Siyu Cheng

Altermagnetism defies conventional classifications of collinear magnetic phases, combining compensated magnetization with momentum‐dependent, nonrelativistic spin splitting. While altermagnetic band splitting has been established in three‐dimensional compounds such as MnTe and CrSb, layered altermagnets remain far less explored spectroscopically. Here, combining termination‐resolved micro‐ARPES, fine photon‐energy mapping, spin‐ARPES, and density functional theory, we map the three‐dimensional fermiology of the layered g ‐wave altermagnet , obtained by ordered Co intercalation of 2H‐. We resolve the spin–split branches of the altermagnetic pairs directly in the raw photoemission spectra and, with spin‐ARPES at these experimentally identified momenta, show the reversal of the spin polarization at opposite momenta and between two regions of the sample, consistent with opposite time‐reversal domains, and its absence along the symmetry‐protected nodal –K direction. We further report a weak low‐temperature anomaly near T ₀ ≈ 50–60 K, detected by magnetometry, temperature‐dependent micro‐ARPES, and ultrafast reflectivity, whose microscopic origin remains open. These results provide a momentum‐resolved benchmark of the electronic structure of a layered altermagnet.

Altermagnets recently emerged as a new class of magnetic materials, arising from specific spin crystal symmetries. They exhibit a spin-polarized electronic band structure similar to ferromagnets, yet possess zero net magnetization, promising exotic properties. Here we study a layered triangular lattice altermagnet, cobalt-intercalated NbSe ₂ using scanning tunneling microscopy and spectroscopy (STM/S). Spectroscopic-imaging STM and spin-polarized STM reveals emergent 2 a ₀ tri-directional charge and spin density modulations on the selenium surface. Density functional theory simulations suggest these modulations reflect the underlying cobalt superstructure. We discover that an out-of-plane magnetic field tunes the modulation amplitudes and the electronic density-of-states in a manner dependent on the field direction and strength. This behavior is attributed to the field-induced tilting of cobalt spins, which can have profound implications on the electronic properties of the altermagnet. Our results provide atomic-scale insights to uncover a magnetic-field tunable altermagnetic band structure, highlight the importance of understanding spin canting in altermagnets.

Charge, spin and Cooper-pair density waves have now been widely detected in exotic superconductors. Understanding how these density waves emerge — and become suppressed by external parameters — is a key research direction in condensed matter physics. Here we study the temperature and magnetic-field evolution of charge density waves in the rare spin-triplet superconductor candidate UTe₂ using scanning tunneling microscopy/spectroscopy. We reveal that charge modulations composed of three different wave vectors gradually weaken in a spatially inhomogeneous manner, while persisting to surprisingly high temperatures of 10–12 K. We also reveal an unexpected decoupling of the three-component charge density wave state. Our observations match closely to the temperature scale potentially related to short-range magnetic correlations, providing a possible connection between density waves observed by surface probes and intrinsic bulk features. Importantly, charge density wave modulations become suppressed with magnetic field both below and above superconducting T_c in a comparable manner. Our work points towards an intimate connection between hidden magnetic correlations and the origin of the unusual charge density waves in UTe₂.

Charge density waves (CDWs) in kagome metals have been tied to many exotic phenomena. Here, using spectroscopic-imaging scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we study the charge order in kagome metal ScV₆Sn₆. The similarity of electronic band structures of ScV₆Sn₆ and TbV₆Sn₆ (where charge ordering is absent) suggests that charge ordering in ScV₆Sn₆ is unlikely to be primarily driven by Fermi surface nesting of the Van Hove singularities. In contrast to the CDW state of cousin kagome metals, we find no evidence supporting rotation symmetry breaking. Differential conductance dI/dV spectra show a partial gap Δ¹_CO ≈ 20 meV at the Fermi level. Interestingly, dI/dV maps reveal that charge modulations exhibit an abrupt phase shift as a function of energy at energy much higher than Δ¹_CO, which we attribute to another spectral gap. Our experiments reveal a distinctive nature of the charge order in ScV₆Sn₆ with fundamental differences compared to other kagome metals.