The transition-metal-based kagome metals provide a versatile platform for correlated topological phases hosting various electronic instabilities. While superconductivity is rare in layered kagome compounds, its interplay with nontrivial topology could offer an engaging space to realize exotic excitations of quasiparticles. Here, we use scanning tunneling microscopy to study a newly discovered $Z_2$ topological kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ with a superconducting ground state. We observe charge modulation associated with the opening of an energy gap near the Fermi level. When across single-unit-cell surface step edges, the intensity of this charge modulation exhibits a $\pi$-phase shift, suggesting a three-dimensional $2\times 2\times 2$ charge density wave ordering. Interestingly, while conventional Caroli–de Gennes–Matricon bound states are observed inside the superconducting vortex on the Sb surfaces, a robust zero-bias conductance peak emerges that does not split in a large distance when moving away from the vortex center on the Cs $2\times 2$ surfaces, resembling the Majorana bound states arising from the superconducting Dirac surface states in ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ heterostructures. Our findings establish ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ as a promising candidate for realizing exotic excitations at the confluence of nontrivial lattice geometry, topology and multiple electronic orders.

The symmetry of Cooper pairs in iron-based superconductors is an issue under continued investigation. A scanning tunnelling study of Fe(Te,Se) reveals a robust zero-energy bound state, providing evidence for a non-trivial pairing symmetry. In superconductors, electrons are paired and condensed into the ground state. An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap. The characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state1. A zero-energy in-gap state is particularly noteworthy, because it can be the consequence of non-trivial pairing symmetry1 or topology2,3. Here we use scanning tunnelling microscopy/spectroscopy to demonstrate that an isotropic zero-energy bound state with a decay length of ∼10 Å emerges at each interstitial iron impurity in superconducting Fe(Te,Se). More noticeably, this zero-energy bound state is robust against a magnetic field up to 8 T, as well as perturbations by neighbouring impurities. Such a spectroscopic feature has no natural explanation in terms of impurity states in superconductors with s-wave symmetry, but bears all the characteristics of the Majorana bound state proposed for topological superconductors2,3, indicating that the superconducting state and the scattering mechanism of the interstitial iron impurities in Fe(Te,Se) are highly unconventional.

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

By using flux method, we have grown single crystals of NdFeAsO0.82F0.18 at ambient pressures. Resistive measurements reveal a surprising discovery that the anisotropy Γ=(mc∕mab)1∕2 is below 5, which is much smaller than the theoretically calculated value. The data measured up to 400K show a curved feature, which prevents a conjectured linear behavior for an unconventional metal. The upper critical fields determined based on the Werthamer–Helfand–Hohenberg formula [N. R. Werthamer et al., Phys. Rev. 147, 295 (1966)] are Hc2ab(T=0K)≈304T and Hc2c(T=0K)≈62–70T. These high values indicate very encouraging applications of the new superconductors.

Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$. It is found that the sharp SH anomaly $\Delta C/T{\mid }_{T_c}$ in our samples reaches an unexpected high value of $98\text{mJ}/\text{mol}{\text{K}}^2$, about 1 order of magnitude larger than that of the F-doped system LnFeAsO ($\text{Ln}=\text{rare}$-earth elements), suggesting a very high normal-state quasiparticle density of states in the FeAs-122 system than in the FeAs-1111 system. Furthermore, we found that the electronic SH coefficient ${\gamma }_e\left(T\right)$ of ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$ is weakly temperature dependent and increases almost linearly with the magnetic field in the low temperature region, which may indicate that the hole-doped FeAs-122 system contains a dominant component with a full superconducting gap, although we cannot rule out the possibility of a small component with an anisotropic or nodal gap. A detailed analysis reveals that the ${\gamma }_e\left(T\right)$ of ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$ cannot be fitted with a single gap of $s$-wave symmetry, probably due to the multigap effect. These results indicate a clear difference between the properties of the superconducting state of the holed-doped ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$ and the F-doped LnFeAsO ($\text{Ln}=\text{rare}$-earth elements) system, which we believe originated from the complex Fermi-surface structures in different systems.