Abstracts - QuMat 2025 Yearly meeting
Paola Gori Giorgi – Accurate and scalable exchange-correlation with deep learning
Talk Monday 2025-10-27 10:20
Density Functional Theory (DFT) is the most widely used electronic structure method for predicting the properties of molecules and materials. Although DFT is, in principle, an exact reformulation of the Schrödinger equation, practical applications rely on approximations to the unknown exchange-correlation (XC) functional. Most existing XC functionals are constructed using a limited set of increasingly complex, hand-crafted features that improve accuracy at the expense of computational efficiency. Yet, no current approximation achieves the accuracy and generality for predictive modeling of laboratory experiments at chemical accuracy — typically defined as errors below 1 kcal/mol. In this talk, I will present Skala, a modern deep learning-based XC functional that bypasses expensive hand-designed features by learning representations directly from data. Skala achieves chemical accuracy for atomization energies of small molecules while retaining the computational efficiency typical of semi-local DFT. This performance is enabled by training on an unprecedented volume of high-accuracy reference data generated using computationally intensive wavefunction-based methods. Notably, Skala systematically improves with additional training data covering diverse chemistry. By incorporating a modest amount of additional high-accuracy data tailored to chemistry beyond atomization energies, Skala achieves accuracy competitive with the best-performing hybrid functionals across general main group chemistry, at the cost of semi-local DFT. As the training dataset continues to expand, Skala is poised to further enhance the predictive power of first-principles simulations.Berkay Kilic – Universal symmetry-enforced persistent spin textures in nonmagnetic crystals
Talk Monday 2025-10-27 11:05
The significance of Mendeleev’s periodic table extends beyond the classification of elements; it lies in its remarkable predictive power for discovering new elements and properties, revealing the underlying symmetrical patterns of nature that were only fully understood with the advent of quantum mechanics. Fundamental material properties, such as electron transport and magnetism, are also governed by crystal symmetry. In particular, spin transport depends on the spin polarization of electronic states, and recently discovered materials where the electron spin polarization is independent of momentum–a property known as persistent spin texture (PST)–promise extended spin lifetime and efficient spin accumulation. In this work, we establish the full classification of the symmetry-protected PST in bulk crystals. By systematically analyzing all noncentrosymmetric crystallographic space groups, similar to elements in the periodic table, we demonstrate that PST is universally present in all nonmagnetic solids lacking inversion symmetry. Using representation theory, we identify the regions within the Brillouin zone that host PST and determine the corresponding directions of spin polarization. Our findings, supported by first-principles calculations of representative materials, open the route for discovering robust spintronic materials based on PST.Lucas Maisel Licerán – Theory of Bose-Einstein condensation of moat-band excitons
Talk Monday 2025-10-27 11:25
We have theoretically investigate Bose-Einstein condensation of excitons in two-dimensional systems where the electron and hole bands exhibit a camelback shape near the Γ point. This band structure arises in certain topological insulators with band inversion, as well as in specific monolayer materials. In these systems, excitons can inherit the camelback feature, leading to a moat-shaped dispersion with a highly degenerate minimum at a nonzero momentum magnitude. These momentum-indirect excitons have inherently long radiative lifetimes, which can be further increased via the use of spatially separated electron-hole bilayers. Under such conditions, a dilute quasiequilibrium gas of interacting excitons can undergo Bose-Einstein condensation into one or more degenerate momentum states. When multiple momentum states are macroscopically occupied, the system transitions into a supersolid—a unique phase of matter that combines superfluidity with broken translational symmetry along one or several directions. To explore this, we model the excitons as interacting bosons with a Mexican-hat dispersion simulating the moat band, and analytically derive conditions under which the effective interaction potential supports supersolid phases. We show that these conditions are met by the T-matrix associated with general potentials approximating realistic interactions between excitons. Solving the Gross-Pitaevskii equation under these conditions, we map out the resulting phase diagram in one and two dimensions, finding in particular stripe and triangular phases. Our findings reveal that moat dispersions can fully eliminate the energy barrier typically required for supersolid formation, allowing such phases to emerge even in weakly interacting regimes. In this talk I will present these results, propose potential candidate systems, and provide experimental parameter estimates highlighting the accessible regions of the phase diagram.William Huddie – Microscopic theory of atomic spin diodes
Talk Monday 2025-10-27 11:45
Ralph Claessen – Surfaces go topological – atomic monolayers as 2D quantum materials
Talk Monday 2025-10-27 13:25
Confining electrons to two dimensions (2D) is known to enhance electronic correlations and promote non-trivial topological phases. Atomic monolayers on semiconductor substrates represent the ultimate 2D limit of such confinement and thus have recently come into focus as third-generation 2D designer quantum materials, following the lead of graphene and monolayer transition metal dichalcogenides. Here I will focus on atomic monolayers as topological insulators hosting 1D metallic and spin-polarized edge states as hallmark of the quantum spin Hall (QSH) effect. My examples range from bismuthene (Bi/SiC(0001)), the 2D-TI with the largest band gap realized to date, to indenene (In/SiC(0001)), a triangular lattice of In atoms with emergent honeycomb physics, to our recent discovery of an antimonene (Sb/SiC(0001)) phase with breathing kagome structure. Using ARPES as well as STM/STS we have studied their electronic structure and especially their topological edge states, revealing interesting insights into their protection (or loss thereof) against single particle backscattering. Time allowing, I will also demonstrate how graphene intercalation can be used to protect these rather delicate monolayer phases against environmental impact.Padraig Maderson – Anomalous Hall Effect in Cr:Mn3Sn
Talk Monday 2025-10-27 14:10
Mn3Sn displays a large anomalous Hall effect (AHE) at room temperature, which can be controlled with a weak external magnetic field. It is therefore an excellent candidate material for spintronic applications. The large AHE is closely linked to the magnetic structure, where the Mn spins in the triangular ab-plane form a 120° structure which breaks inversion symmetry. This leads to the presence of Weyl points near the Fermi level, causing a non zero Berry curvature, which in turns causes the large AHE. Various methods to tune the transport properties of Mn3Sn have been proposed, such as applying external pressure or strain. However, such solutions are impractical for actual applications. It was recently shown that substituting a moderate amount of Mn with Fe drastically changes both the magnetic structure and the AHE. However, other chemical substitutions remain largely unexplored. We therefore focus on single crystal attempting to tune the properties by chemical substitution via Cr doping.Lumen Eek – Electric field-induced spin-valley locking in twisted bilayer buckled honeycomb materials
Talk Monday 2025-10-27 14:30
Twisted honeycomb bilayers form moiré superstructures consisting of hexagonally arranged AB and BA domains separated by domain boundaries. In twisted bilayer graphene, a perpendicular electric field opens inverted band gaps in these domains, giving rise to a triangular network of counterpropagating valley-protected helical states, also known valley-Hall networks.In twisted bilayer silicene and germanene, spin–orbit coupling and structural buckling make this picture richer. We find that within a certain range of electric fields, the spin and valley degrees of freedom become locked in the domain boundary states, leading to enhanced topological protection. Below this range, the system remains topologically trivial, while above it the spin–valley locking is lifted, leaving only valley-protected edge states.
Our results reveal how electric fields can tune the interplay of spin, valley, and topology in moiré bilayers beyond graphene.
Dennis Klaassen – Decorated electronic kagome lattice in twisted bilayer germanene
Talk Monday 2025-10-27 15:25
Kagome lattices have attracted substantial attention because they are the ideal model system to study frustrated magnetism and spin liquids. The occurrence of kagome lattices in nature is, unfortunately, quite rare. Here we show that large-angle twisted bilayer germanene hosts a flat band that exhibits a decorated electronic kagome lattice. The unit cell of this decorated kagome lattice contains four atoms, i.e. one atom more than a conventional kagome unit cell. We show that this additional fourth atom leaves the characteristic kagome flat band intact.Yoran Starmans – Emergent superconductivity in selective-area grown SnTe nanowires
Talk Monday 2025-10-27 15:45
Tin telluride (SnTe) is a topological crystalline insulator predicted to host gapless surface states, making it a promising platform for spintronics and quantum information applications [1,2]. Coupling SnTe nanowires with superconductivity is particularly appealing, as it may enable the realization of topological superconductivity [3]. In this work, we investigate selective-area grown SnTe nanowires on indium phosphide (InP) substrates using low-temperature transport measurements. Remarkably, all devices exhibit signatures of hard-gap superconductivity, systematically characterized under applied magnetic fields. The observations are most consistently explained by the formation of a few-nanometer interfacial layer of In-doped SnTe at the nanowire-substrate interface. The coexistence of superconductivity with the topological surface states of SnTe establishes this hybrid system as a versatile platform for further studies, with ongoing work aimed at assessing its potential for hosting topological superconductivity.[1] Hsieh, T. H., Lin, H., et al. (2012). Nature communications, 3(1), 982.
[2] He, M., Sun, H., et al. (2019). Frontiers of Physics, 14, 1-16.
[3] Cook, A., & Franz, M. (2011).Physical Review B—Condensed Matter and Materials Physics, 84(20), 201105.
Joost Aretz – From strong to weak correlations in breathing-mode kagome van der Waals materials Nb3(F,Cl,Br,I)8
Talk Monday 2025-10-27 16:05
Tunable correlated electron systems are highly desirable to obtain experimentally, for studying both strongly correlated materials and prospective correlation-driven devices. I will present our recent work which demonstrates that the family of van der Waals materials Nb3(F,Cl,Br,I)8 offer such a platform. By using ab initio downfolding we find that alternating hybridization strength between layers is crucial to the physics of this family and enables a description of the low energy physics in terms of Hubbard-dimers. Solving these models using cluster dynamical mean-field theory we explain how correlation effects decrease across the halide series both in the doped and un-doped compounds. The trend is supported by ARPES measurements, which reveal changes in spectral weight consistent with decreasing correlation strength from Nb3Br8 to Nb3I8. The Coulomb-driven magnetic properties lead to the symmetry-breaking effects necessary for the recently observed Josephson diode effect in NbSe2/Nb3Br8/NbSe2 heterostructures.Chiara Cocchi – Quantum oscillations and bulk superconductivity in Weyl semimetal PtBi2
Talk Monday 2025-10-27 16:25
The Type-I Weyl semimetal PtBi2 has recently been in the spotlight as a new platform to study unconventional superconductivity in a topological material.In high magnetic fields, it exhibits a large non-saturating magnetoresistance [1, 2]. Electrical transport measurements have shown the presence of a superconducting phase with critical temperatures between 275 mK (thin flakes) [3] and 400 mK (bulk samples) [4]. Surface superconductivity has also been reported by ARPES [5] and STM measurements [6].
Here, we investigate the Fermi surface of PtBi2 through quantum oscillations in electrical transport and torque magnetometry. Owing to the high resolution of the de Haas-van Alphen oscillation study, we identify new frequencies that could not be previously resolved. The temperature dependence study allows us to perform an analysis of the cyclotron masses at different angles, so to get a complete picture of the material’s Fermi surface.
We also show a thorough study of the superconducting phase at low magnetic fields, including a complete angle dependence which allows us to establish the bulk origin of the superconductivity.
The unprecedented resolution of quantum oscillations and the phase diagram of the superconducting phase help us untangle the interplay between topology and superconductivity in this Weyl semimetal and pave the way to tune its properties.
References:
[1] W. Gao et al., Nat. Comm. 9, 2018
[2] B. Wu et al., Phys. Rev. Reas. 2, 2020
[3] A. Veyrat et al., Nano Lett. 23, 2023
[4] G. Shipunov et al., Phys. Rev. Mat. 4, 2020
[5] A. Kuibarov et al., Nature 626, 2024
[6] S. Schimmel et al., Nature Comm. 15, 2024
Steven Bos – Investigating exciton physics in defected TMD’s using a first-principles tight-binding approach.
Talk Tuesday 2025-10-28 09:00
Krishnaraajan Sundararajan – Tunable magnon contribution to the thermal conductivity in CrPS4 observed via transverse Seebeck Effect in TaIrTe4
Talk Tuesday 2025-10-28 09:20
Understanding and controlling heat flow in low-dimensional magnetic materials is central to thermoelectrics, spin caloritronics, and nanoscale thermal management. Sensitive, spatially resolved thermal probes are therefore essential to detect small, field-tunable changes in thermal transport arising from collective excitations of the magnetic order, namely magnons. We show that TaIrTe 4 , a type-II Weyl semi-metal, serves as a highly sensitive nonlocal thermal probe due to its large transverse Seebeck response. For a temperature gradient applied at 45 o to the principal crystallographic axis the transverse thermopower surpasses 200 µV/K, enabling precise detection of changes in the local thermal gradient. Using this effect in a nonlocal geometry on the magnetic insulator CrPS 4 , we detect a magnetic-field- dependent modulation of the substrate temperature profile, which we ascribe to a field-induced change in the substrate thermal conductivity arising from magnon heat transport. Field sweeps show the magnon-mediated contribution grows with applied field up to the spin-flip transition and then diminishes at higher fields, while out-of-plane fields reveal clear signatures of the spin-flop transition. Quantitatively, the magnon-related modulation amounts to roughly 5% of the total thermal conductivity, with the remainder dominated by phononic transport. Finally, exploiting the transverse Seebeck effect we extract a field-free temperature dependence of the substrate’s thermal conductivity. These results demonstrate the utility of TaIrTe 4 ’s large transverse thermopower as a sensitive nonlocal thermocouple and reveal a measurable, field-tunable contribution of magnetic excitations to heat transport in CrPS 4 . This provides us a senisitive on-chip thermocouple in the family of van der Waals materials that helps detect the contribution of magnons to heat capacity making detection of topological magnons via their heat transport (such as thermal Hall) possible.Biplab Bhattacharyya – Multi-channel second-order topological states in 3D Dirac semimetal Bi0.97Sb0.03
Talk Tuesday 2025-10-28 09:40
Higher-order 1D topological edge states in 3D crystals, protected by crystalline symmetries, form robust hinge-localized channels enabling dissipationless transport and topological quantum computing devices. In this work, we performed comprehensive study of proximity-induced supercurrent in Nb-BiSb-Nb Josephson junctions fabricated on flakes of varying thickness and junction lengths. We find that the critical supercurrent modulates with magnetic field in a SQUID-like pattern, which upon radio-frequency excitation shows missing odd Shapiro steps. Interestingly, we found a strong correlation between fractional Shapiro steps—indicative of a 4π-periodic supercurrent—and the presence of long-ballistic hinge-localized modes.Zeb Osseweijer – Topology in and induced by Fractals
Talk Tuesday 2025-10-28 11:20
Fractals are a new frontier in the research effort on topological materials. In this presentation, I will illustrate the interplay between fractality and topology by going over two recent works. First, I will discuss a two-dimensional topological model, the Haldane model placed on a fractal geometry. Here, I will discuss the spectra and the topological nature of the observed edge states. A much more intricate phase space is obtained, and a in depth analysis revealed the presence of corner states. Understanding these corner states directly leads us to the second work, where the isospectral reduction is employed to show that fractality itself can drive a system into a higher-order topological phase.Semonti Bhattacharyya – Towards Tuning Magnetism and correlation with pressure
Talk Tuesday 2025-10-28 11:40
The interface of stacked 2D materials is an ideal platform for continuous modulation of various quantum and topological phases under external tuning parameters. Although electric and magnetic field has been utilized to realize, control, and harness new quantum phases in such systems, pressure—the key tuning knob that can selectively control the orbital overlap at the van der Waals gap—remains an underutilized tool. In our group, we have developed the capability of pressure-tuning 2D materials stacks. I will first introduce how we customized the pressure cell to make it suitable for measuring van der Waals heterostructures in our cryostat. I will further discuss our investigations of the complex magnetism in a Kagome metal, Yb0.5Co3Ge3, under pressure.[1] Zhiyuan Cheng, Yaojia Wang, Heng Wu, Mazhar N. Ali, Julia Y. Chan, and Semonti Bhattacharyya, arXiv:2410.23872
Bowy La Riviere – Extended Ashkin-Teller transition in two coupled frustrated Haldane chains
Talk Tuesday 2025-10-28 12:00
We investigate transitions out of period-4 ordered phases in the Haldane chain on a zig-zag ladder, with additional next-nearest-neighbor (NNN) couplings, and frustrated by a three-body term. The resulting phase diagram is exceptionally rich, including Ashkin-Teller criticality that persists even at very large nearest neighbor (NN) coupling. Interestingly, we find this transition to separate the period-4 plaquette phase from an exotic topologically-trivial disordered phase. This phase extends up to a Gaussian transition, after which the well-known Haldane phase is retrieved. The intermediate disordered phase vanishes upon decreasing the NN coupling, giving place to a conventional dimerized phase, which is separated from the plaquette phase by an Ising transition. Finally, when the NN coupling is absent, the period-4 phase consists of leg-dimerized states only and at its boundary is a WZW SU(2)₂ × SU(2)₂ critical point that marks the transition to a double Haldane chain.Andrey Bagrov – Superconducting properties of hyperbolic lattices
Talk Tuesday 2025-10-28 12:20
We investigate s-wave superconductivity on hyperbolic lattices, which differ fundamentally from Euclidean systems due to their large boundary-to-bulk ratio that persists even in the thermodynamic limit. Using both Bogoliubov-de Gennes theory for discrete lattices and Ginzburg-Landau theory for the continuous hyperbolic plane, we demonstrate that this geometric property leads to qualitatively new physics.In finite hyperbolic systems with open boundaries, we find enhanced superconducting correlations at the boundary and, strikingly, boundary-only superconducting states that survive above the bulk critical temperature. The boundary geometry plays a crucial role: rough terminations with dangling bonds support zero-energy boundary modes that can raise Tc by several times compared to smooth boundaries.
These results show that hyperbolic geometry provides a new knob for controlling superconductivity through boundary engineering, with potential implications for both fundamental physics and materials design.
Julian Strik – Tuneable spin-orbit coupling in tailored atomic semiconductor quantum dots
Talk Tuesday 2025-10-28 14:00
Julian H. Strik, Hermann Osterhage, Anna M. H. Krieg, Ivan Ado, Mikhail Titov, Daniel Wegner, Alexander A. Khajetoorians Institute for Molecules and Materials, Radboud University, Nijmegen, The Netherlands Quantum simulators are a pathway to study novel physical phenomena which are difficult to theoretically predict or experimentally observe in synthesized materials. Often, these phenomena depend on a specific set of parameters, such as size, symmetry and spin-orbit coupling. Therefore, a quantum simulator platform with a high degree of tunability is desired. In this talk, I will present a highly tuneable quantum simulator based on patterning Cs atoms on the surface of InSb(110) by scanning tunnelling microscopy[1]. Using low-temperature scanning tunnelling microscopy and spectroscopy we pattern quantum dots of various sizes and geometries by sculpting the potential of the underlying 2DEG. I will show that we can tune the spin-orbit coupling in individual artificial atoms by using induced electrostatic fields. Additionally, I will show a magnetic field dependence that can be explained at the hand of the Kane model[2], going beyond the traditional Fock-Darwin description of quantum dots in magnetic field.[1] E. Sierda et al, Science 380, 1048 (2023).
[2] E.O. Kane, Journal of Physics and Chemistry of Solids 1, 249-261 (1957)
Cristina Mier Gonzalez – Nuclear magnetic resonance on a single atom with a local probe
Talk Tuesday 2025-10-28 14:20
Nuclear spins exhibit a high degree of isolation from their environment, resulting in long lifetimes and coherence times. This makes nuclear spins a promising platform for the development of quantum technologies [1]. The combination of electron spin resonance (ESR) with scanning tunneling microscopy (STM) has enabled the indirect measurement of nuclear spins on single atoms via the hyperfine interaction [2]. More recently, single-shot readout on Ti isotopes has demonstrated a nuclear lifetime of several seconds [3]. In this work we controllably address nuclear spin transitions using ESR-STM. We employ electron-nuclear double resonance (ENDOR) to drive and readout the nuclear spin resonances on a 47Ti isotope (𝐼 = 5/2). Our study paves the way for the coherent manipulation of single nuclear spins using STM.References:
[1] Pla, J., Tan, K., Dehollain, J. et al. High-fidelity readout and control of a nuclear spin qubit in silicon. Nature 496, 334–338 (2013).
[2] Willke, P. et al. Hyperfine interaction of individual atoms on a surface. Science 362, 336-339 (2018).
[3] Stolte, E.W., Lee, J., Vennema, H.G. et al. Single-shot readout of the nuclear spin of an on-surface atom. Nat Commun 16, 7785 (2025).
Sebastian Huber – TBA
Talk Tuesday 2025-10-28 14:40
Chrystalla Knekna – Chemical-potential tuning in the type-II Dirac semimetal PtBixTe2−x.
Poster Tuesday 2025-10-28 10:30
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 poster, we introduce 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. We present our latest ARPES data on the series of PtBixTe2-x single crystals and discuss the evolution of the electronic structure. Interestingly, our data – supported by DFT calculations – highlight that Bi doping not only shifts the chemical potential closely to what theory predicts, but also that it further opens a giant (> 1 eV) inverted band gap in Bi-rich samples. Moreover, our latest CD-ARPES data indicate that the band structure in the PtBixTe2-x series can be significantly engineered while preserving a topologically non-trivial band structure.[1] M. Yan et al., Nat. Comm. 8, 257 (2017).
Diego Felipe Munoz Arboleda – Thermodynamics of analogue black holes in a non-Hermitian tight-binding model.
Poster Tuesday 2025-10-28 10:30
Esra van ‘t Westende – The influence of a strain-induced pseudo-magnetic field on topological states in germanene nanobubbles
Poster Tuesday 2025-10-28 10:30
Quantum spin Hall (QSH) insulators are characterized by an insulating bulk and two spin-polarized metallic edge states protected by time-reversal symmetry. The robustness of the edge states to backscattering makes them highly promising for next-generation low-power electronics and spintronic devices. A central open question, however, is how robust are these states to perturbations such as lattice strain. Germanene, a two-dimensional honeycomb Dirac material with strong spin-orbit coupling, provides an ideal platform to study this question [1]. In particular, strain in germanene can generate extremely large pseudomagnetic fields by modulating in-plane electronic hopping. In this study, we investigate the effect of strain-induced pseudomagnetic fields on the topological edge states of germanene nanoribbons, epitaxially grown on Pt(110) on Ge(110) [2] [3]. Scanning tunneling microscopy reveals that repeated annealing of the sample leads to the formation of nanobubbles on the germanene nanoribbons. Low-temperature scanning tunneling spectroscopy is used to probe the electronic states of these nanobubbles. The differential conductivity, dI(V)/dV, exhibits periodic peaks that originate from the interplay of Landau quantization and quantum confinement, and are stable even under very high perpendicular electric fields. Remarkably, our measurements provide signatures that the strain-induced pseudomagnetic field completely destroys the topological edge states, pointing to a breakdown of the QSH phase under extreme lattice deformations. Our findings open new avenues for strain engineering of electronic states in two-dimensional materials and reveal a fundamental limit to the robustness of topological protection in germanene.[1] Bampoulis, P., Castenmiller, C., Klaassen, D. J., van Mil, J., Liu, Y., Liu, C.-C., Yao, Y., Ezawa, M., Rudenko, A. N. & Zandvliet, H. J. W. Quantum Spin Hall States and Topological Phase Transition in Germanene. Phys. Rev. Lett. 130, 196401 (2023).
[2] Klaassen, D. J., Eek, L., Rudenko, A. N., van ’t Westende, E. D., Castenmiller, C., Zhang, Z., de Boeij, P. L., van Houselt, A., Ezawa, M., Zandvliet, H. J. W., Morais Smith, C., Bampoulis, P. Realization of a one-dimensional topological insulator in ultrathin germanene nanoribbons. Nat. Commun. 16, 2059 (2025).
[3] Eek, L., van ’t Westende, E. D., Klaassen, D. J., Zandvliet, H. J. W., Bampoulis, P., & Morais Smith, C. Electric-field control of zero-dimensional topological states in ultranarrow germanene nanoribbons. arXiv:2506.16158 (2025) (accepted in Phys. Rev. Lett.).
Guliuxin Jin – Constant search time algorithm via topological quantum walks
Poster Tuesday 2025-10-28 10:30
It is well-known that quantum algorithms such as Grover’s can provide a quadradic speed-up for unstructured search problems. By adding topological structure to a search problem, we show that it is possible to achieve a constant search-time quantum algorithm with a constant improvement of the search probability over classical search. Specifically, we study the spatial search algorithm implemented by a two-dimensional split-step quantum random walks that realize topologically nontrivial phases and show the asymptotic search behavior is constant with growing system size. Using analytical and numerical calculations, we determine the efficient search regions in the parameter space of the quantum walker. These regions correspond to pairs of trapped states formed near a lattice defect. By studying the spectral properties of the discrete time-evolution-operators, we show that these trapped states have large overlap with the initial state. This correspondence, which is analogous to localization by constructive interference of bound states, makes it possible to reach the best possible search-time asymptotic and produce a disorder-protected fast search in quantum random walks.Jinyu Zhang – Towards Atomically Thin Superconducting Switching Devices Using Monolayer NbSe₂
Poster Tuesday 2025-10-28 10:30
The current computing field is facing significant power consumption issues because CMOS devices always generate heat and consume energy during their logic operations. Superconducting devices are promising candidates for achieving ultra-low-power switching devices. . In contrast, van der Waals materials enable lattice-mismatch-free, atomically thin, and ultra-clean interfaces, as well as compatibility with air-sensitive materials. Theoretically, it is possible to achieve ultrathin, low-power switches in a two-dimensional (2D) superconductor a 2D ferromagnetic insulator stack through magnetic proximity. However, the progress has been limited by unstable contacts and uncertain layer identification. In this project, we aim to create atomically thin superconducting switches with ultrathin NbSe₂ (a 2D superconductor) as the superconducting layer. As a first step, we have successfully prepared hBN/NbSe2 stack using a dry transfer setup under an inert atmosphere and performed preliminary transport measurements.In this poster, we will demonstrate recent progress in developing a glovebox-compatible exfoliation and transfer workflow, an electrode optimization process flow, and a combined Raman and optical method for reliable NbSe2 layer identification, as well as preliminary low-temperature transport measurements of the device.
Karina Hudson – Spin-orbit interaction in lithographically defined 1D quantum wires in planar germanium
Poster Tuesday 2025-10-28 10:30
Quantum point contacts (QPCs) in planar semiconductors have proven a powerful platform for probing spin and many body effects. We use Zeeman spin-splitting transport measurements to probe the g-tensor components of holes in germanium, and are able to link the anisotropy in the g-tensor to spin-orbit Hamiltonian terms arising from strain and cubic crystal effects. We have demonstrated tunable spin-orbit interaction via engineering the strain profile in low-disorder germanium heterostructures. Furthermore, we have recently extended this work from point contacts to demonstrating quantized conductance in lithographically defined quantum wires with spin-orbit interaction in germanium.Kevin Vonk – Height dependent topological properties of Bi(110) on germanium
Poster Tuesday 2025-10-28 10:30
Muhammad Waseem – Band structure investigation of alloyed ternary Mo0.5W0.5Se2 by Angle resolved photo emission spectroscopy and X-ray photoemission electron microscopy
Poster Tuesday 2025-10-28 10:30
Two dimensional transition metal dichalcogenides (TMDCs) MoSe2 and WSe2 have direct band gap of 1.55eV and 1.65eV respectively in a mono-layer limit. Alloying one of these TMDC enables precise control over their bandgap in above mentioned range for application in opto-electronic and electronic devices. Here, we present the electronic band structure and structural investigation of 2D Mo₀.₅W₀.₅Se₂ single crystal alloy flakes exfoliated by Kinetic in-situ single layer synthesis (KISS) exfoliation method in ultra-high vacuum. These measurements were carried out in MAXPEEM end station of the MAX IV Laboratory (Lund, Sweden). Low energy electron diffraction (LEED) was employed to confirm the long range order and hexagonal symmetry in thin flakes. Angle resolved photo emission spectroscopy (ARPES) along with spatially resolved X-ray photo emission electron microscopy (XPEEM) were employed to study the valence band structure along high symmetry direction aiming to notice bandgap modification induced by the Mo/W alloying ratios [2-3]. Intensity-voltage low energy electron microscopy (IV-LEEM) give insight thickness dependent electronic properties.Pim Lueb – TBA
Poster Tuesday 2025-10-28 10:30
Rebecca Gharibaan – Flip-chip devices with 2DEGs
Poster Tuesday 2025-10-28 10:30
Hybrid super-semiconductor 2DEGs in III-V materials are a versatile and flexible platform for studying superconductor-semiconductor devices and offer an alternative way to implement novel qubits [1]. An essential pre-requisite for the realisation of such devices is their integration with high quality microwave resonators for fast control and readout. We utilise the flip-chip architecture and demonstrate our progress towards integrating high quality microwave resonators with InAs/Al hybrid 2DEG based qubits.[1] Pino, D. M., Souto, R. S., & Aguado, R. (2024). Minimal Kitaev-transmon qubit based on double quantum dots. Physical Review. B./Physical Review. B, 109(7). https://doi.org/10.1103/physrevb.109.075101
Shuang Wu – Coherent-phonon and exciton coupling in a layered antiferromagnet CrSBr
Poster Tuesday 2025-10-28 10:30
Coherent-phonon and exciton coupling in a layered antiferromagnet CrSBr Shuang Wu1, *, Pim Witte2, Robin de Hoogh2, Machteld E. Kamminga2, Dmytro Afanasiev1, and Alexey V. Kimel1 1Institute for Molecules and Materials, Radboud University, 6525 AJ Nijmegen, the Netherlands 2Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, 3508 TA Utrecht, The Netherlands Investigating electron-phonon coupling, particularly its coherent form, is essential in condensed matter physics, as it reveals the interaction between the lattice and electronic degrees of freedom in the dynamical regime. Recently, an antiferromagnetic layered semiconductor CrSBr was found to exhibit exciton-magnon coupling [1], opening new opportunities to detect and control magnetism through electronic states. Evidence of strong exciton-phonon coupling was reported previously [2], yet the correlation between excitons and the coherent motion of atoms remains an open question. In this work, we observed coherent modes in CrSBr using optical pump-probe technique. The oscillation frequencies of these modes match the phonon modes identified by polarization-resolved Raman spectroscopy. Clear resonant behavior was observed in the photon energy dependent pump-probe experiments, confirming strong coupling between coherent phonons and excitons. Variable-temperature experiments suggest that this resonance can be affected by the variations in the electronic band structure of CrSBr, with the magnetic interactions potentially playing a role. Our results provide clear evidence of coherent-phonon exciton coupling in CrSBr, and pave the way for the detection of possible short-range magnetic orders. [1] Bae, Y.J., Wang, J., Scheie, A. et al. Exciton-coupled coherent magnons in a 2D semiconductor. Nature 609, 282–286 (2022) [2] Lin, K., Sun, X., Dirnberger, F, et al. Strong Exciton–Phonon Coupling as a Fingerprint of Magnetic Ordering in van der Waals Layered CrSBr. ACS Nano 18, 2898–2905 (2024)Thijs Roskamp – Towards high-resolution scanning SQUID microscopy in a conduction-cooled cryostat
Poster Tuesday 2025-10-28 10:30
Scanning superconducting quantum interference device (SQUID) microscopy (SSM) is a powerful scanning probe technique that enables spatially resolved mapping of local magnetic flux at a surface. It has been used to image diverse phenomena such as ferromagnetism in magnetic materials, Abrikosov vortices in superconductors, and edge currents in topological devices. However, conventional SQUID probes, fabricated on planar silicon substrates using standard wafer processes, are limited to spatial resolutions of several micrometers due to the finite spacing between the pickup loop and the sample surface. Achieving higher spatial resolution and improved magnetic sensitivity requires bringing the SQUID pickup area much closer to the surface. Another challenge in SSM is the reliance on liquid helium to reach the low operating temperatures required for SQUID operation. While liquid helium cooling provides mechanical stability with minimal vibrations, its risen cost in recent years motivates the transition to cryogen-free cooling technologies.To address these limitations, we have developed novel SQUID-on-cantilever probes. By combining conventional silicon processing with corner lithography and focused-ion beam milling, we fabricate SQUID-on-tip devices with dimensions ranging from below 100 nm to several micrometers. In parallel, we have constructed a cryostat based on a commercial cryogen-free cryocooler, designed to host a future high-resolution SSM system. Together, these advances pave the way for cryogen-free, high-resolution SQUID microscopy with nanoscale sensitivity.
Thomas Winkler – Multivalue probabilistic computing with Magnetic Skyrmions
Poster Tuesday 2025-10-28 10:30
Magnetic systems are highly promising for implementing probabilistic computing paradigms because of the fitting energy scales and conspicuous non-linearities. While conventional binary probabilistic computing has been realized, implementing more advantageous multi-value probabilistic computing (MPC) remains a challenge. Here, we report the realization of MPC by leveraging the thermally activated diffusion of magnetic skyrmions through an effectively non-flat energy landscape defined by a discrete number of pinning sites. The time-averaged spatial distribution of the diffusing skyrmions directly realizes a discrete probability distribution, which is tunable by current-generated spin-orbit torques, and can be quantified by non-perturbative electrical measurements. Even a very straightforward implementation with global tuning, already allows us to demonstrate the softmax computation – a core function in artificial intelligence. As a key advance, we demonstrate invertible logic without the need to create a network of probabilistic devices, offering major scalability advantages. Our proof of concept can be generalized to multiple skyrmions and can accommodate multiple locally tunable inputs and outputs using magnetic tunnel junctions, potentially enabling the representation of highly complex distribution functions.Viktoriia Radovskaia – osting the Spin Dynamics in an Antiferromagnet with Competing Anisotropies
Poster Tuesday 2025-10-28 10:30
All-optical control of antiferromagnets (AFM) is highly relevant for next-generation information technologies, including energy-efficient magnetic data storage and processing, antiferromagnetic spintronics, and magnonics. In this study, we present a strategy for engineering a two-dimensional (2D) material that exhibits enhanced coupling between femtosecond laser pulses and antiferromagnetic spins. Specifically, we investigate light-induced ultrafast spin dynamics in the mixed compound Mn0.9Ni0.1PS3. Our findings reveal that the competition between the anisotropies of Mn and Ni ions leads to spontaneous spin reorientation, resulting in nearly an order-of-magnitude enhancement of coherent spin dynamics. Furthermore, we uncover a pronounced probe-wavelength selectivity, indicative of monoclinic symmetry breaking and suggesting a potential coupling to the excitonic states. Together, these results provide direct insight into controlling of ultrafast spin dynamics in mixed MPX3 systems.Yuliia Kreminska – Magnon Transport in Thin CrCl3 Flakes: A First Step Toward Topological Magnon Spintronics in Chromium Trihalides
Poster Tuesday 2025-10-28 10:30
The chromium trihalides (CrX₃, X = Cl, Br, I) are a family of 2D magnetic materials that can be considered a magnetic analogue of graphene. In these systems, the honeycomb arrangement of Cr³⁺ ions gives rise to magnonic excitations that mimic the behavior of Dirac electrons, exhibiting linear band crossings at the K and K′ points of the Brillouin zone. In the heavier halides, CrBr₃ and CrI₃, spin–orbit coupling combined with Dzyaloshinskii–Moriya interactions opens gaps at these Dirac points, producing topologically protected magnon edge modes analogous to the quantum spin Hall effect. Such topological magnons hold promise as robust carriers of spin information, potentially enabling low-dissipation spintronic devices operating at the nanoscale.
While CrCl₃ possesses weaker spin–orbit coupling and negligible DMI (leaving its Dirac magnons topologically trivial) it remains a compelling system for exploration. Its antiferromagnetic in-plane ordering provides a richer magnon spectrum, including acoustic and optical branches, faster THz-range spin dynamics, and hidden order parameters, all of which are experimentally interesting. Moreover, CrCl₃ is far more environmentally stable than its bromide and iodide counterparts, making it an ideal candidate for thin-flake fabrication and device integration. In this sense, CrCl₃ serves both:
1) as a free-standing individual of the CrX₃ family with unique physics and
2) as a toy model for studying the principles underlying topological magnon transport.
The big goal of this project is to establish a foundation for magnon-based spin transport in CrX₃ materials, starting with the most stable CrCl₃. In the first phase, we aim to demonstrate thermally induced magnon transport in thin CrCl₃ flakes. This represents a crucial proof-of-concept, showing that spin information can propagate via the magnonic modes of the system . Building upon this, magnons will be actively injected and detected using the spin Hall and inverse spin Hall effects, enabling controlled studies of spin transport in a non-local device geometry.
At higher energy scales, Dirac magnons at the K points present an exciting frontier; while they are not thermally populated at cryogenic temperatures, they can be accessed using THz resonant pumping or via engineered magnetic gratings that couple low-k magnons to the K-point states. Such approaches set the stage for future studies of topological magnon transport, particularly if CrCl₃ is combined with proximity-induced spin–orbit interactions through heterostructures with heavy-element materials.
Ultimately, CrCl₃ provides a unique combination of stability, rich antiferromagnetic physics, and Dirac magnon modes, making it an ideal platform for both fundamental studies and the long-term development of topological magnonic devices. Its relatively simple magnetic structure allows careful characterization and device fabrication, while its complex magnonic properties including high-frequency spin dynamics and multiple magnon branches, provide avenues for exploring advanced spintronic concepts. By starting with CrCl₃, we aim for future investigations in heavier CrX₃ compounds and engineered heterostructures, with the ultimate aim of realizing low-dissipation, high-speed spin transport devices based on topological magnon modes.