On the Susceptibility of a Mott Insulator

Gang Chen, Hae‐Young Kee, Yong Baek Kim

Motivated by recent experiments on the quantum-spin-liquid candidate material LiZn2Mo3O8, we study a single-band extended Hubbard model on an anisotropic Kagome lattice with the 1/6 electron filling. Due to the partial filling of the lattice, the inter-site repulsive interaction is necessary to generate Mott insulators, where electrons are localized in clusters, rather than at lattice sites. We provide examples of such cluster Mott insulators and study the phase transitions between metallic states and cluster Mott insulators on an anisotropic Kagome lattice. It is shown that these cluster Mott insulators are generally U(1) quantum spin liquids with spinon Fermi surfaces. However, the nature of charge excitations in different cluster Mott insulators could be quite different and we show that there exists a novel cluster Mott insulator where charge fluctuations around the hexagonal cluster induce a plaquette charge order (PCO). The spinon excitation spectrum in this spin-liquid cluster Mott insulator is reconstructed due to the PCO so that only 1/3 of the total spinon excitations are magnetically active. The strong coupling limit of the same model is also analyzed via a Kugel-Khomskii-like model. Based on these results, we propose that the anomalous behavior of the finite-temperature spin-susceptibility in LiZn2Mo3O8 may be explained by finite-temperature properties of the cluster Mott insulator with the PCO as well as fractionalized spinon excitations. Existing and possible future experiments on LiZn2Mo3O8, and other Mo-based cluster magnets are discussed in light of these theoretical predictions.

B. H. Brandow

Florian Gebhard

Y. Mizuno, K. Tsutsui, Takami Tohyama +1 more

We theoretically study the nonlinear optical response and photoexcited states of the Mott insulators. The nonlinear optical susceptibility ${\ensuremath{\chi}}^{(3)}$ is calculated by using the exact diagonalization technique on small clusters. From the systematic study of the dependence of ${\ensuremath{\chi}}^{(3)}$ on dimensionality, we find that the spin-charge separation plays a crucial role in enhancing ${\ensuremath{\chi}}^{(3)}$ in the one-dimensional (1D) Mott insulators. Based on this result, we propose a holon-doublon model, which describes the nonlinear response in the 1D Mott insulators. These findings show that the spin-charge separation will become a key concept of optoelectronic devices.

L. B. Ioffe, Vadim Kalmeyer

We discuss the orbital part of the magnetic susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathit{d}}$ of the doped Mott insulator in two dimensions. Starting from the U(1) gauge theory, we obtain corrections to ${\mathrm{\ensuremath{\chi}}}_{\mathit{d}}$ due to gauge-field fluctuations. We find that these fluctuations reduce the Bose contribution to the susceptibility, implying that the Bose-condensation temperature is strongly suppressed by gauge interactions. The temperature dependence of ${\mathrm{\ensuremath{\chi}}}_{\mathit{d}}$ is obtained by a numerical simulation of a lattice Bose gas interacting with a random magnetic field, and found to be consistent with recent data on high-${\mathit{T}}_{\mathit{c}}$ oxides.

Sindhunil Barman Roy