Utilizing infrared vibrational spectroscopy we have investigated dimerized two-dimensional organic salts in order to search for possible charge redistribution that might constitute electronic dipoles and ferroelectricity: the quantum spin liquid $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ [BEDT-TTF: bis-(ethylenedithio)tetrathiafulvalene], the antiferromagnetic Mott insulator $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Cl, and the superconductor $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br. None of them exhibit any indication of charge disproportionation. Upon cooling to low temperatures all BEDT-TTF molecules remain homogeneously charged within $\pm 0.005e$. No modification in the charge distribution is observed around $T=6$ K where a low-temperature anomaly has been reported for the spin-liquid material $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$. In this compound the in-plane optical response and vibrational coupling are rather anisotropic, indicating that the tilt of the BEDT-TTF molecules in $c$ direction and their coupling to the anion layers has to be considered in the explanation of the electromagnetic properties.

We report on the anisotropic response, the charge and lattice dynamics of normal and charge ordered phases with horizontal stripes in single crystals of the organic conductor $\alpha${}-(BEDT-TTF)2I3 determined by dc resistivity and dielectric and optical spectroscopy. An overdamped Drude response and a small conductivity anisotropy observed in optics is consistent with a weakly temperature-dependent dc conductivity and anisotropy at high temperatures. The splitting of the molecular vibrations $\nu${}27(Bu) evidences the abrupt onset of static charge order below TCO=136 K. The drop of optical conductivity measured within the ab plane of the crystal is characterized by an isotropic gap that opens at approximately 75 meV with several phonons becoming pronounced below. Conversely, the dc conductivity anisotropy rises steeply, attaining at 50 K a value 25 times larger than at high temperatures. The dielectric response within this plane reveals two broad relaxation modes of strength $\Delta${}$\varepsilon${}LD$\approx${}5000 and $\Delta${}$\varepsilon${}SD$\approx${}400, centered at 1$\hskip0.3em${}kHz<$\nu${}LD<100 MHz and $\nu${}SD$\approx${}1 MHz. The anisotropy of the large-mode (LD) mean relaxation time closely follows the temperature behavior of the respective dc conductivity ratio. We argue that this phasonlike excitation is best described as a long-wavelength excitation of a 2kF bond charge density wave expected theoretically for layered quarter-filled electronic systems with horizontal stripes. Conversely, based on the theoretically expected ferroelectriclike nature of the charge ordered phase, we associate the small-mode (SD) relaxation with the motion of domain-wall pairs, created at the interface between two types of domains, along the a and b axes. We also consider other possible theoretical interpretations and discuss their limitations.

We have in detail characterized the anisotropic charge response of the dimer Mott insulator $\kappa \text{−}\left(\text{BEDT-TTF}{)}_2{\mathrm{Cu}}_2\right(\text{CN}{)}_3$ by dc conductivity, Hall effect, and dielectric spectroscopy. At room temperature, the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity $\rho \left(T\right)$. Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K, we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also, the relaxation levels off. The observed behavior bears features of relaxorlike ferroelectricity. Because heterogeneities impede its long-range development, only a weak-tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

By systematic investigations of the magnetic, transport, and thermodynamic properties of single crystals of ${\mathrm{EuFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ ($0\le x\le 1$), we explore the complex interplay of superconductivity and ${\mathrm{Eu}}^{2+}$ magnetism. Below 30 K, two magnetic transitions are observed for all P substituted crystals, suggesting a revision of the phase diagram. In addition to the canted $A$-type antiferromagnetic order of ${\mathrm{Eu}}^{2+}$ at $\sim 20\mathrm{K}$, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of ${\mathrm{EuFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ coexists with superconductivity around $x\approx 0.2$.

The Mott insulator $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{Ag}}_2{\left(\mathrm{CN}\right)}_3$ forms a highly-frustrated triangular lattice of $S=1/2$ dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response, and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasidegenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound $\kappa -(\text{BEDT}-\text{TTF}){}_2{\mathrm{Cu}}_2\left(\mathrm{CN}\right){}_3$.