In quantum materials, spin and phonons always induce many interesting phenomena, including the superconductivity and spin liquids. In this presentation, Shaozhi Li of the University of Michigan will show studies of spin and phonons in complex materials. In the first part of the presentation, he will focus on the spin on the triangular lattice and explain how the spin spectra change in the metal-insulator transition. By examining the dynamical magnetic susceptibility and spin-lattice relax rate using the ladder dual fermion approximation, he finds that the low-energy spin excitations persist into the metallic state, which is consistent with the experimental observations in κ-(ET)2Cu2(CN)3. He also systematically studies the crossover from one to two dimensions on the triangular lattice and its impact on the spin properties. The results can contribute to the understanding of spin properties in Cs2CuCl4-xBrx.
In the second part of the talk, Li will discuss the role of breathing phonons in the high-temperature superconductor Ba1-xKxBiO3. By studying a three-orbital Su-Schrieffer-Heeger model defined on a two-dimensional Lieb Lattice and in the negative charge transfer regime using determinant quantum Monte Carlo, he computes the phase diagram on the plane of doping and temperature. At half-filling, he observes a bipolaron insulating phase, where the ligand oxygen atoms collapse and expand about alternating cation atoms to produce a bond-disproportionated state. This phase is robust against moderate hole doping but is eventually suppressed at large hole concentrations, leading to a metallic polaron-liquid-like state with fluctuating patches of local distortions. He also finds an s-wave superconducting state at finite doping that primarily appears on the oxygen sublattices. All these results are consistent with experimental observations in Ba1-xKxBiO3.