
Chair: Karin EverschorSitte

13:00  14:00

Claire Donnelly
(University of Cambridge)
3D imaging of magnetic systems at the nanoscale
Three dimensional magnetic systems promise significant opportunities for new physics, ranging from ultrahigh domain wall velocities and geometryinduced magnetochirality effects, to 3D topological structures as well as 3D technological devices [1,2]. Experimentally, appropriate techniques are required to map both complex threedimensional magnetic configurations, and the response to external excitations.
For threedimensional magnetic imaging, we have developed Xray magnetic nanotomography [3], combining a new iterative reconstruction algorithm [4] with a dual rotation axis experimental setup, therefore providing access to the threedimensional magnetic configuration at the nanoscale. In a first demonstration, we have determined the complex threedimensional magnetic structure within the bulk of a micrometresized soft magnetic pillar and observed a magnetic configuration that consists of vortices and antivortices, as well as Bloch point singularities [3].
In addition to the static magnetic structure, the dynamic response of the 3D magnetic configuration to excitations is key to our understanding of both fundamental physics, and applications. With our recent development of Xray magnetic laminography [5,6], it is now possible to determine the magnetisation dynamics of a threedimensional magnetic system [5] with spatial and temporal resolutions of 50 nm and 70 ps, respectively.
A final challenge concerns the identification of nanoscale topological objects within the complex reconstructed magnetic configurations. To address this, we have recently implemented calculations of the magnetic vorticity [7,8], that make possible the location and identification of 3D magnetic solitons, leading to the first observation of magnetic vortex rings [8].
These new experimental capabilities of Xray magnetic imaging open the door to the elucidation of complex threedimensional magnetic structures, and their dynamic behaviour.
[1] FernándezPacheco et al., “Threedimensional nanomagnetism” Nat. Comm. 8, 15756 (2017)
[2] Donnelly and V. Scagnoli, “Imaging threedimensional magnetic systems with Xrays” J. Phys. D: Cond. Matt. (2019).
[3] Donnelly et al., “Threedimensional magnetization structures revealed with Xray vector nanotomography” Nature 547, 328 (2017).
[4] Donnelly et al., “Tomographic reconstruction of a threedimensional magnetization vector field” New Journal of Physics 20, 083009 (2018).
[5] Donnelly et al., “Timeresolved imaging of threedimensional nanoscale magnetization dynamics”, Nature Nanotechnology 15, 356 (2020).
[6] Witte, et al., “From 2D STXM to 3D Imaging: Soft Xray Laminography of Thin Specimens”, Nano Lett. 20, 1305 (2020).
[7] Cooper, “Propagating magnetic vortex rings in ferromagnets.” PRL. 82, 1554 (1999).
[8] Donnelly et al., “Experimental observation of vortex rings in a bulk magnet” Nat. Phys. 17, 316 (2021)


MAGNETISM


Chair: Sebastian Eggert

14:00  14:15

Johannes Motruk
(University of California, Berkeley)
Fourspin terms and the origin of the chiral spin liquid in Mott insulators on the triangular lattice
At strong repulsion, the triangularlattice Hubbard model is described by s=1/2 spins with nearestneighbor antiferromagnetic Heisenberg interactions and exhibits conventional 120° order. Using a combination of infinite density matrix renormalization group and exact diagonalization, we study the effect of the additional fourspin interactions naturally generated from the underlying Mottinsulator physics of electrons as the repulsion decreases. Although these interactions have historically been connected with a gapless ground state with emergent spinon Fermi surface, we find that at physically relevant parameters, they stabilize a chiral spinliquid (CSL) of KalmeyerLaughlin (KL) type, clarifying observations in recent studies of the Hubbard model. We also present a selfconsistent solution based on a meanfield rewriting of the fourspin interaction to obtain a Hamiltonian with similarities to the parent Hamiltonian of the KL state, providing a physical understanding for the origin of the CSL.

14:15  14:30

Ciarán Hickey
(University of Cologne)
Offdiagonal Symmetric Exchange on the Triangular and Kagome Lattices
Recent years have seen a surge of interest in spinorbit entangled Mott insulators, largely driven by the tantalizing prospect of realising the physics of Kitaev's celebrated honeycomb model. However, there are also other distinct exchange interactions that can be generated in such Mott insulators, which come with their own unique physics and intriguing properties. Here, we focus on offdiagonal symmetric exchange on the triangular and kagome lattices (with the honeycomb case having already been wellstudied). In the classical limit the model, with an antiferromagnetic sign, leads on both lattices to classical spin liquid behavior. On the other hand, in the quantum spin1/2 limit, quantum fluctuations drive both systems into ordered ground states. We will discuss various elements of these classical and quantum limits using a variety of analytical and numerical techniques, shedding light on these new exchange models within the realm of quantum magnetism.

14:30  14:45

Adhip Agarwala
(Max Planck Instititute for the Physics of Complex Systems)
Gapless state of interacting Majorana fermions in a straininduced Landau level
Mechanical strain can generate a pseudomagnetic field, and hence Landau levels (LL), for low
energy excitations of quantum matter in two dimensions. We study the collective state of the
fractionalised Majorana fermions arising from residual generic spin interactions in the central LL,
where the projected Hamiltonian reflects the spin symmetries in intricate ways: emergent U(1) and
particlehole symmetries forbid any bilinear couplings, leading to an intrinsically strongly interacting
system; also, they allow the definition of a filling fraction, which is fixed at 1/2. We argue that the
resulting manybody state is gapless within our numerical accuracy, implying ultrashortranged
spin correlations, while chirality correlators decay algebraically. This amounts to a Kitaev ‘nonFermi’ spin liquid, and shows that interacting Majorana Fermions can exhibit intricate behaviour
akin to fractional quantum Hall physics in an insulating magnet.

14:45  15:00

Alexandros Metavitsiadis
(Technical University Braunschweig)
Optical phonons coupled to a Kitaev spin liquid
Emergent excitation continua in frustrated magnets are a fingerprint of
fractionalization, characteristic of quantum spinliquid states. Recent
evidence from Raman scattering for a coupling between such continua and
lattice degrees of freedom in putative Kitaev magnets
may provide insight into the nature of the fractionalized quasiparticles.
Here, we report on the renormalization of optical phonons coupled to the
underlying $\mathbb{Z}_{2}$ quantum spinliquid. We show that phonon
lineshapes acquire an asymmetry, observable in light scattering, and
originating from two distinct sources, namely the dispersion of the
Majorana continuum and the Fano effect. Moreover, we find that the
phonon lifetimes increase with increasing temperature due to thermal
blocking of available phase space. Finally, in contrast to lowenergy
probes, optical phonon renormalization is rather insensitive to thermally
excited gauge fluxes and barely susceptible to external magnetic fields.

15:00  15:15

Elio J. König
(Max Planck Institute for Solid State Research)
Detecting and destroying quantum spin liquids with metallic leads
Quantum Spin Liquids have recently enjoyed renewed interest. This is partly driven by synergies with quantum information theory and by the experimental progress, which provides evidence for new material realizations, in particular in 2D vanderWaals materials. The chemical versatility of this platform allows and requires the study of new probes in device geometries combining quantum spin liquids with, e.g., metals or semimetals. In this talk, I present a comprehensive study of electrical tunneling signatures of Kitaev quantum spin liquids in such heterostructures. I argue that momentum conserving tunneling setups, such as planar tunneling and the tunneling between quantum Hall edges are experimentally particularly suitable. In the second part, I will discuss the stability of the spin liquid state with respect to the coupling to an electronic bath. As an exemplary toymodel, I will demonstrate that a simple triangular KondoHeisenberg cluster impurity displays a topological phase and a trivial phase, which are separated by a deconfinement transition driven by the proliferation of monopole like gauge field excitations which is reminiscent of the confinement transition in 2D U(1) quantum spin liquids.
[1] EJ König, MT Randeria, B Jäck; Physical Review Letters 125 (26), 267206 (2020)
[2] EJ König, P Coleman, Y Komijani; arXiv:2002.12338 (2020)

15:15  15:30

Martin Gärttner
(Heidelberg University)
Neural network quantum state tomography with neuromorphic hardware
We employ neural networks to find compressed representations of manybody quantum states. In this talk I focus on the application of this approach to quantum state tomography. I will report on state tomography on experimental data from a twophoton experiment and discuss the scalability of neural state tomography with convolutional neural networks. The most resource consuming task in this approach is the generation of Monte Carlo samples form the learned network representation. This task can potentially be accelerated by emulating the networks physically on neuromorphic hardware. I will report on the successful encoding of fewqubit entangled states on a neuromorphic chip.

15:30  15:45

Huaiyang Yuan
(Utrecht University)
Manipulation of magnonmagnon entanglement in an antiferromagnet
Cavity spintronics is a rising field that manipulates the interaction of magnetic magnons and the photons inside a cavity, which benefits the advantages of the long lifetime and easy tunability properties of magnetic magnons. One promising route of this field is to bridge it with the quantum information science, which utilizes the entanglement of quasiparticles as a computing and information processing resource. A prior and crucial, but seldom studied question is how magnons and photons interplay inside the cavity to manifestate their entanglement properties.
In this talk, I will first outline the recent developments in caivty spintronics and then present the antiferromagnetic magnonmagnon and magnonphoton entanglement inside a microwave cavity as well as their application potential in quantum information science. If time permits, I will further talk about the promising properties of van der Waals magnets as a platform to manipualte the entanglement of magnons.

15:45  16:00

Sebastián Díaz
(Johannes Gutenberg Universität Mainz)
Majorana Bound States Induced by Antiferromagnetic Skyrmion Textures
Majorana bound states are zeroenergy states predicted to emerge in topological superconductors and intense efforts seeking a definitive proof of their observation are still ongoing. A standard route to realize them involves antagonistic orders: a superconductor in proximity to a ferromagnet. Here we show this issue can be resolved using antiferromagnetic rather than ferromagnetic order. We propose to use a chain of antiferromagnetic skyrmions, in an otherwise collinear antiferromagnet, coupled to a bulk conventional superconductor as a novel platform capable of supporting Majorana bound states that are robust against disorder. Crucially, the collinear antiferromagnetic region neither suppresses superconductivity nor induces topological superconductivity, thus allowing for Majorana bound states localized at the ends of the chain. Our model introduces a new class of systems where topological superconductivity can be induced by editing antiferromagnetic textures rather than locally tuning material parameters, opening avenues for the conclusive observation of Majorana bound states.

16:00  16:15

Alessio Chiocchetta
(University of Cologne)
Cavityinduced quantum spin liquids
Recent developments at the interface between quantum materials and photonics are opening novel avenues to engineer hidden phases of matter.
Among these, quantum spin liquids provide paradigmatic examples of highly entangled quantum states of matter, hosting fractionalized excitations and emerging gauge fields.
In this talk, I will propose to engineer these phases by exploiting the coupling of quantum magnets to the quantized light of an optical cavity.
The interplay between the quantum fluctuations of the electromagnetic field and the strongly correlated electrons results in a tunable longrange, frustrating interaction between spins. This cavityinduced interaction robustly stabilizes spin liquid states, which occupy an extensive region in the phase diagram spanned by the range and strength of the tailored interaction. Remarkably, this occurs even in originally unfrustrated systems, as we showcase for the Heisenberg model on the square lattice.
Finally, I will outline perspectives on how this implementation can be used for engineering further exotic states of matter, and for novel measurement protocols

16:15  16:30

Attila Szabó
(University of Oxford)
Neural network wave functions and the sign problem
Neural quantum states are a promising approach to studying manybody quantum physics. However, they face a major challenge when applied to lattice models: Neural networks struggle to converge to ground states with a nontrivial sign structure. Here, I present a neural network architecture with a simple, explicit, and interpretable phase ansatz, which can robustly represent such states and achieve stateoftheart variational energies for both conventional and frustrated antiferromagnets. In the first case, the neural network correctly recovers the Marshall sign rule without any prior knowledge. For frustrated magnets, our approach uncovers lowenergy states that exhibit the Marshall sign rule but does not reach the true ground state, which is expected to have a different sign structure. I discuss the possible origins of this "residual sign problem" as well as strategies for overcoming it, which may allow using neural quantum states for challenging spin liquid problems.

16:30  17:00

Break (via gather.town)


Chair: Stefan Wessel

17:00  17:45

Poster flash talks II

17:45  19:00

Poster Session II
