Magnons, the quanta of spin waves, can serve as information carriers with promising applications in ultralow-power spintronic devices. Specifically, magnon Chern insulators (MCIs) exhibit topological magnon band structures featuring chiral edge states. Direct observations of the topologically protected magnon edge states have long been pursued. Here, we report the spatially resolved detection of magnon edge states in a two-dimensional ferromagnet with a honeycomb lattice (single-layer chromium triiodide). Using scanning tunneling microscopy, we observed magnon-assisted inelastic tunneling conductance and revealed the gapped magnon spectra with enhanced signals at the van Hove singularities. Extra tunneling conductance contributed from the magnon edge states was detected at different edge configurations. Our work provided direct evidence proving the existence of MCI states down to the single-layer limit, initiating spatially resolved explorations of exotic properties arising from topological edge states of MCIs.

Electronic nematicity, the spontaneous breaking of rotational symmetry, has emerged as a key instability in correlated quantum systems. CsTi3Bi5, a kagome metal of the AV3Sb5 (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 CsTi3Bi5, providing a microscopic framework for nematic order in kagome systems.

The recent observation of superconductivity in the vicinity of Fermi surface reconstructed insulating or metallic states has established twisted bilayers of WSe 2 as an exciting platform to study the interplay of strong electron-electron interactions, broken symmetries, and topology. In this work, we use a first-principles, material-specific theoretical treatment that is unbiased with respect to electronic instabilities to study the emergence of electronic ordering in twisted WSe 2 driven by gate-screened Coulomb interactions. We construct exponentially localized moiré Wannier orbitals that faithfully capture the band structure and topology of the system, project the gate-screened Coulomb interaction onto them, and use unbiased functional renormalization group techniques to resolve the momentum and orbital structure of the leading instabilities and the relevant energy scales. We find an interplay between intervalley-coherent antiferromagnetic (IVC-AFM) order and chiral, mixed-parity d / p -wave superconductivity for carrier concentrations near a displacement field and twist-angle-tunable van Hove singularity. Our microscopic approach establishes incommensurate IVC-AFM spin fluctuations as the dominant electronic mechanism driving the formation of superconductivity in θ = 5.08 ° twisted WSe 2 and explains key aspects of recent experiments including the asymmetric density dependence of the spin ordering with respect to the van Hove line, the single- and double-peak structure of the DOS in the ordered (hole-doped) IVC-AFM phase, the emergence of superconductivity as the density is varied across the van Hove line, and the evolution of the displacement field-density phase diagram with twist angles between 3.7 ° … 5 ° . We show that the interplay of electronic correlations and nontrivial quantum geometry in tWSe 2 manifests in orbital-selective order parameters associated to the IVC-AFM and SC states that are detectable by local spectroscopy measurements.

Identifying high-temperature unconventional charge order and superconductivity in kagome systems is crucial for understanding frustrated, correlated electrons and enabling future quantum technologies. Here, we report that the kagome superconductor YRu3Si2 hosts an exceptional interplay of charge order, magnetism, and superconductivity, revealed through a comprehensive suite of muon spin rotation (μSR), magnetotransport, X-ray diffraction, and density functional theory (DFT). We identify a high-temperature charge-ordered state with propagation vector (1/2,0,0) and a record onset temperature of 800 K, unprecedented in kagome systems and quantum materials more broadly. μSR measurements further reveal time-reversal symmetry-breaking below 25 K and field-induced magnetism near 90 K, features mirrored in the magnetoresistance, which reaches 45% at low temperatures. Band-structure calculations show two van Hove singularities near the Fermi level, including one within a flat band. At low temperatures, YRu3Si2 becomes superconducting below Tc = 3.4 K with either two full isotropic gaps or an anisotropic nodeless gap. These results establish YRu3Si2 as a prime platform for studying correlated kagome physics.

Field-effect transistors (FETs) with sub-60 mV/decade subthreshold swing (SS) at room temperature are highly sought after for enabling next-generation ultralow-power integrated circuits (ICs). We present a van Hove source (VHS) FET that exploits the steeply declining density of states (DOS) at the van Hove singularity in a one-dimensional (1D) semiconductor to overcome the Boltzmann limit on switching performance in conventional FETs. The VHS FETs built on individual semiconducting carbon nanotubes (CNT) exhibit a room temperature SS of 49 mV/decade. This steep switching behavior is achieved by electrostatically tuning the source Fermi level through a control gate. Compared with the 22-nanometer-node silicon FETs, a comparable on-state current is obtained in our VHS FETs with a 450 nm gate length but at a reduced supply voltage of 0.5 V (versus 0.75 V for silicon). VHS engineering may offer a generalizable pathway for 1D semiconductors to lower SS and even construct steep-slope transistors that simultaneously deliver ultralow power, high performance, and scalability.

Van Hove (VH) singularities in the single-particle band spectrum are important for interaction-driven quantum phases. Whereas VH points are usually spin-degenerate, in newly proposed altermagnets VH singularities can become spin-dependent, due to momentum-dependent spin polarization of the Fermi surfaces arising from combined rotation and time-reversal symmetry. We consider two altermagnetic models ( d x 2 − y 2 - and d x y -wave) on a square lattice with spin-polarized VH points, and study their stable fixed-point solutions indicating interaction-induced instabilities using parquet renormalization group. For both models, we find new stable fixed-point solutions of the renormalization group equations which are not connected to the solution in the spin-degenerate limit. This implies that on the square lattice, the system with VH singularities is unstable with respect to altermagnetic perturbations. The leading instability for the d x 2 − y 2 model is real transverse spin density wave. For the d x y -wave model, it is found to be real transverse spin density wave at large altermagnetic splitting. At small altermagnetic splitting both imaginary charge density wave and real longitudinal spin density waves are dominant.

The interplay of high-order Van Hove singularities and topology plays a central role in determining the nature of the electronic correlations governing the phase of a system with unique signatures characterizing their presence. Layered van der Waals heterostuctures are ideal systems for band engineering through the use of twisting and proximity effects. Here, we use symmetry to demonstrate how twisted kagome bilayers can host topological high-order Van Hove singularities. We study a commensurate system with a large twist angle and demonstrate how the initial choice of high-symmetry stacking order can greatly influence the electronic structure and topology of the system. Furthermore, we study the possibility of sublattice interference in the system. Our results illustrate the rich energy landscape of twisted kagome bilayers and unveil large Chern numbers (of order 10), establishing twisted bilayer kagome as a natural playground for probing the mixing of strong correlations and topology.

Recent experiments have revealed that many kagome metals exhibit sublinear temperature dependence in resistivity up to room temperature. Here, we develop a minimal semiclassical two-pocket model—comprising a Dirac cone and a Van Hove singularity—and show that, within an extended Fermi liquid scattering framework, internode electron-electron interactions naturally lead to sublinear scaling in both electrical and thermal transport at low temperatures. At higher temperatures, distinct scattering channels for charge and heat currents lead to a violation of the Wiedemann-Franz law. Our work provides a simple and broadly applicable framework for understanding anomalous transport in kagome metals, capturing non-Fermi-liquid behavior without requiring fine-tuning or exotic interactions.

Abstract The kagome metals A V 3 Sb 5 ( A = K, Rb, Cs) feature intertwined Dirac fermions, topological flat bands, and van Hove singularities (vHS) near the Fermi level, which give rise to a range of exotic, strongly correlated phenomena such as charge density waves (CDW) and superconductivity. Although the vHS from V 3d states have been implicated in CDW formation, their three-dimensional nature and temperature evolution remain poorly understood. In this study, we used high-resolution angle-resolved photoemission spectroscopy and density functional theory to reveal pronounced out-of-plane dispersion of vHS and their temperature dependence in KV 3 Sb 5 . The identified c -axis band folding and scattering channels were directly linked to the CDW order. These results demonstrate that the CDW transition in this family involves cooperative coupling between electron correlations and structural modulation along the c axis. This offers new insights into the interplay of topology, correlations, and lattice instabilities in kagome metals.

In this paper, we report an ab-initio investigation on the electronic structure of iron-based ladder compounds AFe[Formula: see text] ([Formula: see text], Ba, Sr; [Formula: see text], Se). By gradually varying the pressure, the studied systems become metallic and the stripe-type antiferromagnetic (AFM) order disappears and is replaced by the nonmagnetic (NM) state. Magnetic moments vanish at different pressures in the studied two-leg ladder materials. The insulator–metal transitions, accompanied by the quenching of local magnetic moments and the reduction of electron correlation effects under pressure, are crucial in elucidating the Mott nature of the system. On the other hand, the density of states at the Fermi level [Formula: see text], the van Hove singularity energy ([Formula: see text]), the band structures and the Fermi surfaces are also studied and are used as criteria in order to select possible superconducting materials. Experiments are strongly solicited to confirm the nature of the observed transitions and the potential metallic or superconductivity character in the studied hypothetical systems under pressure.

Abstract The geometric frustration inherent in the kagome lattice gives rise to exotic electronic structures—combining Dirac dispersions, van Hove singularities, and flat bands. Recent discoveries in stacked kagome materials with strong interlayer coupling have reignited questions about the origin and tunability of these distinctive flat bands. In this work, we propose an exact analytical decimation transformation scheme to explore the coexistence of flat bands and Dirac fermions in three-dimensional coupled kagome systems. This method coarse-grains the parameter space, mapping the original system onto an equivalent reduced lattice. The decimated model identifies a key parameter governing flat-band formation and provides a criterion for absolute flatness. In terms of atomic separations, we define a quantity that primarily controls the flat-band width, enabling prediction and tunability in real materials. We validate our framework for the M 3 X (M = Ni, Mn, Co, Fe; X = Al, Ga, In, Sn, Cr, …) family using materials databases and first-principles calculations. This analytical formalism offers a practical route for accurate prediction and design of flat bands in realistic kagome systems.

The divergence of the density of states (DOS) near the Fermi energy plays a critical role in strengthening electron correlations. van Hove singularities (vHS) are a common source of this DOS divergence, and their characteristics depend on the dimensionality of electron dispersion. The atomic arrangement on the surface can alter the effective dimensionality of electron dispersion and, consequently, vHS. In V-based Kagome metals AV3Sb5 (A = K, Rb, and Cs), the saddle-shaped dispersion results in two-dimensional vHS. Intriguingly, the surface arrangement of atoms in a Kagome lattice can reduce the effective dimensionality of electron dispersion, enhancing electron correlations. By taking advantage of nearly closed shell electronic character of Rb atoms, their linear arrangement imposes a one-dimensional localized potential on the underlying Kagome lattice. As a result, we observe a significantly increased divergence of DOS along with an augmentation of the charge density wave, potentially driven by reinforced electronic correlations.

SummaryRecently, considerable attention has been given to the exploration of superconductivity in low-dimensional materials. By employing unbiased structure search and first-principles calculations, we systematically investigate the structural and electronic properties of the predicted two-dimensional (2D) compounds SiBC4, SiBC6, and SiB4C in the monolayer limit. These structures demonstrate high thermal stability at 1,500 K and meet the criteria for mechanical and dynamic stability. Most importantly, they are identified as intrinsic superconductors with superconducting transition temperatures (Tc) of 17.59, 12.18, and 10.91 K, respectively. The van Hove singularity caused by the high density of states near the Fermi surface plays a crucial role in the emergence of superconductivity. Furthermore, by applying compressive/tensile strains on SiBC4, it is observed that the tensile strain dramatically softens the phonon branch, as well as increases the electron-phonon coupling (EPC) and Tc. Our findings not only contribute to the development of novel 2D superconducting materials but also present an excellent platform to investigate the role of van Hove singularity in emergent superconductivity.

Reaching the van Hove singularity (VHS) in a material enables the emergence of exotic electronic and magnetic phases, such as superconductivity and the quantum anomalous Hall effect. This is demonstrated in cuprates, magic-angle bilayer graphene, and more recently, monolayer graphene interfaced with alkali and rare earth elements. Here, the europium density at the graphene/rhenium interface is modulated to tune the electron doping level in monolayer graphene across the VHS point, forming either a dense or diluted europium phase. The dense phase enables flat bands at the Fermi level, while graphene remains decoupled from the Re(0001) substrate in both cases. The Dirac point is shifted over 1.5eV below the Fermi level, and europium lifts the degeneracy of the Dirac cones: one branch hybridizes with Eu 4f states, the other retains Dirac-like dispersion, as corroborated by density functional theory. X-ray absorption spectroscopy reveals a mixed Eu(II)/Eu(III) valence state in the dense phase and the persistence of Eu magnetic response up to room temperature in both. The intercalated phases exhibit exceptional thermal stability, with the diluted phase stable up to 960 K. These results highlight the potential of rare-earth-doped graphene for engineering flat bands, tunable Dirac-cone splitting, and robust interfacial magnetism.

Charge and pair density waves in a spin- and valley-polarized system at a Van Hove singularity

On the kagome lattice, electrons benefit from the simultaneous presence of band topology, flat electronic bands, and Van Hove singularities, forming competing or cooperating orders. Understanding the interrelation between these distinct order parameters remains a significant challenge, leaving much of the associated physics unexplored. In the kagome superconductor KV_{3}Sb_{5}, which exhibits a charge density wave (CDW) state below T≃78 K, we uncover an unpredicted field-induced phase transition below 6K. The observed transition is marked by a hysteretic anomaly in the resistivity, nonlinear electrical transport, and a change in the symmetry of the electronic response as probed via the angular dependence of the magnetoresistivity. These observations surprisingly suggest the emergence of an unanticipated broken symmetry state coexisting with the original CDW. To understand this experimental observation, we developed a theoretical minimal model for the normal state inside the high-temperature parent CDW phase, where an incommensurate CDW order emerges as an instability subleading to superconductivity. The incommensurate CDW emerges when superconducting fluctuations become fully suppressed by large magnetic fields. Our results suggest that, in kagome superconductors, quantum states can either coexist or are nearly degenerate in energy, indicating that these are rich platforms to expose new correlated phenomena.

Engineering van Hove singularities (vHss) near the Fermi level, if feasible, offers a powerful route to control exotic quantum phases in electronic and magnetic behaviors. However, conventional approaches rely primarily on chemical and electrical doping and focus mainly on local electrical or optical measurements, limiting their applicability to coupled functionalities. In this study, a vHs-induced insulator-metal transition coupled with a ferromagnetic phase transition was empirically achieved in atomically designed quasi-2D SrRuO3 (SRO) superlattices via epitaxial strain engineering, which has not been observed in conventional 3D SRO systems. Theoretical calculations revealed that epitaxial strain effectively modulates the strength and energy positions of vHs of specific Ru orbitals, driving correlated phase transitions in the electronic and magnetic ground states. X-ray absorption spectroscopy confirmed the anisotropic electronic structure of quasi-2D SRO modulated by epitaxial strain. Magneto-optic Kerr effect and electrical transport measurements demonstrated modulated magnetic and electronic phases. Furthermore, magneto-electrical measurements detected significant anomalous Hall effect signals and ferromagnetic magnetoresistance, indicating the presence of magnetically coupled charge carriers in the 2D metallic regime. This study establishes strain engineering as a promising platform for tuning vHss and resultant itinerant ferromagnetism of low-dimensional correlated quantum systems.

Kagome materials serve as a versatile platform where an interplay of flat bands, Dirac Fermions, and Van Hove singularities enables the emergence of exotic strongly correlated phenomena. Recently, it was predicted that an ideal single layer kagome lattice may host high-order Van Hove singularities (HOVHSs) characterized by extremely flat dispersions, leading to drastic changes in electronic behavior. However, experimentally, HOVHSs have been observed up to now only in a narrow range of materials, mostly in graphene layers, but not in metal-semiconductor interfaces. Here, we report the discovery of HOVHSs in the monolayer-thick kagome metal LaTl3 epitaxially synthesized on the Si(111) substrate. The scanning tunneling microscopy observations and ab initio calculations indicate the kagome-like ordering of the LaTl3 layer, while the angle-resolved photoemission spectroscopy measurements and theoretical predictions uncover a rich and complex landscape of various Van Hove singularities emerged in the system, including high-order ones, which can significantly affect the anomalous Hall response and enable the unique high electron-correlation regime in the system. The discovered properties make the LaTl3 kagome monolayer a highly attractive material for ultracompact nanoelectronic devices.

AB-stacked bilayer graphene has emerged as a fascinating yet simple platform for exploring macroscopic quantum phenomena of correlated electrons. Under large electric displacement fields and near low-density van-Hove singularities, it exhibits a phase with features consistent with Wigner crystallization, including negative dR/dT and nonlinear bias behavior. However, direct evidence for the emergence of an electron crystal at zero magnetic field remains elusive. Here, we explore low-frequency noise consistent with depinning and sliding of a Wigner crystal or solid. At large magnetic fields, we observe enhanced noise at low bias current and a frequency-dependent response characteristic of depinning and sliding, consistent with earlier scanning tunnelling microscopy studies confirming Wigner crystallization in the fractional quantum Hall regime. At zero magnetic field, we detect pronounced AC noise whose peak frequency increases linearly with applied DC current-indicative of collective electron motion. These transport signatures pave the way toward confirming an anomalous Hall crystal.

Motivated by recent advances in the realization of Truchet-tiling structures in molecular networks and metal-organic frameworks, we investigate the wave localization issue in this kind of structure. We introduce an electron model based on random Truchet tilings, square lattices with randomly oriented diagonal links, and uncover a rich interplay between spectral and localization phenomena. By varying the strength of diagonal couplings, we explore successive transitions from an extended phase, through a regime with a mobility edge, to a fully localized phase. The energy-resolved fractal dimension analysis captures the emergence and disappearance of mobility edges, while an anomalous shift and asymmetry in the Van Hove singularity are identified as key signatures of the underlying disordered Truchet-tiling structure. Notably, by using the finite-size scaling of level spacing statistics, we clarify that the transition occurs at a finite level of disorder even in the two-dimensional system. Our findings position Truchet-tiled electron systems as a versatile platform for engineering disorder-driven localization and interaction effects in amorphous quantum materials and photonic architectures.