Advent calendar - December 24th - Zernike Institute Papers of the Year 2024

In the Zernike Institute Advent Calendar, we are presenting 24 short spotlights in December. In these specials, we highlight PhD students, postdocs, support staff, and technicians of our research groups and team - providing a glimpse into their typical day at work. In Episode 24 - the final one - we are deviating from this and provide our handpicked Best of 2024 publications - the personal research highlights of our principal investigators. These are original research works, reviews or patents authored or co-authored by members of our team. We provide them together with a short summary on what makes the publication a special one. Enjoy the selection, enjoy the science, learn something new, stay curious and most importantly: stay safe, stay healthy, enjoy the holiday season, and all the best for 2025!

The principal investigators are listed in alphabetcial order

Use ctrl+f (Windows)/ cmd+f (Mac) to search for keywords of your interest

Domains with Varying Conductance in Tensile Strained SrMnO₃ Thin Films Using Out-of-Plane Electric Fields | Job J. L. van Rijn, Ishitro Bhaduri, Majid Ahmadi, Beatriz Noheda, Bart J. Kooi, and Tamalika Banerjee | Here we have used a suite of new and complementary microscopy techniques to unravel the origin of conductive domains in strained SrMnO₃ films on a degenerate semiconductor of Nb:SrTiO₃, We study domains with varying conducting properties with these techniques that sheds intriguing insights into their temporal evolution. Besides using conducting AFM, we have used HAADF scanning transmission electron microscope that establishes that the observed domains are formed by cracks driven by inhomogeneous strain relaxation throughout the film. Our in house microscopy maestro, Majid Ahmadi, thereafter introduced a new technique of using secondary electrons in Scanning electron microscope to detect domain dependent contrast over a large area that correlates with the conductive properties of the domains and serves as a new direction to probe domains and domain walls in multiferroic materials.

Giant magnetocaloric effect in a rare-earth-free layered coordination polymer at liquid hydrogen temperatures | Joshua J.B. Levinsky, Benedikt Beckmann, Tino Gottschall, David Koch, Majid Ahmadi, Oliver Gutfleisch, Graeme R. Blake | In this paper, a collaboration with the Technical University of Darmstadt and the Dresden High Magnetic Field Laboratory, we report on a new material that exhibits excellent magnetocaloric properties in the temperature range relevant for the liquefaction of hydrogen for energy storage. Magnetocaloric cooling technology promises to be much more energy-efficient than compression-based cooling methods. Until now, most suitable materials for this temperature range have been based on scarce and expensive rare-earth elements. Our work shows that comparable performance is also achievable using magnetic transition metals, which are generally cheaper and more abundant.

Finding the gap: neuromorphic motion-vision in dense environments | Thorben Schoepe, Ella Janotte, Moritz B. Milde, Olivier J. N. Bertrand, Martin Egelhaaf & Elisabetta Chicca | This work stands out as a rare example of neuromorphic engineering applied to hypothesize about biological computation. It offers concrete ideas on potential neural substrates underlying behavior, providing testable hypotheses for neuroscientists. Additionally, it exemplifies a closed-loop behaving agent that utilizes spiking activity from sensors to actuators, demonstrating a novel integration of neuromorphic principles.

Coincidence Detection with an Analog Spiking Neuron Exploiting Ferroelectric Polarization | Paolo Gibertini; Luca Fehlings; Thomas Mikolajick; Elisabetta Chicca; David Kappel; Erika Covi | In our group, one of our research lines focuses on replicating the way biological neural networks detect correlated events in the environment, a key capability for efficient sensory processing. Our work in neuromorphic computing aims to achieve this by designing more efficient neuron models. In this work, we propose a HfO₂-based ferroelectric capacitor (FeCap) combined with complementary metal oxide semiconductor (CMOS) technology to create a leaky integrate-and-fire (LIF) neuron. This neuron is capable of detecting highly correlated events by leveraging two distinct temporal dynamics.

By using two time constants, our neuron becomes more versatile and adaptable, which is crucial for detecting both transient and sustained coincidences. This feature not only improves the neuron’s ability to handle different types of events but also makes it more compact and efficient. Additionally, the time constants we’ve chosen align with biologically relevant timescales, meaning our neuron can tackle real-time tasks like keyword spotting or sensory processing.

Overall, the FeCap-based LIF (FeLIF) neuron that we have developed extends the dynamics of a traditional LIF neuron, offering a powerful and elegant solution for event-driven neuromorphic computing. Our FeLIF paves the way for the development of more advanced analog neuromorphic hardware for a variety of sensory processing applications.

Bidispersity improves the toughness and impact resistance of star-polymer thin films| Utku Gürel, Sinan Keten, Andrea Giuntoli | While all polymeric systems present complex multiscale heterogeneities, these are rarely taken into account in theoretical and computational models. Defects and heterogeneities are often considered detrimental to the performance of a material, but in this paper we show that molecular weight bidispersity in the arms of star polymers leads to enhanced mechanical properties of the glassy thin films formed by the stars.

Nano-ARPES investigation of twisted bilayer tungsten disulfide | Giovanna Feraco, Oreste De Luca, Przemysław Przybysz, Homayoun Jafari, Oleksandr Zheliuk, Ying Wang, Philip Schädlich, Pavel Dudin, José Avila, Jianting Ye, Thomas Seyller, Paweł Dąbrowski, Paweł Kowalczyk, Jagoda Sławińska, Petra Rudolf, Antonija Grubišić-Čabo | Twisted bilayer systems are rich platforms for exploring exotic quantum phenomena and novel quantum phases In this paper we investigate bilayer WS2 twisted at an angle of 4.4° using nanoscale angle-resolved photoemission spectroscopy (nanoARPES), a technique that enables direct imaging of the electronic band structure. Despite theoretical predictions that this system should have flat bands and host strongly corelated states, our results revealed that the electronic structure resembles that of a regular, untwisted bilayer WS2. This unexpected behaviour indicates that significant atomic relaxation occurs at 4.4°, resulting in large untwisted domains dominating the structure. Density functional theory calculations further support this conclusion, aligning closely with experimental observations. Our findings emphasize the critical role of atomic relaxation in determining the properties of twisted bilayer systems and highlight nano-ARPES as a powerful tool for unraveling such structural effects.

Efficient Magnon Injection and Detection via the Orbital Rashba-Edelstein Effect | J. A. Mendoza-Rodarte, M. Cosset-Chéneau, B. J. van Wees, and M. H. D. Guimarães | In this paper we show how we can harness the orbital angular momentum of the electron to drive and detect spin waves in a magnetic insulator. The orbital angular momentum per electron can be much higher than just its spin -- think of d-orbitals in an atom, which can carry 2*hbar of angular momentum compared to hbar/2 for the spin. Using this we were able to excite and detect spin waves with a nearly 10-fold increase in efficiency. We also observed that the conversion between orbital accumulation-to-spin current and the inverse (spin current-to-orbital accumulation) in these systems is not the same, i.e. it is asymmetric. We don't fully understand the underlying microscopic mechanisms yet and this is a very new phenomenon, so our work provides the first steps towards the understanding and the harnessing of orbital currents for new spintronic devices.

Spatial Correlations Drive Long-Range Transport and Trapping of Excitons in Single H‑Aggregates: Experiment and Theory | Carta, A., Wittmann, B., Kreger, K., Schmidt, H.-W., Jansen, T. L. C., & Hildner, R. | Trapping of energy (or charges) in low-lying excited states of functional materials is usually associated with poor transport efficiencies and ultimately bad device performance. In this collaborative work between the Optical Spectroscopy of Functional Nanosystems and Computational Spectroscopy groups, we looked at a model system - supramolecular nanofibres based on small molecules - and found that energy can move over micrometer distances (a world record) and at the same time being trapped. We found that the nanofibres possess an energy landscape that is locally, i.e., on a length scale of some tens of molecules, very flat. Transport is then exceptionally efficient over long distances, but you buy this advantage by ultimately trapping energy. Nevertheless, our joint work shows the potential of 'energy-landscape engineering' of nanostructures to optimise function. This joint publication is the result of the Small Research Project of the former Topmaster Nanoscience student Alberto Carta, who is now a PhD student at ETH Zurich.

Power-Law Scaling Relating the Average Charge State and Kinetic Energy in Expanding Laser-Driven Plasmas | J. Sheil, L. Poirier, A. C. Lassise, D. J. Hemminga, S. Schouwenaars, N. Braaksma, A. Frenzel, R. Hoekstra, and O. O. Versolato | The paper presents a universal power-law scaling between the average ion charge state and the ions kinetic energy in expanding laser-driven plasmas. Universality here refers to an insensitivity to all experimental conditions: target geometry, expansion direction, laser wavelength, and power density. The power law is based on analytical descriptions of the dependence of the charge state on temperature and the transfer of internal to kinetic energy. Experimentally we used tin plasma based 13.6-nm EUV sources for nanolithography to validate the power-law scaling, which is expected to hold also for other dense plasma containing heavy, complex ions.

What in addition to the science makes the paper special to me is the process towards the paper. Several years ago we observed the first signs of the relation between charge state and energy at the EUV source at ARCNL. The evidence grew over the years but an explanation evaded us. John Sheil, Oscar Versolato and I, we scheduled regularly meetings to unveil the physics underlying the power-law without success till early this year during a coffee conservation all pieces fell together.

Observation of Dark States in Two-Dimensional Electronic Spectra of Chlorosomes | Vesna Eric, Xinmeng Li, Lolita Dsouza, Sean K Frehan, Annemarie Huijser, Alfred R Holzwarth, Francesco Buda, GJ Agur Sevink, Huub JM De Groot, Thomas LC Jansen | This paper is the result of a collaboration between our group and the optical spectroscopy group at University of Twente and the NMR group at Leiden University. This study focuses on understanding the light harvesting mechanism in Chlorosomes. Chlorosomes are the photosynthetic antenna of green-sulphur bacteria. They contain thousands of strongly interacting bacteriochlorophyll molecules allowing them to perform the most efficient ultrafast excitation energy transfer of all natural light-harvesting complexes. It was a mystery how they efficiently prevent fluorescence, which would result in a significant loss of efficiency. In this paper, we find and explain the mechanism behind. In essence, the couplings between the chlorophyll molecules in the two-dimensional rolled up structures are tuned to ensure the presence of a relatively narrow band of dark states slightly below the optical active one. These states are populated within 100 fs of the absorption of light and as the states are dark and no fluorescence can take place. We speculate that the important transfer towards the reaction center, where the energy is converted to chemical energy, is happening via the so-called base plate which is close enough to break the dipole-dipole transfer rule and allow efficient transfer even if the donor states have very small transition dipole moments. We are currently studying this dark state transfer step.

Effect of Polyelectrolyte Charge Density on the Linear Viscoelastic Behavior and Processing of Complex Coacervate Adhesives | Larissa van Westerveld, Théophile Pelras, Anton H Hofman, Katja Loos, Marleen Kamperman, Julien Es Sayed | In this work we combined our knowledge on complex coacervates with our polymer synthesis expertise. We systematically studied the effect of charge density of a strong polyanion on the properties of complex coacervates. To control this charge density, statistical anionic/charge-neutral hydrophilic copolymers were synthesized by means of an elegant protection/deprotection strategy and subsequently complexed with a strong polycation.

Atomically Resolved Phase Coexistence in VO2 Thin Films | Masoud Ahmadi, Atul Atul, Sytze de Graaf, Ewout van der Veer, Ansgar Meise, Amir Hossein Tavabi, Marc Heggen, Rafal E Dunin-Borkowski, Majid Ahmadi, Bart J Kooi | VO₂ is an interesting material for a variety of applications, because it shows a well-accessible insulator to metal transition at about 70 °C. Despite the importance of understanding the structure of VO₂ thin films, a detailed real-space atomic structure analysis in which the oxygen atomic columns are also resolved is lacking. Moreover, intermediate atomic structures have remained elusive due to the lack of robust atomically resolved quantitative analysis. Here, we directly resolve both V and O atomic columns and use a new approach to discover the presence of intermediate monoclinic (M2) phase nanolayers (less than 2 nm thick) in epitaxially grown VO₂ films on a TiO₂ (001) substrate, where the dominant part of VO2 undergoes a transition from the tetragonal (rutile) phase to the monoclinic M1 phase. Our new approach can be used for quantifying the detailed local atomic structures in a wide range of oxides.

Carrier–Carrier Repulsion Limits the Conductivity of N-Doped Organic Semiconductors | Xuwen Yang, Gang Ye, Jian Liu, Ryan C. Chiechi, L. Jan Anton Koster | This paper challenges a long-held paradigm in the field of organic semiconductors. Organic semiconductors are unique as they offer a radically different approach to electronics, they are flexible, light-weight, and their properties can be tailored by changing their chemical structure. Molecular doping of organic semiconductors, however, is a challenging process that is not fully understood. Typically, the electrical conductivity increases upon doping, but only up to a certain doping level. Upon further increasing the doping level, the electrical conductivity decreases. This decrease in conductivity is often attributed to morphological changes brought about by the introduction of dopants. In this study, we show that carrier-carrier repulsion, rather than morphology, plays a crucial role in the observed decrease in conductivity.

Magnet-superconductor hybrid quantum systems: a materials platform for topological superconductivity | Roberto Lo Conte, Jens Wiebe, Stephan Rachel, Dirk K. Morr, Roland Wiesendanger | Magnet-superconductor hybrid (MSH) materials are man-made materials where we can understand and engineer the interplay of magnetism and superconductivity. Such interplay is expected to give origin to emergent quantum properties, such as topological superconductivity, which can play a crucial role in the design of novel quantum technologies. In this manuscript, the reader can find an extensive review of this fast growing field of research. The focus is on MSHs crafted at the atomic scale and investigated via scanning tunneling microscopy and spectroscopy. Apart from a summary of all the most relevant experimental work done to date, the reader can also find an introduction to tight-binding models used to describe the emergent physics of MSHs and explain some of the main experimental observations. The scope of this article is two-fold. On the one hand, we hope it will serve as an introduction to MSHs as platforms for the establishment of topological superconductivity to the new generations of physicists, motivating them to join this exciting and fast developing field of modern condensed matter physics. On the other hand, we expect this review manuscript to stimulate new efforts in the study of MSHs which go beyond the current state-of-the-art, allowing us to reach an higher understanding of the fascinating new physics emerging from the interplay of magnetism and superconductivity.

Enzymatic bulk synthesis, characterization, rheology, and biodegradability of biobased 2,5-bis(hydroxymethyl)furan polyesters | Cornelis Post, Dina Maniar, Jesse A. Jongstra, Daniele Parisi, Vincent S. D. Voet, Rudy Folkersma and Katja Loos | We synthesized biodegradable BHMF-based polyesters through a solvent-free bulk polymerization process, highlighting the unique ability to perform this challenging synthesis in bulk. By varying the number of methylene units in the aliphatic comonomer, we tailored the thermal, rheological, and biodegradability properties, showcasing the potential for sustainable material design.

Biobased Networks from Lignin/Cellulose via Diels-Alder Click Chemistry | Luan Moreira Grilo, Sara Faoro, Rudy Folkersma, Talita Martins Lacerda, Laura Mazzocchetti, Katja Loos, Dina Maniar | Thermosets are widely used in plastics and coatings due to their exceptional strength and resistance to chemicals and heat. However, their permanent cross-linked structure makes recycling and reprocessing nearly impossible. This study presents a novel approach to creating fully biobased thermosets using lignin and cellulose, two renewable resources, through the Diels–Alder (DA) click reaction. By attaching furan and maleimide groups to these biopolymers, the resulting materials form dynamic networks capable of reversible thermal de-cross-linking and re-cross-linking. While combining lignin and cellulose in the same network did not yield thermoreversibility, the findings underscore the potential of DA chemistry to develop sustainable and adaptable materials, paving the way for greener alternatives in material science.

Reentrant multiple-q magnetic order and a “spin meta-cholesteric” phase in Sr₃Fe₂O₇ | N. D. Andriushin, J. Muller, N. S. Pavlovskii, J. Grumbach, S. Granovsky, Y. V. Tymoshenko,O. Zaharko, A. Ivanov, J. Ollivier, M. Doerr, B. Keimer, M. Mostovoy, D. S. Inosov & D. C. Peets | Josse Muller and Maxim Mostovoy explained the origin of unusual multiply-periodic states found in Sr_3Fe_2O_7 by the experimental group from the Dresden Technical University. At high temperatures, this material shows two periodic modulations with equal amplitudes and orthogonal wave vectors. As temperature goes down, this state is replaced by a single spin spiral, which at low temperatures turns into a periodic array of vortices and antivortices.

Tertiary structure and conformational dynamics of the anti-amyloidogenic chaperone DNAJB6b at atomistic resolution | Vasista Adupa, Elizaveta Ustyantseva, Harm H. Kampinga & Patrick R. Onck | The molecular chaperone DNAJB6b plays a key role in prohibiting neurodegenerative diseases by blocking protein aggregation in nerve cells. Here we use atomistic molecular dynamics simulations to show that DNAJB6b is a transiently interconverting protein cycling between three different conformational states. Interestingly, in all these states autoinhibition is preserved, suggesting a highly intriguing allosteric mechanism in which auto-inhibition is only released when DNAJB6b is loaded by disease-related disordered proteins.

MXene Surface Engineering Enabling High-Performance Solid-State Lithium Metal Batteries | Xiaolong He, Yinyu Xiang, Wenjiao Yao, Feng Yan, Yongsheng Zhang, Dominic Gerlach, Yutao Pei, Petra Rudolf, Giuseppe Portale | The development of efficient solid state polymeric electrolytes is an important step towards safer and durable Li-metal batteries. Often, polymers alone are not sufficient to fulfill this task due to limited ionic conductivity and limited mechanical properties when mixed with lithium salts and other battery-related additives. Here, we have developed a novel polymer composite based on poly(vinylidene fluoride)-hexafluoropropylene containing 2D fillers such as MXenes. In order to increase the compatibility between the polymer and the 2D filler, as well as to boost the ionic conductivity of the solid electrolyte membrane, we have engineered the surface of the MXene with polyethyeleneglycol (PEG) side chains. The surface modified MXene are easily dispersible in the polymer matrix and show a synergistic effect capable of boosting the mechanical and transport properties of the material, when only 2% of MXene is added. We have tested this novel solid polymer electrolyte inside coin cell batteries Li/LiFePO4 and Li/LiNi0.6Co0.2Mn0.2O2 reaching impressive capacities and high cycling stability. Following this approach of filler surface engineering, we are currently developing other new polymer electrolyte composites that can improve battery safety and performances not only at the level of the solid electrolyte part, but also as binders for the electrode materials and protective coating layers at the Li metal anode.

Glycan-induced structural activation softens the human papillomavirus capsid for entry through reduction of intercapsomere flexibility | Yuzhen Feng, Dominik van Bodegraven, Alan Kádek, Ignacio L.B. Munguira, Laura Soria-Martinez, Sarah Nentwich, Sreedeepa Saha, Florian Chardon, Daniel Kavan, Charlotte Uetrecht, Mario Schelhaas, Wouter H. Roos | Together with collaborators from the CSSB Hamburg and the Universität Münster we studied the molecular interactions taking place during the initial steps of Human Papillomavirus (HPV) infection. The editors of the scientific journal Nature Communications were excited about our findings and decided to publish our results. HPVs are a large family of DNA viruses that cause diseases ranging from asymptomatic infections to throat or genital cancers. While vaccines are available to prevent HPV infection, the development of additional anti-viral strategies is highly desirable. We investigated the role played by cellular heparan sulphates (HS) in facilitating HPV entry. Employing a combination of virological assays, hydrogen/deuterium exchange mass spectromerty and atomic force microscopy we determined the effect of viral capsid-HS bonding and structural activation. We showed that the engagement of several binding sites by a minimum length of HS polysaccharide is needed for structural activation of the viral particle. A surprising finding was that the binding of HS to HPV capsomeres imparts a reversible pincer-like force. This force then stabilizes the viral capsid in a conformation with extended capsomer linkers and the capsid is thereby both enlarged and softened which ultimately facilitates viral entry into the host cell. In our lab we performed the atomic force microscopy (AFM) experiments for the mechanical characterisation of the viral particles. With this work we revealed new mechanistic insights into the initial steps of HPV infection that will likely advance fundamental science, and moreover may help further the design of novel small compound inhibitors to help combat HPV infections.

Efficient spin accumulation carried by slow relaxons in chiral tellurium | E. Barts, K. Tenzin, J. Sławińska | One of the cornerstones of spintronics is the electrical generation of spin signals and their conversion back to electric voltage, a process known as charge-to-spin interconversion. Efficient charge-to-spin conversion is only possible in materials with strong spin-orbit interaction. However, a significant drawback is that the generated spin signals quickly decay during diffusive transport through the material, as the spin-orbit interaction acts like an effective magnetic field, causing momentum-dependent spin precession and its dephasing upon scattering. In the paper, we discover slow collective relaxation modes that reduce spin relaxation in crystals with strong spin-orbit coupling. These modes, which we term "slow relaxons," are responsible for generating strong spin signals and enabling their transfer over long distances – two effects that were previously thought to be contradictory. We focus on chiral tellurium to explain the physical origin of slow relaxons, attributing their existence to spin-momentum entanglement at the Fermi surface, which partially prevents the back- scattering of charge carriers. We report a record charge-to-spin conversion efficiency of 50% for Te crystals and the ability to transmit the spin signals over long distances. We anticipate that slow relaxons exist in other materials, facilitating long spin lifetimes, which opens the route for the discovery of other suitable materials for spintronics devices.

Quantum communication networks with defects in silicon carbide| Sebastian Ecker, Matthias Fink, Thomas Scheidl, Philipp Sohr, Rupert Ursin, Muhammad Junaid Arshad, Cristian Bonato, Pasquale Cilibrizzi, Adam Gali, Péter Udvarhelyi, Alberto Politi, Oliver J. Trojak, Misagh Ghezellou, Jawad Ul Hassan, Ivan G. Ivanov, Nguyen Tien Son, Guido Burkard, Benedikt Tissot, Joop Hendriks, Carmem M. Gilardoni, Caspar H. van der Wal, Christian David, Thomas Astner, Philipp Koller, Michael Trupke| The field of Quantum Information Science aims to get devices that function with fully quantum mechanical states (rather than classical states) for the signals that are processed. An application that is most promising for getting a wide impact in the coming years is quantum communication. This can outperform all classical versions of communication in offering better security against eavesdropping. For this development, one of the most important material systems consists of transparent crystals that contain atomic impurities (at very low density) that can act as optical emitters, while they also have electronic spin. For this direction, our group investigates new impurity-crystal combinations. We co-discovered suitable impurities where the optical emission is at a standard telecom wavelength, as now used in the fiber networks of the internet. At the same time, the crystal is silicon carbide, which is a high-band gap semiconductor with existing industrial routes for applying it in control electronics and optical detectors. This paper presents how these advantages can work out in a full-system analysis.

Resolving Atomic-Level Dynamics and Interactions of High Molecular-Weight Hyaluronic Acid by Multidimensional Solid-State NMR | Pushpa Rampratap, Alessia Lasorsa, Abinaya Arunachalam, Marleen Kamperman, Marthe T. C. Walvoort, and Patrick C. A. van der Wel | The polysaccharide hyaluronic acid plays an important role in the human body, both in health and in disease. In this paper we show how the combination of isotope enrichment and advanced NMR spectroscopy allowed us to see the behavior of these dynamic biomolecules in great detail. With these methods one can study these large polymers in complex contexts, enabling investigations of the mechanisms behind diseases ranging from osteoarthritis to cancer.

Mechanism for Electrostatically Generated Magnetoresistance in Chiral Systems without Spin-Dependent Transport | Sytze H. Tirion and Bart J. van Wees | Chiral materials have attracted attention due to the large magnetoresistance (MR) that is generated when they are coupled to a ferromagnet owing to the chirality-induced spin selectivity (CISS) effect. Commonly, this MR is interpreted to originate from spin transport, which could make chiral materials suitable for fundamental research as well as spintronics applications. However, in our paper, we propose a new mechanism that does not require spin transport to generate an MR and is based on an electrostatic modification of the transport barrier. In the past year our work has already generated significant interest and stimulated further discussion in the field of CISS worldwide

Image processing device and medical treatment device including the same |Pieter Jan van der Zaag, Bas Keizers, Thomas Sebastiaan Nijboer, Floris Jan Voskuil| Fluorescent-guided surgery is a technique which is being used to help surgeons find tumors during cancer surgery. We discovered that within fluorescent images of these tumor, the tumor boundary can be identified consistent with the pathology analysis. This is remarkable as a pathologist determines what tumor is during pathology analysis, days after after the surgery, based on cell morphology. Whereas our results suggest that the tumor edge can be identified from a fluorescent signal. This could help to indicate during surgery already until where tissue has to be removed, ensuring that all tumor tissue is removed precisely. Potentially this could help reducing the chance of having a tumor positive margin, and improve outcomes of cancer surgery.

See all Advent Calendar items 2024 here!