The Kramers-Kronig relations between the real and imaginary parts of a response function are widely used in solid-state physics to evaluate the corresponding quantity if only one component is measured. They are among the most fundamental statements since only based on the analytical behavior and causal nature of the material response [Phys. Rev. 104, 1760–1770 (1956)]. Optical losses, for instance, can be obtained from the dispersion of the dielectric constant at all wavelengths, and vice versa [Handbook of optical constants of solids, Vol. 1, p. 35]. Although the general validity was never casted into doubt, it is a longstanding problem that Kramers-Kronig relations cannot simply be applied to anisotropic crystalline materials because contributions from different directions mix in a frequency-dependent way. Here we present a general method to identify frequency-independent principal polarizability directions for which the Kramers-Kronig relations are obeyed even in materials with lowest symmetry. Using generalized spectroscopic ellipsometry on a single crystal surface of triclinic pentacene, as an example, enables us to evaluate the complex dielectric constant and to compare it with band-structure calculations along the crystallo-graphic directions. A general recipe is provided how to proceed from a macroscopic measurement on a low symmetry crystal plane to the microscopic dielectric properties of the unit cell, along whose axes the Kramers-Kronig relations hold.

Infrared reflection measurements of the half-filled two-dimensional organic conductors $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]{\mathrm{Br}}_x{\mathrm{Cl}}_{1-x}$ [BEDT-TTF denotes bis-(ethylenedithio)tetrathiafulvalene] were performed as a function of temperature $(5\mathrm{K}<T<300\mathrm{K})$ and Br substitution ($x=0\%$, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant and localized charge carriers. The broad midinfrared absorption has two contributions: transitions between the two Hubbard bands and intradimer excitations from the charges localized on the ${\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2$ dimer. Since the latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both electronic and vibrational features provides a tool to disentangle these contributions and to follow their temperature and electronic-correlation dependence. Calculations based on the cluster model support our interpretation.

A gapped spin liquid: Quantum spin liquids avoid conventional magnetic ordering down to the lowest temperatures. Among the candidates for this state of matter, organic salts such as κ-(BEDT-TTF) ₂ Cu ₂ (CN) ₃ have been prominent. Miksch et al. studied this material using electron spin resonance to elucidate the nature of its ground state. Instead of the expected gapless state, the temperature dependence of spin susceptibility suggests the formation of a spin gap. Science , this issue p. 276

In conventional ferroelectrics the polarization is induced either by the relative displacement of positive and negative ions due to a lattice distortion or by the collective alignment of permanent electric dipoles. Strongly correlated materials with the inversion-symmetry-broken ground states feature electronic ferroelectricity, a phenomenon which has recently caught the attention of condensed matter physicists due to its great fundamental and technological importance. The discovery of electronic ferroelectricity in one and two-dimensional molecular solids is an exciting development because they show a rich variety of nonlinear properties and complex electrodynamics, including nontrivial emergent excitations. We summarize key experimental results, sketch the current theoretical understanding and outline promising prospects of this phenomenon which have great potential for future electronic devices.

Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m^⋆. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe–Regel–Mott limit is accompanied by a ‘displaced Drude peak’ in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition.