Martin Lee – Quantum twisting microscope reveals electron interactions in graphene at 300 K.

26/05/2025 – 13:30

In the single particle picture, graphene has a famously linear band structure with a dispersion that only depends on momentum k and its Fermi velocity v_F. However, in ultra-clean devices at low temperatures, the single particle picture breaks down. Logarithmic nonlinearities in the dispersion of graphene leading to k-dependence of v_F(k) have been reported in earlier studies using ARPES, STM, capacitance, and Shubnikov-de Haas oscillations, demonstrating the significance of electron-electron interactions . However, it was unclear to which temperature this electron-electron interaction persists, since the signatures from the abovementioned techniques become smeared as the temperature increases. In this talk I will present the observation of nonlinearities in the energy dispersion of monolayer graphene revealed through the quantum twisting microscope (QTM). The QTM is a new form of a scanning probe technique that compliments conventional band structure probes such as ARPES. By performing momentum-energy resolved elastic tunneling spectroscopy, where the momentum resolution arises from in-situ twisting, we can extract the information about the band structure in the technologically relevant window of ~±0.5 eV around the Fermi level.

Pim Lueb – Towards Low Disorder in In-Plane Pb_1-xSn_xTe Nanowire Devices for Topologica Quantum Applications

26/05/2025 – 14:15

Esra van ‘t Westende – Quantized energy levels in strain-induced germanene nanobubbles

26/05/2025 – 14:45

Under normal circumstances, Landau levels emerge for two-dimensional carriers under the influence of a perpendicular magnetic field. However, previous studies have shown that in materials exhibiting a Dirac fermion-like band structure strain can be used to create a pseudo-magnetic field, thereby enabling these energy levels due to in-plane hopping modulation of electrons. [1]

In this study, we show the observation of quantized energy levels on germanene nanobubbles. These nanobubbles are formed at the ends of 2D germanene nanoribbons, which are grown by segregation of Germanium atoms on top of platinum [110] after annealing. We expect the bubbles to have a highly strained crystal lattice structure, which could lead to enormous pseudo-magnetic fields of 100 Tesla or even higher. [2]

Low-temperature scanning tunneling spectroscopy (LT_STS) is used to experimentally measure the electronic states in the nanobubbles. Periodic peaks emerge in the differential conductivity that could be caused by Landau levels, quantum confinement, or a combination of both. The quantized energy levels remain constant for very high electric fields.

Our findings could create new possibilities in the field of strain engineering electronic states, as well as the study of charge carriers in high magnetic fields.

References

[1] N. levy et al, Science, 329 (5991), 1191700 (2010).

[2] D. Kang et al, Nat. Commun. 12 (5087), 10.1038 (2021).

Jara Vliem – The influence of surface state bands on the optical properties of 2D Bi2Se3 nanoplatelets

26/05/2025 – 16:00

Three-dimensional bismuth selenide (Bi2Se3), a layered topological insulator, features an insulating bulk and gapless surface states characterized by spin-momentum locking. Upon reducing the crystal thickness below 7 quintuple layers, these surface states hybridize, resulting in the opening of a gap. The original gapless surface states of 3D Bi2Se3 become an integral part of the 2D band structure by forming regions with a high surface contribution, which affects both electronic and optical properties. The role of these Surface State Bands in the photogeneration, cooling, and recombination of charge carriers was investigated using pump-probe spectroscopy on 2D bismuth selenide nanoplatelets. A significant delay of several picoseconds in carrier recombination is observed when surface state transitions are excited, which is attributed to carrier accumulation in the valleys of the Rashba-shaped surface-state valence band and in higher-lying surface states of the conduction band.

Rebecca Gharibaan – Flip-chip integration of semiconductor 2DEGs with microwave circuits

26/05/2025 – 16:30

Two-dimensional electron gases (2DEGs) in III-V materials offer a versatile and flexible platform to study hybrid superconductor-semiconductor devices. Such 2DEGs have been used to create gate-tunable superconducting qubits and are a potential platform to explore topological systems. In order to study these hybrid devices it is desirable to control them at short timescales. However, in general, these materials are not ideal for microwave frequency experiments, since dielectric losses significantly lower the quality factor of on-chip microwave resonators. Moreover, the fabrication steps required to create high quality microwave circuitry is often incompatible with the processing of hybrid devices in the 2DEGs.

These issues can be resolved by using flip-chip architecture [1], allowing the microwave resonators and the superconductor-semiconductor hybrid devices to have their own separate substrates and fabrication procedures. We will discuss and show the fabrication process for creating flip-chip devices. This includes the electrochemical deposition of indium bumps using electroplating techniques, and integration of hardstops to target inter-chip distances. In order to characterize our devices, we study the quality factors of microwave resonators (fabricated on the bottom chip) in proximity to different materials on the top chip.

[1] M. Hinderling, D. Sabonis, S. Paredes, D. Haxell, M. Coraiola, S. Kate, E. Cheah, F. Krizek, R. Schott, W. Wegscheider, F. Nichele, Phys. Rev. Appl., 19, 054026. (2023).

Max van der Schans – The fully photonic formation of topological textures

26/05/2025 – 17:00

A skyrmion, a chiral spin texture with non-trivial topology, not only offers great potential for applications in non-conventional computing – from a skyrmion racetrack, to neuromorphic computing via stochastic skyrmion ensembles – but also provides great insights in the fundamentals of topological lattices and textures. There are numerous approaches to writing and annihilating skyrmions, but doing so all-optically is an interesting endeavour because of its potential for higher energy efficiency, easy control, and versatility. To fully profit from its potential, a deterministic method for writing and annihilating individual skyrmions is required.

As to their optical generation, previous research has succeeded in stochastically nucleating ensembles of skyrmions under the influence of an external magnetic field with ultrashort (femtosecond) laser pulses. Moreover, recently, similar ensembles were generated in field-free conditions. However, a fully deterministic all-optical toggle switch of a single skyrmion remains an elusive goal.

In this work, we present an approach using the well-established concept of All-Optical Switching (AOS) of the magnetization. We successfully demonstrate the field-free AOS of single isolated sub-micron skyrmions on a Co/Gd based magnetic thin film heterostructure. On top of this, we investigate the threshold fluence (energy per area) for various processes as a function of the thickness of the ferromagnetic material in our heterostructure. This gives rise to a rich state diagram filled with competing effects that determine whether the switching is stochastic or deterministic.

It is discovered that, as one increases the amount of ferromagnetic material, increasingly smaller skyrmions can be written. However, at the same time, the fluence regime in which this is possible decreases, eventually ridding the system of true toggle switching as stochastic effects take over. We finally answer these challenges via the addition of a heat sink to the magnetic stack, which, in accordance with research from M. Verges et al., shows that longer timescale stochastic effects are suppressed, increasing the regime in which deterministic toggle switching is possible.

Guangze Chen – Topological Zero Modes and Correlation Pumping in an Engineered Kondo Lattice

26/05/2025 – 17:30

Topological phases of matter provide a flexible platform to engineer unconventional quantum excitations in quantum materials. Beyond single particle topological matter, in systems with strong quantum many-body correlations, many-body effects can be the driving force for non-trivial topology. Here [1], we propose a one-dimensional engineered Kondo lattice where the emergence of topological excitations is driven by collective many-body Kondo physics. We first show the existence of topological zero modes in this system by solving the interacting model with tensor networks, and demonstrate their robustness against disorder. To unveil the origin of the topological zero modes, we analyze the associated periodic Anderson model showing that it can be mapped to a topological non-Hermitian model, enabling rationalizing the origin of the topological zero modes. We finally show that the topological invariant of the many-body Kondo lattice can be computed with a correlation matrix pumping method directly with the exact quantum many-body wavefunction. Our results provide a strategy to engineer topological Kondo insulators, highlighting quantum magnetism as a driving force in engineering topological matter.

[1] Z. Lippo, E. L. Pereira, J. L. Lado and G. Chen, Phys. Rev. Letters 134 (11), 116605 (2025)

Dominik Juraschek – Emerging functionalities from phonon angular momentum

27/05/2025 – 09:30

Chiral phononics is an emerging field, in which the angular momentum of circularly polarized lattice vibrations is utilized to control the properties of materials. When chiral phonons are driven coherently with an ultrashort laser pulse, the light makes the ions in the material behave like electromagnetic coils, producing circular ionic currents around their equilibrium positions in the crystal. This induces real and effective magnetic fields that can reach the tesla scale, providing an unprecedented means for the manipulation of magnetic order. Here, I present recent theoretical predictions and measurements of novel phenomena arising from chiral phonon excitation. These predictions highlight phonon angular momentum and chirality as fundamental degrees of freedom that enable new functional properties in solids.

Krishnaraajan Sundararajan – Two dimensional Van Der Waals material WTe2 as an alternative magnons spin injector/detector electrode

27/05/2025 – 10:15

One of the bottlenecks in the development of all two-dimensional material-based magnonic devices is the absence of a two-dimensional material for the excitation and detection of magnon spin currents. This issue becomes more crucial when attempting to study the transport properties of topological magnons, as many proposed topological magnon insulators (like CrI3) are air-unstable and conventional device fabrication techniques are not easily adaptable to these systems. We observe WTe2, a layered, non-magnetic van der Waals material, to function as a two-dimensional spin injector and detector for magnon spins polarized both in-plane and out-of-plane. Using an insulating two-dimensional antiferromagnet, CrPS4 as the magnon transport media, we employ a hybrid non-local device geometry where magnon spins are injected and detected via platinum and WTe2 contacts. We find the efficiency of WTe2 to be around 50 percent and 150 percent for injecting in-plane and out-of-plane polarized spins in comparison to the in-plane spin injection of the standard in the field, platinum. These results pave the way for a fully two-dimensional magnon transport device and enable the study of magnon transport aided by a non-invasive platform for arbitrarily magnetized van der Waals magnets.

Lumen Eek – Realization of a one-dimensional topological insulator in ultrathin germanene nanoribbons

27/05/2025 – 11:00

Realizing a one-dimensional (1D) topological insulator and identifying the lower-dimensional limit of two-dimensional (2D) behavior are crucial steps toward developing high-density quantum state networks, advancing topological quantum computing, and exploring dimensionality effects in topological materials. Although 2D topological insulators have been experimentally realized, their lower dimensional limit and 1D counterparts remain elusive. Here, we fabricated and characterized arrays of zigzag-terminated germanene nanoribbons, a 2D topological insulator with a large topological bulk gap. The electronic properties of these nanoribbons strongly depend on their width, with topological edge states persisting down to a critical width (∼2 nm), defining the limit of 2D topology. Below this threshold, contrary to the tenfold way classification, we observe zero-dimensional (0D) states localized at the ends of the ultrathin nanoribbons. These end states, topologically protected by time-reversal and mirror symmetries, indicate the realization of a 1D topological insulator with strong spin-orbit coupling.

Yoran Starmans – Electrical detection of current-induced spin polarization in PbSnTe nanowires

27/05/2025 – 11:30

SnTe is a topological crystalline insulator predicted to host gapless surface states with a helical spin-texture. These exotic states make SnTe a compelling candidate for spintronic and quantum information applications. Despite this promise, unambiguous detection of these states via electrical transport measurements has remained challenging. In this work, we investigate electrical transport in devices based on single-crystalline SnTe nanowires grown via selective area growth. Using ferromagnetic tunnel contacts, we probe for current-induced spin polarization, an essential signature of spin-momentum locking associated with topological surface states. Our measurements reveal a robust spin polarization signal with the expected polarity for topological surface states. Intriguingly, we also observe a substantial current-independent contribution, underscoring the complexity of interpreting spin signals in such systems. Comparative studies with topologically trivial Pb_1-xSn_xTe nanowires will be essential to clarify these effects and to further establish SnTe nanowires as a promising material platform for spintronic and quantum information technologies.

Marcus Bäcklund – Abelian Spectral Topology of Multifold Exceptional Points

27/05/2025 – 13:00

Despite their ubiquity, multifold exceptional points, n-fold non-Hermitian eigenvalue degeneracies (EPns), where just recently topologically classified [1]. Still, the fundamental origin of this topological remains an open question. In this talk, I aim to provide this fundamental origin by explaining how, and why, the underlying vector bundle construction is essential when connecting Hamiltonians to topology [2]. Albeit comprising a mathematically technical subject, I aim to provide more of an overview, explain and motivate why abstract classification schemes are important, and in such a way try to connect to experimentally relevant setups in a more direct way.

[1] T. Yoshida, J.L.K. König, L. Rødland, E.J. Bergholtz, and M. Stålhammar (Bäcklund), Winding Topology of Multifold Exceptional Points, Phys. Rev. Research 7, L012021 (2025).
[2] M. Stålhammar (Bäcklund), L. Rødland, Abelian Spectral Topology of Multifold Exceptional Points, arXiv.2412.15323.

Mark Golden – mm-sized monolayers of 2D materials and ARPES of chiral Dirac fermions

27/05/2025 – 13:30

• The first is using gold assisted (a.k.a. template stripped gold) exfoliation to generate mm-sized monolayer samples in ultrahigh vacuum, looking to enable – for example – ARPES of thin 2D quantum materials without needing to win nanoARPES beamtime.
• The second came out of discussions with Alexander and Chuan at UT and is on the chiral material CoSi which has parallel spin-momentum locking and potential for quantum battery applications. The fun then really started when we found a pencil-sized single crystal labelled “CoSi, grown 12/04/96” in a drawer in Amsterdam….