The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer salt $\kappa \text{−}{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_2]\mathrm{Br}$ induce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperature $T^{*}\simeq 50\mathrm{K}$, far higher than the equilibrium transition temperature $T_{\mathit{C}}=12.5\mathrm{K}$. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.

Coexisting density-wave and superconducting states along with the large anomalous Hall effect in the absence of local magnetism remain intriguing and enigmatic features of the AV₃Sb₅ kagome metals (A = K, Rb, Cs). Here, we demonstrate via optical spectroscopy and density-functional calculations that low-energy dynamics of KV₃Sb₅ is characterized by unconventional localized carriers, which are strongly renormalized across the density-wave transition and indicative of electronic correlations. Strong phonon anomalies are prominent not only below the density-wave transition, but also at high temperatures, suggesting an intricate interplay of phonons with the underlying electronic structure. We further propose the star-of-David and tri-hexagon (inverse star-of-David) configurations for the density-wave order in KV₃Sb₅. These configurations are strongly reminiscent of p-wave states expected in the Hubbard model on the kagome lattice at the filling level of the van Hove singularity. The proximity to this regime should have intriguing and far-reaching implications for the physics of KV₃Sb₅ and related materials.

Temperature-dependent reflectivity measurements on the kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ in a broad frequency range of $50-20000{\mathrm{cm}}^{-1}$ down to $T$=10 K are reported. The charge-density wave (CDW) formed below $T_{\mathrm{CDW}}$ = 94 K manifests itself in a prominent spectral-weight transfer from low to higher energy regions. The CDW gap of 60–75 meV is observed at the lowest temperature and shows significant deviations from an isotropic BCS-type mean-field behavior. Absorption peaks appear at frequencies as low as 200 ${\mathrm{cm}}^{-1}$ and can be identified with interband transitions according to density-functional calculations. The change in the interband absorption compared to ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ reflects the inversion of band saddle points between the K and Cs compounds. Additionally, a broader and strongly temperature-dependent absorption feature is observed below 1000 ${\mathrm{cm}}^{-1}$ and assigned to a displaced Drude peak. It reflects localization effects on charge carriers.

Temperature- and frequency-dependent infrared spectroscopy identifies two contributions to the electronic properties of the magnetic kagome metal ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$: two-dimensional Dirac fermions and strongly correlated flat bands. The interband transitions within the linearly dispersing Dirac bands appear as a two-step feature along with a very narrow Drude component due to intraband contribution. Low-lying absorption features indicate flat bands with multiple van Hove singularities. Localized charge carriers are seen as a Drude peak shifted to finite frequencies. The spectral weight is redistributed when the spins are reoriented at low temperatures; a sharp mode appears suggesting the opening of a gap due to the spin reorientation as the sign of additional Weyl nodes in the system.

The presence of magnetism in potentially altermagnetic RuO 2 has been a subject of intense debate. Using broadband infrared spectroscopy combined with density-functional band-structure calculations, we show that the optical conductivity of RuO 2 , the bulk probe of its electronic structure, is best described by a nonmagnetic model. The sharp Pauli edge demonstrates the presence of a Dirac nodal line lying 45 meV below the Fermi level. An excellent match between the experimental and plasma frequencies underpins the weakness of electronic correlations. The intraband part of the optical conductivity indicates Fermi-liquid behavior with two distinct scattering rates below 150 K. Fermi-liquid theory also accounts for the temperature-dependent magnetic susceptibility of RuO 2 and allows a consistent description of this material as a paramagnetic metal.