In transition metal compounds, due to the interplay of charge, spin, lattice and orbital degrees of freedom, many intertwined orders exist with close energies. One of the commonly observed states is the so-called nematic electron state, which breaks the in-plane rotational symmetry. This nematic state appears in cuprates, iron-based superconductor, etc. Nematicity may coexist, affect, cooperate or compete with other orders. Here we show the anisotropic in-plane electronic state and superconductivity in a recently discovered kagome metal CsV₃Sb₅ by measuring c-axis resistivity with the in-plane rotation of magnetic field. We observe a twofold symmetry of superconductivity in the superconducting state and a unique in-plane nematic electronic state in normal state when rotating the in-plane magnetic field. Interestingly these two orders are orthogonal to each other in terms of the field direction of the minimum resistivity. Our results shed new light in understanding non-trivial physical properties of CsV₃Sb₅.

Kagome superconductors with $T_{\mathrm{C}}$ up to 7 K have been discovered for over 40 y. Recently, unconventional chiral charge order has been reported in kagome superconductor $\mathrm{K}{\mathrm{V}}_3{\mathrm{Sb}}_5$, with an ordering temperature of one order of magnitude higher than the $T_{\mathrm{C}}$. However, the chirality of the charge order has not been reported in the cousin kagome superconductor $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$, and the electronic nature of the chirality remains elusive. In this paper, we report the observation of electronic chiral charge order in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ via scanning tunneling microscopy (STM). We observe a 2 × 2 charge modulation and a 1 × 4 superlattice in both topographic data and tunneling spectroscopy. 2 × 2 charge modulation is highly anticipated as a charge order by fundamental kagome lattice models at van Hove filling, and is shown to exhibit intrinsic chirality. We find that the 1 × 4 superlattices form various small domain walls, and can be a surface effect as supported by our first-principles calculations. Crucially, we find that the amplitude of the energy gap opened by the charge order exhibits real-space modulations, and features 2 × 2 wave vectors with chirality, highlighting the electronic nature of the chiral charge order. STM study at 0.4 K reveals a superconducting energy gap with a gap size $2\mathrm{\Delta }=0.85\mathrm{meV}$, which estimates a moderate superconductivity coupling strength with $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{C}}=3.9$. When further applying a $c$-axis magnetic field, vortex core bound states are observed within this gap, indicative of clean-limit superconductivity.

Structural investigations by means of transmission electron microscopy (TEM) on ${\mathrm{K}}_{0.8}$Fe${}_x$Se${}_2$ and KFe${}_x$Se${}_2$, with 1.5 ⩽ $x$ ⩽ 1.8, have revealed a rich variety of microstructure phenomena. Materials with 1.5 ⩽ $x$ ⩽ 1.6 often show a superstructure modulation along the [310] zone-axis direction, and this modulation can be well interpreted by the Fe-vacancy order, which likely yields a superstructure phase of ${\mathrm{K}}_2$Fe${}_4$Se${}_5$. The superconducting ${\mathrm{K}}_{0.8}$Fe${}_x$Se${}_2$ and KFe${}_x$Se${}_2$ (1.7 ⩽ $x$ ⩽ 1.8) materials contain clear phase separation, in particular, along the $c$-axis direction, recognizable as visible parallel lamellae in the crystals; this fact suggests that the superconducting phase could have the Fe-vacancy disordered state.

We have performed angle-resolved photoemission spectroscopy on HfSiS, which has been predicted to be a topological line-node semimetal with square Si lattice. We found a quasi-two-dimensional Fermi surface hosting bulk nodal lines, alongside the surface states at the Brillouin-zone corner exhibiting a sizable Rashba splitting and band-mass renormalization due to many-body interactions. Most notably, we discovered an unexpected Dirac-like dispersion extending one dimensionally in $k$ space—the Dirac-node arc—near the bulk node at the zone diagonal. These novel Dirac states reside on the surface and could be related to hybridizations of bulk states, but currently we have no explanation for its origin. This discovery poses an intriguing challenge to the theoretical understanding of topological line-node semimetals.

We report on a study of the optical properties of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ for a large number of temperatures above and below the charge-density-wave (CDW) transition. Above the CDW transition, the low-frequency optical conductivity reveals two Drude components with distinct widths. An examination of the band structure allows us to ascribe the narrow Drude component to a light-electron-like and multiple Dirac bands, and the broad Drude component to heavy-hole bands near the $M$ points which form saddle points near the Fermi level. Upon entering the CDW state, the opening of the CDW gap is clearly observed. A large portion of the broad Drude component is removed by the gap, whereas the narrow Drude component is not affected. Meanwhile, an absorption peak associated with interband transitions involving the saddle points shifts to higher energy and grows in weight. These observations attest to the importance of saddle point nesting in driving the CDW instability in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.