Ilija Zeljkovic

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.

Electronic nematicity, the spontaneous breaking of rotational symmetry, has emerged as a key instability in correlated quantum systems. CsTi ₃ Bi ₅ , a kagome metal of the AV ₃ Sb ₅ (A = K, Rb, Cs) family, hosts rich unconventional electronic phases, yet the origin of its nematicity remains unsettled. Here, we combine polarization-dependent angle-resolved photoemission spectroscopy with functional renormalization group calculations on a fully interacting ab initio model. We reveal an orbital-selective nematic deformation in the low-energy band structure and identify a finite angular momentum ( d -wave) Pomeranchuk instability driven by electronic correlations in specific orbital channels and detuning from Van Hove singularities. Our results establish a direct link between orbital selectivity and symmetry-breaking instabilities in CsTi ₃ Bi ₅ , providing a microscopic framework for nematic order in kagome systems.

Three important driving forces for creating qualitatively new phases in quantum materials are the topology of the materials' electronic band structures, frustration in the electrons' motion or magnetic interactions, and strong correlations between their charge, spin, and orbital degrees of freedom. In very few material systems do all of these aspects come together to contribute on an equal footing to stabilize new electronic states with unprecedented properties; however the search for such systems can be guided by models of configurational motifs or key sublattices that can host such physics. One of the most fascinating structural motifs for realizing this rich interplay of frustration, electronic topology, and electron correlation effects is the kagome lattice. In this review, we provide an overview of the theoretical underpinnings driving the physics of kagome lattices, and we then discuss experimental progress in realizing novel states enabled by kagome networks in crystalline materials. Different material classes are discussed with an emphasis on the phenomenologies of their electronic states and how they map to interactions arising from their kagome lattices.