A beautiful twist on condensed matter

Vortex knots have been seen decaying in many physical systems. Here we describe topologically protected vortex knots, which remain stable and undergo fusion and fission while conserving a topological invariant analogous to that of baryon number. While the host medium, a chiral nematic liquid crystal, exhibits intrinsic chirality, cores of the vortex lines are structurally achiral regions where twist cannot be defined. We refer to them as "dischiralation" vortex lines, in analogy to dislocations and disclinations in ordered media where, respectively, positional and orientational order is disrupted. Fusion and fission of these vortex knots, which we reversibly switch by electric pulses, vividly reveal the physical embodiments of knot theory's concepts like connected sums of knots. Our findings provide insights into related phenomena in fields ranging from cosmology to particle physics and can enable applications in electro-optics and photonics, where such fusion and fission processes can be used for controlling light.

Professor Ivan Smalyukh

University of Colorado Boulder, US

Professor Ivan Smalyukh

University of Colorado Boulder, US

Ivan I Smalyukh is a tenured professor at the Department of Physics, University of Colorado at Boulder, which he joined in 2007 (promoted from Assistant to Associate Professor with tenure in 2014 and from Associate to Full Professor in 2017). He is also the Founding Director of the International Institute for Sustainability with Knotted Chiral Meta Matter, as well as the founding fellow of Renewable Sustainable Energy Institute, a joint institute of CU-Boulder and NREL. He is an elected fellow of APS, AAAS, Optica and SPIE. He received many awards, including the Bessel and Glenn Brown Awards, Gray Medal, NASA iTech award and Mid-Career Award of the International Liquid Crystal Society, the PECASE Award from the Office of Science and Technology of the White House and the GSoft Award from the American Physical Society.

Blue phases (BPs) of chiral liquid crystals are formed by skyrmion filaments, which spontaneously arrange in the crystal-like BP I and BP II phases or the fully disordered BP III phase, thereby forming crystals or a fluid made of topological defects. Optical microscopy studies of thin films of BP liquid crystals reveal a variety of 2D skyrmion structures, depending on the liquid crystal's thickness and temperature. Individual half-skyrmions are observed at very small thicknesses of around ~50 nm, which above the critical thickness of ~120 nm transform into a dense liquid of half-skyrmion filaments, forming the BP III phase in bulk. We observe that pretransitional, symmetry-breaking order-parameter fluctuations of a skyrmion-forming chiral nematic liquid crystal are slowed-down by 4 orders of magnitude if confined to ≲100 nm thin films. Fluctuating fragments of half-skyrmions are observed in a narrow temperature interval and are explained by thermally activated hopping between the various energy states. Skyrmion fluctuations are accompanied by imbalanced topological charge: positive charges appear at higher temperatures and dominate in the fluctuating region until skyrmions fully condense and negative charges appear at lower temperatures.

Professor Igor Muševič

Jozef Stefan Institute, Slovenia

Professor Igor Muševič

Jozef Stefan Institute, Slovenia

Igor Muševič is a professor of physics at the University of Ljubljana and Head of the Soft Matter Laboratory, which he established in 1995, at the Jožef Stefan Institute. He received his BSc (1977) and PhD (1993) degrees in physics from the University of Ljubljana. He was a visiting scientist at the High Magnetic Field Laboratory at the Radbout University in Nijmegen, The Netherlands, the Max-Planck-Institut fur Festkoerperforschung Hochfeld Magnetlabor, Grenoble, France, and was an adjunct professor at the Raman Research Institute, Bangalore, India. He is the recipient of several international awards for the science and technology of liquid crystals and has received the Slovenian national Zois Award for outstanding scientific achievements. He was a supervisor of numerous PhD students, several of whom had theses that were awarded national and international prizes, and he was recognized as Mentor of the Year in 2011. He has held several positions of responsibility at the national and international levels, and he was the Head of the Condensed Matter Department at the Jožef Stefan Institute in the period 2006‒2020. In 2020 he was awarded a prestigious ERC Advanced Grant on soft-matter photonics to develop all-optical logical gates based on liquid crystals. He is a soft-matter experimental physicist, and his recent research topics include experimental topology and the photonics of soft matter. He has published more than two hundred and sixty articles in peer-reviewed journals, written three books, including a recent book entitled Liquid Crystal Colloids. Among his many publications, he has co-authored two articles in Science and several articles in Nature Physics and Nature Photonics.

Electrons play a pivotal role in stabilizing matter, but they are also tools that can reveal the underlying physics of complex systems from high energy physics to condensed matter. Electrons can be used as imaging probes, where properties of matter such as ferroelectricity, magnetism or topology can be observed atom-by-atom. In this talk, I will discuss a new type of electron probe which can image the chiral order of centrosymmetric magnetic skyrmions called Lorentz electron ptychography, an iterative phase retrieval imaging technique for magnetic materials. In particular, my research focuses on amorphous layered thin films of FeGdPt, which have been shown to form centrosymmetric skyrmions with a predicted internal Bloch wall and Néel caps. Using simulation and experimental results from Lorentz electron ptychography and four-dimensional scanning transmission electron microscopy, I show that this structure does indeed have a hybrid Bloch and Néel skyrmion structure, as predicted from micromagnetic simulations. Finally, I will also show how electron ptychography can improve resolution beyond the numerical aperture of the electromagnetic lenses to the sub-angstrom limit in a conventional electron microscope. Using this technique, I essentially develop a ‘computation lens’ approach to imaging, opening opportunities to explore new physics in emergent materials beyond physical lenses in a cost-effective manner, and thus expanding access to high-resolution imaging approaches to a broader range of institutions.

Professor Kayla Nguyen

University of Oregon, US

Professor Kayla Nguyen

University of Oregon, US

Dr Kayla Nguyen has made a tremendous impact in the field of transmission electron microscopy. She earned her undergraduate degree in Physics from the University of California Santa Barbara, and PhD from Cornell University. At Cornell, she provided a critical role in the development of a novel pixel array detector for electron microscopes with unprecedented dynamic range, sensitivity, and speed. This new detector has been licensed and sold around the world by Thermo Fisher Scientific. During her postdoctoral fellowship at the University of Illinois Urbana-Champaign, she won the L’Oreal For Women in Science Postdoctoral Fellowship and was named a promising Asian researcher by The Japan Times. In 2023, she became an Assistant Professor at the University of Oregon where she received a coveted Arnold and Mabel Beckman Young Investigator, the Army Research Office Early Career Program Award, the National Scientific Foundation MRI for a new TEM, and industry sponsored research from Intel to continue her cutting edge work. Her passion for science extends beyond the laboratory setting, towards developing accessible pathways for young scientists in STEM.

The magnetic phase diagrams of cubic chiral magnets follow a similar pattern consisting of the helical spiral, conical spiral, and the skyrmion lattice phase, which appears in a narrow pocket of the phase diagram, the so-called A-phase, just below the magnetic ordering temperature. A remarkable exception to this universality is the Mott insulator Cu2OSeO3, where robust skyrmionic states can be produced over large areas of the magnetic phase diagram from the lowest temperatures up to the A-phase. Furthermore, when the field is applied along the [001] easy crystallographic axis and its value is just below the critical field at which the conical spiral state disappears, the spiral wave vector rotates away from the magnetic field direction leading to a multidomain tilted spiral state. This phase occurs where it is least expected, at low temperatures, where thermal spin fluctuations are suppressed, and at magnetic fields strong enough to align all spirals along their direction.

Nascent and disappearing (tilted) spirals catalyze topological charge changing processes, leading to the formation of skyrmionic states at low temperatures, which are thermodynamically stable or metastable, depending on the orientation and strength of the magnetic field. The metastable low temperature skyrmions are extremely robust and surprisingly resilient to high magnetic fields: the memory of skyrmion states persists in the field polarized state, even when the skyrmion lattice signal has disappeared. A in depth comparison between experiment and theory leads to the conclusion that the driving forces behind the observed unconventional behaviour are temperature dependent competing anisotropies, generic to chiral magnets. These competing anisotropies and may stabilize novel skyrmionics states in a wide range of magnetic fields and temperatures, beyond the A-phase, and thus provide an additional lever for tailoring the properties of chiral magnets.

Professor Catherine Pappas

Delft University of Technology, The Netherlands

Professor Catherine Pappas

Delft University of Technology, The Netherlands

Catherine (Katia) Pappas joined 2009 Delft University of Technology to lead the section Neutron and Positron Methods in Materials (NPM2), within the Faculty of Applied Sciences. Her field of expertise is in neutron scattering science and techniques, with focus on high-resolution (neutron spin echo) spectroscopy and polarized neutrons. Besides neutron instrumentation, her scientific interests are in the field of magnetism and chiral magnetism, and the field of skyrmions. Before Delft Katia spent several years at the Hahn-Meitner Institute – nowadays Helmholtz Zentrum Berlin – where she was involved in numerous large scale neutron instrumentation projects. She was deputy director of the Berlin Neutron Scattering Center and head of the "Neutron Instruments and Methods" department. In Delft, she was again the initiator of several big instrumentation projects, such as the neutron powder diffractometer PEARL or the multipurpose instrument LARMOR, a Dutch-UK collaboration, which is being built at the UK neutron source ISIS and is supported by the Dutch Science Foundation (NWO).

Extending spin systems to three dimensions promise significant opportunities for applications, for example providing higher density devices and new functionalities associated with complex topology and greater degrees of freedom [1,2]. Until now, however, insight into three dimensional spin systems has mainly been limited to ferromagnetic and ferrimagnetic systems through X-ray magnetic tomography [3] – where a variety of topological textures [3,4], as well as 3D dynamics [5,6], have been observed. In this talk I will describe our recent work mapping topological textures in compensated systems. I will first describe the development of X-ray linear orientation tomography [7], which we have harnessed to map three-dimensional orientation fields – both crystallographic [7], and antiferromagnetic [8] – at the nanoscale. Second, I will present our recent mapping of topological textures in altermagnets [9,10], harnessing both X-ray circular and linear magnetic dichroism. These insights into the formation of topological textures in compensated magnets not only paves the way not only for enhanced understanding of these systems, but also towards the next generation of technological devices.

[1] Fernández-Pacheco et al., Nature Communications 8, 15756 (2017)
[2] C. Donnelly and V. Scagnoli, J. Phys. D: Cond. Matt. 32, 213001 (2020)
[3] C. Donnelly et al., Nature 547, 328 (2017)
[4] C. Donnelly et al., Nat. Phys. 17, 316 (2021)
[5] C. Donnelly et al., Nature Nanotechnology 15, 356 (2020)
[6] S. Finizio et al., Nano Letters (2022)
[7] A. Apseros et al., Nature 636, 354 (2024)
[8] A. Apseros et al., In Prep
[9] R. Yamamoto et al., Phys. Rev. Appl. 24, 034037 (2025)
[10] R. Yamamoto et al., In Prep

Dr Claire Donnelly

Max Planck Institute for Chemical Physics of Solids, Germany

Dr Claire Donnelly

Max Planck Institute for Chemical Physics of Solids, Germany

Following her MPhys at the University of Oxford, Claire went to Switzerland to carry out her PhD studies at the Paul Scherrer Institute and ETH Zurich. She was awarded her PhD in 2017 for her work on three dimensional magnetic systems, in which she developed X-ray magnetic tomography, work that was recognised by a number of awards.

After a postdoc at the ETH Zurich, she moved to the University of Cambridge and the Cavendish Laboratory as a Leverhulme Early Career Research Fellow, where she focused on the behaviour of three dimensional magnetic nanostructures.

Since September 2021 she is a Lise Meitner Group Leader of Spin3D at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany. Her group focuses on the physics of three dimensional magnetic and superconducting systems, and developing synchrotron X-ray-based methods to resolve their structure in three dimensions.

The resonance frequency of the lattice polarization of most ferroelectric materials is in the terahertz (THz) range. Therefore, ferroelectric polarization can resonantly interact with a THz electromagnetic wave. In this talk, the speaker will discuss how such resonant interaction can be utilized to (1) modulate the amplitude, phase, and even the chirality of a THz wave transmitting through a freestanding ferroelectric membrane, using BaTiO₃, strained SrTiO₃, LiNbO₃, and ScAlN as examples; (2) create new hybridized states from the strong coupling between the quanta of coherent polarization waves (aka coherent ferrons) and standing bulk acoustic waves (ie, cavity acoustic phonons), using a freestanding van der Waals (vdW) ferroelectric CuInP₂S₆ membrane as an example; (3) achieve the resonant coupling between ferroelectric domain walls and acoustic phonons, using a strained BaTiO₃ membrane as an example; and (4) activate new, acoustically amplified, topological modes of the periodically aligned flux closures in strained cation-doped BaTiO₃ thin films. These predictions are based on dynamical phase-field simulations and complementary analytical calculations. An outlook for the development of thermodynamic theory and dynamical phase-field model for predicting the optical, electro-optic, and elasto-optic properties of oxide, nitride, and vdW ferroelectric materials will also be presented.

Professor Jiamian Hu

University of Wisconsin-Madison, US

Professor Jiamian Hu

University of Wisconsin-Madison, US

Dr Jiamian Hu is an Associate Professor in the Department of Materials Science and Engineering at the University of Wisconsin (UW)-Madison. Dr Hu received the Vilas Associate Award for research from UW-Madison, the Innovation Award from the Wisconsin Alumni Research Foundation, the Robert L Coble Award for Young Scholars from the American Ceramic Society, and the National Science Foundation CAREER award. Dr Hu has published over 100 peer-reviewed articles and is the lead inventor of five granted US Patents. His current research activities include mesoscale modeling of ferroic (magnetic, ferroelectric, and multiferroic) materials, polar semiconductors, and the resulting quantum and microelectronic devices, microstructure formation and evolution under nonequilibrium conditions, and microstructure informatics. Dr Hu served as an Associate Editor for the Journal of Materials Research and an Editorial Board Member of Journal of Physics D: Applied Physics.

The dynamics of homochiral domain walls in ultra thin ferromagnetic (FM) layers deposited on a heavy metal (HM) in asymmetric stacks is strongly modified by the presence of interfacial Dzyaloshinskii-Moriya interaction (DMI). In particular, the Walker field beyond which the field-driven mobility strongly decreases can be pushed to larger fields, allowing reaching very large velocities, proportional to the strength of the DMI. Similarly, DW velocities driven by spin-orbit torque (SOT) are also enhanced in systems with large DMI.

In HM/Co/oxide trilayers the DMI strength and the magnetic anisotropy energy results from contributions of both the bottom and the top Co interfaces. When the latter are manipulated by magneto-ionic effects tuning the oxidation degree of the Co top interface, the resulting DW dynamics is strongly affected. This will be demonstrated for the case of a Pt/Co/AlOx trilayer. The propagation direction of SOT-driven chiral textures depends also on the sign of the DMI. We will show that this can be reversed by finely tuning the degree of oxidation of the FM layer with a gate voltage, which in Ta/CoFeB/TaOx trilayers can lead to a reversal of skyrmion direction of motion. Hard x-ray photoelectron spectroscopy measurements on Pt/Co/oxide/HfO2 capacitor-like devices allowed us to prove that in our integrated devices, the gate voltage modifies the PMA and the DMI through the modification of the oxidation degree of the Co layer, driven by oxygen-ion migration through the HfO2 dielectric layer.

This local degree of freedom at the nanometer scale controlled with gate voltages compatible with applications could lay the foundations for efficient architectures involving domain walls or magnetic skyrmions as information carriers.

Dr Stefania Pizzini

Institut Néel, CNRS, France

Dr Stefania Pizzini

Institut Néel, CNRS, France

Stefania Pizzini obtained her Physics Degree from Università degli Studi di Milano (Italy) in 1986. She then moved to the UK where she worked on the structural characterisation of condensed interfaces using x-ray absorption spectroscopy at the Daresbury synchrotron radiation sources, and obtained her PhD from the University of Strathclyde in Glasgow in 1990. In 1991 she moved to the French synchrotron radiation source laboratory in Paris with a Marie Curie PostDoc fellowship, where she worked on the characterization of magnetic properties of ultrathin magnetic films using x-ray circular magnetic dichroism (XMCD). In 1994 she obtained a research position at Laboratoire Louis Néel in Grenoble, laboratory associated to the Centre National de la Recherche Scientifique. In the 1990s she was involved in the implementation of XMCD at the energy-dispersive x-ray absorption beamline of ESRF and the development of time-resolved XMCD and XMCD-PEEM techniques. In more recent years she has specialised in the study of domain wall (DW) dynamics in chiral thin film heterostructures and the manipulation of magnetic properties with magneto-ionic effects.