#### Talks

###### Andrei Kirilyuk – Dynamic pattern formation in phonon-mediated magnetic switching

Strong light-matter interaction constitutes the basis of all photonic applications, empowering material elements to create and mediate interactions of light with light. Among others, phonon-amplified interactions in magnetic crystals were shown to bring a specific twist into this, in the infrared frequency range. Thus, phono-magnetic effects are the low-frequency analogues of inverse Faraday and Cotton-Mouton effects [1,2] where phonons, not electrons, mediate the interaction between light and spins. In this case, light couples to the spins indirectly by exciting coherent vibrations of the crystal lattice (phonons) that transfer angular momentum to the magnetic ions [3,4].

Following these ideas, we have recently demonstrated that an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching [5]. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only. The mechanism appears to be very universal, and is shown to work in samples with very different crystallographic symmetry and magnetic properties, including weak ferromagnets and antiferromagnets [6], but also completely different systems such as ferroelectrics [7].

The dynamics of the magnetic domains in the reversal process show very intriguing patterns, that reveal very strong nonlinearities in the magneto-elastic interactions. Novel phenomena appear ranging from the condensation of magnon-polaron waves resulting in a self-organization of periodic domain patterns [8], and to the appearance of rapidly-propagating stable magneto-elastic solitons.

References:

- A.V. Kimel, A. Kirilyuk, P.A. Usachev, R.V. Pisarev, A.M. Balbashov, and Th. Rasing, Nature
**435**, 655 (2005). - A.M. Kalashnikova, A.V. Kimel, R.V. Pisarev, V.N. Gridnev, A. Kirilyuk, and Th. Rasing, Phys. Rev. Lett.
**99**, 167205 (2007). - T. F. Nova, A. Cartella, A. Cantaluppi, M. Först, D. Bossini, R.V. Mikhaylovskiy, A.V. Kimel, R. Merlin, A. Cavalleri, Nature Phys.
**13**, 132 (2017). - D. M. Juraschek, M. Fechner, A. V. Balatsky, N. A. Spaldin, Phys. Rev. Mater.
**1**, 014401 (2017). - A. Stupakiewicz, C.S. Davies, K. Szerenos, D. Afanasiev, K.S. Rabinovich, A.V. Boris, A. Caviglia, A.V. Kimel & A. Kirilyuk, Nature Phys.
**17**, 489 (2021). - P. Stremoukhov, C.S. Davies A. Safin, S.A. Nikitov, and A. Kirilyuk, New J. Physics
**24**, 023009 (2022). - M. Kwaaitaal, D.G. Lourens, C.S. Davies, and A. Kirilyuk, arXiv:2305.11714 (2023).
- M. Gidding, T. Janssen, C.S. Davies, and A. Kirilyuk, Nature Commun.
**14**, 2208 (2023).

###### Andres Botello – Defects on hBN for quantum emitters: A DFT and BSE study

Hexagonal boron nitride (hBN) has emerged as a promising platform in the pursuit of enhancing quantum emitters, due to its remarkable optical properties and quantum states. It is a wide gap semiconductor, mechanically and chemically stable, and exhibits single photon emission at ambient conditions[1,2]. However, the detailed nature of the defects involved in the photoemission process is hitherto not fully understood. In this study, we delve into a Density Functional Theory-based characterization of defects in hBN using a comprehensive approach to solve the Bethe-Salpeter Equation (BSE) within the framework of a model potential. We investigate a range of defects, including nitrogen and boron vacancies, the BN bi-vacancy, and vacancy-substitutional complexes (V$_N$ N$_X$) where $X$=B, C, O. The groundstate wavefunctions of the fully relaxed systems (positions and cell)[3] are used to construct a $p_z$ Hamiltonian from their projection onto Wannier fucntions[4]. These localized single particle wavefucntions are used to construct a BSE hamiltonian using a model potential for the interaction kernel[5], allowing us to explore optical excitations. Our integrated approach provides a deeper understanding of the impact of defects on the quantum emission properties of hBN. The results not only contribute to the fundamental understanding of defect-induced quantum states but also hold significant implications for the design and optimization of hBN-based quantum devices.

1. G. Grosso, H. Moon, B. Lienhard, S. Ali, D. K. Efetov, M. M. Furchi, P. Jarillo-Herrero, M. J. Ford, I. Aharonovich, and D. Englund, “Tunable and high-purity room temperature single-photon emission from atomic defects in hexagonal boron nitride,” Nature Communications, **8**, 705, (2017).

2. A. Sajid, M. J. Ford, and J. R. Reimers, “Single-photon emitters in hexagonal boron nitride: a review of progress,” Reports on Progress in Physics, **83**, 044501, (2020).

3. P. Giannozzi, et al., “QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials,” Journal of Physics: Condensed Matter, ** 21**, 395502, (2009).

4. G. Pizzi, et al., “Wannier90 as a community code: new features and applications,” Journal of Physics: Condensed Matter, **32**, 165902, (2020).

5. A. C. Dias, J. F. R. V. Silveira, and F. Qu, “WanTiBEXOS: A Wannier based Tight Binding code for electronic band structure, excitonic and optoelectronic properties of solids,” Computer Physics Communications, ,**285**, 108636, (2023).

###### Andrey Bagrov – Strong enhancement of superconductivity in fractal lattices

Using the Sierpinski gasket (triangle) and carpet (square) lattices as examples, we theoretically study properties of fractal superconductors. For that, we focus on the phenomenon of $s$-wave superconductivity in the Hubbard model with attractive on-site potential and employ the Bogoliubov-de Gennes approach and the theory of superfluid stiffness. For the case of the Sierpinski gasket, we demonstrate that fractal geometry of the underlying crystalline lattice can be strongly beneficial for superconductivity, not only leading to a considerable increase of the critical temperature $T_c$ as compared to the regular triangular lattice but also supporting macroscopic phase coherence of the Cooper pairs. In contrast, the Sierpinski carpet geometry does not lead to pronounced effects we find no substantial difference We conjecture that the qualitative difference between these cases are caused by different ramification properties of the fractals.

###### Daniel Muis – Optical Energy Localization at a Valley Photonic Crystal Waveguide Termination

We demonstrate energy localization at suitably designed terminations of a topological non-trivial photonic crystal interface. Such an interface is formed by two mirror inverted valley photonic crystals (VPCs) with distinct topology and is terminated by a topologically trivial photonic crystal that effectively acts as a mirror. Using phase- and polarization resolved scanning near-field microscopy we experimentally confirm the existence of a no-surface-mode band gap where the edge state cannot scatter into trivial surface modes or the edge modes of the VPCs. In the absence of other loss channels, the dominant scattering mechanism for light incident on the termination must be inter-valley scattering. However, a topologically non-trivial edge state along the VPC is robust against inter-valley scattering due to symmetry protection. We observe a broadband energy localization at such a symmetry protected waveguide termination, facilitated by the low inter-valley scattering rate.

###### Dennis Klaassen – Moiré-modulated band gap and van Hove singularities in twisted bilayer germanene

Twisting bilayers of two-dimensional topological insulators, such as germanene, has the potential to create unique quantum states of matter. Here, we successfully synthesized a twisted bilayer of germanene with a 21.8o degrees twist angle, corresponding to a commensurate (√7×√7) structure. Using scanning tunneling microscopy and spectroscopy, we unraveled the structural and electronic properties of this configuration, revealing a moiré-modulated band gap. This band gap opens at AB/BA regions and closes at AA regions, a phenomenon attributed to the electric field induced by the scanning tunneling microscopy tip. Our study further revealed two van Hove singularities at -0.8 eV and +1.04 eV, resulting in a Fermi velocity of (8±1)×10^5 m/s. These observations illuminate a unique quantum state, where the topological properties could be regulated through an electric field, potentially triggering two topological phase transitions.

###### Duncan Haldane – Geometry and fluid dynamics of the Quantum Hall effect.

A reinterpretion (arXiv:2302.14991) of the “Hall viscosity” of incompressible quantum Hall fluids reveals that their fundamental fluid property is an intrinsic (primitive, not “traceless”) electric quadrupole density that couples to gradients of the electric field. In the integer quantum Hall fluids, this is the second moment of the charge distribution of the Landau orbit relative to its guiding center, but in the fractional case it is an additional emergent dynamical quadrupole density associated with the charge distribution in the correlation hole or “composite boson” that accompanies “flux attachment”. This may be considered to be the analog of the “goldstone boson” of flux attachment, albeit one that is gapped, as no symmetry is broken. A number of new results such as a relation between the edge dipole density and bulk quadrupole density emerge; the local bound charge that accompanies spatial variation of the quadrupole density is also an important feature.

###### Karina Hudson – Planar Ge: a new host for topological superconductivity

Highly robust topologically-protected quantum states may be realised in hybrid solid state devices where proximitising a conventional superconductor to a semiconducting quantum wire induces topological superconductivity. Historically this technology has been pursued in III-V semiconducting quantum wires that host spin-orbit interaction. However signatures of topologically protected states such as bound Majorana zero modes are often indistinguishable from those of disorder even in relatively clean devices. For the field to move forward, ultra-clean and reproducible semiconductor-superconductor hybrid platforms with carefully engineered spin-orbit properties are essential.

The development of ultra-low disorder germanium quantum wells that can host very high mobility hole transport with strong spin-orbit interaction has opened a new avenue of investigation, with the added benefit of CMOS-compatibility. Very recently, a hard superconducting gap was demonstrated in germanium by proximitising platinum to a germanium quantum well. I will give an overview of what has been achieved so far in planar germanium and the materials challenges that remain to reliably engineer and observe topologically-protected quantum states.

###### Lucas Maisel Licerán – Topology of Bi_{2}Se_{3} nanosheets

Recently, a group of the QuMat consortium directly observed the quantum spin-Hall edge channels of two-dimensional colloidal nanocrystals of the topological insulator Bi₂Se₃. Motivated by this development, we reconsider the four-band effective model usually employed to describe thin nanosheets of this material. Derived from a three-dimensional **k · p** model, it physically describes the top and bottom surface states that become gapped due to the material’s small thickness. However, we find that the four-band model for the surface states, as derived directly from the three-dimensional theory, is inadequate for the description of thin films of a few quintuple layers and even yields an incorrect topological invariant within a significant range of thicknesses. To address this limitation, we propose an eight-band model that also incorporates the effects of the bulk bands closest to the Fermi level. We find that the eight-band model not only captures most of the experimental observations, but also agrees with previous first-principles calculations of the Z₂ invariant. Moreover, we demonstrate that the topological properties of thin Bi₂Se₃ nanosheets emerge as a result of the intricate interplay between the surface and bulk states, which in fact results in nontrivial Chern numbers for the latter.

###### MengXIng Na – Non-thermal electron dynamics and electron-phonon coupling in graphite

Pump-probe spectroscopies have extended many well-established equilibrium techniques into the time domain. Among them, time-resolved ARPES is especially exciting, as it provides direct access to the electronic structure and many-body interactions on an ultrafast timescale. In the past, the applicability of TR-ARPES has been curtailed by the limited photon energy, energy resolution, and repetition rate of available laser sources, which generally confine experiments to the low-momentum, high-fluence regime. However, the continued maturity of high-harmonic sources now enables detailed, low-fluence TR-ARPES studies over the full Brillouin zone of quantum materials.

We apply such a source to the study of electron-phonon coupling on graphite. While many experimental techniques are sensitive to electron-phonon coupling, the measured quantity is usually averaged over electron degrees of freedom or bosonic degrees of freedom. Using a high energy resolution, high repetition rate cavity-based high-harmonic source[1], we observe quantized energy-loss processes that correspond to the emission of strongly coupled optical phonons, which allow for the quantitative extraction of the mode-projected electron-phonon matrix element, for specific initial and final electron states[2].

The features that we observe come from the non-thermal occupation of electrons. In high-fluence experiments, electron dynamics are often described by the evolution of the electronic temperature, which masks the underlying microscopic scattering processes that are rich with information. Using the study of graphite as a benchmark, we use numerical simulations to elucidate non-thermal regimes at high and flow fluences, and further highlight the need for detailed perturbative experiments to exploit non-equilibrium electron properties in materials[3].

[1] Rev. Sci. Instrum. 90, 083001 (2019)

[2] Science 366, 1231 (2019)

[3] PRB **102**, 184307 (2020)

###### Riccardo Reho – Identification of high-energy excitations in two-dimensional topological Bi_{2}Se_{3} nanoplatelets

**R. Reho$^{1,2}$**, **P. M. M. C. de Melo$^{1,2}$**, **A. R. Botello-Méndez$^{1,2}**, **D. Vanmaekelbergh$^{1}$**, **Z. Zanolli$^{1,2}**

^1 Chemistry Department, Debye Institute for Nanomaterials Science, Condensed Matter and Interfaces, Utrecht University, PO Box 80.000, 3508 TA Utrecht, The Netherlands

^2 European Theoretical Spectroscopy Facility (ETSF)

Layered topological materials, with Bi$_₂$Se$_₃$ leading the way, have experienced renewed interest over the past decade due to their potential optoelectronic applications.

In its three-dimensional configuration, Bi$_₂$Se$_₃$ is a robust topological insulator [^1^][^2^]; characterized by a fundamental inverted gap that hosts a helical Dirac-cone surface state with spin-momentum locking. Two-photon photoemission spectroscopy and photo-luminescence excitation spectroscopy showed the importance of high-energy (IR – visible) optical excitations involving surface states several eV below and above the Fermi level [^3^]. While extensive research has focused on its 3D configuration, a comprehensive analysis of optical properties of two-dimensional Bi$_₂$Se$_₃$ is currently lacking. In the few-layers 2D case, the system becomes gapped [^4^]. However, the nature of this gap is currently debated. Moreover, the group of D. Vanmaekelbergh has measured absorption for six quintuple layers (6QL) Bi$_₂$Se$_₃$ nanoplatelets finding several unexpected transitions [^5^]. To explain the nature of the gap and identify the high-energy excitations,

we exploit first principles density functional theory (DFT) and many-body [^6^][^7^] techniques to compute the band structure and absorption of a 2D slab composed of 6QL Bi$_₂$Se$_₃$. Our approach allows us to calculate surface and k-resolved optical transitions, essential for interpreting absorption and quenching experiments. In the energy-range between 1 and 3.5 eV, we identify 10 important transitions that collectively shape the absorptance spectrum. These transitions can be categorized into three distinct groups within the two-dimensional Brillouin zone; the dominant group encompasses surface-state transitions. Remarkably, our results elucidate the persistent and strong absorption quenching at 3.1 eV, attributing it to the persistent hole accumulation at the top of the valence bands that is separated, in momentum space, from the electronic states in the conduction bands. The fine details of the band structure also explain the photoluminescence at 2.3 eV and 1.5 eV, previously reported [^3^].

In conclusion, our study reveals that few-layers Bi$_₂$Se$_₃$ not only qualifies as a topological insulator [^8^] but also exhibits intriguing high-energy surface state transitions. These transitions result in photogenerated carriers that exhibit prolonged lifetimes away from the lowest-energy fundamental state, holding promise for innovative designs in quantum optoelectronics.

[**^1^**]: Zhang, H., et al. (2009). Topological insulators in Bi$_₂$Se$_₃$, Bi$_₂$Te$_₃$ and Sb$_₂$Te$_₃$ with a single Dirac cone on the surface. *Nature Physics*, 5(6), 438-442.

[**^2^**]: Aguilera, I., et al. (2019). Many-body corrected tight-binding Hamiltonians for an accurate quasiparticle description of topological insulators of the Bi$_₂$Se$_₃$ family. *Physical Review B*, 100(15), 155147.

[**^3^**]: Kung, H.-H., et al. (2019). Observation of chiral surface excitons in a topological insulator Bi$_₂$Se$_₃$. *Proceedings of the National Academy of Sciences*, 116(10), 4006-4011.

[**^4^**]: Zhang, Y., et al. (2010). Crossover of the three-dimensional topological insulator Bi$_₂$Se$_₃$ to the two-dimensional limit. *Nature Physics*, 6(8), 584-588.

[**^5^**]: Vliem, J., Cayan, S. A., Reho, R., Botello-M ́endez, A. R., Melo, P., Geiregat, P., Zanolli, Z. & Vanmaekelbergh, D. Identification of high energy excitations in two-dimensional topological Bi$_2$Se$_3$ nanoplatelets, in preparation

(2023)

[**^6^**]: Giannozzi, P., & Cavazzoni, C. (2009). Large-scale computing with Quantum ESPRESSO. *Il Nuovo Cimento C*, 32(2), 49-52.

[**^7^**]: Sangalli, D., et al. (2019). Many-body perturbation theory calculations using the yambo code. *Journal of Physics: Condensed Matter*, 31(32), 325902.

[**^8^**]: Moes, J., Vliem, J., Melo, P., Wigmans, T., Botello-M ́endez, A., Mendes, R., van Brenk, E. F., Swart, I., Licer ́an, L., Stoof, H., Delerue, C., Zanolli, Z. & Vanmaekelbergh, D. Colloidal Bi2Se3 platelets with topological edgestates, under review, NanoLetters (2023).

###### Srijit Goswami – Majorana bound states in artificial Kitaev chains

In the past decade superconductor-semiconductor hybrids have been studied intensively, with significant efforts towards studying Majorana zero modes (MZMs). While there have been several claims of the observation of MZMs, a convincing demonstration of these states is still missing. In this talk I will discuss a relatively new approach to realize MZMs using quantum dot-superconductor hybrids. I will focus on how MZMs can be systematically and reliably engineered in a two-dimensional electron gas by tuning the relative strengths of the different kinds of interactions between the quantum dots.

###### Thijs Roskamp – Scanning SQUID microscopy: A tool to study the smallest and weakest magnets

Scanning superconducting-quantum-interference-device (SQUID) microscopy (SSM) uses SQUIDs, the most sensitive magnetic flux sensors, to spatially map the local magnetic flux from a surface, making it a powerful scanning probe technique. The ability to directly measure magnetic flux sets out SSM as opposed to other magnetic scanning probe microscopies such as magnetic force microscopy and diamond nitrogen-vacancy magnetometry.

Scanning SQUID microscopy at the University of Twente has a rich history of scientific achievements, such as its use in investigating the occurrence of ferromagnetism in thin film oxides [1], studying the occurrence of fractional vortices in pi-loop arrays based on the d-wave symmetry of high-Tc superconductors [2] and more recently to studying the effects of oxygen scavenging layers on the suppression of ferromagnetism in LaMnO3 [3]. The current instrument makes use of a SQUID chip extended with a micron-sized pickup loop which is placed in contact with the sample under an angle of 15 degrees. By using ex-situ driving motors the sample can be scanned over a millimeter-large surface area at 4.2 K. The current capabilities of the system will be discussed to give an overview of possible measurements that can be performed and examples of previous work will be shown.

Furthermore, state-of-the-art scanning SQUID techniques such as SQUID-on-tip will be discussed to give an idea of what future improvements of the instrument will be. In addition to getting a clear overview of how scanning SQUID microscopy at the University of Twente will progress over the coming years.

[1] Wang et al. Imaging and control of ferromagnetism in LaMnO3/SrTiO3 heterostructures. Science **349**, 716-719 (2015).

[2] Hilgenkamp et al. Ordering and manipulation of the magnetic moments in large-scale superconducting π-loop arrays. Nature **422**, 50-53 (2003).

[3] Folkers et al. Imaging the suppression of ferromagnetism in LaMnO3 by oxygen scavenging layers. In preperation (2023).

###### Thomas Blank – 2D THz Spectroscopy of Nonlinear Phononics in the Toplogical Insulator MnBi_{2}Te_{4}

Although the lattice dynamics of real crystals are rather complex and imply multiple mutual correlations between movements of individual atoms, the modern theory of condensed matter has successfully managed to describe the dynamics in terms of linear superpositions of mutually independent phononic modes. However, if the amplitude of the lattice oscillations is large, this approximation fails and the lattice dynamics enter a poorly explored regime of nonlinear phononics where new channels of energy transfer open up between otherwise non-interacting modes.

The family of topological insulators originating from the parent compound Bi2Te3 (or Bi2Se3) forms an excellent playground to explore nonlinear phononics experimentally. Because of the heavy bismuth and tellurium ions, these materials feature several phononic modes in the terahertz spectral range, which could enable nonlinear terahertz light-lattice interactions. The topological antiferromagnet MnBi2Te4 is, in this respect, an especially interesting compound in the family, given the evidence of strong magneto-phononic coupling and theoretically predicted possibilities to control their topological and magnetic properties using the principles of nonlinear phononics [1,2]. Previously, it was demonstrated that Raman-active phonons in Bi2Se3, which by symmetry cannot be excited by the electric field of light directly, may still be driven by terahertz electric field via a nonlinear mechanism [3]. Theoretically, three nonlinear excitation pathways could exist: the purely phononic, purely photonic, and the recently suggested [4] combined photo-phononic pathway. In principle, the responsible pathway can be resolved using the state-of-the art technique of 2D THz pump-probe spectroscopy, which allows us to isolate the nonlinear part from a pump and probe signal and analyze the nonlinear interaction pathways in detail.

We applied this technique to MnBi2Te4 where, similar to Bi2Se3, a Raman-active Eg phonon could be excited with by THz light. Using the outcome of 2D spectroscopy, we show that the excitation of this phonon actually proceeds predominantly via the combined photo-phononic route [5]. In particular, a first photon excites the “hidden” infrared-active Eu phonon, lifting inversion-symmetry and thereby opens up the excitation of the Raman phonon by a second photon. The discovery of this nonlinear photo-phononic mechanism in the topological antiferromagnet MnBi2Te4 in the terahertz spectral range, where the lattice is tightly connected to topological and magnetic properties, opens up a plethora of opportunities for the field of nonlinear phononics and the lattice engineering of topological matter, enabling new routes for the ultrafast manipulation of complex quantum phases.

[1] H. Padmanabhan et al., Nat. Commun. 13, 1929 (2022).

[2] M. Rodriguez-Vega et al., J. Phys. Chem. Lett. 13, 4152 (2022).

[3] A. A. Melnikov et al., Phys. Rev. B 97, 214304 (2018).

[4] G. Khalsa, N. A. Benedek, and J. Moses, Phys. Rev. X 11, 021067 (2021).

[5] T.G.H. Blank et al., Phys. Rev. Lett. 131, 026902 (2023).

###### Thomas Metzger – THz magnetism of antiferromagnetic CoF_{2} and NiO/Pt in the non-collinear phase

The booming fields of antiferromagnetic spintronics and THz magnonics urge to understand the ultrafast dynamics triggered in antiferromagnets by ultrashort (sub-100 ps) stimuli. The interest in the ultrafast magnetism of antiferromagnets has led to new and vastly counter-intuitive findings in experimental and theoretical research [1-2]. However, the majority of the studies was dedicated to antiferromagnets in their ground state. Here, we report on ultrafast dynamics in the poorly explored, non-collinear phase of NiO and CoF_{2} antiferromagnets, in which the spins of the sublattices are canted by high magnetic fields up to 7 T.

We show that tuning the spin canting by an external magnetic field, one can create conditions for the impulsive magnon-phonon Fermi resonance between otherwise non-interacting modes in CoF_{2}. In particular, if the frequencies of the magnon f_{m }and the phonon mode f_{ph} obey the condition of the Fermi resonance [3] f_{ph}=2f_{m} selective pumping of the magnon results in an excitation of the phonon via a nonlinear mechanism [4]. Measuring THz-induced rotation of the probe polarization in magnetic fields up to 7 T, we reveal these two distinct modes in the time domain data. By supporting our findings with numerical calculations following the sigma-model, we reveal that a large amplitude of lattice vibrations is not necessarily a requirement for initiating the nonlinear excitation of phononic modes and thus nourishes new ways for coherent control of magnon-phonon energy exchange [5].

Inspired by the recent discoveries of ultrafast spin currents in NiO/Pt heterostructures [6-7], we performed THz emission spectroscopy experiments on ultrathin (<50 nm) heterostructures in high magnetic fields. Our measurements as a function of temperature, sample rotation θ, pump polarization α, magnetic field μH_{ext} and polarization of the emitted THz waves reveal that the magnetic field initiates a new and unprecedented source of THz emission and spin current in this material [8].

*[1] P. Němec, M. Fiebig, T. Kampfrath et al., Nature Phys 14, 229–241 (2018)*

*[2] A. Kimel, B. Ivanov, R. Pisarev et al., Nature Phys 5, 727–731 (2009).*

*[3] E. Fermi, Zeitschrift fuer Physik 71(3-4), 250–259 (1931)*

*[4] E. Mashkovich et al., Science 374,1608-1611 (2021)*

*[5] T.W.J. Metzger, K.A. Grishunin, C. Reinhoffer, R.M. Dubrovin, A. Arshad, I. Ilyakov, T.A.G. de Oliveira, A. Ponomaryov, JD. Deinert, S. Kovalev, R.V. Pisarev, M.I. Katsnelson, B.A. Ivanov, P.H.M. van Loosdrecht, A.V. Kimel, and E.A. Mashkovich https://arxiv.org/abs/2308.01052 (2023)*

*[6] E. Rongione, O. Gueckstock, M. Mattern et al., Nature Communications, 14, 1818 (2023)*

*[7] H. Qiu, L. Zhou, C. Zhang et al., Nature Physics, 17, 388–394 (2021) *

*[8] T.W.J. Metzger, P. Fischer, T. Kikkawa, E. Saitoh, A.V. Kimel, and D. Bossini (in preparation)*

###### Viktoriia Radovskaia – Light-Driven Control of Spin-Wave Damping in an Antiferromagnet

Employing antiferromagnetic magnons instead of their conventional ferromagnetic counterparts in future magnonics devices promises THz rates of data processing and novel relativistic physics, but crucially lacks efficient sources of the THz spin-wave excitation [1]. Femtosecond pulses of light, the shortest and thus the most broadband stimuli in experimental condensed matter physics, have recently become a game changer in the field of antiferromagnetic magnonics. It has been shown that the light pulses can be interconverted into broadband propagating wavepackets of coherent magnons with a continuous bandwidth of more than 0.2 THz [2].

Here we investigate the many-body interaction of the light-driven magnons in prototypical antiferromagnet DyFeO3. Using femtosecond pulses of light, we excite the magnon wavepacket and show that increase in the population of the light-driven magnons leads to a dramatic suppression of the lifetime of the uniform spin precession, corresponding to the zone-center magnon. We demonstrate that the magnon lifetime can be easily reduced to more than two orders of magnitude lower than one in the equilibrium state. We argue that the stimulated processes of magnon-phonon scattering govern the renormalization of the magnon’s lifetime. Our findings not only emphasize the importance of scattering processes in redistributing energy and momentum in wavepackets of antiferromagnetic magnons, but also present an opportunity for efficient control of spin-wave damping.

[1] A. Barman, G. Gubbiotti, S. Ladak, A. O. Adeyeye, M. Krawczyk, J. Grafe, C. Adelmann, S. Cotofana, A. Naeemi, V. I. Vasyuchka, et al., “The 2021 magnonics roadmap”, Journal of Physics: Condensed Matter 33, 413001 (2021).

[2] J. Hortensius, D. Afanasiev, M. Matthiesen, R. Leenders, R. Citro, A. Kimel, R. Mikhaylovskiy, B. Ivanov, and A. Caviglia, “Coherent spin-wave transport in an antiferromagnet”, Nature physics 17, 1001–1006 (2021).

###### Xiaoli Peng – Ion-gating established metallic contact for achieving high mobility in transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDs) have attracted significant attention due to their rich physics and potential for next-generation nano electronic devices. However, high impurity, low carrier mobility, and high electrical contact resistance have impeded their potential in real applications. To address these issues, we present a new configuration for establishing metallic contact in h-BN-encapsulated TMDs field-effect transistors (FETs) through ionic gating and a revert dry transfer method. By utilizing the variation of ionic gating as a function of the thickness of the Helmholtz layer, metallic contact is achieved by ion-gating directly on the exposed part of the TMDs where the metal contact located and meanwhile the high-mobility conduct channel is achieved by ion-gating through the top ultrathin h-BN layer. A high Hall mobility of 11166 cm2V-1s-1achieved at MoS2-based FETs at 2 K. Prominent SdH oscillations have been observed and investigated in all measured high-quality TMDCs devices. For the established metallic contact through ion-gating, the contact even can show a sign of superconducting transition in IDS-T curve. The high-frequency Shubnikov-de Haas (SdH) quantum oscillations arise from the directly exposed contact area is consistent high doping and formation of metallic contacts.

###### Yoichi Ando – Superconducting proximity effects in topological insulators

n this talk, I will present our recent experiments on the superconducting proximity effect in topological insulators. The first part concerns the ferromagnetic topological insulator in the quantum anomalous Hall (QAH) regime, which is called a QAH insulator. We contacted the chiral edge state of a QAH insulator with a Nb finger electrode and measured the nonlocal voltage downstream of the grounded Nb finger. We found that when the finger width is less than 500 nm, the nonlocal downstream resistance is negative, which points to the occurrence of crossed Andreev reflection (CAR) across the Nb finger. Since the CAR occurs for a much longer length scale than the superconducting coherence length of Nb (about 30 nm), it is most likely that the surface of the QAH insulator is proximitized and mediates the CAR process. Theoretically, the proximitized QAH insulator surface can become a 2D topological superconductor hosting chiral Majorana fermions, and hence this result is an important step towards the generation and detection of chiral Majorana fermions (arXiv:2307.08578).

The second part concerns the conductance spectroscopy experiments on proximitized topological insulator nanowires (TINWs). We were able to measure both the local and nonlocal difference conductance of a TINW proximitized by a grounded Nb electrode, and we observed signatures of extended Andreev bound states and crossed Andreev reflections. Most importantly, we found that the local and nonlocal conductances oscillate with the axial magnetic flux with a periodicity of h/e. This means that the proximity-induced superconducting state bears the signature of the topological subband structure of a TINW, which is a prerequisite to realize topological superconductivity in TINWs.

This work was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 741121) and was also funded by the German Research Foundation (DFG) under CRC 1238 – 277146847 as well as under Germany’s Excellence Strategy – Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) EXC 2004/1 – 390534769.

###### Yuriy Mokrousov – Orbital Magnetism out of Equilibrium

#### Posters

###### Arnold Kole – First-principles study of Yu-Shiba-Rusinov states in superconductors

In recent years there has been an increased interest in materials with topological properties. These materials are characterised by a topological phase, which is separated from the normal phase by a discontinuous transformation. Topological phases and their related properties are therefore protected from disorder. Topological superconductors are a subclass of topological materials where a superconducting material exhibits a topological phase. They might exhibit exciting new forms of physics such as edge states that behave like Majorana fermions [1]. One route to topological superconductivity could be by creating hybrid structures that combine s-wave superconductivity with spin-orbit coupling and magnetism [1]. Examples are structures where chains of magnetic atoms (Fe, Mn) are put on top of superconducting subtrates (Nb, Re). Experiments suggest that the presence of the magnetic atoms induces Yu-Shiba-Rusinov (YSR) states in the superconducting gap which might be used to engineer Majorana Zero Modes (MZM) [2,3]. Attempts have been made to explain these experiments using theoretical models, but a treatment that combines superconductivity with the full complexity of the electronic state of these systems is still missing. We have recently implemented an approach to describe superconductivity through the Bogoliubov-deGennes formalism in the Density Functional Theory code SIESTA [4-6]. We validate our implementation against known results from experiments and KKR calculations by studying the YSR states of magnetic adatoms on top of superconducting Nb [7,8]. This will allow us to predict new materials and their superconducting properties.

newline

newline [1] Sato et al., Reports on Progress in Physics~textbf{80}: 076501 (2017)

newline [2] Schneider et al., Nat. Com.~textbf{11}: 4707 (2020).

newline [3] Schneider et al., Nature Nanotechnology~textbf{17}: 384 (2022)

newline [4] Soler et al., Journal of Physics: Condensed Matter~textbf{14}: 2745–2779 (2002)

newline [5] García et al., J. Chem. Phys.~textbf{152}: 204108 (2020)

newline [6] Reho et al., textit{in preparation} (2023)

newline [7] Nyári et al., Phys. Rev. B~textbf{104}: 235426 (2021)

newline [8] Schneider et al., Nature Physics~textbf{17}: 943–48 (2021)

###### Arwin Kool – The effect of uniaxial strain on the ultra-quantum limit of narrow gap semiconductors

At high enough magnetic field, all the charge carriers can be confined to the lowest Landau level, which is called the quantum limit. There, the kinetic energy of the charge carriers is quenched in the direction transverse to the applied magnetic field which can give rise to various electronic transitions, such as the magnetic-freeze-out metal-insulator transition (MIT) in narrow gap semi-conductors [1]. We study the effect of uniaxial strain on the MIT in low doped, narrow-gap semiconductors InAs and InSb in high magnetic fields up to 35 T. We find that the behaviour of the MIT under strain is qualitatively similar in both materials, whereas there are large quantitative differences in the ultra quantum limit. In particular, in low doped InSb, we get a sign change of the carrier density. We explain this behaviour in the context of an anomalous Hall effect due to the skew scattering of the spin-polarized charge carrier on ionized impurities.

###### Auke Vlasblom – On the Edge States of Finite 2D Bi2Se3 Crystals

Topological insulators in two or three dimensions exhibit an insulating bulk, but possess gapless conducting edge or surface states that are protected by time-reversal symmetry. One property of the surface state is spin momentum locking, which results in two types of surface currents, each with opposite spin (up or down) and opposite momentum (direction). In the 2D limit, the edge currents are fully protected from backscattering, as this would require a forbidden spin flip.

Here, we study thin (3 – 6 quintuple layers) finite sized Bi2Se3 crystals, prepared via wet-chemical synthesis, using scanning tunnelling microscopy and spectroscopy. Measurements are performed on Bi2Se3 nanoplatelets with varying thickness to study the transition between 2D and 3D topological insulators. Additionally, the robustness of the edge states is investigated with respect to magnetic field and deposition of magnetic impurities. These measurements provide new insights into the topological nature of 2D topological insulators.

###### Bowy La Riviere – Critical properties of a quantum loop model on a zig-zag ladder

Motivated by recent discovery of the non-magnetic Ising transition, at which the energy gap of magnetic excitations remains open, we search for other types of non-magnetic phase transitions that can be realized in quantum spin chains. In this work we focus on a new chiral transition recently reported in the context Rydberg atoms between the period-four and disorder phase. To explore whether chiral transitions can also be realized in quantum spin chains, we look at the quantum loop model, i.e. an effective model of spin-1 ladder with a constrained Hilbert space limited to the singlet sector only. We use extensive density-matrix renormalization group simulations to show the presence of chiral perturbations and to unveil how this affect the nature of the quantum phase transitions between the plaquette (period-four) and the next-nearest-neighbor Haldane (disordered) phases.

###### Caroline Bauer – THz electro-magneto optical effect in Holmium Iron Garnet (Ho3Fe5O12)

THz electro-magneto-optical effect in Holmium Iron Garnet

C. Bauer, T. G. H. Blank, A. V. Kimel

Until now, the field of ultrafast magnetism has mainly focused on spin dynamics triggered in magnetic materials using ultrashort stimuli. However, it is reasonable to expect that the dynamics in rare-earth ions must be more complex due to their strong spin-orbit coupling. For a long time, ferrimagnetic iron garnets offered a rich playground in ultrafast magnetism. Hence, ultrafast dynamics triggered by ultrashort stimuli in rare-earth iron garnets is a natural and exciting next research subject to explore.

Here we study the ultrafast dynamics triggered by a nearly single-cycle THz pulse in a slab of single-crystal Ho3Fe5O12 with a thickness of 470μm. Performing time-resolved measurements of the Faraday effect with the pump-probe technique revealed the excitation of two modes, which correlate with f-f transitions of the Ho3+ ion at 0.64THz and 0.88THz [1]. Additionally, we discovered a drastic increase in the amplitude of the THz-induced dynamics with decreasing temperature. Above 40K, the detected signals are relatively small, while its amplitude starts to increase and eventually becomes nearly ten times larger below 40 K, reaching 0.5° rotation at the lowest temperature of 6 K.

Performing the measurements as a function of external magnetic field, THz polarization, and probe polarization, we conclude that the observations must be interpreted in terms of the THz electro-magneto-optical effect. Above 40 K, the effect is symmetry forbidden in all iron garnets. However, according to earlier reports on static neutron diffraction, several members of the rare-earth iron garnets, amongst which Ho3Fe5O12, develop a so-called ‘double umbrella structure’ [2-4] below a typical temperature. This structure is a non-colinear phase in which the spins of the rare earth ion on two magnetically inequivalent sites are canting in two different angles, and the unit cell distorts from a cubic to a rhombohedral symmetry. We argue that the huge THz electro-magneto-optical effect discovered here can be interpreted as a signature of the ‘double umbrella and can be employed as a probe of ultrafast spin-orbital dynamics in future studies.

[1] Sievers III, A. J., and M. Tinkham. “Far infrared spectra of rare-earth iron garnets.” Physical Review 129.5 (1963): 1995.

[2] Guillot, M., et al. “Temperature evolution of the umbrella structure in holmium iron garnet.” Zeitschrift für Physik B Condensed Matter 56.1 (1984): 29-39.

[3] Tcheou, F., E. F. Bertaut, and H. Fuess. “II—Neutron diffraction study of some rare earth iron garnets RIG (R= Dy, Er, Yb, Tm) at low temperatures.” Solid State Communications 8.21 (1970): 1751-1758.

[4] Hock, Rainer, et al. “Crystallographic distortion and magnetic structure of terbium iron garnet at low temperatures.” Journal of solid state chemistry 84.1 (1990): 39-51.

###### Chao Chen Ye – First-principles studies of quantum oscillations in bulk ZrTe5

First-principles studies of quantum oscillations in bulk ZrTe5

Topological insulators have been the subject of intensive study over the last decades. Zirconium

pentatelluride (ZrTe5) stands out as a prominent material for experimental investigations into

topological phase transitions. Although this material has been studied for many years, the results of

various experiments have varied due to its inherent complexity and sensitivity to the details of the

crystal structure. On the experimental front, a common practice involves applying a strong magnetic

field to observe quantum oscillatory effects in order to determine the Fermi surface. In this work,

we employ density functional theory (DFT) to computationally simulate ZrTe5, and provide the

numerical values of the oscillation frequencies.

###### Davide Pizzirani – Thickness-dependent electronic properties of the Dirac nodal line semimetal ZrSiSe

Nodal line semi-metals (NLSMs) are materials in which the Dirac band crossing takes place along a one-dimensional line or loop in momentum space, where the properties of charge carriers with a linear dispersion relation and topological correlated matter can be investigated. Among them, ZrSiSe and ZrSiS possess linearly dispersing Dirac bands over ~2 eV energy range, as well as hybridized surface-bulk states [1,2], making them perfect candidates for the development of macroscopically ordered states [3,4,5].

We present a thickness-dependent magneto-transport study on exfoliated high-quality thin flakes of ZrSiSe with thicknesses ranging from 28 to 112 nm. Changes in the magnetoresistance (MR) as well as in the onset, amplitude and frequency of the quantum oscillations (QOs) are observed as the flakes become thinner.

Notably, we find that the MR in thin samples strongly deviates from the quadratic MR of compensated semimetals, and the Fermi surface is altered as seen from the decrease of the area of the hole-pocket.

With the preparation of these high-quality thin flakes, we pave the way to investigate the carrier’s response to strong electric fields (gate tunability) and to ultimately search for ordered states in Dirac NLSMs.

[1] Hu et al., Phys. Rev. Lett. 117, 016602 (2016).

[2] S. Pezzini et al., Nat. Phys. 14, 178 (2018).

[3] J. Liu and L. Balents, Phys. Rev. B 95, 075426 (2017).

[4] B. Roy, Phys. Rev. B 96, 041113(R) (2017).

[5] M. M. Scherer, Phys. Rev. B 98, 241112(R) (2018).

###### Dinar Khusyainov – Observation of stochastic domain networks during cumulative all-optical helicity-dependent switching in ferromagnetic films

All-optical helicity-dependent switching (AO-HDS) of magnetization is a fascinating phenomenon with a high potential to revolutionize data storage technologies. The discovery of AO-HDS in ferromagnetic thin films of Co/Pt has shown that although a single optical pulse might be not sufficient to switch the magnetization, the switching still can be achieved by a successive action of multiple pulses1–3. This cumulative switching has been explained in terms of a two-step process, where thermally assisted micrometer domains grow out of laser-induced multi-domain states by thermally induced domain-wall motion2. However, due to the limited optical resolution, such spatially inhomogeneous multi-domain states’ direct visualization has remained elusive. Here we study AO-HDS by combining femtosecond lasers excitation with nanoscale imaging using a magnetic force microscope. This allows us to directly probe light-induced changes of the magnetization with a spatial resolution down to 30 nm. Our measurements on a Pt(3 nm) /Co(0.6 nm)/ Pt(3 nm) trilayer reveal that the first few optical pulses trigger the emergence of a magnetic domain networks with characteristic size being ~1 μm. Using a cross-correlation analysis we unambiguously show that the formation of the domain network is partly stochastic, with different domain networks forming under the same excitation conditions. Only after about 10 pulses, switching kinetics lead to a more deterministic switched domain pattern, which acquires a stable shape determined by the laser profile. We thus show that stochastic domain networks play an important role in the cumulative AO-HDS switching of Pt/Co/Pt thin films.

References

- Lambert, C.-H. et al. All-optical control of ferromagnetic thin films and nanostructures. Science (1979)
**345**, 1337–1340 (2014). - Medapalli, R. et al. Multiscale dynamics of helicity-dependent all-optical magnetization reversal in ferromagnetic Co/Pt multilayers. Physical Review B
**96**, 224421 (2017). - Yamada, K. T. et al. Efficient All-Optical Helicity Dependent Switching of Spins in a Pt/Co/Pt Film by a Dual-Pulse Excitation. Frontiers in Nanotechnology
**4**, (2022).

.

###### Ekaterina Kochetkova – Structural variations of the magnetic topological insulators Mn1+xSb2-2x/3Te4

The recently discovered semiconductors with the general formula Mn_{1+x}Sb_{2-2x/3}Te_{4} (MST), where x varies from –0.1 to 0.15, are promising candidates for topological ferromagnetic insulators with a high *T*_{C} ranging from 27 to 45 K [1–3].

The MST system is very compositionally flexible because the ionic radii of Mn^{2+} (0.83 Å) and Sb^{3+} (0.90 Å) are quite similar, which helps to create strong intermixing up to several tens % between the manganese and antimony atoms in the crystal structures of the compounds. Thus, it is possible to vary both the content of manganese in the composition of the compound and in the crystallographic positions, which leads to a change in the physical properties.

In our experiments x varies in a broad range from −0.1 to 1.7. Up to x ≈ 0.8, the compounds crystallize in the GeAs_{2}Te_{4} structure type (sp. gr. *R**m*1) with a layered structure, in which the septuple layers are stacked in a trigonal lattice and connected by weak van der Waals interactions. At x larger than 0.8, the van der Waals gap is gradually filled, and the lattice subsequently transforms in a more close-packed structure with an *F*-centered cubic symmetry. As we can see from the single-crystal X-ray diffraction data, the crystals with the formula Mn_{1.87(6)}Sb_{1.31(5)}Te_{4} (sp. gr. *R**m*1, *a* = 4.1955(1) Å, *c* = 41.5366(2) Å) have a partially occupied van der Waals gap by 17% of antimony atoms. The crystals with the maximum value of x have the formula Mn_{2.72}Sb_{1.28}Te_{4. }and an *F*-centered cubic lattice (sp. gr. *Fd**m*, *a* = 11.9147(6) Å) with all cationic sites occupied. The crystal structure of the cubic phase is derived from the cubic spinel structure type AB_{2}X_{4}, where the B site (16d) is fully occupied by the Mn atoms, the X site (32e) is occupied by Te, and the position with intermixing by antimony and manganese atoms (16c) has a lower symmetry than the A site (8a) in the spinel structure type. The difference is that in the spinel crystal structure, the A cation is tetrahedrally coordinated, while in our structure this site is octahedrally coordinated by the Te atoms.

In this series of MST compounds, we observe that increasing x leads to the changes in the magnetic properties of compounds from ferri- to ferromagnetic and *T*_{C} varies from 32 to 73 K.

References

[1] Y. Liu et al. *Phys. Rev.* X **11**, 021033 (2021)

[2] S. Wimmer et al. *Adv. Mater*., **33**, 2102935 (2021)

[3] L. Folkers et al. *Z. Krist.* DOI: https://doi.org/10.1515/zkri-2021-2057 (2021)

###### Fabio Salvati – Stability of a quantum skyrmion: projective measurements and the quantum Zeno effect

Magnetic skyrmions are vortex-like quasiparticles characterized by long lifetime and remarkable topological properties.

That makes them a promising candidate for the role of information carriers in magnetic information storage and processing devices.

Although considerable progress has been made in studying skyrmions in classical systems, little is known about the quantum case: quantum skyrmions cannot be directly observed by probing the local magnetization of the system, and the notion of topological protection is elusive in the quantum realm.

Here, we explore the potential robustness of quantum skyrmions in comparison to their classical counterparts.

We theoretically analyze the dynamics of a quantum skyrmion subject to local projective measurements and demonstrate that the properties of the skyrmionic quantum state change very little upon external perturbations.

We further show that by performing repetitive measurements on a quantum skyrmion, it can be completely stabilized through an analog of the quantum Zeno effect.

###### Falk Pabst – Band structure engineering in topological semimetals Pt(Bi_{x}Te_{1−x})_{2} (0 ≤ x ≤ 2/3)

F. Pabst,^{[a]} T. Menshchikova,^{[b]} I. Rusinov,^{[b]} A. Isaeva^{[a, c]}

The platinum group ditellurides constitute a family of topological semimetals that have extensively been studied for the realization of type-II Dirac fermions,^{[1,2]} or type-I superconductivity as in the case of PdTe_{2}.^{[3]} In those compounds, the contribution of the topological Dirac band crossing to e.g., magneto-transport properties, is often limited by its unfavourable position above or below the Fermi level.

Here we report on the growth, crystal structure and band-structure calculations of Pt(Bi_{x}Te_{1−x})_{2}

(0 ≤ *x* ≤ 2/3). Trigonal prismatic, mm-sized crystals have been grown by flux synthesis under inert gas conditions. Adjusting the synthetic conditions gives excellent control over a wide compositional range of *x*. Our investigation of the crystal structure by X-ray and electron diffraction (SCXRD and TEM) revealed an almost isostructural lattice to PtTe_{2} in *P*3*m* (*a* = 4.0533(3) Å, *c* = 5.4040(5) Å for Pt(Bi_{5/8}Te_{3/8})_{2}) with the mixed Bi/Te occupancy lifting the inversion symmetry. Pt(Bi_{x}Te_{1−x})_{2} is structurally located between the two parent compounds PtTe_{2} and *γ*-PtBi_{2}. The former is a Dirac semimetal with CdI_{2}-structure type,^{[2]} the latter a candidate for topological superconductivity with a slightly distorted variant of the same structure type.^{[4]} In PtTe_{2}, the bulk Dirac cone and the surface state Dirac cone are buried below the Fermi level (about −1 and −2.5 eV, respectively).^{[2]} However, our *ab initio* DFT calculations of Pt(Bi_{x}Te_{1−x})_{2} predict a shift of the Fermi level towards these crossings with increasing Bi-substitution. This will allow to experimentally verify the tuning of the band structure and potentially investigate the exotic states close to *E*_{F} in the van-der-Waals material Pt(Bi_{x}Te_{1−x})_{2}. Magnetotransport and angle-resolved photoemission experiments are underway.

^{[a]} University of Amsterdam, 1098XH Amsterdam, The Netherlands, E-Mail: f.pabst@uva.nl

^{[b]} Tomsk State University, 634050 Tomsk, Russia

^{[c]} Leibniz Institute for Solid State and Materials Research Dresden, 01069 Dresden, Germany

##### References

[1] C. Xu, B. Li, W. Jiao, W. Zhou, B. Qian, R. Sankar, N. D. Zhigadlo, Y. Qi, D. Qian, F.-C. Chou, X. Xu, *Chemistry of Materials* **2018**, *30*, 4823.

[2] M. Yan, H. Huang, K. Zhang, E. Wang, W. Yao, K. Deng, G. Wan, H. Zhang, M. Arita, H. Yang, Z. Sun, H. Yao, Y. Wu, S. Fan, W. Duan, S. Zhou, *Nat. Commun.* **2017**, *8*, 257.

[3] H. Leng, C. Paulsen, Y. K. Huang, A. de Visser, *Phys. Rev. B* **2017**, *96*, 220506.

[4] G. Shipunov, I. Kovalchuk, B. R. Piening, V. Labracherie, A. Veyrat, D. Wolf, A. Lubk, S. Subakti, R. Giraud, J. Dufouleur, S. Shokri, F. Caglieris, C. Hess, D. V. Efremov, B. Büchner, S. Aswartham, *Phys. Rev**.** Mater**.* **2020**, *4*, 124202.

###### Feike van Veen – Measuring a Hybridization Gap in Ultrathin Topological Insulator (Bi_{1-x}Sb_{x})_{2}Te_{3}

The measurement of quantized conductance in HgTe/(Hg,Cd)Te quantum wells [1] ignited the search for more candidates showing the quantum spin Hall effect (QSHE). One of the material candidates for realizing this effect are 3D topological insulators (TIs). The conducting surfaces of ultrathin TIs hybridize such that a gap is opened at the Dirac point. The size of the gap is dependent on the film-thickness in an oscillatory fashion, where a negative (non-trivial) gap indicates the presence of QSH states around the perimeter of the measured device [2]. We deposited ultrathin (Bi_{0.28}Sb_{0.72})_{2}Te_{3} films using molecular beam epitaxy (MBE) and measured a positive (trivial) hybridization gap in transport measurements. Also, we are able to close the gap by applying a magnetic field perpendicular to the device. These measurements provide solid ground for developing thin films in the topologically non-trivial regime, bringing us one step forward towards measuring the QSHE.

[1] König, M. et al., Science, 318, 5851 (2007)

[2] Liu, C.-X. et al., PRB, 81, 041307 (2010)

###### Hrvoje Vrcan – With great driving comes great instability

Understanding magnetism at the shortest length and time scales inevitably leads to the

appearance of quantum effects. Due to the complexity of the quantum many-body problem,

no exact solution is available, and numerical approaches are challenging as well. Even for

one of the simplest models, the antiferromagnetic Heisenberg model on a square lattice,

no numerically exact solutions are available. Recently, new variational approaches inspired

by machine learning have emerged, which go beyond some of the limitations of existing

methods. These Neural Quantum States (NQS) ansätze have proven to be a powerful tool to

accurately represent the many-body wave function for a wide class of physical systems [1].

Extensive research has been done to explore linear dynamics of collective magnetic excitation

such as magnons using NQS [2]. However, the nonlinear regime of excitations at the very

edge of the antiferromagnetic Brillouin zone has not been explored at all.

The dynamics of NQS models are governed by the time-dependent variational principle

(TDVP) equation of motion. However, TDVP is known to suffer from numerical instabilities

for non-linear driving, or otherwise induced dynamical complexity. While the precise origin

of instabilities is not well understood, they are related to the overdetermination of the parameter

basis. So far, these instabilities have been addressed with regularization, but this proved

to be useful only in the linear response regime [3].

In order to identify the cause of numerical instabilities, we study the Heisenberg antiferromagnet

on a 2 × 2 square lattice, using the Restricted Boltzmann Machine (RBM) neural

network ansatz. Dynamics are induced by a perturbation of the exchange interaction along

the vertical bonds. We investigate the numerical accuracy and possible breakdown due to

instabilities, as a function of the perturbation strength. The advantage of this simple system

is the availability of the exact diagonalization (ED), which we use as a physical benchmark.

The dynamical properties of this system are monitored through the energy and spin-spin correlation

function. We compare the exact dynamics with those obtained by three different

numerical procedures designed to treat the instabilities. Interestingly, distinct from previous

investigations for larger lattice models and in the presence of noise, we identify a specific

value of quench strength in which RBM solutions break down, regardless of the stabilization

procedure.

[1] G. Carleo, M. Troyer: Solving the quantum many-body problem with artificial neural

networks, Science 355, 602–606 (2017), DOI: 10.1126/science.aag2302

[2] G. Fabiani: Quantum dynamics od 2D antiferromagnetic: predictions from theory and

machine learning, PhD thesis (2022), link: https://hdl.handle.net/2066/250503

[3] D. Hofmann, G. Fabiani, J. H. Mentink, G. Carleo, M. A. Sentef: Role of stochastic noise

and generalization error in the time propagation of neural-network quantum states,

SciPost Physics 12, 165 (2022), DOI: 10.21468/SciPostPhys.12.5.165

###### Isidora Araya Day – Pymablock, a python package for effective models

Effective models are a useful tool to describe physical systems. To construct one, we identify a relevant low energy subspace and apply perturbation theory. While systematic, this procedure is cumbersome and time consuming when applied to high perturbative orders or with multiple perturbations. Here we develop an efficient algorithm to produce symbolic and numeric low energy models. Our algorithm is fast, versatile, and well tested. https://pymablock.readthedocs.io/en/latest/

###### Joost Aretz – Correlated Physics in Kagome Mott Insulator Heterostructures

We study correlation effects of layered superconductors at interfaces with other correlated layered materials. One such interface which is actively being studied is between NbSe2, a multi-band superconductor, and Nb3Br8, a putative Mott-Insulator. Using these materials, the first field-free Josephson diode was recently created. The origin of the Josephson diode lacks a microscopic description as of yet. The measurements demonstrate the potential these interfaces have as a platform for studying the interesting many-body physics of correlated heterostructures. Using ab initio DFT and GW calculations we construct many-body lattice models for these layered heterostructures from first principles. By solving these constructed models within many-body perturbation theory frameworks (such as DMFT or Eliashberg theory) we study how Mott-Insulators affect Superconductors and vice versa. With this approach we aim to predict the behavior of correlated interfaces from first principles in device-like structures which is crucial in understanding device properties and for proposing future devices. I will present results of first principles calculations on the properties of Nb3Br8 and discuss future plans.

###### Julian Strik – Aging in the self-induced spin glass Nd(0001)

Elemental neodymium has been shown to be a self-induced spin glass, where glassy behaviour stems solely from the frustrated nature of the magnetic interactions [1]. This is in contrast to traditional spin glasses, where the presence of disorder is essential toward realizing glassy behaviour. The magnetic state of Nd(0001) is characterized by a lack of long range order, but exhibits local non-collinear order (Q-states). Upon increasing the temperature, neodymium displays an unusual magnetic phase transition from a self-induced spin glass to a long-range ordered multi-Q phase [2]. Here, we explore the aging behaviour of Nd(0001) in its self-induced spin glass state using spin-polarized scanning tunneling microscopy in varying magnetic fields and variable temperature. We explore how the favourability of the Q-states evolves as we age the system and relate these changes to the preferred structure of the ordered phase. These observations indicate that neodymium may be a multi-well system, which deviates from the traditional energy landscape expected of prototypical spin glass systems, thus providing a new platform to study aging dynamics as well as dynamic heterogeneity.

[1] U. Kamber et al., Science **368** (2020).

[2] B. Verlhac et al., Nat. Phys. **18** (2022).

###### Kevin Vonk – Growth of Bismuth on HOPG

We show our progress so far in growing few-layered bismuth on HOPG substrates in UHV and room temperature. Scanning Tunnelling Microscopy (STM) allows us to quickly and accurately observe the properties and quality of the bismuth layers. Exploring this system is a first step towards more complicated samples, where we aim to observe the quantum spin Hall effect at room temperature.

###### Lumen Eek – Emergent Non-Hermitian Models

By employing the isospectral reduction (ISR) technique, we can design and study systems with gains/loss and complex hoppings that reduce to the paradigmatic Hatano-Nelson and non-Hermitian Su-Schrieffer-Heeger (NH SSH) models. Our approach reveals that these models may exhibit energy (or frequency)-dependent non-Hermitian skin effect, where eigenstates can simultaneously localize on either ends of the systems, with different localization lengths. Moreover, we predict the existence of various topological edge states, pinned at non-zero energies, with different exponential envelopes, depending on their energy. Overall, our work sheds new light on the nature of topological phases and the non-Hermitian skin effect in one-dimensional systems. This poster shows an example of our results with the NH SSH model.

###### Maarten van Delft – From orbital to paramagnetic pair breaking in layered superconductor 2H-NbS2 upon thinning down

Davide Pizzirani^{1,2}, Thom Ottenbros^{1,2}, Maró van Rijssel^{1,2}, Jasper Linnartz^{1,2}, Nigel Hussey^{1,2,3}_{,} Steffen Wiedmann^{1,2}, and __Maarten van Delft__^{1,2}

^{1}* High Field Magnet Laboratory (HFML-EMFL), Radboud University, 6525 ED Nijmegen, The Netherlands. *^{2}* Institute for Molecules and Materials, Radboud University, Nijmegen 6525AJ, Netherlands. *^{3 }*H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom*

High quality transition metal dichalcogenides (TMDs) are intensively studied on account of their wide range of tunability and unique electronic properties. The superconductors 2H‐NbSe_{2} and 2H‐NbS_{2} are of particular interest since monolayers of these materials exhibit Ising superconductivity with upper critical fields greatly exceeding the Pauli limit of superconductivity [1]. However, interest in these materials is not limited to monolayers. Even in bulk crystals, there exist reports of multiband superconductivity and exotic states, such as a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase [2]. Up to now, magnetotransport studies of these states in 2H-NbS_{2} are limited and the phase diagram has not been established down to low temperatures. Here, we report a complete mapping of the phase diagram of bulk 2H-NbS_{2} between 0.3 and 6 K using both magnetotransport and magnetostriction for different angles between the applied magnetic field and the layered structure. We compare this phase diagram with that acquired for a 6 nm thick flake of 2H‐NbS_{2 }and find a drastically modified Maki parameter, signifying a change of the relevant pair breaking mechanism upon thinning down.

[1] X. Xi *et al., *Nature Physics **12**, 139 (2016).

[2] C.W. Cho *et al., *Nature Communications 12, 3676 (2021).

###### Marieke Altena – Vanadium doping as a carrier modulator in the topological insulator Bi4Te3

__Marieke Altena ^{1}__, Thies Jansen

^{1}, Daan Wielens

^{1}, Martina Tsvetanova

^{1}and Alexander Brinkman

^{1}

^{1} Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands

Bi_{2}Te_{3} is a 3D topological insulator with a single Dirac cone on the surface [1]. This surface state can be gapped by means of magnetic doping, resulting in the quantum anomalous hall state [2]. Recently, there has been an increasing interest in natural superlattices containing Bi_{2}Te_{3}, such as Bi_{4}Te_{3 }[3]. This compound consists of alternating Bi_{2 }and Bi_{2}Te_{3} layers. The exact topological nature of these compounds is still under debate. We fabricated (V-doped) Bi_{4}Te_{3} thin films with molecular beam epitaxy and characterized the films with X-ray diffraction, transmission electron microscopy and electrical transport measurements. We show that the level of V-doping influences the dominant carrier type in transport and observed the presence of magnetism in the films with the highest level of V-doping.

[1] Y. L. Chen, *et al., Science*, **325, **5937. (2009).

[2] C.-Z. Chang*,* *et al, Science, ***340, **6129. (2013)

[3] D. Nabok, *et al, Phys. Rev. Mat. ***6, **034204. (2022)

###### Max van der Schans – Deterministic All-Optical writing of topologically protected skyrmions

Several approaches have been published to generate topologically protected skyrmions. A very small number addresses the use of ultrashort laser pulses to generate them, but so far only stochastically. We intend to go beyond, by deterministically writing individual skyrmions using All-Optical Switching (AOS), for which we will explore novel methods to apply stabilizing effective magnetic fields.

###### Montserrat Navarro Espino – Electronic transport in Bi nanostructures through its hinge states

The electronic structure of Bi is topological and follows a generalized bulk–boundary correspondence of higher- order: the hinges of the material host topologically conducting modes. This affirmation is based on experimental detection of the hinge states [1] and analysis in the framework of Topological Quantum Chemistry [2] (based on Band Representation Theory) [3]. The hinge states provide intriguing features to the material. The approach of this project is calculating the electronic structure of Bi using the tight-binding Hamiltonian proposed in 2018 [2], which describes the topological states of the material. Then, electronic transport calculations were performed in KWANT [5] using two and three contacts in a Bi nanostructure shaped as a hexagonal prism. This allows us to explore the electronic transport through the hinge states where six conduction states are identified in the device with two contacts. However, by adding a third contact, electronic decoherence takes place and one of the channels is removed from the device. This observation opens the possibility of working with new Bi-based devices, where the addition or deformation of contacts in the nanostructure enables the manipulation of the hinge states.

[1] Murani, Anil, et al. 2017. “Ballistic Edge States in Bismuth Nanowires Revealed by SQUID Interferometry.” Nature Communications 8 (July): 15941.

[2] Schindler, Frank, et al. 2018. “Higher-Order Topology in Bismuth.” Nature Physics 14 (9): 918–24. [3] Bradlyn, Barry, et al. 2017. “Topological Quantum Chemistry.” Nature 547 (7663): 298–305.

[4]Cano, Jennifer, and Barry Bradlyn. 2021. “Band Representations and Topological Quantum Chemistry.” Annual Review of Condensed Matter Physics 12 (1): 225–46.

[5] Groth, Christoph W., Michael Wimmer, Anton R. Akhmerov, and Xavier Waintal. 2014. “Kwant: A Software Package for Quantum Transport.” New Journal of Physics 16 (6): 063065.

###### Nikolai Khokhlov – Electric-field-assisted laser-induced optical switching in iron garnet

Control of magnetic order with femtosecond optical pulses offers an important route towards future magneto-photonic hybrid devices for data processing. The essential role in the practical realization of the devices is going to be addressed to electric field and its impact on the laser-induced magnetization dynamics. In the work, we experimentally studied the impact of an electric field on the magneto-optical switching in epitaxial iron garnet film (BiLu)3(FeGa)5O12 with (210) orientation. We have found, the applied electric field leads to an increase of the frequency of magnetization precession, accompanied by a reduction of the oscillations’ amplitude. In turn, the reduction of amplitude depends strongly on the magnitude of external magnetic field as well. For instance, the increment of the field leads to reversing the amplitude behavior: it starts to be higher with electric field. Moreover, at low magnetic fields the electric field decreases the damping parameter by a factor of 1.5. Finally, we found the impact of electric field is even, i.e., its sign change leads to the same effect on the magnetization. The study shows new possibilities to control the ultrafast optically induced processes in multiferroic media.

###### Pim Lueb – Towards low disorder in-plane PbTe nanowire networks for quantum applications

Hybrid semiconductor-superconductor nanowires are promising candidates as quantum information processing devices. The need for scalability and complex designs calls for the development of high quality, low disorder, selective area growth techniques. We introduce the route and developments towards growth of low disorder large-scale lead telluride (PbTe) networks by molecular beam epitaxy. The group IV-VI lead-salt semiconductor is an attractive material choice due to its large dielectric constant, strong spin-orbit coupling, and high carrier mobility. The scalable approach of our methods show the potential of our system as a basis for research of topological quantum devices.

###### Rebecca Gharibaan – Towards microwave experiments in III-V 2DEGs

Two-dimensional electron gases (2DEGs) in III-V materials offer a versatile and flexible platform to study hybrid superconductor-semiconductor devices. Recently such 2DEGs have been used to create gate-tunable superconducting qubits (gatemons) and are a promising platform to explore topological systems. In order to study these systems it is desirable to control them at short timescales. However, in general, these 2DEGs are not ideal for microwave frequency experiments, since they significantly lower the quality factor of on-chip microwave resonators due to dielectric losses. This limits the ability to perform fast operations and to study these hybrid systems on short timescales.

Flip-chip architecture resolves this issue by separating resonators and qubits on their own respective chips, allowing the use of different substrates. A necessary component is a galvanic connection between chips, provided by indium bonds, used for gatemon control. Here I’ll present our current progress on developing flip-chip devices containing this component.

###### Robert Cañellas Núñez – Topological edge and corner states in Bi fractals on InSb

###### Sergii Grytsiuk – Nb3Cl8: A Prototypical Layered Mott-Hubbard Insulator

The Hubbard model provides an idealized description of electronic correlations in solids. Despite its simplicity, the model features a competition between several different phases that have made it one of the most studied systems in theoretical physics. Real materials usually deviate from the ideal of the Hubbard model in several ways, but the monolayer of Nb3Cl8 has recently appeared as a potentially optimal candidate for the realization of such a single-orbital Hubbard model. Here, we show how this single orbital Hubbard model can be indeed constructed within a “molecular” rather than atomic basis set using ab initio constrained random phase approximation calculations. This way, we provide the essential ingredients to connect experimental reality with ab initio material descriptions and correlated electron theory, which clarifies that monolayer Nb3Cl8 is a Mott insulator with a gap of about 1 to 1.2eV depending on its dielectric environment. By comparing with an atomistic three-orbital model, we show that the single molecular orbital description is indeed adequate. Furthermore, we comment on the expected electronic and magnetic structure of the compound and show that the Mott insulating state survives in the low-temperature and bulk phases of the material.

###### Thom Ottenbros – Exploring and Tuning of Magnetic Order in Rare-Earth Tritellurides

The family of rare-earth tri-tellurides (RTe3) consists of square-net tellurium sheets with van-der-Waals (vdW) gaps and rare-earth – tellurium (RTe) slabs stacked along the b-axis, with the rare-earth atoms R = La-Nd, Sm, Gd-Tm, Y. While the quasi-two-dimensional electronic structure originates mainly from the 5p orbitals of the tellurium planes, the magnetic order (if present) stems from the R3+ ions. Currently, there is a renewed interest on these materials to understand their charge transport properties in the presence of a charge density wave. For an overview, see [1]. For the compound GdTe3 the highest carrier mobility of any magnetic vdW-layered material has been reported recently [2].

In this work, we present thermal expansion and high-field magnetostriction studies on the compound GdTe3, the perfect candidate material to investigate the antiferromagnetic ordered phases due to the relatively high transition temperatures. Below 15 K, this material exhibits several magnetic phases [2]. We present magnetostriction data on this material when the field is applied along different high-symmetry orientations and establish a complete phase diagram of this material when the field is applied out-of-plane (along the b-axis).

Motivated by recent electrocaloric and elasto-resistance measurements on sister compounds [3], we demonstrate that out-of-plane uniaxial strain alters the magnetic phase transitions in GdTe3, as seen in thermal expansion. Meanwhile the magnetostriction signal is strongly enhanced under uniaxial strain. From the quantum oscillations analysis, we do find a strain-induced Fermi surface reconstruction. Our results demonstrate the complexity of magnetic order in this system and its tunability due to its layered structure.

[1] K. Yumigeta et al., Adv. Sci 8, 2004762 (2021).

[2] S. Lei et al., Science Advances 6, eaay6407 (2020).

[3] J. A. W. Straquadine, M. S. Ikeda and I. R. Fisher, Phys. Rev. X 12, 021046 (2022).

###### Timur Gareev – Coherent THz Spin Dynamics in RbMnF3 Antiferromagnet

Femtosecond (fs) pulses of light have been shown to generate spin-waves (magnons) in practically all classes of magnetically ordered materials, including antiferromagnets characterized by the fastest possible THz spin dynamics. While a generation of long-wavelength antiferromagnetic magnons close to the center of the Brillouin zone (BZ) is well understood, the understanding of the highest-frequency and shortest-wavelength magnons at the edge of the BZ is significantly less understood. This is because classical long-wavelength macroscopic theories of magnons fail when the wavelength approaches the distance between two atoms. Including quantum effects becomes crucial at nm length scales and fs time scales.

Here, using the Heisenberg antiferromagnet RbMnF3 and fs light pulses we experimentally demonstrate an optical excitation of coherent two-magnon (2M) mode, corresponding to pairs of the mutually counter-propagating coherent spin-waves with the wavevectors up to the edge of the BZ. We show that the 2M excitation cannot be understood in terms of macroscopic magnetization and Neel vectors, conventionally used to describe spin-waves in the classical macrospin approximation. Instead, we propose to model such spin dynamics using the microscopic spin correlations function. We derive a quantum-mechanical equation of motion for the latter and emphasize that unlike the magnetization and the antiferromagnetic vectors the spin correlations in antiferromagnets do not possess inertia.

###### Tjacco Koskamp – Semiclassical theory for plasmons in inhomogeneous two-dimensional systems

We consider plasmons, quantized collective oscillations of conduction electrons in metals or semiconductors, in inhomogeneous two-dimensional systems. Plasmons can be used to manipulate and control light, using heterostructures of nanometer size. This requires structures that are spatially inhomogeneous, which are difficult to describe analytically. Although these systems can be studied numerically, this approach is limited to relatively small system sizes. Here, we present a novel semi-analytical method to describe plasmons in two-dimensional inhomogeneous media within the framework of the Random Phase Approximation (RPA). Our approach is based on the semiclassical approximation, which is formally applicable when the length scale of the inhomogeneity is much larger than the plasmon wavelength. Our first main result is an effective classical Hamiltonian for quantum plasmons. We obtain this result by first separating the in-plane and out-of-plane degrees of freedom, and then employing the semiclassical Ansatz for the induced plasmon potential. To illustrate our method, we develop a theory for scattering of plasmons by radially symmetric inhomogeneities. We compute the (differential) scattering cross section for a specific model of the inhomogeneity and compare it to the classical trajectories.

###### Yann in ‘t Veld – Discovery of interlayer plasmon polaron in graphene/WS2 heterostructures

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get “dressed”. This leads to the formation of polaronic quasiparticles that dramatically impact charge transport, surface reactivity, thermoelectric and optical properties, as observed in a variety of crystals and interfaces composed of polar materials. Similarly, when oscillations of the charge density couple to conduction electrons the more elusive plasmon polaron emerges, which has been detected in electron-doped semiconductors. However, the exploration of polaronic effects on low energy excitations is still in its infancy in two-dimensional (2D) materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of SL WS2. By using micro-focused angle-resolved photoemission spectroscopy (microARPES) during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the SL WS2 conduction band minimum (CBM). Our results are explained by an effective many-body model in terms of a coupling between SL WS2 conduction electrons and graphene plasmon modes. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides (TMDs).