Holon Wigner Crystal in a Lightly Doped Kagome Quantum Spin Liquid

On-Surface Synthesis of Novel Kagome Lattices Coordinated via Four-Fold N-Ag Bonding

The Kagome lattice structures based on metal-organic coordination have garnered widespread interest because of their topologically Dirac/flat bands and other exotic electronic structures. However, the experimental fabrication of large-area two-dimensional (2D) Kagome lattice structures of metal-organic frameworks (MOFs) via on-surface synthesis remains limited. Herein, we successfully construct two kinds of large-scale 2D Kagome-type lattices stabilized by 4-fold N-Ag coordination on the Ag(111) surface. With the aid of scanning tunneling microscopy (STM) and synchrotron radiation photoemission spectroscopy (SRPES), we clearly elucidate the reaction pathway and mechanism of fabrication of the two Kagome lattices. This work provides a novel platform for investigating related intriguing physical properties.

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.

Quantum Phase Diagram and Spontaneously Emergent Topological Chiral Superconductivity in Doped Triangular-Lattice Mott Insulators

The topological superconducting state is a highly sought-after quantum state hosting topological order and Majorana excitations. In this Letter, we explore the mechanism to realize the topological superconductivity (TSC) in the doped Mott insulators with time-reversal symmetry (TRS). Through large-scale density matrix renormalization group study of an extended triangular-lattice t-J model on the six- and eight-leg cylinders, we identify a d+id-wave chiral TSC with spontaneous TRS breaking, which is characterized by a Chern number C=2 and quasi-long-range superconducting order. We map out the quantum phase diagram with by tuning the next-nearest-neighbor (NNN) electron hopping and spin interaction. In the weaker NNN-coupling regime, we identify a pseudogaplike phase with a charge stripe order coexisting with fluctuating superconductivity, which can be tuned into d-wave superconductivity by increasing the doping level and system width. The TSC emerges in the intermediate-coupling regime, which has a transition to a d-wave superconducting phase with larger NNN couplings. The emergence of the TSC is driven by geometrical frustrations and hole dynamics which suppress spin correlation and charge order, leading to a topological quantum phase transition.

Self-Intercalated 1T-FeSe2 as an Effective Kagome Lattice

In kagome lattice, with the emergence of Dirac cones and flat band in electronic structure, it provides a versatile ground for exploring intriguing interplay among frustrated geometry, topology and correlation. However, such engaging interest is strongly limited by available kagome materials in nature. Here we report on a synthetic strategy of constructing kagome systems via self-intercalation of Fe atoms into the van der Waals gap of FeSe₂ via molecular beam epitaxy. Using low-temperature scanning tunneling microscopy, we unveil a kagome-like morphology upon intercalating a 2 × 2 ordered Fe atoms, resulting in a stoichiometry of Fe₅Se₈. Both the bias-dependent STM imaging and theoretical modeling calculations suggest that the kagome pattern mainly originates from slight but important reconstruction of topmost Se atoms, incurred by the nonequivalent subsurface Fe sites due to the intercalation. Our study demonstrates an alternative approach of constructing artificial kagome structures, which envisions to be tuned for exploring correlated quantum states.

Observation of Topological Flat Bands in the Kagome Semiconductor Nb3Cl8

The destructive interference of wavefunctions in a kagome lattice can give rise to topological flat bands (TFBs) with a highly degenerate state of electrons. Recently, TFBs have been observed in several kagome metals, including Fe₃Sn₂, FeSn, CoSn, and YMn₆Sn₆. Nonetheless, kagome materials that are both exfoliable and semiconducting are lacking, which seriously hinders their device applications. Herein, we show that Nb₃Cl₈, which hosts a breathing kagome lattice, is gapped out because of the absence of inversion symmetry, while the TFBs survive because of the protection of the mirror reflection symmetry. By angle-resolved photoemission spectroscopy measurements and first-principles calculations, we directly observe the TFBs and a moderate band gap in Nb₃Cl₈. By mechanical exfoliation, we successfully obtain monolayer Nb₃Cl₈, which is stable under ambient conditions. In addition, our calculations show that monolayer Nb₃Cl₈ has a magnetic ground state, thus providing opportunities to study the interplay among geometry, topology, and magnetism.

Cascade of correlated electron states in the kagome superconductor CsV3Sb5

The kagome lattice of transition metal atoms provides an exciting platform to study electronic correlations in the presence of geometric frustration and nontrivial band topology^1-18, which continues to bear surprises. Here, using spectroscopic imaging scanning tunnelling microscopy, we discover a temperature-dependent cascade of different symmetry-broken electronic states in a new kagome superconductor, CsV₃Sb₅. We reveal, at a temperature far above the superconducting transition temperature T_c ~ 2.5 K, a tri-directional charge order with a 2a₀ period that breaks the translation symmetry of the lattice. As the system is cooled down towards T_c, we observe a prominent V-shaped spectral gap opening at the Fermi level and an additional breaking of the six-fold rotational symmetry, which persists through the superconducting transition. This rotational symmetry breaking is observed as the emergence of an additional 4a₀ unidirectional charge order and strongly anisotropic scattering in differential conductance maps. The latter can be directly attributed to the orbital-selective renormalization of the vanadium kagome bands. Our experiments reveal a complex landscape of electronic states that can coexist on a kagome lattice, and highlight intriguing parallels to high-T_c superconductors and twisted bilayer graphene.

Possible Superconductivity with a Bogoliubov Fermi Surface in a Lightly Doped Kagome U(1) Spin Liquid

Whether the doped t-J model on the Kagome lattice supports exotic superconductivity has not been decisively answered. In this Letter, we propose a new class of variational states for this model and perform a large-scale variational Monte Carlo simulation on it. The proposed variational states are parameterized by the SU(2)-gauge rotation angles, as the SU(2)-gauge structure hidden in the Gutzwiller-projected mean-field Ansatz for the undoped model is broken upon doping. These variational doped states smoothly connect to the previously studied U(1) π-flux or 0-flux states, and energy minimization among them yields a chiral noncentrosymmetric nematic superconducting state with 2×2-enlarged unit cell. Moreover, this pair density wave state possesses a finite Fermi surface for the Bogoliubov quasiparticles. We further study experimentally relevant properties of this intriguing pairing state.

High Temperature Superconductivity in a Lightly Doped Quantum Spin Liquid

We have performed density-matrix renormalization group studies of a square lattice t-J model with small hole doping, δ≪1, on long four and six-leg cylinders. We include frustration in the form of a second-neighbor exchange coupling, J_{2}=J_{1}/2, such that the undoped (δ=0) "parent" state is a quantum spin liquid. In contrast to the relatively short range superconducting (SC) correlations that have been observed in recent studies of the six-leg cylinder in the absence of frustration, we find power-law SC correlations with a Luttinger exponent, K_{SC}≈1, consistent with a strongly diverging SC susceptibility, χ∼T^{-(2-K_{SC})} as the temperature T→0. The spin-spin correlations-as in the undoped state-fall exponentially suggesting that the SC "pairing" correlations evolve smoothly from the insulating parent state.

Li doped kagome spin liquid compounds

Herbertsmithite and Zn-doped barlowite are two compounds for experimental realization of two-dimensional kagome spin liquids. Theoretically, it has been proposed that charge doping a quantum spin liquid gives rise to exotic metallic states, such as high-temperature superconductivity. However, one recent experiment on herbertsmithite with successful Li-doping surprisingly showed an insulating state even under a heavily doped scenario, which cannot be explained by previous theories. Using first-principles calculations, we performed a comprehensive study on the Li intercalation doping effect of these two compounds. For the Li-doped herbertsmithite, we identified the optimized Li position at the Cl-(OH)3-Cl pentahedron site instead of the previously speculated Cl-(OH)3 tetrahedral site. With increasing Li doping concentration, saturation magnetization decreases linearly due to charge transfer from Li to Cu ions. Moreover, we found that Li forms chemical bonds with nearby (OH)- and Cl- ions, which lowers the surrounding chemical potential and traps electrons, as evidenced by the localized charge distribution, explaining the insulating behavior measured experimentally. Though a different structure from herbertsmithite, Zn-doped barlowite shows the same features upon Li doping. We conclude that Li doping this family of kagome spin liquids cannot realize exotic metallic states, and other methods should be further explored, such as element substitution with those having different valence electrons.

1T-TaS2 as a quantum spin liquid

1T-TaS2 is unique among transition metal dichalcogenides in that it is understood to be a correlation-driven insulator, where the unpaired electron in a 13-site cluster experiences enough correlation to form a Mott insulator. We argue, based on existing data, that this well-known material should be considered as a quantum spin liquid, either a fully gapped [Formula: see text] spin liquid or a Dirac spin liquid. We discuss the exotic states that emerge upon doping and propose further experimental probes.