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.

In the second-order response regime, the Hall voltage can be nonzero without time-reversal symmetry breaking but inversion symmetry breaking. Multiple mechanisms contribute to the nonlinear Hall effect. The disorder-related contributions can enter the NLHE in the leading role, but experimental investigations are scarce, especially the exploration of the contributions from different disorder sources. Here, we report a giant nonlinear response in twisted bilayer graphene, dominated by disorder-induced skew scattering. The magnitude and direction of the second-order nonlinearity can be effectively tuned by the gate voltage. A peak value of the second-order Hall conductivity reaching $8.76\mu \mathrm{m}{\mathrm{SV}}^{-1}$ is observed close to the full filling of the moiré band, four order larger than the intrinsic contribution detected in ${\mathrm{WTe}}_2$. The scaling shows that the giant second-order nonlinear Hall effect in twisted bilayer graphene stems from the collaboration of the static (impurities) and dynamic (phonons) disorders. It is mainly determined by the impurity skew scattering at 1.7 K. The phonon skew scattering, however, has a much larger coupling coefficient, and becomes comparable to the impurity contribution as the temperature rises. Our observations provide a comprehensive experimental understanding of the disorder-related mechanisms in the nonlinear Hall effect.

The recently discovered coexistence of superconductivity and charge-density wave order in the kagome systems $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) has stimulated enormous interest. It is predicted that a vanadium-based kagome system may host a flat band, nontrivial linear dispersive Dirac surface states, and electronic correlation. Although much research has been carried out, the origin of the charge-density wave (CDW) order, the relationship between the superconductivity and the CDW, and whether the anomalous Hall effect (AHE) arises primarily from the kagome lattice or the CDW order, remain controversial. Here, we report an extensive investigation of $\mathrm{Cs}{\left({\mathrm{V}}_{1\text{−}x}{\mathrm{Nb}}_x\right)}_3{\mathrm{Sb}}_5$ samples with systematic Nb doping. Our results show that the Nb doping induces apparent suppression of CDW order and promotes superconductivity; meanwhile, the AHE and magnetoresistance (MR) are weakened significantly together with the CDW order. Combining with density-functional calculations, we interpret these effects by an antiphase shift of the Fermi energy with respect to the saddle points near $M$ and the Fermi surface centered around Γ. It is found that the former depletes the filled states for the CDW instability and worsens the nesting condition for CDW order, while the latter lifts the Fermi level upward and enlarges the Fermi surface surrounding the Γ point, and thus promotes superconductivity. Our results uncover a delicate but unusual competition between the CDW order and superconductivity.

As unique perovskite transparent oxide semiconductors, high-mobility La-doped BaSnO₃ films have been successfully synthesized by molecular beam epitaxy and pulsed laser deposition. However, it remains a big challenge for magnetron sputtering, a widely applied technique suitable for large-scale fabrication, to grow high-mobility La-doped BaSnO₃ films. In this work, we developed a method to synthesize high-mobility epitaxial La-doped BaSnO₃ films (with mobility up to 121 cm² V⁻¹ s⁻¹ at the carrier density of ∼4.0 × 10²⁰ cm⁻³ at room temperature) directly on SrTiO₃ single crystal substrates using high-pressure magnetron sputtering. The structural and electrical properties of La-doped BaSnO₃ films were characterized by combined high-resolution x-ray diffraction, x-ray photoemission spectroscopy, and temperature-dependent electrical transport measurements. The room-temperature electron mobility of La-doped BaSnO₃ films achieved in this work is two to four times higher than the reported values of the films grown by magnetron sputtering. Moreover, in the high carrier density range (n > 3 × 10²⁰ cm⁻³), the electron mobility value of 121 cm² V⁻¹ s⁻¹ achieved in our work is among the highest values for all reported doped BaSnO₃ films. It is revealed that high argon pressure during sputtering plays a vital role in stabilizing the fully relaxed films and inducing oxygen vacancies, which facilitates high mobility at room temperature. Our work provides an easy and economical way to massively synthesize high-mobility transparent conducting films for transparent electronics.