Observation of a linked-loop quantum state in a topological magnet.

Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state1-13. Over the past decades, these invariants have come to play a central role in describing matter, providing the foundation for understanding superfluids5, magnets6,7, the quantum Hall effect3,8, topological insulators9,10, Weyl semimetals11-13 and other phenomena. Here we report an unusual linking-number (knot theory) invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet14-20. Using state-of-the-art spectroscopic methods, we directly observe three intertwined degeneracy loops in the material's three-torus, T3, bulk Brillouin zone. We find that each loop links each other loop twice. Through systematic spectroscopic investigation of this linked-loop quantum state, we explicitly draw its link diagram and conclude, in analogy with knot theory, that it exhibits the linking number (2, 2, 2), providing a direct determination of the invariant structure from the experimental data. We further predict and observe, on the surface of our samples, Seifert boundary states protected by the bulk linked loops, suggestive of a remarkable Seifert bulk-boundary correspondence. Our observation of a quantum loop link motivates the application of knot theory to the exploration of magnetic and superconducting quantum matter.

Revealing the Heavy Quasiparticles in the Heavy-Fermion Superconductor CeCu_{2}Si_{2}.

The superconducting order parameter of the first heavy-fermion superconductor CeCu_{2}Si_{2} is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here, we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy-fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.

A five-axis parallel kinematic mirror unit for soft X-ray beamlines at MAX†IV.

With the introduction of the multi-bend achromats in the new fourth-generation storage rings the emittance has decreased by an order of magnitude resulting in increased brightness. However, the higher brightness comes with smaller beam sizes and narrower radiation cones. As a consequence, the requirements on mechanical stability regarding the beamline components increases. Here an innovative five-axis parallel kinematic mirror unit for use with soft X-ray beamlines using off-axis grazing-incidence optics is presented. Using simulations and measurements from the HIPPIE beamline at the MAX IV Laboratory it is shown that it has no Eigen frequencies below 90 Hz. Its positioning accuracy is better than 25 nm linearly and 17-35 µrad angularly depending on the mirror chamber dimensions.

Emergence of Weyl fermions by ferrimagnetism in a noncentrosymmetric magnetic Weyl semimetal.

Condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics. Here, using angle-resolved photoemission spectroscopy, we provide an example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the emergence of new Weyl fermions in the ferrimagnetic state. Our results establish NdAlSi as a magnetic Weyl semimetal and provide an experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.

A Room-Temperature Spin-Valve with van der Waals Ferromagnet Fe5 GeTe2 /Graphene Heterostructure.

The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe5 GeTe2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe5 GeTe2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe5 GeTe2 /graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe5 GeTe2 along with the presence of negative spin polarization at the Fe5 GeTe2 /graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

Modeling engagement in long-term, in-home socially assistive robot interventions for children with autism spectrum disorders.

Socially assistive robotics (SAR) has great potential to provide accessible, affordable, and personalized therapeutic interventions for children with autism spectrum disorders (ASD). However, human-robot interaction (HRI) methods are still limited in their ability to autonomously recognize and respond to behavioral cues, especially in atypical users and everyday settings. This work applies supervised machine-learning algorithms to model user engagement in the context of long-term, in-home SAR interventions for children with ASD. Specifically, we present two types of engagement models for each user: (i) generalized models trained on data from different users and (ii) individualized models trained on an early subset of the user's data. The models achieved about 90% accuracy (AUROC) for post hoc binary classification of engagement, despite the high variance in data observed across users, sessions, and engagement states. Moreover, temporal patterns in model predictions could be used to reliably initiate reengagement actions at appropriate times. These results validate the feasibility and challenges of recognition and response to user disengagement in long-term, real-world HRI settings. The contributions of this work also inform the design of engaging and personalized HRI, especially for the ASD community.

A simple electron counting model for half-Heusler surfaces.

Heusler compounds are a ripe platform for discovery and manipulation of emergent properties in topological and magnetic heterostructures. In these applications, the surfaces and interfaces are critical to performance; however, little is known about the atomic-scale structure of Heusler surfaces and interfaces or why they reconstruct. Using a combination of molecular beam epitaxy, core-level and angle-resolved photoemission, scanning tunneling microscopy, and density functional theory, we map the phase diagram and determine the atomic and electronic structures for several surface reconstructions of CoTiSb (001), a prototypical semiconducting half-Heusler. At low Sb coverage, the surface is characterized by Sb-Sb dimers and Ti vacancies, while, at high Sb coverage, an adlayer of Sb forms. The driving forces for reconstruction are charge neutrality and minimizing the number of Sb dangling bonds, which form metallic surface states within the bulk bandgap. We develop a simple electron counting model that explains the atomic and electronic structure, as benchmarked against experiments and first-principles calculations. We then apply the model to explain previous experimental observations at other half-Heusler surfaces, including the topological semimetal PtLuSb and the half-metallic ferromagnet NiMnSb. The model provides a simple framework for understanding and predicting the surface structure and properties of these novel quantum materials.