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 κ-ðBEDTÀTTFÞ 2 Cu½NðCNÞ 2 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 Ã ≃ 50 K, far higher than the equilibrium transition temperature T C ¼ 12.5 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.

STEM study of Cu kagome lattice in claringbullite.

Since the first observation of weak ferromagnetism in the charge-transfer salt κ-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840 (1992)], no further evidence of a ferromagnetic state in this class of organic materials has been reported. Here static and dynamic spin susceptibility measurements on κ-(BEDT-TTF)2Hg(SCN)2Br exhibit weak ferromagnetism below 20 K on the geometrically frustrated background. Our experimental results suggest that frustrated spins in the molecular dimers suppress long-range antiferromagnetic order, forming a spin-glass type ground state of the triangular lattice in the insulating phase which locally contains ferromagnetic polarons. Moreover, specific heat data reveal an excess peak located around 5 K indicating the glassy nature of the electrons as well. PACS numbers: 33.35.+r, 75.40.Gb, 75.50.Gg The search for ferromagnetism (FM) in lowdimensional organic charge-transfer salts still attracts attention in physics and chemistry. However, pure ferromagnetic organic π-electron systems containing only sand p-valence electrons remain rare and their synthesis is a challenging problem [1], while in inorganic materials FM usually arises from transition metals or transitionmetal ions (3d, 4f ), which in case of direct exchange may fulfill the Stoner criterion or in case of indirect exchange are subject to Goodenough-Kanamori-Anderson rules or Zener double exchange [2].The organic radical salts κ-(BEDT-TTF) 2 X, where BEDT-TTF is the abbreviation of bis-(ethylenedithio)tetrathiafulvalene, consist of alternating layers of the electron donor BEDT-TTF and electron acceptor X. In the κ-phase crystal structure the BEDT-TTF molecules stack in pairs as depicted in Fig. 1. Here the (BEDT-TTF) 2 dimers are arranged in a two-dimensional structure rather than in chains. Within the BEDT-TTF layers, the molecular dimers are close to each other, allowing substantial overlap of the molecular orbitals. Since one electron is transferred from each (BEDT-TTF) 2 dimer to the anion, the conduction band is half-filled. For weak electronic correlations, this implies that these organic compounds are metallic enabling nearly isotropic electron motion within the layer; perpendicular to the plane the resistivity is larger by more than one order of magnitude [3,4].The BEDT-TTF-based salts can be easily tuned by hydrostatic and uniaxial pressure, deuteration, or chemical substitution such that a wide range of electronic phases is obtained including paramagnetic, antiferromagnetic (AFM), spin-liquid, and superconducting ground FIG. 1: (Color online) Top view on the conductive bc planes of the dimerized BEDT-TTF molecules of κ-(BEDT-TTF)2Hg(SCN)2Br; each dimer hosts one spin. The dimer pattern can be modeled by an almost isosceles triangular lattice that is characterized by frustration effects. An artistic view of the spin arrangement illustrates that below the metalinsulator transition at TMI ≈ 90 K, spin frustration becomes dominant and suppresses the magnetic order. states [5]. The best studied examples a...