The Mott transition is investigated in three different organic insulators with triangular lattices and evidence of quantum criticality in an intermediate temperature regime is uncovered. A many-body quantum system on the verge of instability between two competing ground states may exhibit quantum-critical phenomena1,2, as has been intensively studied for magnetic systems. The Mott metal–insulator transition3, a phenomenon that is central to many investigations of strongly correlated electrons, is also supposed to be quantum critical, although this has so far not been demonstrated experimentally. Here, we report experimental evidence for the quantum-critical nature of the Mott instability, obtained by investigating the electron transport of three organic systems with different ground states under continuously controlled pressure. The resistivity obeys the material-independent quantum-critical scaling relation bifurcating into a Fermi liquid or Mott insulator, irrespective of the ground states. Electrons on the verge of becoming delocalized behave like a strange quantum-critical fluid before becoming a Fermi liquid.

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.

The Coulomb interaction among massless Dirac fermions in graphene is unscreened around the isotropic Dirac points, causing a logarithmic velocity renormalization and a cone reshaping. In less symmetric Dirac materials possessing anisotropic cones with tilted axes, the Coulomb interaction can provide still more exotic phenomena, which have not been experimentally unveiled yet. Here, using site-selective nuclear magnetic resonance, we find a non-uniform cone reshaping accompanied by a bandwidth reduction and an emergent ferrimagnetism in tilted Dirac cones that appear on the verge of charge ordering in an organic compound. Our theoretical analyses based on the renormalization-group approach and the Hubbard model show that these observations are the direct consequences of the long-range and short-range parts of the Coulomb interaction, respectively. The cone reshaping and the bandwidth renormalization, as well as the magnetic behaviour revealed here, can be ubiquitous and vital for many Dirac materials.

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

Quantum spin liquids, which are spin versions of quantum matter, have been sought after in systems with geometrical frustration. We show that disorder drives a classical magnet into a quantum spin liquid through conducting NMR experiments on an organic Mott insulator, $\kappa \text{−}({\mathrm{ET})}_2\mathrm{Cu}[\mathrm{N}({\mathrm{CN})}_2]\mathrm{Cl}$. Antiferromagnetic ordering in the pristine crystal, when irradiated by x rays, disappears. Spin freezing, spin gap, and critical slowing down are not observed, but gapless spin excitations emerge, suggesting a novel role of disorder that brings forth a quantum spin liquid from a classical ordered state.

Finding correlations in a Dirac-cone material: Researchers have long been on the lookout for signatures of electron-electron interactions in materials whose electrons have linear energy dispersions represented by Dirac cones, such as graphene. However, these effects have remained frustratingly small. Hirata et al. used nuclear magnetic resonance to study the layered organic material α-(BEDT-TTF) ₂ I ₃ , in which a phase featuring Dirac cones is known to be adjacent to one with enhanced electronic correlations. The unusual temperature dependence of spin-related properties in this material indicated strong correlations among the linearly dispersing electrons. Science , this issue p. 1403