The localization of charge carriers by electronic repulsion was suggested by Mott in the 1930s to explain the insulating state observed in supposedly metallic NiO. The Mott metal-insulator transition has been subject of intense investigations ever since-not least for its relation to high-temperature superconductivity. A detailed comparison to real materials, however, is lacking because the pristine Mott state is commonly obscured by antiferromagnetism and a complicated band structure. Here we study organic quantum spin liquids, prototype realizations of the single-band Hubbard model in the absence of magnetic order. Mapping the Hubbard bands by optical spectroscopy provides an absolute measure of the interaction strength and bandwidth-the crucial parameters that enter calculations. In this way, we advance beyond conventional temperature-pressure plots and quantitatively compose a generic phase diagram for all genuine Mott insulators based on the absolute strength of the electronic correlations. We also identify metallic quantum fluctuations as a precursor of the Mott insulator-metal transition, previously predicted but never observed. Our results suggest that all relevant phenomena in the phase diagram scale with the Coulomb repulsion U, which provides a direct link to unconventional superconductivity in cuprates and other strongly correlated materials.
The Mott insulator κ-(BEDT-TTF)2Ag2(CN)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 quasi-degenerate 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 κ-(BEDT-TTF)2Cu2(CN)3.PACS numbers: 75.10. Kt, 77.22.Gm, Electronic ferroelectricity and multiferroicity attracts great attention of condensed matter physicists due to their fundamental and technological importance. 1-3 They are identified in systems with strong electronic correlations such as transition-metal oxides and low-dimensional charge-transfer molecular solids. In the latter category, electric polarization arises from valence instability and charge ordering. In both cases, breaking the inversionsymmetry results in the concurrence of non-equivalent charge-sites and bonds. 4 There is no doubt that electron correlations are fundamental for stabilizing the ferroelectric ground state, nevertheless, experimental evidence indicates that the delicate interplay of Coulomb forces and structural changes within the coupled molecular-anion system have to be taken into account. Along these lines a solid understanding of electronic ferroelectricity was achieved for the families of quasi-one-dimensional organic charge-transfer salts: (TMTTF) 2 X and TTF-X, but also some layered (BEDT-TTF) 2 X systems. [5][6][7] However, no consensus has been reached yet on the origin of the ferroelectric signatures detected in the strongly dimerized κ-(BEDT-TTF) 2 X salts. [8][9][10][11][12] In these compounds, the BEDT-TTF dimers are arranged in a triangular lattice with a relatively high geometrical frustration. In some of them, indications of charge-ordering phenomena have been reported, but in-depth studies are missing 13,14 . On the other hand, the Mott dimer insulators κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl and κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , called κ-CuCN, have been thoroughly studied because they are discussed as prototypes of a molecular multiferroic and quantum spin liquid (QSL) systems. 9,15 It turns out to be extremely challenging to reconcile the idea of quantum electric dipoles on molecular dimers interacting via dipolar-spin coupling [16][17][18][19] with the experimentally evidenced absence of any considerable charge imbalance. So far no global structural changes and no charge disproportionation between molecular dimer sites larger than 2δ ρ ≈ ±0.01e that could break the symmetry have been found. 20,21 In the case of the QSL κ-...
The air-stable phosphide, Ag6Ge10P12, was synthesized from its elements in gram amounts. As its structure is closely related to high-performance thermoelectric tetrahedrites (Ag6□Ge4Ge6P12 ≡ Cu6SSb4Cu6S12), we studied temperature dependent single-crystal X-ray diffraction experiments, quantum chemical calculations, and thermoelectric transport properties of spark plasma sintered and pristine, single crystalline samples, in order to give a comprehensive picture of its thermoelectric performance and its origin. The semiconducting character of this material is reflected in band structure calculations. Measurements of the thermal diffusivity exhibit a very low thermal conductivity, κ < 1 W m–1 K–1, which is close to a phonon glass-like behavior, and has its origin in a strong local bonding asymmetry, induced by strong bonding of the phosphorus–germanium (Ge4+) covalent framework and weak bonding of lone-pair electrons (Ge2+). This chemical bond hierarchy creates a pronounced anisotropic behavior of the silver atoms leading to low-frequency vibrations and thermal damping. Combining this with a moderate electrical resistivity (ρ ∼ 15 mΩ cm) and a high Seebeck coefficient (S ∼ 380 μV K–1) results in a remarkably high figure of merit (zT) of about 0.6 at 700 K. These results demonstrate that Ag6Ge10P12 is one of the best thermoelectric phosphides and a promising new platform for the future development of thermoelectrics.
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