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.

Kagome metals possess peculiar optical spectra consisting of contributions from free charge carriers in a Drude-type response, localized carriers seen as a strongly temperature-dependent localization peak, and, in some cases, phonons displaying strong anomalies. The rare-earth kagome metal series, $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$, provides a marvelous playground to study the electronic properties of kagome metals in the presence of variable magnetic order. Here, we report temperature-dependent reflectivity studies on two members of the $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ family, ${\mathrm{GdMn}}_6{\mathrm{Sn}}_6$ (in-plane ferrimagnet) and ${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$ (out-of-plane ferrimagnet), in a broad energy range (50–18 000 ${\mathrm{cm}}^{-1}$, equivalent to 6.2 meV–2.23 eV) down to 10 K. At high temperatures, a phonon mode at approximately 160 ${\mathrm{cm}}^{-1}$ is observed, which becomes screened out in ${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$ below $\sim$150 K as the localization peak linearly passes through the mode. In ${\mathrm{GdMn}}_6{\mathrm{Sn}}_6$, the disappearance of the phonon is accompanied by the onset of saturation of the peak position, suggesting an unusual interplay between the two features.

The two-dimensional organic superconductor $\lambda -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ exhibits pronounced charge fluctuations below $T\approx 150$ K, in contrast to the sibling compound $\kappa -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ that remains metallic down to milli-Kelvin. Infrared spectroscopy reveals only minor splitting in the vibrational features of the latter compound, common to other strongly dimerized $\kappa$-type salts. When the organic molecules are arranged in the $\lambda$-type pattern, however, a strong vibrational ${\nu }_{27}\left(b_{1u}\right)$ mode is present that forms a narrow doublet. Most important, when cooling $\lambda -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ below 150 K, two weak side modes appear due to charge disproportionation that amounts to $2\delta =0.14e$. In analogy to the ${\beta }^{\prime \prime }$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in $\lambda -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ at $T_c=4.7$ K. We discuss the possibility of a charge-density wave that coexists with the proposed spin-density-wave state.

We performed angular and temperature-dependent electron-spin-resonance measurements in the quasi-two-dimensional organic conductor $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$. The interlayer spin diffusion is much weaker compared to the Cl and Br analogs, which are antiferromagnetic insulator and paramagnetic metal, respectively; $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2\mathrm{Cu}[\mathrm{N}{\left(\mathrm{CN}\right)}_2]\mathrm{I}$ behaves insulating when cooled below $T=200\mathrm{K}$. A spin gap ($\mathrm{\Delta }\approx 18\mathrm{K}$) opens at low temperatures leading to a spin-singlet state. Due to intrinsic disorder a substantial number of spins ($\sim 1\%$) remains unpaired. We observe additional signals below $T=4\mathrm{K}$ with a pronounced anisotropy indicating the presence of local magnetic moments coupled to some fraction of those unpaired spins.

To understand the physical properties of the series of organic conductors $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]X$ with $X=\mathrm{Cl}$, Br, and I, not only electronic correlations, but also the effect of disorder has to be taken into account. While for Cl- and Br-containing salts the influence of both parameters were investigated and a universal phase diagram was proposed, the position of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{I}$ is still not settled. Here we conducted transport, infrared, and dielectric measurements on single crystals of the title compound to clarify its electronic state at low temperatures. The correlation strength was determined as $U/W\approx 2.2$; thus this salt is placed deeper in an insulating state compared to the two sister compounds. We found that inherent disorder leads to a Coulomb localized insulating state similar to the moderately x-ray-irradiated $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$.