Coherent Spin Excitations and Correlated Fermi Liquid in Genuine Mott Systems

Recent success in determining the electronic correlation strength in Mott insulators by means of optical spectroscopy [1] enables a quantitative comparison among (i) different materials and (ii) between experiment and theory. We identify frustrated molecular conductors as ideal realizations of the single-band Hubbard model in contrast to transition-metal oxides with a more complicated band structure, such as the charge-transfer insulator Herbertsmithite [2]. This finally allows us to selectively probe specific regions in the unified phase diagram and unveil exotic phenomena in absence of magnetic order [1,3]. Deep in the insulating state of a quantum spin liquid, coherent spin excitations show up in the optical conductivity when the charge degrees of freedom are sufficiently suppressed within the Mott gap [3,4]. Turning to the correlated metallic state, we can unambiguously identify the spectral features of coherent and bad-metallic transport. We observe a pronounced quadratic temperature and frequency dependence of the scattering rate in the Fermi-liquid state. All data collapse on a generalized energy scale in excellent agreement with the theoretical framework of Landau and Gurzhi from the 1950's. We monitor the effective mass enhancement as correlations increase and infer that the Fermi liquid retains its intrinsic properties towards the Mott transition.
[1] Nat. Mater. 17, 773–777 (2018)
[2] Phys. Rev. B 96, 241114(R) (2017)
[3] J. Phys. Condens. Matter 30, 203001 (2018)
[4] Phys. Rev. Lett. 121, 056402 (2018)