Andrej Pustogow

Anion effects on the electronic structure and electrodynamic properties of the Mott insulator kappa

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. Ou

-(BEDT-TT

In a comprehensive infrared study, the molecular vibrational features of (TMTTF)2SbF6, (TMTTF)2AsF6 and (TMTTF)2PF6 single crystals have been measured down to temperatures as low as 7 K by applying hydrostatic pressure up to 11 kbar. We follow the charge disproportionation below the critical temperatures T CO as pressure increases, and determine the critical pressure values p CO at which the charge-ordered phase is suppressed. The coexistence of the spin-Peierls phase with charge order is explored at low temperatures, and the competition of these two phases is observed. Based on our measurements we construct a generic phase diagram of the Fabre salts with centrosymmetric anions. The pressure-dependent anion and methyl-group dynamics in these quasi-one-dimensional charge transfer compounds yields information about the interplay of the organic molecules in the stacks and the anions, and how this interaction varies upon the transition to the charge-ordered state.

_2Ag2(CN)3

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 since1–3—not least for its relation to high-temperature superconductivity4. 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.A thorough analysis of the optical and transport properties of several two-dimensional organic conductors and insulators with varying on-site correlation strengths and bandwidths led to a quantitative phase diagram for pristine Mott insulators.

Pressure-dependent optical investigations of Fabre salts in the charge-ordered state

Optical conductivity measurements are combined with density functional theory calculations in order to understand the electrodynamic response of the frustrated Mott insulators Herbertsmithite

Quantum spin liquids unveil the genuine Mott state

\mathrm{ZnCu_{3}(OH)_{6}Cl_{2}}

Nature of optical excitations in the frustrated kagome compound herbertsmithite

and the closely-related kagome-lattice compound

Strong magnetic frustration in Y3Cu9(OH)19Cl8: a distorted kagome antiferromagnet

\mathrm{Y_{3}Cu_{9}(OH)_{19}Cl_{8}}

Electrodynamics in Organic Dimer Insulators Close to Mott Critical Point

. We identify these materials as charge-transfer rather than Mott-Hubbard insulators, similar to the high-

Raman spectroscopy evidence of domain walls in the organic electronic ferroelectrics (TMTTF)2X (X=SbF6,AsF6,PF6)

T_c

Structural and Electronic Properties of (TMTTF)2X Salts with Tetrahedral Anions

cuprate parent compounds. The band edge is at 3.3 and 3.6 eV, respectively, establishing the insulating nature of these compounds. Inside the gap, we observe dipole-forbidden local electronic transitions between the Cu