase transitions in quantum materials

M. Calandra, G. Marini, M. Furci and S. Mocatti
In this talk I will demonstrate the capability of constrained density functional perturbation theory [1] coupled with a non-perturbative treatment of quantum anharmonicity to describe light-induced phase transition in GeTe and SnSe [2] monochalcogenides and in 2D materials [3,4].

In SnSe I will show that ultrafast lasers can permanently transform the topologically-trivial orthorhombic structure of SnSe into the topological crystalline insulating rocksalt phase via a firstorder non-thermal phase transition. I will describe the reaction path and evaluate the critical fluence and the possible decay channels after photoexcitation.

Our simulations of the photoexcited structural and vibrational properties are in excellent agreement with recent pump-probe data in the intermediate fluence regime below the transition with an error on the curvature of the quantum free energy of the photoexcited state that is smaller than 2%.
In GeTe I will investigate the non-thermal phase transition from a rhombohedral to a rocksalt
crystalline phase. The microscopic mechanism and nature of the transition are unclear. I will show
that the non-thermal phase transition is strongly first order and does not involve phonon softening, in contrast to the thermal one. The transition is driven by the closure of the single-particle gap in the photoexcited rhombohedral phase. Finally, I will show that ultrafast XRD data are consistent with the coexistence of the two phases, as expected in a first order transition.

Finally I will show how similar structural phase transitions occurs in single layer MoTe2 and « Te2.
Furthermore, I will show how it is possible to induce a magnetic state in layered V2O5 by using ultrafast pulses.
The high accuracy of our results demonstrates that an approach based on a complete self-consistent constrained density functional perturbation theory in the presence of an electron-hole plasma captures the light-induced structural deformations and transitions extremely accurately.

Acknowldegements: Funded by the European Union (ERC, DELIGHT, 101052708)
References:
[1] Lattice dynamics of photoexcited insulators from constrained density-functional perturbation theory, G. Marini and M. Calandra, Phys. Rev. B 104, 144103 (2021)
[2] Light-Induced Nonthermal Phase Transition to the Topological Crystalline Insulator State in SnSe, S. Mocatti, G. Marini and M. Calandra, J. Phys. Chem. Lett., 14, 41, 9329 (2023)
[3] Light-Tunable Charge Density Wave Orders in MoTe2 and WTe2 Single Layers, Giovanni Marini and Matteo Calandra Phys. Rev. Lett. 127, 257401 (2021)
[4] Theory of ultrafast magnetization of nonmagnetic semiconductors with localized conduction bands, Giovanni Marini and Matteo Calandra Phys. Rev. B 105, L220406 (2022)

Abstract:
Molecular spin switches on surfaces

Magnetic molecules on surfaces have attracted considerable interest, in particular, in view of potential (quantum) technological applications. The ligands around the metal center essentially determine the electronic configuration of the ion, which is closely interconnected with the electronic, optical, and magnetic properties of the molecule. Molecules with multiple stable ligands configurations are particularly interesting as they exhibit a palette of physical properties.

We will report on different strategies to controllably modify the magnetic properties of molecules. Spin-crossover complexes exhibit two stable configurations with different ligand-field strengths. We will present the local and remote reversible spin-state switching of such complexes on a metal surface via electron injection [1]. For a second class of system, a ligand is displaced relative to the metal center to change
the symmetry of the ligand field, and thereby the spin [2]. In a last example, the orbital moment of a dinuclear complexes is modified by addressing the peripheral ligands [3].

References
[1] Johannsen et al., ACS Nano 15, 11770 (2021)
[2] Köbke et al., Nat. Nanotechnol. 15, 18 (2020)
[3] Li et al., ACS Nano 17, 10608 (2023)

Abstract:
Electrically driven cascaded photon-emission in a single molecule

In STM, electroluminescence from a single molecule adsorbed on a thin insulating film was first demonstrated almost 20 years ago [1]. Although the basic mechanisms leading to excited state formation in STM-induced luminescence (STML) are by now generally understood, the intricate interplay between charging dynamics and excited state (i.e. exciton) formation and decay has remained elusive due to the limited time resolution in such experiments. This limitation can be circumvented by studying correlations between consecutively emitted photons in the STML of single molecules, for example using Hanbury Brown-Twiss interferometry (HBT). This not only provides access to the time constants of the processes involved in excited state formation and decay, but also sheds light on the quantum mechanical nature of a single molecule as an emitter. Using this method in combination with STML, it has for example been demonstrated that electrically driven single-photon emission from a single molecule located in a well-controlled picocavity can be achieved in an STM [2].
Until now, such studies have been mainly focusing on one transition only. In STML on molecules adsorbed on thin insulating films, however, there are many states that are accessible, depending on the STM setpoint (i.e. bias voltage and tunnel current) due to the fact that the molecule is transiently charged at resonant tunnel conditions. Thus, by applying sufficiently high bias voltages, it is possible to bring for example ZnPc molecules adsorbed on 4 ML NaCl on Ag(111) into charged and even charged excited states (i.e. trion) [3].
Taking advantage of this, we investigate the interplay between the exciton and trion formation and decay in individual ZnPc molecules using HBT (Fig. 1). We show that, by bringing the molecule into the D1+ state (i.e., the charged excited state), we can initiate an emission cascade in which the radiative decay of the trion is followed by the formation and decay of the exciton, generating correlated emission of two photons from two different excited states within one molecule. In addition, by tuning the rate at which the molecule is driven to its charged state we can control the population dynamics of the states involved. This allows us to disentangle which states play a role in the formation of excited states in a molecule in STML, and how this is affected by the population dynamics of these states.

References
[1] X. H. Qiu, G. V. Nazin, and W. Ho, Science 2003, 299, 542.
[2] L. Zhang et al., Nat. Commun. 2017, 8, 580.
[3] J. Doležal, S. Canola, P. Merino, and M. Švec, ACS Nano 2021, 15, 7694.

Abstract:
Electron quantum optics with graphene nanoribbons

I will present computational studies that propose the utilization of a junction formed by two crossed graphene nanoribbons (GNRs) as an electron beam splitter. This concept explores the coherent splitting of incoming electron waves in one GNR into two outgoing terminals, exhibiting nearly equal amplitudes and zero back-scattering. Moreover, the inclusion of GNRs hosting spin-polarized edge states can be used to introduce a spin-dependent scattering potential, resulting in spin-polarization in the outputs. By leveraging multiple GNR junctions to construct networks, the realization of Mach-Zehnder-like circuitry becomes possible, enabling the exploration of electron interference phenomena. The talk will introduce the theoretical framework and discuss the potential implications of this approach to electron manipulation.

Abstract:
High Order Commensurate zwitterionic supramolecular network on HOPG

Imposing a moiré effect by superposition of two-dimentional (2D) materials has recently emerged as an efficient approach to modify the band structure in van der Waals heterostructures. This moiré effect, or superlattice, induces a periodic modulation of the electrostatic surface potential, which could lead to the emergence of materials with strongly correlated electronic states. [1] Until now, these experiments have only been done on inorganic heterostructures in which the spatial modulation and the amplitude of the electrostatic potential are fixed by the lattice parameters of the inorganic 2D layers used. The concept of these heterostructures is versatile enough to consider the replacement of an inorganic layer by a supramolecular layer of dipolar organic molecules. The modulation of the amplitude of the surface potential would be ensured by the dipolar nature of the molecules and the periodicity by the lattice parameters of the supramolecular network. Our methodology consists in using the degree of chemical synthesis to introduce precise control on the structural parameters of the superlattice. The first part of this work is to find a candidate molecule that can perform these tasks. We have chosen molecules from the Zwitterionic Quinone (QZ) family [2]. These molecules have a strong internal dipole (of about 10 Debye) and are derived from synthesis methods that make it possible to modify the peripheral functional groups to promote their self-assembly on the surface. To select the most promising molecules, we conducted our work on HOPG (Highly Oriented Pyrolytic Graphite) before moving to a graphene surface.
These studies are carried out by scanning tunneling microscopy at the liquid nitrogen temperature in ultrahigh vacuum. Our first results show that the Di-Phenyl Zwitterionic Quinones (DPQZ) form supramolecular domains probably guided by hydrogen bonds. As a result to an image processing method by cross correlation (Figure 1c); it is possible to show that these supramolecular domains form a high-order commensurate phase on HOPG.

References
[1] Cao, Y.,. et al. Nature 556, 2018.
[2] Siri, O., & Braunstein, P. Chemical communications 3, 2002.

Abstract:
Atomic-Scale Optical Spectroscopy at Surfaces

Optical spectroscopy is a powerful tool for chemical analysis, providing a wealth of information on structure, dynamics, and electronic properties. However, the diffraction limit of light does not allow resolve nanoscale structures. This physical limitation can be overcome by the use of near-field optics. In particular, localized surface plasmon resonance of metal nanostructures yields strong confinement and enhancement of electromagnetic fields, enabling ultrasensitive optical spectroscopy. Surface- and tip-enhanced spectroscopy, benefiting from extreme confinement and enhancement of gap-mode plasmon, has demonstrated nanoscale and even single-molecule spectroscopy [1-3], which will be a promising approach to study surface chemistry such as electrochemistry [4] and heterogeneous catalysis [5].
More recently, tip-enhanced spectroscopy in plasmonic junctions showed optical spectroscopy with sub-molecular resolution [6-9]. This emerging technique will allow for investigation of light-matter interaction at atomic scales [10]. I will discuss our recent development toward atomic-scale optical spectroscopy by a combination of quantum plasmonics with low-temperature scanning tunneling microscopy [11-22].

References
[1] Chem. Rev. 117, 7583 (2017). [2] Chem. Rev. 117, 6447 (2017). [3] Chem. Soc. Rev. 46, 4020 (2017). [4] Acc. Chem. Res. 49, 2023 (2016). [5] J. Phys. Chem. Lett. 7, 1570 (2016). [6] Nature 568, 78 (2019). [7] Natl. Sci. Rev. 6, 1169 (2019). [8] Nat. Photo. 14, 693 (2020). [9] Science 373, 95 (2021). [10] Nat. Rev. Phys. 3, 441 (2021). [11] Phys. Rev. Lett. 128, 206803 (2022). [12] Nano Lett. 22, 2170 (2022). [13] ACS Photonics 8, 2610 (2021). [14] Nano Lett. 21, 4057 (2021). [15] Nano Lett. 20, 5879 (2020). [16] Nano Lett. 19, 5725 (2019). [17] Nano Lett. 19, 3597 (2019). [18] Nano Lett. 21, 4057 (2021). [19] Nano Lett. 22, 2170 (2022). [20] Phys. Rev. Lett. 128, 206803 (2022). [21] ACS Nano 16, 16443 (2022). [22] Sci. Adv. 8, eabq5682 (2022).

Abstract:
Lattices of Shiba states in supramolecular architectures

Interaction between local moments and itinerant electrons may give rise to the magnetic polarization of the Fermi sea and/or to more exotic phenomena near Fermi level such as a mass renormalisation of the conduction electrons or a non-magnetic ground state due to the Kondo effect. Once superconductivity is introduced into the system, the magnetic moment of the impurity, if rigid enough, can induce Yu-Shiba-Rusinov (YSR) states which may form new topological states when coupled to each others. These phenomena have been widely studied in recent years, as they are central to design emerging spintronics devices and quantum-based technology. In this context, magnetic molecules forming hybrids with superconductors are promising building blocks of these kinds of devices. However the fundamental properties of supramolecular assemblies of magnetic molecules on normal or superconducting metal surfaces are still to be studied.

We have used scanning tunneling microscopy/spectroscopy (STM/STS) techniques to study organization, electronic, magnetic and superconducting properties of magnetic self-assembled supramolecular systems on normal and superconducting metal surfaces. We have found that the magnetic anistorpopy, which, always presents in magnetic impurities adsorbed on metal surfaces, can be well controlled through charge fluctuation. By tuning magnetic anisotropy, we are able to drive magnetic molecule across different regimes determined by the competition between magnetic anisotropy and coupling strength between molecule with substrate. For a S = 1 magnetic molecule, when interacting with superconducting metal, it can show in-gap YSR excitation, out-gap inelastic spin-flip (SF) excitation or coexsistence of them in different regimes, while interacting with normal metal, it can show Kondo feature or SF feature. The observation of crossover between these regimes allows us to have deeper understanding of interaction between local moments with intinerant electrons.

The thesis work has been performed in QuantumSpecs group of Laboratoire de Physique et d’Étude des matériaux (LPEM) under
the supervision of Dimitri Roditchev and Stéphane Pons.

Abstract:
Searching for unconventional electronic states by high-resolution spectroscopic-imaging scanning tunneling microscopy

Abstract:
An important feature of scanning tunneling microscopy (STM) is its ability to operate under combined extreme conditions, allowing us to search for unknown electronic states. It is often necessary to acquire a tunneling spectrum at each pixel of the atomic-resolution STM image to construct spectroscopic images at many different energies. Such a measurement, called spectroscopic-imaging STM (SI-STM), demands high instrument stability because it typically takes more than a few days to complete. To investigate the electronic states of various emergent materials, we have developed ultra-low-temperature high-magnetic-field SI-STM systems. In this seminar, I will describe the technical aspects of our STM [1] and present recent results on the putative Majorana quasiparticle in the iron-based superconductor Fe(Se,Te) [2], as well as the spectroscopic features of 1T-TaS2 in relation to the strong electron-correlation physics [3-5].

[1] T. Machida, Y. Kohsaka, and T. Hanaguri, Rev. Sci. Instrum. 89, 093707 (2018).
[2] T. Machida et al., Nature Mat. 18, 811 (2019).
[3] C. J. Butler et al., Nature Commun. 11, 2477 (2020).
[4] C. J. Butler et al., Phys. Rev. X 11, 011059 (2021).
[5] C. J. Butler et al., arXiv:2301.11537.

Abstract:
Mesoscale-ordered 2D polymers by on-surface photopolymerization

Lukas Grossmann [a], Benjamin T. King [b], Jonas Björk [c], Markus Lackinger [a] [a] Deutsches Museum, Museumsinsel 1, 80538 München and Physics Department, Technische Universität München, James-Franck-Strasse 1, 85748 Garching (Germany)
[b] Department of Chemistry, University of Nevada, North Virginia Street, 1664, Reno (USA)
[c] Department of Physics Chemistry and Biology Linköping University, 83, 581, Linköping (Sweden)

We attained the synthesis of mesoscale-ordered 2D polymers by the topochemical on-surface photopolymerization of fluorinated anthracene triptycene (fantrip) monomers. The underlying protocol is two-staged: (1) Self-assembly of the monomers into a photopolymerizable monolayer structure, where the photoactive anthracene moieties are face-to-face stacked; (2) cross-linking of the self-assembled monolayer into a covalent 2D polymer by photochemically excited [4+4] cycloadditions between the antiparallel aligned anthracene blades. Thereby, the long-range order attained in step (1) is transferred into the covalent state. Yet, the topochemical approach crucially depends on achieving the reactive packing with the appropriate mutual alignment of the anthracene blades. For the self-assembly the underlying surface plays a decisive role. We used graphite substrates, but additional passivation with an alkane monolayer was necessary to weaken molecule-surface interactions. As a result, the desired monolayer that is determined by molecule-molecule interactions became thermodynamically favored. STM has proven as ideal analytical tool for monitoring intermediate and final structures of the photochemical linking with the ultimate single-linkage resolution. The [4+4] cycloadditions induce a sizable increase of the HOMO-LUMO gap, which translates into a characteristic change of STM contrast. This possibility to identify individual newly formed covalent linkages facilitated studies of the polymerization progression and allowed to assess the temperature dependence of polymerization rates, where an increase with temperature indicated a small energy barrier in the photoexcited state. Ongoing experiments indicate a decisive influence of the underlying substrate, not only on the initial self-assembly, but also on the subsequent photochemistry.

[1] Nat. Chem. 13, 730-736 (2021)
[2] Eur. J. Org. Chem. 2021, 5478–5490 (2021)
[3] Trends Chem. 4, 471-474 (2022)
[4] Angew. Chem. Int. Ed. 61, e202201044 (2022)

Abstract:
C60 on Cu(111): a model platform to study complex hetero-epitaxy & disordered phases of matter Fullerenes (C60 molecules) have been extensively studied as candidates to develop molecular electronics [1]. How these molecules bind to metal surfaces is a key aspect of the problem, including the geometrical structure [2] and electronic properties of such binding [3].
In this talk I will present a part of my PhD work concerning the observation of C 60/Cu(111) samples by scanning probe microscopy (STM, nc-AFM). In this work, we demonstrate a puzzling coexistence of ten different molecular phases in samples prepared at room temperature (Fig. 1), while only five of them have been described in the literature [4].

In the first part of the talk I will concentrate on the binding of fullerenes to the copper surface. Combining STM observations and extensive DFT calculations, we elucidate the details of the molecule-substrate binding. We consider both individual degrees of freedom (molecular orientation and height, which is due to the formation of vacancies on the copper surface [5]) and collective order in the molecular lattices (epitaxy with the substrate). Considering all these degrees of freedom and competing interactions (fullerene-fullerene vs. copper-
fullerene interactions), we build a simplified, parameterized model which allows to estimate the internal energy of each phase and to characterize the complex energy landscape accounting for phase coexistence.

In the second part of the talk, I will briefly address the complex non-periodic height patterns observed for some of the fullerene’s phases In particular, I will show that these patterns can be interpreted in the framework of an effective model which is usually studied in statistical physics and magnetism to describe frustrated phases of matter [6]. These frustrated phases correspond to non-ordered but correlated arrangements of atoms or spins which cannot simultaneously satisfy all the interactions with neighbor particles.

[1] Guldi et al., Chem. Soc. Rev., 38, 1587 (2009)
[2] Altman et al., Surf. Sci., 295, 13 (1993); Sakurai et al., Prog. Surf. Sci., 51, 263 (1996)
[3] Hunt et al., Phys. Rev. B 51, 10039 (1995)
[4] Pai et al., Phys. Rev. B, 69, 125405 (2004)
[5] Pai et al., Phys. Rev. Lett, 104, 036103-1 (2010)
[6] Wannier, Phys. Rev., 79, 357 (1950)

Abstract:
Accessing non-equilibrium states at atomic scales

J. Repp
Department of Physics, University of Regensburg, 93040 Regensburg, Germany

Scanning probe microscopy (SPM) has revolutionized our understanding of the atomistic world. Conventional SPM, however, is an inherently slow technique – too slow to capture transition states in excitation processes in most cases. While ultra-fast non-equilibrium phenomena is enabled by terahertz (THz) scanning tunneling microscopy (STM) [1], another approach gives us access to intermediate timescales that are relevant for spin precession and relaxations. We introduce a novel variant of SPM by combining principles of STM and atomic force microscopy (AFM). Instead of the usual direct current in conventional STM, we drive a tiny alternating current between the microscope’s tip and a single molecule under study. We exploit the single-electron sensitivity of AFM [2] in detecting the current which consists of only a single electron per AFM-cantilever oscillation cycle, tunneling back and forth between tip and molecule. This enables operation in absence of any conductance of the underlying substrate, while retaining the capability of imaging electronic states with sub-angstrom resolution. Thereby, we can access out-of-equilibrium charge states that are out of reach for conventional STM [3]. Extending this technique by electronic pump-probe spectroscopy [4], we measured the triplet lifetime of an individual pentacene molecule on an insulating surface [5] and lifetime quenching by nearby oxygen molecules. Combined with radio-frequency magnetic-field driving we introduce AFM-based electron spin resonance and spin manipulation showing long spin coherence in single molecules [6].

References
[1] T. L. Cocker et al., Nature 539, 263 (2016).
[2] J. Klein, C. C. Williams, Appl. Phys. Lett. 79, 1828 (2001).
[3] L. L. Patera et al., Nature 566, 245 (2019).
[4] S. Loth et al., Science 329, 1628 (2010).
[5] J. Peng et al., Science 373, 452 (2021).
[6] L. Sellies et al., arXiv:2212.12244 (2022).

Abstract:
High-resolution SPM imaging of molecules with a functionalized probe

P. Jelínek
Institute of Physics of the Czech Academy of Sciences, Czech Republic

High-resolution AFM/STM/IETS imaging of molecules acquired functionalized tips [1] created a lot of excitement among researchers from many fields including material science, physics and chemistry. Here we will briefly describe a common underlying mechanism responsible for the unprecedented AFM/STM/IETS submolecular contrast [2]. The first results were obtained using CO-tips, which became very widespread due to easy handling and the high spatial distribution. However, these tips show a weak electrostatic signal and do not allow the acquisition of magnetic contrast. Recently, other alternatives have also been used, such as a metallocene tips, which allows for magnetic contrast [3,4]. Another interesting alternative is tip functionalization with a single Xenon atom, which, thanks to its strong polarization, allows us to image the anisotropic distribution of the atomic charge [4]. We will provide some more theoretical insight into the imaging mechanism of these probes too.

[1] R. Temirov et al, New J. Phys. 10, 053012 (2008); L. Gross et al, Science 325, 1110 (2009); Ch. Chiang et al, Science 344, 885 (2014); P. Jelinek J. Phys. Cond. Matt 29, 166001 (2017).
[2] P. Hapala et al, Phys. Rev. B 90, 085421 (2014); P. Hapala et al, Phys. Rev. Lett. 113, 226101 (2016); B. de la Torre et al Phys. Rev. Lett. 119, 166001 (2017).
[3] Ormaza et al, Nano Lett. 2017, 17, 3, 1877–1882, (2016); Verlhac et al., Science 366, 623–627 (2019)
[4] Ch. Wackerlin et al, arxiv :2201.03627v1 (2022).
[5] B. Mallada et al, Science 374, 863 (2021).

Abstract:
Metal Halide Perovskites: From Surface Properties to Material Stability and Device Stability

Jeremy Hieulle
Department of Physics and Materials Science, University of Luxembourg, Luxembourg City L-1511, Luxembourg

Metal halide perovskite solar cells are currently under the spotlight. But the commercialization is hampered by certain drawbacks such as device degradation and hysteresis effects [1]. Atomic-scale effects such as doping, vacancies, ion migration, and the organic cations’ orientation are currently under intensive investigation [2-3]. However, a fundamental understanding of the surface atomic structure and its impact on device stability and efficiency is still lacking.
In this work, we combined scanning tunneling microscopy (STM, AFM, KPFM), X-ray, and ultraviolet photoelectron spectroscopies (XPS, UPS) to investigate the surface properties and intrinsic stability of metal-halide perovskite interface down to the nanometer-scale. We demonstrate that metal-halide perovskites are degraded by releasing some of the organic cations, while metallic lead is produced at the perovskite surface. Additionally, the Pb(0) formation was found to be triggered by white light. Importantly we demonstrated that by tuning Pb-halide bond strength, the perovskite could be stabilized [4-6]. Finally, we show that cooling down the sample, helps to reduce the release of the organic cation during sample analysis. Those findings were further tested and corroborated on full device solar cells.
Metal Halide Perovskites: From Surface Properties to Material Stability and Device Stability

References:
[1] P Wang et al., Adv. Funct. Mater. 29 (47), 1807661 (2019).
[2] C. Stecker et al., ACS Nano, 13 (10), 12127–12136 (2019).
[3] R. Ohmann et al., J. Am. Chem. Soc., 137, 51, 16049–16054 (2015).
[4] J. Hieulle, et al., J. Am. Chem. Soc., 141, 8, 3515–3523 (2019).
[5] J. Hieulle, et al., J. Phys. Chem. Lett., 11, 3, 818–823 (2020).
[6] A. Jamshaid, et al., Energy Environ. Sci., 14, 4541–4554 (2021).

Abstract:
Atomically-resolved surface studies on In2O3(111): clean, hydroxylated, and with water

Ulrike Diebold,
Institute of Applied Physics, TU Wien, Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria

Author Email:

In2O3 is a post-transition metal oxide with a wide band-gap. The material combines optical transparency with a high electrical conductivity that is sensitive to gas adsorption. Because of these attractive prop-erties, In2O3 finds extensive use as a transparent conductive oxide and chemical sensor [1]. More re-cently, interesting catalytic properties were identified, e.g., efficient hydrogenation of CO2 [2].
From a surface science perspective, In2O3 is peculiar because reduction does not result in oxygen vacan-cies [3] but in an ordered array of single indium adatoms [4]. Its most stable (111) surface has a simple (1×1) bulk termination but an unusually large unit cell that provides an appealing playground for atom-ically resolved experiments. The figure shows In2O3(111) in top and side views, with bulk-like In(6c)/O(4c) and surface In(5c)/O(3c) atoms. The surface has three-fold symmetry, and the areas around the high-symmetry points labelled A, B, and C show different chemical reactivity.
We have combined surface science techniques (ncAFM/STM, XPS, TPD, etc. in UHV) with DFT calcu-lations to study the interaction with water. Single crystals are rare, small, and (to our knowledge) not commercially available; we have perfected the growth of epitaxial thin films [5] and used both types of samples in our experiments.
Water dissociates at room temperature with saturation coverage of 3 mol/u.c. located around B [6]. This provided the basis for establishing a method to probe the proton affinity of individual surface hydroxyls [7]. At higher coverages, water accumulates around B and C, but A stays water-free; the unit cell exhib-its both, hydrophobic and hydrophilic behaviour.

References
[1] Egdell, R. G., Dopant and Defect Induced Electronic States at In2O3 Surfaces. In Defects at Oxide Surfac-es, Springer International Publishing: Cham, 2015; Vol. 58, pp 351-400.
[2] Martin, O.; Martín, A. J.; Mondelli, C.; Mitchell, S.; Segawa, T. F.; Hauert, R.; Drouilly, C.; Curulla-Ferré, D.; Pérez-Ramírez, J., Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation. Angewandte Chemie 2016, 128 (21), 6369-6373.
[4] Setvin, M.; Wagner, M.; Schmid, M.; Parkinson, G. S.; Diebold, U., Surface point defects on bulk oxides: atomically-resolved scanning probe microscopy. Chemical Society Reviews 2017, 46, 1772-1784.
[3] Wagner, M.; Seiler, S.; Meyer, B.; Boatner, L. A.; Schmid, M.; Diebold, U., Reducing the In2O3(111) sur-face results in ordered Indium adatoms. Advanced Materials Interfaces 2014, 1 (8), 1400289-6.
[5] Franceschi, G.; Wagner, M.; Hofinger, J.; Krajňák, T.; Schmid, M.; Diebold, U.; Riva, M., Growth of In2O3 (111) thin films with optimized surfaces. Physical Review B 2019, 3 (10), 103403.
[6] Wagner, M.; Lackner, P.; Seiler, S.; Brunsch, A.; Bliem, R.; Gerhold, S.; Wang, Z.; Osiecki, J.; Schulte, K.; Boatner, L. A.; Schmid, M.; Meyer, B.; Diebold, U., Resolving the Structure of a Well-Ordered Hydroxyl Overlayer on In2O3(111): Nanomanipulation and Theory. ACS nano 2017, 11 (11), 11531-11541.
[7] Wagner, M.; Meyer, B.; Setvin, M.; Schmid, M.; Diebold, U., Direct assessment of the acidity of individual surface hydroxyls. Nature 2021, 592 (7856), 722-725.

Abstract:
Spin-Polarized Surface States of Pb Monolayers on Si(111)
Christian Brand 1,2, Herbert Pfnür 1, Hugo Dil 3,4, Stefan Muff 3,4, Mauro Fanciulli 3,4, Michael C. Tringides 5, Christoph Tegenkamp 1,6

1 Leibniz Universität Hannover, Germany
2 Universität Duisburg-Essen, Germany
3 Swiss Light Source, Villigen, Switzerland
4 École Polytechnique Fédérale de Lausanne, Switzerland
5 Ames Laboratory and Iowa State University, Ames, USA
6 Technische Universität Chemnitz, Germany

Atomic monolayers (ML) of Pb/Si(111) have been found to be superconducting below Tc ≈ 1.8 K [1], but the mechanism behind the evolution of these 2D states is not understood yet. In the so-called striped-incommensurate (SIC) phase close to 4/3 ML stripes with local H3- or T4-centered (√3×√3) reconstruction are separated by (√7×√3) domain walls. Here we present STM and (SR)-ARPES measurements at low temperatures (> Tc) to evaluate the influence of the spin-orbit interaction on the Pb surface states. As it turns out the local adsorption geometry and symmetry of the atomic structure play important roles for the understanding of the measured spin-polarization by SR-ARPES showing strongly spin-polarized metallic surface states [2]. The experimental results are in very good agreement with DFT calculations [3] and reveal beside a complex spin-texture at EF large Rashba-type and Zeeman-like spin-splittings of the Pb surface states. At lower coverage of 1.2 ML Pb a (√7×√3) reconstruction is found which exhibits quasi-1D electronic properties rather than 2D.

References:
[1] Nature Phys. 6, 104 (2010)
[2] Phys. Rev. B 96, 035432 (2017)
[3] Phys. Rev. B 94, 075436 (2016)

Abstract:
Single-molecule laser nanospectroscopy in STM for studying fundamental optical and energy conversion processes

Laser spectroscopy is an important tool in materials science, used to identify materials, determine emission and absorption energies, elucidate dynamic phenomena occurring in materials. In this lecture, I introduce single-molecule nanospectroscopy techniques that has been developed by combining scanning tunneling microscopy (STM) and near-field optics, which are based on plasmon-enhanced light-matter interaction at the molecular scale. STM allows us to observe materials at the atomic level, and various kinds of near-field spectroscopy (Raman [1], photoluminescence [2], photocurrent [3]) now allows us to directly investigate the electronic, optical, and vibrational properties of the materials with submolecular spatial resolution.

References:
[1] R. B. Jaculbia, H. Imada, K. Miwa, T. Iwasa, M. Takenaka, B. Yang, E. Kazuma, N. Hayazawa, T. Taketsugu and Y. Kim, Nat. Nanotechnol. 15 (2020) 105-110.
[2] H. Imada, M. Imai-Imada, K. Miwa, H. Yamane, T. Iwasa, Y. Tanaka, N. Toriumi, K. Kimura, N. Yokoshi, A. Muranaka, M. Uchiyama, T. Taketsugu, Y. K. Kato, H. Ishihara and Y. Kim, Science 373 (2021) 95-98.
[3] M. Imai-Imada, H. Imada, K. Miwa, Y. Tanaka, K. Kimura, I. Zoh, R. B. Jaculbia, H. Yoshino, A. Muranaka, M. Uchiyama and Y. Kim, Nature 603 (2022) 829-834

Abstract:
Designer materials have emerged as a new direction for exploring exotic quantum phenomena, which are rarely realized in naturally occurring materials. This approach can be illustrated by topological superconductivity [1], which is interesting both from fundamental point of view and as the building block for topological quantum bit. Here, we engineer topological superconductivity in an artificial van der Waals heterostructure combining ferromagnetic CrBr3 and superconducting NbSe2. We create this heterostructure by growing monolayer CrBr3 islands on bulk NbSe2 using molecular beam epitaxy (MBE) and investigate it using scanning tunneling microscopy (STM) and spectroscopy (STS) [2]. This heterostructure contains all the key ingredients conventionally required for topological superconductivity: superconductivity, out of plane ferromagnetism and Rashba spin-orbit coupling. We show that the designed system realizes the expected response for topological superconductivity [3]. Detailed understanding of this system requires going beyond the conventional picture and taking into account the moiré modulation associated with the twist angle between the two materials [4]. Our results may lead to incorporation of the heterostructure in future device structures and the designer approach allows further control of the properties through e.g. electrostatic gating or twist engineering.

References
[1] M. Sato, and Y. Ando, Topological superconductors: a review. Rep. Prog. Phys. 80, 076501 (2017).
[2] S. Kezilebieke, M.N. Huda, O.J. Silveira, V. Vaňo, J. Lahtinen, R. Mansell, S. van Dijken, A.S. Foster, P. Liljeroth, Electronic and magnetic characterization of epitaxial CrBr3 monolayers, Adv. Mater. 33, 2006850 (2021).
[3] S. Kezilebieke, M. N. Huda, V. Vaňo, M. Aapro, S. C. Ganguli, O. J. Silveira, S. Głodzik, A. S. Foster, T. Ojanen, P. Liljeroth, Topological superconductivity in a designer van der Waals heterostructure, Nature 588, 424 (2020).
[4] S. Kezilebieke, V. Vaňo, M.N. Huda, M. Aapro, S.C. Ganguli, P. Liljeroth, J.L. Lado, Moiré-enabled topological superconductivity, arxiv:2011.09760.

Abstract:
Scanning probe microscopy has brought us experimental access to the world of single atoms. The atomic force microscope (AFM), offspring of the scanning tunneling microscope (STM), has rapidly found wider applications than the STM because it allows to image any sample without requiring electrical conduction. However, the spatial resolution of AFM initially was inferior to the one of the STM. In the last years, the AFM’s resolution has been boosted far beyond STM, e.g. when imaging organic molecules and even revealing electron clouds within single atoms. Height resolution reaches sub-picometer values, and forces beyond piconewtons can be measured. The origin of the forces covers chemical bonding forces, Pauli repulsion forces as well as exchange forces.
Is this the end of the development of AFM? The answer is a clear no, as there are some inspiring challenges that require great future developmental efforts.

Abstract:
Full insight into the dynamics of a coupled quantum system depends on the ability to follow the effect of a local excitation in real-time. Here, we use scanning tunneling microscopy to precisely design a coupled system of two atomic spins and control their interaction of the tip. This enable us to precisely tune the relative precession of the spins and thereby their level of entanglement. Using a pump-probe scheme we are able to induce a local excitation in the spin system and follow the spin magnetization over time. The time evolution of the magnetization can be directly related to the level of entanglement between the two spins and we show that, at maximal entanglement, when the Larmor frequencies of the two spins match, the angular momentum is swapped back and forth. These results provide insight into the locality of electron spin scattering and set the stage for controlled migration of a quantum state through an extended spin lattice.

Abstract:
Graphene nanostructures can spontaneously develop intrinsic paramagnetism due to the stabilization of open shell configurations in its electronic structure. Radical states of the
conjugated lattice, as singly occupied states, respond to the presence of finite Coulomb correlations by localizing electrons with a net spin polarization. An interesting aspect of such
unconventional form of (para)magnetism is it lays in a the conjugated lattice of graphene. Therefore, it extends for nanometers length scales, and interacts with others with exchange
coupling strengths of tens of millielectronvolts. The challenge of fabricating atomically precise graphene nanostructures with custom shapes for localizing spins and tuning their
interactons became possible with the development of complex on-surface synthesis strategies [1].
In this presentation, I will show results on spin-hosting nanographenes, including their syntesis routes, their magnetic fingerprints and the origin of such unconventional form of
magnetism. We use scanning tunneling microscopy and spectroscopy to detect and spatially localized the spin density by mapping the amplitude of a Kondo resonance [2,3,4,6] or spin
excitations [2,5]. The structures we investigated range from the one-dimensional graphene nanoribbons [3,7], where their band structure can be tuned between different symmetry
protected topological phases, to zero-dimensional system like triangulene, where robust spin states are found to survive over a metal surface.

[1] Cai, J. et al., Nature 466, 470–473 (2010
[2] J. Li, S. Sanz, M. Corso, D.J. Choi, D. Peña, T. Frederiksen, J.I. Pascual, Nature Commun. 10, 200 (2019).
[3] N. Friedrich, P. Brandimarte, J. Li, S. Saito, S. Yamaguchi, I. Pozo, D. Pena, T. Frederiksen, A. Garcia-Lekue, D. Sanchez-Portal and J.I. Pascual, Physical Review Letters 125, 146801 (2020)
[4] J. Li, S. Sanz, J. Castro-Esteban, M. Vilas-Varela, N. Friedrich, T. Frederiksen, D. Peña and J.I. Pascual,Physical Review Letters 124, 177201 (2020)
[5] J. Hieulle, S. Castro, N. Friedrich, A. Vegliante, F. Romero Lara, S. Sanz, D. Rey, M. Corso, T. Frederiksen, J.I. Pascual and D. Pena, Angewandte Chemie-International Edition 60, 25224 (2021)
[6] Tao Wang, Alejandro Berdonces-Layunta, Niklas Friedrich, Manuel Vilas-Varela, Jan Patrick Calupitan, Jose Ignacio Pascual, Diego Peña, David Casanova, Martina Corso, Dimas G. de Oteyza.
arXiv:2111.15302
[7] J. Li, S. Sanz, N. Merino-Diez, M. Vilas-Varela, A. Garcia-Lekue, M. Corso, D. de Oteyza, T. Frederiksen, D. Pena and J.I. Pascual, , Nature Communications 12, 5538 (2021)

Abstract:
Organic molecules may become the building blocks of tomorrow’s electronics. Their size (~1nm) is one order of magnitude smaller than the smallest components of silicon devices (~10nm). An organic molecule can absorb light and transfer its energy to its neighbors via short-scale (~1 to 10nm) resonant dipole-dipole coupling, a process that may be used to transfer information in highly miniaturized devices. Such a device could be a 1D nanowire along which molecular excitations (excitons) can propagate. Molecular self-assembly into 1D chains is therefore a critical aspect for this purpose. In this work, we study the growth of self-assembled 1D chains of a prochiral molecule (quinacridone) under ultrahigh vacuum. Under these conditions, the formation of chains results from multiple interactions, which include intermolecular hydrogen bonds and interactions with a monocrystalline Cu(111) surface. To recover the intrinsic optoelectronics properties of the molecule, the molecular electronic states are decoupled from the metallic substrate using thin films of alkali halides (less than 5 ML) or a single layer of boron nitride (a wide-bandgap 2D semiconductor). We study the growth modes and the structure of the chains using scanning tunneling microscopy (STM) and high-resolution low energy electron diffraction (SPA-LEED).

Abstract:
In recent years, the synthesis of atomically-precise nanographene surface [1] have led to a plethora of novel functionalities such as the emergence of topological electronic states or magnetic edge states. However, proximity-induced superconductivity in nanographene is rather scarce in literature [2], although it could lead to the emergence of new class of topological superconductors [3]. Extending the
on-surface chemistry toolbox on superconducting surfaces is therefore a crucial step in this quest.
In this context, our recent works have explored synthetic routes for silicene [4], porous GNRs [5] and Kagome graphene [6] on Ag(111) characterized by low temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM). I will also briefly discuss our first experimental attempts to extend on-surface reactions to superconducting substrates such as Pb(111) or Nb(100) covered by thin Ag films [7].

References
[1] J. Cai et al. Nature 466, 470 (2010)
[2] Rio et al. Adv. Materials. 33, 2008113 (2021)
[3] S. Kezilebieke et al. Nature 588, 424-428 (2020)
[4] R. Pawlak et al. PNAS 117, 228-237 (2020)
[5] R. Pawlak et al. J. Am. Chem. Soc. 142, 12568–12573 (2020)
[6] R. Pawlak et al. Angew. Chem. Int. Ed. 60, 8370-8375 (2021)
[7] C. Drechsel et al. and J.-C. Liu et al. In preparation

After studying physics at Munich’s Ludwig Maximilians University, J. Barth received his doctorate in physical chemistry with G. Ertl at the Fritz Haber Institute of the Max Planck Society (1992). He was an IBM Postdoctoral Fellow at the IBM Almaden Research Center in San Jose, and spent over a decade at the École Polytechnique Fédérale de Lausanne, where he received the venia legendi. Prior to his engagement as a TUM full professor in 2007, he was a Canada Research Chair at the University of British Columbia in Vancouver.

He is currently Invited Professor at UMR8640 PASTEUR

Abstract:
The judicious use of metal-ligand interactions providing a versatile strategy to control transition and other metal centers in unique environments. Interfacial coordination chemistry, using solid supports as anchoring or even construction platforms, emphasizes the full involvement of the surface atomic lattice in the metal-ligand interactions and coordination spheres. Individual functional molecules and their metal-directed assembly are characterized by scanning tunneling microscopy and spectroscopy, as well as complementary x-ray spectroscopy studies and computational modeling. The atomistic insight gained is used to systematically steer the formation of nano-architectures with special structural features and novel physicochemical properties. We explore the presented coordinatively unsaturated sites in terms of their electronic nature, magnetic characteristics and chemical reactivity. Furthermore surface-mounted rotator modules as well as switchable complexes have been fabricated, with intriguing dynamic phenomena that were monitored and analyzed. The described approach constitutes an intriguing domain in coordination chemistry, wherein special environments provide a unique setting for metal centers, and nanoscale control prospects for distinct and tunable functionalities.

Contact:
Abstract:

Magnetic chains on s-wave superconductors hosting spin spirals or spin-orbit coupling may realize one-dimensional topological superconductors with Majorana modes on their edges. We study artificial spin chains, built atom-by-atom [1], with respect to such phenomena. By variation of substrate and adatom species and interatomic distances in the chain [2-5], we adjust the energies of multi-orbital Yu-Shiba Rusinov states induced by the adatoms [2,3], their hybridizations [4], as well as the chains’ spin structures [5]. This enables us to tailor the emerging multi-orbital Shiba bands such that topologically nontrivial gaps open [6] and precursors of Majorana modes appear [7]. We measure the length-dependent energy oscillations of these precursors in short chains and predict the chain length needed to drive the modes into isolated unpaired Majorana bound states [7].

We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via the Cluster of Excellence ’Advanced Imaging of Matter’ (EXC 2056-project ID 390715994), via the SFB-925-project 170620586, and by the ERC via the Advanced Grant ADMIRE (No. 786020).

[1] D.-J. Choi, N. Lorente, J. Wiebe, K. von Bergmann, A. F. Otte, A. J. Heinrich, Rev. Mod. Phys. 91, (2019) 041001.
[2] L. Schneider, M. Steinbrecher L. Rózsa, J. Bouaziz, K. Palotás, M. dos Santos Dias, S. Lounis, J. Wiebe, R. Wiesendanger, npj Quantum Materials 4, (2019) 42.
[3] L. Schneider, S. Brinker, M. Steinbrecher, J. Hermenau, Th. Posske, M. dos Santos Dias, S. Lounis, R. Wiesendanger, J. Wiebe, Nature Commun. 11, (2020) 4707.
[4] P. Beck, L. Schneider, L. Rózsa, K. Palotás, A. Lászlóffy, L. Szunyogh, J. Wiebe, R. Wiesendanger, Nature Commun. 12, (2021) 2040.
[5] L. Schneider, P. Beck, J. Wiebe, R. Wiesendanger, Science Advances 7, (2021) eabd7302.
[6] L. Schneider, P. Beck, T. Posske, D. Crawford, E. Mascot, S. Rachel, R. Wiesendanger, J. Wiebe, Nat. Phys. 17 (2021) 943.
[7] L. Schneider, P. Beck, J. Neuhaus-Steinmetz, T. Posske, J. Wiebe, R. Wiesendanger, arXiv:2104.11503 [cond-mat.supr-con] (2021).

Abstract:

In common thermodynamic systems, disorder can be induced by temperature which leads to phase transitions from order to disorder, exemplified by the well-known ferromagnetic-paramagnetic phase transition. However, disorder can also be induced by the existence of numerous metastable states due to frustration. This was recently exemplified by elemental neodymium for which a self-induced spin glass phase was reported between 30 mK and 4 K[1]. It was shown that this phase is solely caused by magnetic frustration within the dhcp lattice of neodymium and possesses the aging dynamics of a spin glass.

In this talk, I will show that neodymium undergoes an unusual magnetic transition, where long range multi-Q order emerges from this spin-Q glass phase as temperature is increased from 5 K to 15 K[2]. By means of a temperature study using spin-polarized scanning tunneling microscopy, we characterize the very local order of the spin-Q glass phase and the long-range nature of the multi-Q order phase present at higher temperatures. From the measured magnetization maps, we quantify the phase transition and determine its critical temperature using two distinct analyses. These findings are supported by atomistic spin dynamics simulations, in which the phase transition is qualitatively reproduced and explained by a weakened frustration in the multi-Q order phase compared to the spin-Q glass phase.

References:
[1]          Kamber, U. et al. Self-induced spin glass state in elemental and crystalline neodymium. Science 368, eaay6757, (2020).
[2]          Verlhac, B. et al. Thermally-induced magnetic order from glassiness in elemental neodymium. arXiv:2109.04815 [cond-mat], (2021).

Abstract:
Affiliations:
Donostia International Physics Center, 20018 San Sebastián, Spain
Centro de Física de Materiales, CSIC-UPV/EHU, 20018 San Sebastián, Spain
Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain
Nanomaterials and Nanotechnology Research Center (CINN), CSIC-UNIOVI-PA, 33940 El Entrego, Spain

Carbon nanostructures with zigzag edges exhibit unique properties with exciting potential applications. However, we show here that such nanostructures are often unstable under ambient conditions even if they display a predominantly closed-shell character. In fact, exemplified with narrow chiral graphene nanoribbons (chGNRs), we show how low pressures of pure oxygen gas readily oxidize the ribbons1 in spite of their predominantly closed-shell character.2 The oxidation has dramatically disruptive effects on their electronic properties, which severely jeopardizes their potential applicability. That is, the lack of stability is a barrier that must be surmounted to allow for a scalable exploitation of this type of materials. We prove the viability of chemical protection/deprotection strategies for this aim on the same chGNRs.3 Upon hydrogenation, the chGNRs survive an exposure to air, after which they are easily converted back to their original structure via annealing. We also approach the problem from another angle. Determination of the most reactive sites and of the nature of the main oxidation products allowed us synthesizing a chemically stable oxidized form of the chGNRs that can be subsequently converted to the pristine hydrocarbon form via hydrogenation and annealing. These findings can be extrapolated also to other carbon nanostructures with zigzag edges and may open new doors toward their integration in devices.

References
[1] A. Berdonces-Layunta, J. Lawrence, S. Edalatmanesh, J. Castro-Esteban, T. Wang, M. S. G. Mohammed, L. Colazzo, D. Peña, P. Jelinek, D. G. de Oteyza, ACS Nano 2021, 15, 5610−5617
[2] J. Li, S. Sanz, N. Merino-Díez, M. Vilas-Varela, A. Garcia-Lekue, M. Corso, D. G. de Oteyza, T. Frederiksen, D. Peña, J. I. Pascual, Nat. Commun. 2021, 12, 5538
[3] J. Lawrence, A. Berdonces-Layunta, D. Rey, T. Wang, S. Edalatmanesh, M. S. G. Mohammed, P. Jelinek, D. Peña, D. G. de Oteyza, submitted

Abstract:

Scanning tunneling spectroscopy (STS) is a tool that allows to address individual molecules in a precisely known surrounding. However, it bears the drawback that it requires a conductive substrate. Deposition of organic molecules on a metal substrate leads to strong hybridization of the electronic states. Preservation of the molecular character requires the inclusion of thin band-gapped materials.
In my presentation I will show that MoS2 can act as an effective electronic decoupling layer that exhibits also a small electron-phonon coupling strength. Differential conductance spectra of 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) molecules exhibit a multitude of sharp characteristic peaks, originating from vibronic states. These vibronic fingerprints of different molecules allow for an identification of different rotamers. DFT calculations of the molecules in gas phase provide all details for an accurate simulation of the vibronic spectra of both investigated rotamers.
Moreover, we will make use of the vibronic fingerprints to investigate the influence of the tip potential on the apparently shifted molecular states along the extended molecular backbone of BTTT. Finally, the impact of molecular vibrations on spatial variations of the molecular conductance on BTTT and H2Pc will be discussed and will allow us to understand a vibrational excitation mechanism in tunneling spectroscopy beyond the Franck-Condon model.

Abstract:
A small AC bias applied between an STM tip and a sample, has been shown to be effective in driving electron-spin resonance (ESR) of magnetic adatoms [1-3]. The electric field is able to interact with the atomic spin and make it precess and oscillate. This is somewhat startling because electric fields are known to be particularly inefficient to drive ESR. However, experiments clearly show that ESR signals are now available, which has originated a lot of activity exploring the minute energy scales of atomic and nuclear magnetism [4,5]. Combining ESR with the STM gives rise to unprecedented energy and spatial resolution. Yet, it is not clear how the AC bias is able to drive the ESR.
I will present a concise overview of experimental results, and explore the theoretical possibilities to make ESR a reality with STM. We have recently put together a theoretical tool, that allows us to reproduce the experimental conditions. These involve non-equilibrium transport calculations and spin quantum dynamics [6].
References:
[1] Stefan Müllegger, Stefano Tebi, Amal K. Das, Wolfgang Schöfberger, Felix Faschinger, and Reinhold Koch, Phys. Rev. Lett. 113, 133001 (2014)
[2] S. Baumann, W. Paul, T. Choi, C. P. Lutz, A. Ardavan, and A. J. Heinrich, Science 350, 417 (2015)
[3] P. Willke, W. Paul, F. D. Natterer, K. Yang, Y. Bae, T. Choi, J. Fernández-Rossier, A. J. Heinrich, and C. P. Lutz, Science Advances 4 (2018)
[4] F. D. Natterer, K. Yang, W. Paul, P. Willke, T. Choi, T. Greber, A. J. Heinrich, and C. P. Lutz, Nature 543, 226 (2017)
[5] P. Willke, Y. Bae, K. Yang, J. L. Lado, A. Ferrón, T. Choi, A. Ardavan, J. Fernández-Rossier, A. J. Heinrich, and C. P. Lutz, Science 362, 336 (2018)
[6] Jose reina-Gálvez, Nicolás Lorente, Fernando Delgado and Liliana Arrachea, arXiv:2108.01011

Abstract:

The adsorption of atomic or molecular species on metal nanoparticles (NP), the absorption of atomic species like carbon, hydrogen or oxygen inside NPs and chemical reactions at NPs are of key interest in heterogeneous catalysis. In most cases, ultra-high vacuum (UHV) based surface science techniques like temperature programmed desorption (TPD) as well as molecular beam techniques (MBRS) and photoelectron spectroscopy (XPS, UPS) are used to study the above mentioned reactivity related phenomena. However, the phenomena strongly depend on the NP’s size and shape so that a specific phenomenon observed by the latter techniques cannot be precisely assigned to one type of NP. A characterization at the single NP level is therefore desired.

Because adsorbed or absorbed species almost always create a surface dipole, their presence can be directly put into evidence by measuring the change of work function (WF) at the NP, which can be done by UHV Kelvin probe force microscopy (KPFM). This has been recently demonstrated with oxygen adsorption experiments at palladium NPs (PdNPs) [1] (see Figure 1) and experiments that could reveal the presence of subsurface carbon below the facets of PdNPs [2].

In this contribution, we go a step further and try to answer the question if KPFM can also be used to monitor a chemical reaction that takes place at a single NP, by measuring WF changes. For this, we consider the water forming reaction, which is of general importance in catalysis, and single PdNPs and nano-islands made from Pd. We present our first experiments and discuss the experimental protocols and limits of such experiments.

References
[1] Grönbeck, H.; Barth, C. J. Phys. Chem. C 2019, 123, 24615–24625.
[2] Grönbeck, H.; Barth, C. J. Phys. Chem. C 2019, 123, 4360–4370.

Contact:

Abstract:
It is well-established that Atomic Force Microscopy (AFM) can determine the atomic structure of surfaces and molecules. I will give an overview of some of our current research aimed at achieving and measuring properties with ultrafast temporal resolution using AFM by combining fs lasers with UHV AFM. This opens the door to understanding charge dynamics at the relevant fundamental time and length scale on surfaces or in molecules, and in particular the role of defects.
As a second topic, I will discuss our most recent results on mechanically detecting single electrons, allowing us to perform electron energy spectroscopy of a single molecule on a metallic electrode interface. I will briefly touch on our future plans to adapt these methods to elucidate the energetics of organic charge transfer systems as well as quantum properties of single and coupled dopants in Si.
Unfortunately, the title does not refer to a proposed rapid solution for the current pandemic, the talk will thus be given virtually.

Left image: Frequency shift spectra, with increasing AFM oscillation amplitude (i.e. coupling strength), taken 10 nm above a single ferrocene molecule at 4.8 K. From this data one can directly deduce (without assumptions) the molecular reorganization energy, nuclear-electron coupling constants and molecular vibration frequencies. [Nano Lett. 19, 6104 (2019].
Right image: Optical autocorrelation function measured by AFM with spatial resolution. [Proc. Natl. Acad. Sci. USA 117, 19773 (2020)]

Abstract:
Because of their potential use for Quantum information processing, the study of the spin properties of dopants in semiconductors such as Phosphorous in Si is of intense interest.

For P, As dopants in silicon, STM measurements [1] have shown that the spin-carrying orbital is hybridized with the electronic levels in the conduction band valleys, a phenomena that has important implications for their use as quantum bits (QBits). This hybridization leads to a characteristic spatial modulation of the dopant wavefunction at the valley frequency[1], which controls the spin-exchange interaction between two dopants[2]. Furthermore, it has also been suggested theoretically that the spin-valleys Kramer states could be preferable to pure spin as Qbits.

In this context, TMD 2H-(Mo,W)(Te,S,Se)2 semiconductors are of particular interest because of their large spin-orbit coupling that lift the spin-valley degeneracy. However, the study of dopants in these materials is still in its infancy.

Through STM, Electron Spin Resonance and transport measurements, we have studied the properties of the Bromine dopant in 2H-MoTe₂. We found that at low temperature, in the Mott-Anderson insulating regime, a large spin resonance signal is present that results from the localized electron at the Bromine site. The spin lifetime increases tremendously upon crossing from the actived regime to the hopping regime. STM measurements show that the corresponding electronic orbital is hybridized with the valleys. Through Fourier analysis, the envelop function and the interference between multiple dopants can be extracted. These observations suggest a path toward using dopants in TMDs as Kramer spin-valleys QBits.

[1] Salfi, J., Mol, J., Rahman, R. et al. Spatially resolving valley quantum interference of a donor in silicon. Nature Mater 13, 605–610 (2014).
[2] Voisin, B., Bocquel, J., Tankasala, A. et al. Valley interference and spin exchange at the atomic scale in silicon. Nat Commun 11, 6124 (2020).

Abstract:
Quantum designer materials that realize electronic responses not found in naturally occurring materials have recently attracted intense interest. For example, topological superconductivity [1] – a key ingredient in topological quantum computing – may not exist in any single material. However, using designer van der Waals (vdW) heterostructures, it is possible to realize the desired physics through the engineered interactions between the different components. We use molecular-beam epitaxy (MBE) to grow islands of ferromagnetic CrBr3 on a superconducting NbSe2 substrate [2]. This combines out of plane ferromagnetism with Rashba spin-orbit interactions and s-wave superconductivity and allows us to realize topological superconductivity in a van der Waals heterostructure [3,4]. We characterize the resulting one-dimensional edge modes using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Achieving topological superconductivity in a vdW heterostructure facilitates its incorporation in future device structures and potentially allows further control through e.g. electrostatic gating.

References
[1] M. Sato, and Y. Ando, Topological superconductors: a review. Rep. Prog. Phys. 80, 076501 (2017).
[2] S. Kezilebieke, M.N. Huda, O.J. Silveira, V. Vaňo, J. Lahtinen, R. Mansell, S. van Dijken, A.S. Foster, P. Liljeroth, Electronic and magnetic characterization of epitaxial CrBr3 monolayers, Adv. Mater. 33, 2006850 (2021).
[3] S. Kezilebieke, M. N. Huda, V. Vaňo, M. Aapro, S. C. Ganguli, O. J. Silveira, S. Głodzik, A. S. Foster, T. Ojanen, P. Liljeroth, Topological superconductivity in a designer van der Waals heterostructure, Nature 588, 424 (2020).
[4] S. Kezilebieke, V. Vaňo, M.N. Huda, M. Aapro, S.C. Ganguli, P. Liljeroth, J.L. Lado, Moiré-enabled topological superconductivity, arxiv:2011.09760.

Chemical transformations in molecules are a consequence of valence electron motion and its eventual coupling to atomic motion, hence, tracking valence electron motion at the orbital level is the key to understanding and taming such transformations. Scanning tunnelling microscopy (STM) can passively and locally probe the valence electron density in molecules. Contemporary techniques in attosecond science, on the other hand, can generate and track the temporal evolution of a coherent superposition of quantum states of valence electrons by using strong laser fields, which can be probed only non-locally. In absence of the capability to trigger and probe electron dynamics at the single-orbital level, electron motion could only be inferred by reconstruction. In the talk, I will show you how dynamics
of coherent superposition of valence electron states generated by < 6 femtosecond long carrier-envelope-phase (CEP) stable laser pulses, can be locally probed with picometer
spatial resolution and 300 attosecond temporal resolution simultaneously, at the single orbital-level with the help of an STM, defying the previously established fundamental spacetime limit. Near fields of optical pulses confined to the apex of nanotip of an STM enable orbital imaging of electronic levels of molecules with pm resolution. We envisage that it will be possible to see a chemical bond formation dynamics through a transition state at the orbital level in the near future.

Molecules can be created using atomic manipulation. Atomic force microscopy (AFM) with functionalized tips achieves atomic and bond resolved resolution providing insights into the structure, aromaticity, charge states and bond-order relations of the molecules created and into the reactions performed [1]. Elusive molecules as the carbon allotrope cyclo[18]carbon could be created and studied [2].

On insulating substrates, we can control the charge states of molecules. We can measure the reorganization energy [3] and excited state energies of individual molecules [4]. With AFM we resolved the structure of molecules in different charge states [5].

References
[1] L. Gross et al. Angew. Chem Int. Ed 57, 3888 (2018)
[2] K. Kaiser et al. Science 365, 1299 (2019)
[3] S. Fatayer et al. Nat. Nano. 13, 376 (2018)
[4] S. Fatayer et al. arXiv:2011.09870 (2020)
[5] S. Fatayer et al. Science 365, 142 (2019)