Topological surface states and flat bands in the kagome superconductor CsV3Sb5

Fully-gapped superconductivity with rotational symmetry breaking in pressurized kagome metal CsV3Sb5

The discovery of the kagome metal CsV3Sb5 has generated significant interest in its complex physical properties, particularly its superconducting behavior under different pressures, though its nature remains debated. Here, we performed low-temperature, high-pressure 121/123Sb nuclear quadrupole resonance (NQR) measurements to explore the superconducting pairing symmetry in CsV3Sb5. At ambient pressure, we found that the spin-lattice relaxation rate 1/T1 exhibits a kink at T ~ 0.4 Tc within the superconducting state and follows a T3 variation as temperature further decreases. This suggests the presence of two superconducting gaps with line nodes in the smaller one. As pressure increases beyond Pc ~ 1.85 GPa, where the charge-density wave phase is completely suppressed, 1/T1 shows no Hebel-Slichter peak just below Tc, and decreases rapidly, even faster than T5, indicating that the gap is fully opened for pressures above Pc. In this high pressure region, the angular dependence of the in-plane upper critical magnetic field Hc2 breaks the C6 rotational symmetry. We propose the s + id pairing at P > Pc which explains both the 1/T1 and Hc2 behaviors. Our findings indicate that CsV3Sb5 is an unconventional superconductor and its superconducting state is even more exotic at high pressures.

Electron correlation and incipient flat bands in the Kagome superconductor CsCr3Sb5

Correlated kagome materials exhibit a compelling interplay between lattice geometry, electron correlation, and topology. In particular, the flat bands near the Fermi level provide a fertile playground for novel many-body states. Here we investigate the electronic structure of CsCr3Sb5 using high-resolution angle-resolved photoemission spectroscopy and ab-initio calculations. Our results suggest that Cr 3d electrons are intrinsically incoherent, showing strong electron correlation amplified by Hund's coupling. Notably, we identify incipient flat bands close to the Fermi level, which are expected to significantly influence the electronic properties of the system. Across the density-wave-like transition at 55 K, we observe a drastic enhancement of the electron scattering rate, which aligns with the semiconducting-like property at high temperatures. These findings establish CsCr3Sb5 as a strongly correlated Hund's metal with incipient flat bands near the Fermi level, which provides an electronic basis for understanding its novel properties compared to the weakly correlated AV3Sb5.

Discovery of a New Phase in Thin Flakes of KV3Sb5 under Pressure

Results of magnetotransport measurements are reported on KV3Sb5 thin flakes under pressure. The zero-field electrical resistance reveals an additional anomaly emerging under pressure (p), marking a previously unidentified phase boundary T*(p). Together with the established TCDW(p) and Tc(p), denoting the charge-density-wave transition and a superconducting transition, respectively, the temperature-pressure phase diagram of KV3Sb5 features a rich interplay among multiple phases. The Hall coefficient evolves reasonably smoothly when crossing the T* phase boundary compared with the variation when crossing TCDW, indicating the preservation of the pristine electronic structure. The mobility spectrum analysis provides further insights into distinguishing different phases. Finally, the high-pressure quantum oscillation studies up to 31 T combined with the density functional theory calculations further demonstrate that the new phase does not reconstruct the Fermi surface, confirming that the translational symmetry of the pristine metallic state is preserved.

Diverse Manifestations of Electron-Phonon Coupling in a Kagome Superconductor

Recent angle-resolved photoemission spectroscopy (ARPES) experiments on the kagome metal CsV_{3}Sb_{5} revealed distinct multimodal dispersion kinks and nodeless superconducting gaps across multiple electron bands. The prominent photoemission kinks suggest a definitive coupling between electrons and certain collective modes, yet the precise nature of this interaction and its connection to superconductivity remain to be established. Here, employing the state-of-the-art ab initio many-body perturbation theory computation, we present direct evidence that electron-phonon (e-ph) coupling induces the multimodal photoemission kinks in CsV_{3}Sb_{5}, and profoundly, drives the nodeless s-wave superconductivity, showcasing the diverse manifestations of the e-ph coupling. Our calculations well capture the experimentally measured kinks and their fine structures, and reveal that vibrations from different atomic species dictate the multimodal behavior. Results from anisotropic GW-Eliashberg equations predict a phonon-mediated superconductivity with nodeless s-wave gaps, in excellent agreement with various ARPES and scanning tunneling spectroscopy measurements. Despite the universal origin of the e-ph coupling, the contributions of several characteristic phonon vibrations vary in different phenomena, highlighting a versatile role of e-ph coupling in shaping the low-energy excitations of kagome metals.

Chirality in the Kagome Metal CsV_{3}Sb_{5}

Using x-ray photoelectron diffraction (XPD) and angle-resolved photoemission spectroscopy, we study photoemission intensity changes related to changes in the geometric and electronic structure in the kagome metal CsV_{3}Sb_{5} upon transition to an unconventional charge density wave (CDW) state. The XPD patterns reveal the presence of a chiral atomic structure in the CDW phase. Furthermore, using circularly polarized x-rays, we have found a pronounced nontrivial circular dichroism in the angular distribution of the valence band photoemission in the CDW phase, indicating a chirality of the electronic structure. This observation is consistent with the proposed orbital loop current order. In view of a negligible spontaneous Kerr signal in recent magneto-optical studies, the results suggest an antiferromagnetic coupling of the orbital magnetic moments along the c axis. While the inherent structural chirality may also induce circular dichroism, the observed asymmetry values seem to be too large in the case of the weak structural distortions caused by the CDW.

Revealing the Orbital Origins of Exotic Electronic States with Ti Substitution in Kagome Superconductor CsV_{3}Sb_{5}

The multiband kagome superconductor CsV_{3}Sb_{5} exhibits complex orbital textures on the Fermi surface, making the orbital origins of its cascade of correlated electronic states and superconductivity a major scientific puzzle. Chemical doping of the kagome plane can simultaneously tune the exotic states and the Fermi-surface orbital texture and thus offers a unique opportunity to correlate the given states with specific orbitals. In this Letter, by substituting V atoms with Ti in the kagome superconductor CsV_{3}Sb_{5}, we reveal the orbital origin of a cascade of its correlated electronic states through the orbital-resolved quasiparticle interference. We analyze the quasiparticle interference changes associated with different orbitals, aided by first-principles calculations. We have observed that the in-plane and out-of-plane vanadium 3d orbitals cooperate to form unidirectional coherent states in pristine CsV_{3}Sb_{5}, whereas the out-of-plane component disappears with doping-induced suppression of charge density wave and global electronic nematicity. In addition, the Sb p_{z} orbital plays an important role in both the pseudogap and superconducting states in CsV_{3}Sb_{5}. Our findings offer new insights into multiorbital physics in quantum materials that are generally manifested with intriguing correlations between atomic orbitals and symmetry-encoded correlated electronic states.

Quantum Phase Transition as a Promising Route to Enhance the Critical Current in Kagome Superconductor CsV3Sb5

Developing strategies to systematically increase the critical current, the threshold current below which the superconductivity exists, is an important goal of materials science. Here, the concept of quantum phase transition is employed to enhance the critical current of a kagome superconductor CsV3Sb5, which exhibits a charge density wave (CDW) and superconductivity that are both affected by hydrostatic pressure. As the CDW phase is rapidly suppressed under pressure, a large enhancement in the self-field critical current (Ic, sf) is recorded. The observation of a peak-like enhancement of Ic, sf at the zero-temperature limit (Ic, sf(0)) centered at p* ≈ 20 kbar, the same pressure where the CDW phase transition vanishes, further provides strong evidence of a zero-temperature quantum anomaly in this class of pressure-tuned superconductor. Such a peak in Ic, sf(0) resembles the findings in other well-established quantum-critical superconductors, hinting at the presence of enhanced quantum fluctuations associated with the CDW phase in CsV3Sb5.

Accessing bands with extended quantum metric in kagome Cs2Ni3S4 through soft chemical processing

Flat bands that do not merely arise from weak interactions can produce exotic physical properties, such as superconductivity or correlated many-body effects. The quantum metric can differentiate whether flat bands will result in correlated physics or are merely dangling bonds. A potential avenue for achieving correlated flat bands involves leveraging geometrical constraints within specific lattice structures, such as the kagome lattice; however, materials are often more complex. In these cases, quantum geometry becomes a powerful indicator of the nature of bands with small dispersions. We present a simple, soft-chemical processing route to access a flat band with an extended quantum metric below the Fermi level. By oxidizing Ni-kagome material Cs2Ni3S4 to CsNi3S4, we see a two orders of magnitude drop in the room temperature resistance. However, CsNi3S4 is still insulating, with no evidence of a phase transition. Using experimental data, density functional theory calculations, and symmetry analysis, our results suggest the emergence of a correlated insulating state of unknown origin.

Atomically Thin Two-Dimensional Kagome Flat Band on the Silicon Surface

In condensed matter physics, the Kagome lattice and its inherent flat bands have attracted considerable attention for their prediction and observation to host a variety of exotic physical phenomena. Despite extensive efforts to fabricate thin films of Kagome materials aimed at modulating flat bands through electrostatic gating or strain manipulation, progress has been limited. Here, we report the observation of a d-orbital hybridized Kagome-derived flat band in Ag/Si(111) 3 × 3 as revealed by angle-resolved photoemission spectroscopy. Our findings indicate that silver atoms on a silicon substrate form an unconventional distorted breathing Kagome structure, where a delicate balance in the hopping parameters of the in-plane d-orbitals leads to destructive interference, resulting in double flat bands. The exact quantum destructive interference mechanism that forms the flat band is uncovered in a rigorous manner that has not been described before. These results illuminate the potential for integrating metal-semiconductor interfaces on semiconductor surfaces into Kagome physics, particularly in exploring the flat bands of ideal 2D Kagome systems.

Electronic structure of superconducting VN(111) films

Vanadium nitride (VN) is a transition-metal nitride with remarkable properties that have prompted extensive experimental and theoretical investigations in recent years. However, there is a current paucity of experimental research investigating the temperature-dependent electronic structure of single-crystalline VN. In this study, high-quality VN(111) films were successfully synthesized on α -Al2 O3 (0001) substrates using magnetron sputtering. The crystal and electronic structures of the VN films were characterized by a combination of high-resolution X-ray diffraction, low-energy electron diffraction, resonant soft X-ray absorption spectroscopy, and ultraviolet photoelectron spectroscopy. The electrical transport measurements indicate that the superconducting critical temperature of the VN films is around 8.1 K. Intriguingly, the temperature-dependent photoelectron spectroscopy measurements demonstrate a weak temperature dependence in the electronic structure of the VN films, which is significant for understanding the ground state of VN compounds.

Designer artificial chiral kagome lattice with tunable flat bands and topological boundary states

The kagome lattice is a well-known model system for the investigation of strong correlation and topological electronic phenomena due to the intrinsic flat band, magnetic frustration, etc. Introducing chirality into the kagome lattice would bring about new physics due to the unique symmetry, which is still yet to be fully explored. Here we report the investigation on a two-dimensional chiral kagome lattice utilizing tight binding band calculation and topological index analysis. It is found that the periodic chiral kagome lattice would bring about a robust zero-energy flat band. Furthermore, in the Su-Schrieffer-Heeger type dimer-/trimerized breathing chiral kagome lattice with particular edge terminations, topological corner states or metallic edge states would appear, implying new candidates for the second-order topological insulator. We also proposed the construction strategy for such lattices employing the scanning tunneling microscope atom manipulation technique.

Superconducting, Topological, and Transport Properties of Kagome Metals CsTi3Bi5 and RbTi3Bi5

The recently discovered ATi3Bi5 (A=Cs, Rb) exhibit intriguing quantum phenomena including superconductivity, electronic nematicity, and abundant topological states. ATi3Bi5 present promising platforms for studying kagome superconductivity, band topology, and charge orders in parallel with AV3Sb5. In this work, we comprehensively analyze various properties of ATi3Bi5 covering superconductivity under pressure and doping, band topology under pressure, thermal conductivity, heat capacity, electrical resistance, and spin Hall conductivity (SHC) using first-principles calculations. Calculated superconducting transition temperature (Tc) of CsTi3Bi5 and RbTi3Bi5 at ambient pressure are about 1.85 and 1.92 K. When subject to pressure, Tc of CsTi3Bi5 exhibits a special valley and dome shape, which arises from quasi-two-dimensional compression to three-dimensional isotropic compression within the context of an overall decreasing trend. Furthermore, Tc of RbTi3Bi5 can be effectively enhanced up to 3.09 K by tuning the kagome van Hove singularities (VHSs) and flat band through doping. Pressures can also induce abundant topological surface states at the Fermi energy (EF) and tune VHSs across EF. Additionally, our transport calculations are in excellent agreement with recent experiments, confirming the absence of charge density wave. Notably, SHC of CsTi3Bi5 can reach up to 226ℏ ·(e· Ω ·cm)-1 at EF. Our work provides a timely and detailed analysis of the rich physical properties for ATi3Bi5, offering valuable insights for further experimental verifications and investigations in this field.

Topological Flat Bands in 2D Breathing-Kagome Lattice Nb3 TeCl7

Flat bands (FBs) can appear in two-dimensional (2D) geometrically frustrated systems caused by quantum destructive interference (QDI). However, the scarcity of pure 2D frustrated crystal structures in natural materials makes FBs hard to be identified, let alone modulate FBs relating to electronic properties. Here, the experimental evidence of the complete electronic QDI induced FB contributed by the 2D breathing-kagome layers of Nb atoms in Nb3 TeCl7 (NTC) is reported. An identical chemical state and 2D localization characteristics of the Nb breathing-kagome layers are experimentally confirmed, based on which NTC is demonstrated to be a superior concrete candidate for the breathing-kagome tight-binding model. Furthermore, it theoretically establishes the tunable roles of the on-site energy over Nb sites on bandwidth, energy position, and topology of FBs in NTC. This work opens an aveanue to manipulate FB characteristics in these 4d transition-metal-based breathing-kagome materials.

Flat bands, non-trivial band topology and rotation symmetry breaking in layered kagome-lattice RbTi3Bi5

A representative class of kagome materials, AV3Sb5 (A = K, Rb, Cs), hosts several unconventional phases such as superconductivity, [Formula: see text] non-trivial topological states, and electronic nematic states. These can often coexist with intertwined charge-density wave states. Recently, the discovery of the isostructural titanium-based single-crystals, ATi3Bi5 (A = K, Rb, Cs), which exhibit similar multiple exotic states but without the concomitant charge-density wave, has opened an opportunity to disentangle these complex states in kagome lattices. Here, we combine high-resolution angle-resolved photoemission spectroscopy and first-principles calculations to investigate the low-lying electronic structure of RbTi3Bi5. We demonstrate the coexistence of flat bands and several non-trivial states, including type-II Dirac nodal lines and [Formula: see text] non-trivial topological surface states. Our findings also provide evidence for rotational symmetry breaking in RbTi3Bi5, suggesting a directionality to the electronic structure and the possible emergence of pure electronic nematicity in this family of kagome compounds.

Emergent topological quantum orbits in the charge density wave phase of kagome metal CsV3Sb5

The recently discovered kagome materials AV3Sb5 (A = K, Rb, Cs) attract intense research interest in intertwined topology, superconductivity, and charge density waves (CDW). Although the in-plane 2 × 2 CDW is well studied, its out-of-plane structural correlation with the Fermi surface properties is less understood. In this work, we advance the theoretical description of quantum oscillations and investigate the Fermi surface properties in the three-dimensional CDW phase of CsV3Sb5. We derived Fermi-energy-resolved and layer-resolved quantum orbits that agree quantitatively with recent experiments in the fundamental frequency, cyclotron mass, and topology. We reveal a complex Dirac nodal network that would lead to a π Berry phase of a quantum orbit in the spinless case. However, the phase shift of topological quantum orbits is contributed by the orbital moment and Zeeman effect besides the Berry phase in the presence of spin-orbital coupling (SOC). Therefore, we can observe topological quantum orbits with a π phase shift in otherwise trivial orbits without SOC, contrary to common perception. Our work reveals the rich topological nature of kagome materials and paves a path to resolve different topological origins of quantum orbits.

Tunable Van Hove Singularity without Structural Instability in Kagome Metal CsTi_{3}Bi_{5}

In kagome metal CsV_{3}Sb_{5}, multiple intertwined orders are accompanied by both electronic and structural instabilities. These exotic orders have attracted much recent attention, but their origins remain elusive. The newly discovered CsTi_{3}Bi_{5} is a Ti-based kagome metal to parallel CsV_{3}Sb_{5}. Here, we report angle-resolved photoemission experiments and first-principles calculations on pristine and Cs-doped CsTi_{3}Bi_{5} samples. Our results reveal that the van Hove singularity (vHS) in CsTi_{3}Bi_{5} can be tuned in a large energy range without structural instability, different from that in CsV_{3}Sb_{5}. As such, CsTi_{3}Bi_{5} provides a complementary platform to disentangle and investigate the electronic instability with a tunable vHS in kagome metals.

Observation of flat band, Dirac nodal lines and topological surface states in Kagome superconductor CsTi3Bi5

Kagome lattices of various transition metals are versatile platforms for achieving anomalous Hall effects, unconventional charge-density wave orders and quantum spin liquid phenomena due to the strong correlations, spin-orbit coupling and/or magnetic interactions involved in such a lattice. Here, we use laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations to investigate the electronic structure of the newly discovered kagome superconductor CsTi3Bi5, which is isostructural to the AV3Sb5 (A = K, Rb or Cs) kagome superconductor family and possesses a two-dimensional kagome network of titanium. We directly observe a striking flat band derived from the local destructive interference of Bloch wave functions within the kagome lattice. In agreement with calculations, we identify type-II and type-III Dirac nodal lines and their momentum distribution in CsTi3Bi5 from the measured electronic structures. In addition, around the Brillouin zone centre, [Formula: see text] nontrivial topological surface states are also observed due to band inversion mediated by strong spin-orbit coupling.

Observation of Electronic Nematicity Driven by the Three-Dimensional Charge Density Wave in Kagome Lattice KV3Sb5

Kagome superconductors AV₃Sb₅ (A = K, Rb, Cs) provide a fertile playground for studying intriguing phenomena, including nontrivial band topology, superconductivity, giant anomalous Hall effect, and charge density wave (CDW). Recently, a C₂ symmetric nematic phase prior to the superconducting state in AV₃Sb₅ drew enormous attention due to its potential inheritance of the symmetry of the unusual superconductivity. However, direct evidence of the rotation symmetry breaking of the electronic structure in the CDW state from the reciprocal space is still rare, and the underlying mechanism remains ambiguous. The observation shows unconventional unidirectionality, indicative of rotation symmetry breaking from six-fold to two-fold. The interlayer coupling between adjacent planes with π-phase offset in the 2 × 2 × 2 CDW phase leads to the preferred two-fold symmetric electronic structure. These rarely observed unidirectional back-folded bands in KV₃Sb₅ may provide important insights into its peculiar charge order and superconductivity.

Band splitting and enhanced charge density wave modulation in Mn-implanted CsV3Sb5

Kagome metal CsV₃Sb₅ has attracted unprecedented attention due to the charge density wave (CDW), Z₂ topological surface states and unconventional superconductivity. However, how the paramagnetic bulk CsV₃Sb₅ interacts with magnetic doping is rarely explored. Here we report a Mn-doped CsV₃Sb₅ single crystal successfully achieved by ion implantation, which exhibits obvious band splitting and enhanced CDW modulation via angle-resolved photoemission spectroscopy (ARPES). The band splitting is anisotropic and occurs in the entire Brillouin region. We observed a Dirac cone gap at the K point but it closed at 135 K ± 5 K, much higher than the bulk value of ∼94 K, suggesting enhanced CDW modulation. According to the facts of the transferred spectral weight to the Fermi level and weak antiferromagnetic order at low temperature, we ascribe the enhanced CDW to the polariton excitation and Kondo shielding effect. Our study not only offers a simple method to realize deep doping in bulk materials, but also provides an ideal platform to explore the coupling between exotic quantum states in CsV₃Sb₅.

Nodeless Superconductivity in Kagome Metal CsV3Sb5 with and without Time Reversal Symmetry Breaking

The kagome metal CsV₃Sb₅ features an unusual competition between the charge-density-wave (CDW) order and superconductivity. Evidence for time reversal symmetry breaking (TRSB) inside the CDW phase has been accumulating. Hence, the superconductivity in CsV₃Sb₅ emerges from a TRSB normal state, potentially resulting in an exotic superconducting state. To reveal the pairing symmetry, we first investigate the effect of nonmagnetic impurity. Our results show that the superconducting critical temperature is insensitive to disorder, pointing to conventional s-wave superconductivity. Moreover, our measurements of the self-field critical current (I_c,sf), which is related to the London penetration depth, also confirm conventional s-wave superconductivity with strong coupling. Finally, we measure I_c,sf where the CDW order is removed by pressure and superconductivity emerges from the pristine normal state. Our results show that s-wave gap symmetry is retained, providing strong evidence for the presence of conventional s-wave superconductivity in CsV₃Sb₅ irrespective of the presence of the TRSB.

Kagome superconductors AV3Sb5 (A = K, Rb, Cs)

The quasi-two-dimensional kagome materials AV3Sb5 (A = K, Rb, Cs) were found to be a prime example of kagome superconductors, a new quantum platform to investigate the interplay between electron correlation effects, topology and geometric frustration. In this review, we report recent progress on the experimental and theoretical studies of AV3Sb5 and provide a broad picture of this fast-developing field in order to stimulate an expanded search for unconventional kagome superconductors. We review the electronic properties of AV3Sb5, the experimental measurements of the charge density wave state, evidence of time-reversal symmetry breaking and other potential hidden symmetry breaking in these materials. A variety of theoretical proposals and models that address the nature of the time-reversal symmetry breaking are discussed. Finally, we review the superconducting properties of AV3Sb5, especially the potential pairing symmetries and the interplay between superconductivity and the charge density wave state.

Charge order landscape and competition with superconductivity in kagome metals

In the kagome metals AV₃Sb₅ (A = K, Rb, Cs), three-dimensional charge order is the primary instability that sets the stage for other collective orders to emerge, including unidirectional stripe order, orbital flux order, electronic nematicity and superconductivity. Here, we use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensional charge order in AV₃Sb₅ and its interplay with superconductivity. Our approach is based on identifying an unusual splitting of kagome bands induced by three-dimensional charge order, which provides a sensitive way to refine the spatial charge patterns in neighbouring kagome planes. We found a marked dependence of the three-dimensional charge order structure on composition and doping. The observed difference between CsV₃Sb₅ and the other compounds potentially underpins the double-dome superconductivity in CsV₃(Sb,Sn)₅ and the suppression of T_c in KV₃Sb₅ and RbV₃Sb₅. Our results provide fresh insights into the rich phase diagram of AV₃Sb₅.

Topological superconductivity and large spin Hall effect in the kagome family Ti6X4 (X = Bi, Sb, Pb, Tl, and In)

Topological superconductors (TSC) become a focus of research due to the accompanying Majorana fermions. However, the reported TSC are extremely rare. Recent experiments reported kagome TSC AV3Sb5 (A = K, Rb, and Cs) exhibit unique superconductivity, topological surface states (TSS), and Majorana bound states. More recently, the first titanium-based kagome superconductor CsTi3Bi5 with nontrivial topology was successfully synthesized as a perspective TSC. Given that Cs contributes little to electronic structures of CsTi3Bi5 and binary compounds may be easier to be synthesized, here, by first-principle calculations, we predict five stable nonmagnetic kagome compounds Ti6X4 (X = Bi, Sb, Pb, Tl, and In) which exhibit superconductivity with critical temperature Tc = 3.8 K - 5.1 K, nontrivialZ 2 band topology, and TSS close to the Fermi level. Additionally, large intrinsic spin Hall effect is obtained in Ti6X4, which is caused by gapped Dirac nodal lines due to a strong spin-orbit coupling. This work offers new platforms for TSC and spintronic devices.

Chern Fermi pocket, topological pair density wave, and charge-4e and charge-6e superconductivity in kagomé superconductors

The recent discovery of novel charge density wave (CDW) and pair density wave (PDW) in kagomé lattice superconductors AV3Sb5 (A = K, Rb, Cs) hints at unexpected time-reversal symmetry breaking correlated and topological states whose physical origin and broader implications are not understood. Here, we make conceptual advances toward a mechanism behind the striking observations and new predictions for novel macroscopic phase coherent quantum states. We show that the metallic CDW state with circulating loop currents is a doped orbital Chern insulator near van Hove filling. The emergent Chern Fermi pockets (CFPs) carry concentrated Berry curvature and orbital magnetic moment. We find that the pairing of electrons on the CFPs leads to a superconducting state with an emergent vortex-antivortex lattice and the formation of a complex triple-Q PDW. A plethora of correlated and topological states emerge, including a never-before-encountered chiral topological PDW superconductor, a loop-current pseudogap phase, and vestigial charge-4e and charge-6e superconductivity in staged melting of the vortex-antivortex lattice and hexatic liquid crystal. Our findings reveal previously unknown nature of the superconducting state of a current-carrying Chern metal, with broad implications for correlated and topological materials.

Tunable topological Dirac surface states and van Hove singularities in kagome metal GdV6Sn6

Transition-metal-based kagome materials at van Hove filling are a rich frontier for the investigation of novel topological electronic states and correlated phenomena. To date, in the idealized two-dimensional kagome lattice, topologically Dirac surface states (TDSSs) have not been unambiguously observed, and the manipulation of TDSSs and van Hove singularities (VHSs) remains largely unexplored. Here, we reveal TDSSs originating from a ℤ₂ bulk topology and identify multiple VHSs near the Fermi level (E_F) in magnetic kagome material GdV₆Sn₆. Using in situ surface potassium deposition, we successfully realize manipulation of the TDSSs and VHSs. The Dirac point of the TDSSs can be tuned from above to below E_F, which reverses the chirality of the spin texture at the Fermi surface. These results establish GdV₆Sn₆ as a fascinating platform for studying the nontrivial topology, magnetism, and correlation effects native to kagome lattices. They also suggest potential application of spintronic devices based on kagome materials.

Rich nature of Van Hove singularities in Kagome superconductor CsV3Sb5

The recently discovered layered kagome metals AV3Sb5 (A = K, Rb, Cs) exhibit diverse correlated phenomena, which are intertwined with a topological electronic structure with multiple van Hove singularities (VHSs) in the vicinity of the Fermi level. As the VHSs with their large density of states enhance correlation effects, it is of crucial importance to determine their nature and properties. Here, we combine polarization-dependent angle-resolved photoemission spectroscopy with density functional theory to directly reveal the sublattice properties of 3d-orbital VHSs in CsV3Sb5. Four VHSs are identified around the M point and three of them are close to the Fermi level, with two having sublattice-pure and one sublattice-mixed nature. Remarkably, the VHS just below the Fermi level displays an extremely flat dispersion along MK, establishing the experimental discovery of higher-order VHS. The characteristic intensity modulation of Dirac cones around K further demonstrates the sublattice interference embedded in the kagome Fermiology. The crucial insights into the electronic structure, revealed by our work, provide a solid starting point for the understanding of the intriguing correlation phenomena in the kagome metals AV3Sb5.

Electronic nature of charge density wave and electron-phonon coupling in kagome superconductor KV3Sb5

The Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have received enormous attention due to their nontrivial topological electronic structure, anomalous physical properties and superconductivity. Unconventional charge density wave (CDW) has been detected in AV3Sb5. High-precision electronic structure determination is essential to understand its origin. Here we unveil electronic nature of the CDW phase in our high-resolution angle-resolved photoemission measurements on KV3Sb5. We have observed CDW-induced Fermi surface reconstruction and the associated band folding. The CDW-induced band splitting and the associated gap opening have been revealed at the boundary of the pristine and reconstructed Brillouin zones. The Fermi surface- and momentum-dependent CDW gap is measured and the strongly anisotropic CDW gap is observed for all the V-derived Fermi surface. In particular, we have observed signatures of the electron-phonon coupling in KV3Sb5. These results provide key insights in understanding the nature of the CDW state and its interplay with superconductivity in AV3Sb5 superconductors.