11:10 - Erik van Loon (Radboud University ← Lund University) - Modelling electronic correlations in quantum materials

Predicting the properties of quantum materials is difficult due to the competing interactions between electronic, magnetic and lattice degrees of freedom. Relatively simple models can capture the basic phenomenology of these materials, but how do you find the right model for your material, and how do you get the model parameters? What impact does the choice of model have on the physics that can be captured? I will discuss these questions in the context of strongly correlated electrons and the impact of beyond-Hubbard interactions on phase transitions and collective excitations.

11:35 - Pim Witte (Utrecht University) - TBA

TBA

13:00 - Onima Bishit (TU Delft) - 4D-STEM Mapping of Phase-Matched Modes in Photonic Crystal and Cavity Structures

Free-electron coupling to nanophotonic modes is governed by phase matching between the electron velocity and the modal phase evolution in the structure. In photonic crystals, this condition selects the field distributions that contribute most strongly to the interaction, but direct experimental visualization of these phase-matched fields remains limited. Here, we apply four-dimensional scanning transmission electron microscopy (4D-STEM) to fabricated photonic crystals designed for phase matching at 100, 200, and 300 kV, together with cavity photonic crystals designed for 200 and 300 kV. By measuring all structures at 300 kV, we directly compare how design-specific phase matching controls the measured electric-field response. The reconstructed maps reveal phase-matched mode profiles with spatial structure and relative intensity in agreement with simulation. Cavity integration produces stronger confinement and enhanced localized response, highlighting the role of resonant photonic engineering in strengthening electron–field coupling. Our results show that 4D-STEM can directly probe the electrodynamic signatures of phase matching in photonic crystals and cavities, providing an experimental framework for the design and validation of nanophotonic structures for relativistic free-electron interactions

13:25 - Feike van Veen (University of Twente) - Trivial or topological? Edge states and YSR-like features in ultrathin Bi2Se3 (Josephson) junctions

Candidate materials for exhibiting Majorana bound states (MBS) are of significant interest to the physics community, for both fundamental curiosity and for the development of fault tolerant quantum computing. The development of a poor man’s Majorana (PMM) state has made significant leaps in the last few years, by exploiting Yu-Shiba-Rusinov (YSR) states in quantum dots [1]. Alternatively, the quantum spin Hall (QSH) effect delivers great prospects in bringing the observation of MBS one step closer, if only superconductivity could be induced in these states [2]. In a previous study, an enhanced density of states is observed at the edges of ultrathin colloidal Bi2Se3 nanoplatelets (NPLs) with scanning tunnelling spectroscopy, interpreted as QSH states at the perimeter due the surface hybridization effect [3, 4]. This observation is the key motivation for this project to explore the superconducting transport characteristics in ultrathin Bi2Se3 junctions. For this aim, we fabricated Nb - Bi2Se3 - Nb samples to create a Josephson junction (JJ). In the superconducting transport properties of NPLs with a thickness of 6 nm, a Josephson effect was found, and here, we observed strong signatures of a 1D contribution localized at the edges. Furthermore, we can deplete these NPLs into an insulating regime hereby destroying the supercurrent. When this insulating regime is interpreted as a band gap, this indicates a trivial character of the system, which requires a different theoretical framework to explain for the edge states present. On the poorly conducting NPLs with a thickness of 5 nm, we have performed spectroscopy on the sub-gap states present in the induced superconducting gap. This revealed accidental but intriguing results, indicative of a YSR state present in the system. The latter result is not yet fully understood and therefore still intensively studied.

[1] Zatelli, F., et al., (2024). Nature Communications, 15, 7933.
[2] Hasan, M. Z.  et al., (2010) Rev. Mod. Phys. 82, 3045–3067
[3] Moes, J. R. et al., (2024) Nano Lett. 24, 5110-5116
[4] Asmar, M. M. et al., (2018) Phys. Rev. B 97, 075419
Authors:
Feike van Veen, Femke Witmans, Jara Vliem, Daniel Vanmaekelbergh, Chuan Li, Alexander Brinkman

13:50 - Chrystalla Knekna (University of Groningen) - Chemical-potential tuning in the type-II Dirac semimetal PtBixTe2-x

Type-II Dirac semimetals are topological semimetals featuring Lorentz-violating Dirac fermions in their electronic band structures, leading to unique transport properties and novel quantum effects due to the presence of tilted Dirac cones. The first direct experimental identification of a type-II Dirac semimetal was achieved in PtTe2 single crystals by means of angle-resolved photoemission spectroscopy (ARPES) [1]. Nevertheless, the deep-lying energy position of the type-II Dirac cone hinders exploitation of its exotic properties. In this talk, I will discuss the approach of tuning the chemical potential of PtTe2 by Bi doping in order to bring the bulk type-II Dirac cone closer to the Fermi level. I will present our latest ARPES data on a series of PtBixTe2-x single crystals and discuss the evolution of the electronic structure. Interestingly, our data highlight that Bi doping not only shifts the chemical potential on the eV scale - close to what DFT predicts - but also that it opens further a giant (> 1 eV) inverted band gap in Bi-rich samples. Finally, our latest CD- and spin-ARPES data indicate that while m in the PtBixTe2-x series can be significantly tuned, this band structure engineering preserves the topologically non-trivial character of the system. Time allowing, I'll also give a quick update on nano-ARPES experiments on germanene nanoribbon samples, carried out in collaboration with the Bampoulis/Zandvliet groups.

[1] M. Yan et al., Nat. Comm. 8, 257 (2017).

14:55 - Talieh Sadat Ghiasi (TU Delft) - TBA

TBA

15:20 - Marieke Altena (University of Twente) - A low-temperature transport study on the superconductivity in PtBi2

In this talk, we present our work on PtBi₂, an intrinsic superconductor that also hosts a topological Weyl semimetal phase. The Weyl semimetal state gives rise to topologically protected surface states in the form of Fermi arcs. Recent angle-resolved photoemission spectroscopy (ARPES) studies suggest that the superconductivity in PtBi₂ may originate from these surface states, highlighting the material as a promising platform for exploring topological superconductivity. To probe its superconducting properties, we perform low-temperature transport measurements in a dilution fridge using normal metallic contacts on PtBi₂ flakes. These devices are studied in an out-of-plane oriented magnetic field, where they show oscillatory behavior. In addition, we fabricate and study superconducting SQUID devices using Nb and PtBi₂, enabling phase-sensitive measurements of the superconducting order parameter. By analysing the potential phase shifts between different SQUID orientations with respect to the PtBi2 flakes, the goal is to distinguish between different pairing symmetries, such as s-wave and p-wave states.

15:45 - Zhiying Dan (University of Groningen) - Ultra-high vacuum exfoliation method for the preparation of large-area single layer TMDC films

Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising candidates for next-generation electronic, optical, and spintronic devices. While mechanical exfoliation yields high-quality 2D flakes, their lateral size is typically limited to tens of micrometers. Here, we report the preparation of 2D WS2 and WSe2 using a recently developed kinetic in situ single-layer synthesis (KISS) method, performed in ultra-high vacuum and tailored for surface science studies1. We examine how substrate choice and chalcogen species affect film size and quality using X-ray photoelectron spectroscopy, low-energy electron diffraction, atomic force microscopy, and X-ray standing waves (XSW). Our results show that the quality of the bulk TMDC crystal is crucial for successful KISS exfoliation. Moreover, preliminary XSW data suggest that KISS does not degrade the underlying substrate, highlighting its potential as a non-destructive approach for 2D material synthesis2.  

1. Grubisic-Cabo, A. et al. In Situ Exfoliation Method of Large-Area 2D Materials. Adv Sci 10, e2301243 (2023).
2. Dan, Z. et al. Role of chalcogen atoms in in situ exfoliation of large-area 2D semiconducting transition metal dichalcogenides. Frontiers in Nanotechnology 7(2025).

 

16:50 - Auke Vlasblom (Utrecht University) - Moiré Effects in Twisted Topological Insulator Heterostructures

Moiré superlattices can be created by stacking two van der Waals layers with a twist angle, producing a periodic interference pattern. These superlattices can lead to exciting phenomena, such as unconventional superconductivity and topological states. Moiré effects have been studied in twisted graphene and transition metal dichalcogenides, but an alternative system, namely the surface state of 3D topological insulators, has received little attention. What happens to a topological surface state in a Moiré superlattice?  Here, I will present our results on heterostructures of twisted Bi2Se3, produced by mechanically transferring 2D Bi2Se3 nanoplatelets on a 3D Bi2Se3 crystal. Each nanoplatelet has a random twist angle relative to the substrate, generating a large variety in twist angles. We observe energy gaps, as well as van Hove-like singularities. The occurrence of energy gaps of several hundreds of meV offers the prospect of stable quantum phases at room temperature.

17:15 - Pim Lueb (Eindhoven University of Technology) - PbTe Nanowires on CdTe: Growth and Structural Characterization

Lead telluride (PbTe) is a narrow-gap semiconductor with strong spin–orbit coupling and a large dielectric constant, making it a promising platform for topological quantum devices. Selective area growth (SAG) offers a scalable route toward realizing such systems in nanowire geometries. In this work, we explore the growth of PbTe nanowires on CdTe, a nominally lattice-matched substrate with a different crystal structure (rock-salt for PbTe and zincblende for CdTe). We examine how different growth conditions and crystallographic coherence influence nanowire formation and structural properties. Structural characterization is performed using transmission electron microscopy and nano-focused X-ray diffraction (nano-XRD). These results provide a basis for assessing CdTe as a platform for PbTe nanowire growth and for guiding the development of low-disorder nanostructures for future quantum device applications.

17:40 - Mazhar Ali (TU Delft) - TBA

TBA

09:00 - Thijs Roskamp (University of Twente) - Towards nanoscale topographic and magnetic imaging with a wireframe SQUID on a self-sensing cantilever

Superconducting quantum interference devices (SQUIDs) are the most sensitive magnetic flux sensors and are used in scanning SQUID microscopy (SSM) to spatially resolve and map magnetism. Conventional SSM probes make use of planar silicon substrates which limit their spatial resolution to several micrometers due to an increased sample-pickup area spacing. To increase both the probes spatial resolution and magnetic sensitivity the SQUID pickup area must be brought in closer proximity to the surface. By takin inspiration from other scanning probe techniques like atomic force microscopy moving the SQUID to the apex of tip on a scanning probe can significantly reduce the scanning height and thus increase the spatial resolution. We have combined corner lithography with wafer scale molding to create self-aligned superconducting cantilever probes with a shadow-effect deposition of Nb. Using a focused-ion beam we can create SQUIDs at the apex of these wireframes with sizes from sub-100 nm to several micrometers large. In this talk I will show our efforts in further progressing our bottom-up wafer scale fabrication approach. Where we integrate metallic strain gauges on the cantilever to provide an on-chip fully electronic readout of the cantilever deflection, allowing for topographic readout. Furthermore, I’ll show our work for integrating our SQUID on cantilever approach in a conduction-cooled scanning SQUID microscope.

09:25 - Mohamad Farhan Tanzim (University of Amsterdam) - Relativistic GW for magnetic materials: Application to MnTe2 and MnSb2Te4

The $GW$ approximation is widely used to calculate excited-state properties, such as quasiparticle energies and collective excitations of solids. Spin-orbit coupling (SOC) can be incorporated into the implementation of the method in two different ways: it may be treated perturbatively through a second variation step ($GW$+SOC), or included directly in the non-interacting reference system ($G^{mathrm{SOC}}W^{mathrm{SOC}}$). The latter approach is particularly important when SOC qualitatively modifies the electronic structure. For example, in topological insulators of the $mathrm{Bi_2Se_3}$ family, $G^{mathrm{SOC}}W^{mathrm{SOC}}$ has been shown to describe the quasiparticle band structure more reliably than perturbative $GW$+SOC. The $G^{mathrm{SOC}}W^{mathrm{SOC}}$ approach has not been extended to magnetic systems with strong SOC or to non-collinear magnets. The reason for this limitation is that the broken time-reversal symmetry (TRS) in these systems renders the density-density response function, $P(bold{r},bold{r'};omega)$ asymmetric in $omega$ and non-Hermitian for purely imaginary $omega$. This results in the breaking of standard symmetry relations that are routinely used in $GW$ codes. Here, we will discuss the deviation from these standard symmetry relations and present symmetries which are still valid for magnetic systems, and which in turn can be used to efficiently calculate quantities related to the method. Afterwards, we briefly present the implementation of the method using the all-electron full-potential linearized augmented plane wave (FLAPW) formalism and show first results for non-collinear antiferromagnetic $mathrm{MnTe_2}$ and ferromagnetic $mathrm{MnSb_2Te_4}$.

09:50 - Xiaojing Liu (University of Groningen) - Seeing correlation strength in Nb3X8 kagome family by Angle-Resolved Photoemission Spectroscopy

There is growing interest in van der Waals (vdW) materials in which lattice geometry and electron correlations cooperate to generate robust quantum states. In particular, kagome lattices intrinsically host flat electronic bands arising from destructive interference of electron hopping1. When kinetic energy is suppressed, for example in flat bands, electron-electron interactions can dominate, stabilizing correlation-driven phenomena such as high-temperature superconductivity, unconventional magnetism, and Mott insulating states2,3. The Nb3X8 (X = Cl, Br, I) family is a novel class of vdW kagome materials in which Nb trimers form a distorted kagome network. The halogen substitution offers a chemically controllable way to tune bandwidth and interlayer coupling, while preserving the underlying lattice geometry. This makes Nb3X8 an attractive platform for exploring how geometrically derived flat bands evolve under varying correlation strength. Despite significant interest, the electronic ground state across the Nb3X8 family remains unresolved, with proposals ranging from weakly correlated band insulators to strong correlated Mott phases4,5. In this talk, I will present a systematic experimental investigation of the bulk electronic structure of Nb3X8 using angle-resolved photoemission spectroscopy (ARPES). By directly mapping the momentum-resolved band structure, we observe a systematic evolution of correlation-driven spectral features as the halogen is varied. Combining ARPES measurements with first-principles and correlated electronic structure calculations, we uncover clear signatures of interaction variation in Nb3X8. A central result of our study is the identification of a secondary lower Hubbard band, observed experimentally for the first time in this system. Its presence and systematic evolution establish a clear and experimentally accessible spectral fingerprint for quantifying many-body interactions in kagome materials. Together, our results demonstrate that Nb3X8 provides a platform in which controlled substitution enables systematic modulation of electron correlation strength, while lattice geometry ensures robust flat band. By experimentally linking specific band-structure features to correlation effects, this work establishes practical design metrics for engineering correlated kagome materials.

References
[1] Regmi, S. et al. Physical Review B 108, L121404 (2023)
[2] Jiang, K. et al. National Science Review 10 (2022)
[3] Gao, S. et al. Physical Review X, 13, 041049 (2023)
[4]  Aretz, J. et al. Physical Review X, 15, 041042 (2025).
[5] Sun, Z. et al. Nano Letters 22, 4596-4602 (2022).

10:55 - Max van der Schans (Eindhoven University of Technology) - Investigating the chirality of all-optically generated magnetic textures using SEMPA

Taking an all-optical approach to write (and annihilate) magnetic textures proves to be an interesting endeavor for novel forms of computing. The magnetic skyrmion, a whirling magnetic texture consisting of a transition from up- to down-magnetization or vice versa, tends to be at the core of these endeavors.  One the one hand, it is possible to take a classical approach, defining the existence of a skyrmion as a 1-bit and the non-existence as a 0-bit, which directly allows for classical forms of computation. However, it is also possible to work with semi-stochastically generated ensembles of skyrmions, which prove to be useful for applications in neuromorphic computing. Something that is however overlooked at times, due to it being relatively challenging to measure, is the exact structure of the domain walls of these domains, and thus their exact chirality. In relation to skyrmions, the chirality is extra important, as it also determines whether important topological properties (e.g. topological protection and the skyrmion Hall effect) are present. In our work, to tackle this issue, we have combined our laser setup with our high-vacuum system, which directly leads to a Scanning Electron Microscope with Polarization Analysis (SEMPA). SEMPA is a direct measurement of the magnetization with a resolution going down to 8 nanometers, that allows one to resolve the magnetization in all 3 dimensions. With this novel setup, we investigate the chirality of textures generated from different optical responses in our magnetic thin film structures, ranging from deterministically generated domains through All-Optical Switching (AOS) to semi-stochastically generated ensembles of skyrmions. More specifically, we show that the skyrmions we are working with in our systems have a Counter-ClockWise (CCW) Néel domain wall, ensuring their topological properties. We also show the importance of magnetic stack composition, as minor changes can significantly affect the chirality present in a magnetic stack.

11:20 - Kevin Vonk (University of Twente) - Topologically trivial states at the edges of Bi(110) on Ge(111)

In the family of topological materials, bismuth is regarded as one of the prime candidates owing to its large spin-orbit coupling. Recent reports show that the rectangular Bi(110) phase may host topological edge or higher-order topological hinge states. In this talk, we examine the rectangular Bi(110) phase on Ge(111) using scanning tunneling microscopy and spectroscopy. We find Bi(110) to grow in three distinct phases: irregularly shaped bilayer high islands, well-defined rectangular islands and mesa-shaped clusters. The Bi(110) islands grow in two sets of three-fold high-symmetry directions, which are 11.2° rotated with respect to the high-symmetry directions of Ge(111). All three phases exhibit similar electronic behaviour, with a low, non-zero density of states at the Fermi energy and an increased density of states at step edges near -0.3 eV. We find this state to emerge from the Bi wetting layer on the Ge(111) surface rather than the edge itself. Significant downward buckling of the Bi atom in the enter of the rectangular Bi(110) unit cell on Ge(111), yields a topologically trivial, semi-metallic phase. Our experimental observations reveal that care must be taken when investigating the topological nature of Bi(110), as trivial states at the edges might be misinterpreted as topological edge states.

11:45 - Hai Wang (Utrecht University) - Snapshotting the Ultrafast Dynamics of Excitons and Their Complexes in Low-Dimensional Quantum Materials

Layered two-dimensional (2D) materials are emerging quantum materials for next-generation electronics and optoelectronics. Since the discovery of graphene, the 2D materials family has expanded significantly, now including metallic materials (e.g., graphene and its nanostructures), semiconductors (e.g., transition metal dichalcogenides, TMDs), and insulating phases (e.g., hexagonal boron nitride). Due to the reduced charge screening and confinement effects in monolayer semiconductors, photogenerated electron-hole pairs experience strong Coulomb interactions, leading to the formation of bound excitons with binding energies on the order of 100s of meV. These neutral states can further become charged, forming "trions," with binding energies comparable to thermal excitation at room temperature, in the THz frequency range. Understanding the photophysics, such as how excitons or charge carriers are generated, is essential for improving the energy conversion efficiency of devices based on 2D quantum materials and hybrids. In this talk, I will begin by providing an overview of how we can quantify excitons’ energetics and tracking their dynamics in the ultrafast time scale (e.g. from sub-ps to ns) employing THz spectroscopy, in a contact-free manner. I will then discuss our recent research on unveiling giant exciton effects(1) and hot exciton dissociation(2) in graphene nanoribbons (and briefly on how excitons are charged to form trions in WS2 monolayers if time allows(3)).

References:
1. Alexander Tries……Hai Wang, Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons, Nano Lett. 20, 2993–3002 (2020).
2. Guanzhao Wen……Hai Wang, Hot Exciton Dissociation in Graphene Nanoribbons, in revision.
3. Lei Gao……Hai Wang, How Excitons Get Charged: Tracking Trion Formation Dynamics in Monolayer Semiconductors via Ultrafast Photoconductivity Studies, in preparation.