Quantum spin liquids unveil the genuine Mott state.

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.

Pressure cell for radio-frequency dielectric measurements at low temperatures.

We present the design, test, and performance of a piston type pressure cell for low-temperature dielectric measurements up to 10 kbar with particular emphasis on the electrical feedthrough for four coaxial cables and four conventional copper wires. The coaxial cables provide proper shielding of the applied test signal; a commercial continuous flow cryostat allows us to minimize the total cable length enabling temperature and pressure-dependent dielectric spectroscopy measurements down to 8 K and up to 5 MHz. We performed open compensation measurements, i.e., background measurements of the response originating from the pressure setup without a sample, to obtain its high frequency characteristics. The stray capacitance of the pressure setup is determined as Cstray = 40 fF, making it possible to measure small single crystals with a weak dielectric response. The proper operation is verified by comparing measurements of a test sample in the pressure setup at ambient pressure and in a standard dielectric spectroscopy setup.