Magnonics promises low-dissipation information processing, yet spin-polarized magnon transport requires magnetic fields or spin–orbit couplings. Altermagnets exhibit spin-polarized electronic states and zero net magnetization. However, achieving large magnon spin splitting and robust magnonic spin currents remains challenging. Here we show that twisted van der Waals antiferromagnets provide a symmetry-tunable platform for the altermagnetic magnons. Alternating intralayer exchange arises in twisted bilayers lacking inversion and horizontal mirror symmetries, rendering nonrelativistic magnon spin splitting. Breaking out-of-plane rotational symmetries of a constituent monolayer significantly enhances low-energy splittings. We illustrate general conclusions in twisted CrPS₄ (d-wave) and CrI₃ (i-wave) bilayers. Moreover, pronounced field-free spin currents, characterized by robust spin Seebeck and spin Nernst effects, emerge in CrPS₄. Remarkably, the spin transport is efficiently tuned by twist angle and exceeds that of conventional altermagnets by orders of magnitude. Our work provides novel insights into controlling magnons, deepening our fundamental understanding of altermagnetic spintronics.

For several decades, it was widely believed that a noninteracting disordered electronic system could only undergo an Anderson metal-insulator transition due to Anderson localization. However, numerous recent theoretical works have predicted the existence of a disorder-driven non-Anderson phase transition that differs from Anderson localization. The frustration lies in the fact that this non-Anderson disorder-driven transition has not yet been experimentally demonstrated in any system. Here, using angle-resolved photoemission spectroscopy, we present a case study of observing the non-Anderson disorder-driven transition by visualizing the electronic structure of the Weyl semimetal NdAlSi on surfaces with varying amounts of disorder. Our observations reveal that strong disorder can effectively suppress all surface states in the Weyl semimetal NdAlSi, including the topological surface Fermi arcs. This disappearance of surface Fermi arcs is associated with the vanishing of the topological invariant, indicating a quantum phase transition from a Weyl semimetal to a diffusive metal. These observations provide direct experimental evidence of the non-Anderson disorder-driven transition occurring in real quantum systems, a finding long anticipated by theoretical physicists.

Non-Hermitian physics, studying systems described by non-Hermitian Hamiltonians, reveals unique phenomena not present in Hermitian systems. Unlike Hermitian systems, non-Hermitian systems have complex eigenvalues, making their effects less directly observable. Recently, significant efforts have been devoted to incorporating the non-Hermitian effects into condensed matter physics. However, progress is hindered by the absence of a viable experimental approach. Here, the discovery of the surface-selectively spontaneous reconstructed Weyl semimetal NdAlSi provides a feasible experimental platform for studying non-Hermitian physics. Utilizing angle-resolved photoemission spectroscopy (ARPES) measurements, surface-projected density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements, it is demonstrated that surface reconstruction in NdAlSi alters surface Fermi arc (SFA) connectivity and generates new isolated non-topological SFAs (NTSFAs) by introducing non-Hermitian terms. The surface-selective spontaneous reconstructed Weyl semimetal NdAlSi can be viewed as a Hermitian bulk – non-Hermitian boundary system. The isolated non-topological SFAs on the reconstructed surface act as a loss mechanism and open boundary condition (OBC) for the topological electrons and bulk states, serving as non-Hermitian boundary states. This discovery provides a good experimental platform for exploring new physical phenomena and potential applications based on boundary non-Hermitian effects, extending beyond purely mathematical concepts. Furthermore, it provides important enlightenment for constructing topological photonic crystals with surface reconstruction and studying their topological properties.

Altermagnets constitute a novel, third fundamental class of collinear magnetic ordered materials, alongside with ferro- and antiferromagnets. They share with conventional antiferromagnets the feature of a vanishing net magnetization. At the same time they show a spin-splitting of electronic bands, just as in ferromagnets, caused by the atomic exchange interaction. On the other hand, topology has recently revolutionized our understanding of condensed matter physics, introducing new phases of matter classified by intrinsic topological order. Here we connect the worlds of altermagnetism and topology, showing that the electronic structure of the altermagnet CrSb is topological. Using high-resolution angle-resolved photoemission spectroscopy, we observe the large momentum-dependent spin-splitting in CrSb that induces altermagnetic Weyl nodes. We observe the related topological Fermi-arcs, which in electronic structure calculations are spin polarized. This indicates that in altermagnets the large energy scale intrinsic to their spin-splitting creates its own realm of robust electronic topology.

Electron-doped cuprates consistently exhibit strong antiferromagnetic correlations, leading to the prevalent belief that antiferromagnetic spin fluctuations mediate Cooper pairing in these unconventional superconductors. However, early investigations showed that although antiferromagnetic spin fluctuations create the largest pseudogap at hot spots in momentum space, the superconducting gap is also maximized at these locations. This presented a paradox for spin-fluctuation-mediated pairing: Cooper pairing is strongest at momenta where the normal-state low-energy spectral weight is most suppressed. Here we investigate this paradox and find evidence that a gossamer—meaning very faint—Fermi surface can provide an explanation for these observations. We study Nd2–xCexCuO4 using angle-resolved photoemission spectroscopy and directly observe the Bogoliubov quasiparticles. First, we resolve the previously observed reconstructed main band and the states gapped by the antiferromagnetic pseudogap around the hot spots. Within the antiferromagnetic pseudogap, we also observe gossamer states with distinct dispersion, from which coherence peaks of Bogoliubov quasiparticles emerge below the superconducting critical temperature. Moreover, the direct observation of a Bogoliubov quasiparticle permits an accurate determination of the superconducting gap, yielding a maximum value an order of magnitude smaller than the pseudogap, establishing the distinct nature of these two gaps. We propose that orientation fluctuations in the antiferromagnetic order parameter are responsible for the gossamer states.

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.

Here, we present a high repetition rate, narrow bandwidth, extreme ultraviolet photon source for time- and angle-resolved photoemission spectroscopy. The narrow bandwidth pulses Δ E = 9, 14, and 18 meV for photon energies h ν = 10.8, 18.1, and 25.3 eV are generated through high harmonic generation using ultra-violet drive pulses with relatively long pulse lengths (461 fs). The high harmonic generation setup employs an annular drive beam in tight focusing geometry at a repetition rate of 250 kHz. Photon energy selection is provided by a series of selectable multilayer bandpass mirrors and thin film filters, thus avoiding any time broadening introduced by single grating monochromators. A two stage optical-parametric amplifier provides < 100 fs tunable pump pulses from 0.65 μm to 9 μm. The narrow bandwidth performance of the light source is demonstrated through angle-resolved photoemission measurements on a series of quantum materials, including high-temperature superconductor Bi-2212, WSe2, and graphene. 

Materials with a kagome lattice structure display a wealth of intriguing magnetic properties due to their geometric frustration and intrinsically flat band structure. Recently, topological and superconducting states have also been observed in kagome systems. The kagome lattice may also host a "breathing" mode that leads to charge density wave (CDW) states, if there is strong electron-phonon coupling, electron-electron interaction, or external excitation of the material. This "breathing" mode can give rise to candidate distortions such as the star of David (SoD) or its inverse structure [trihexagonal (TrH)]. To date, in most materials, only a single type of distortion has been observed. Here, we present angle-resolved photoemission spectroscopy measurements on the kagome superconductor CsV3Sb5 at multiple temperatures and photon energies to reveal the nature of the CDW in this material. It is shown that CsV3Sb5 displays two intertwined CDW orders corresponding to the SoD and TrH distortions. These two distinct types of distortions are stacked along the c direction to form a three-dimensional CDW order where the two 2-fold CDWs are phase shifted along the c axis. The presented results provide not only key insights into the nature of the unconventional CDW order in CsV3Sb5, but also an important reference for further studies on the relationship between the CDW and superconducting order.

The preparation of extensive, atomically flat surfaces remains a central challenge in modern quantum materials research, as many crystals lack natural cleavage planes suitable for advanced surface-sensitive investigations. Here, we demonstrate that laser scribing guided by an ultrafast laser can be applied to facilitate easy cleavage along a desired crystallographic plane under ultra-high vacuum. The method is validated on two brittle materials, SrTiO3 and Si. The technique allows precise spatial localization of the cleaving site and produces extensive, uniformly oriented, and atomically flat surfaces, as verified by scanning electron microscopy (SEM) and atomic force microscopy (AFM). When applied to SrTiO3, the technique enables angle-resolved photoemission spectroscopy (ARPES) measurements of surface electronic states characteristic of the two-dimensional electron liquid (2DEL) hosted at its bare (100) surface. Moreover, ultrafast laser scribing is significantly faster than focused ion beam (FIB) techniques for preparing cleavable planes, offering a more accessible and efficient approach. Owing to its broad applicability, this method establishes a powerful and general framework to prepare high-quality surfaces for advanced photoemission and microscopic investigations of quantum phenomena.

Controlling the carrier density and band alignment in three-dimensional topological insulators remains a central challenge due to intrinsic defect-induced n-type doping. In Bi2Se3, selenium vacancies typically shift the Fermi level into the bulk conduction band, complicating access to the intrinsic surface transport regime. Here, we investigate the effect of chlorine incorporation on the electronic structure of Bi₂Se₃ single crystals using angle-resolved photoemission spectroscopy (ARPES). Across nominal Cl concentrations of 0.25%-1.0%, the topological surface state remains gapless and well-defined, while the band structure shifts systematically toward higher binding energy, indicating enhanced electron doping. Quantitative analysis of the Fermi surface areas reveals an increase in the enclosed momentum-space area of both the topological surface state and conduction-band-derived states with increasing Cl content. Hall measurements further confirm an increase in bulk carrier density, consistent with donor behavior of Cl incorporation. Time-dependent ARPES measurements tracking the surface electronic evolution after cleaving demonstrate a significantly reduced energy shift under prolonged XUV exposure in Cl-doped samples compared to pristine Bi₂Se₃, indicating reduced sensitivity to adsorption-induced band bending and suggesting a modification of the near-surface defect landscape. Despite the increased carrier density, the bulk band gap remains essentially unchanged, while the relative energy position of the Dirac point within the gap evolves modestly with Cl concentration.