Studying Electron Dynamics for Quantum Materials with Real Space Resolution: A Wannier Orbital Approach to Spectroscopy using High-Performance Supercomputers

Quantum materials have a promising future for energy and security applications which will lay the bedrock for material science research for decades to follow. Partic- ularly, ‘one-dimensional’ Mott-insulating cuprates such as SrCuO 2 and (Ca)Sr 2 CuO 3 have been deemed to fall under a ‘fractionalization’ paradigm in which the electrons disintegrate into bosonic collective excitations of their fundamental constituents— spin, charge, and ‘orbital’ degrees of freedom— due to the anisotropic crystalline structure, deeming them outside the band theory of solids. Here, I provide ab initio theory for the ‘one-dimensional’ cuprates SrCuO 2 and (Ca)Sr 2 CuO 3 using no adjustable parameters with excellent agreement in absolute units with optical conductivity, dielectric function, and loss function measurements. In addition, we i) notice an overlooked ‘discontinuity’ in the loss dispersion at the antiferromagnetic zone boundary and provide theory to fill this void; ii) predict a novel Mott-gapped longitudinal spin excitation that can be verified via inelastic neutron scattering measurements; and iii) predict a re-emergence of the charge density excitations in higher Brillouin zones which can be validated with non-resonant inelastic X-ray scattering measurements. To understand the microscopic physics, it was necessary to downfold exact time- dependent density functional theory to a low energy space of Wannier orbitals. This required developing a rigorous disentanglement procedure to partition the Wannier basis from the rest of the Hilbert space, a necessity for the cuprates due to the strong entanglement between the oxygen p and copper d derived content of the band vstructure. By doing so, I attest that the Mott-gapped collective excitations require a proper treatment of the solid state chemistry inherit to the electronic structure, and they are triggered by the long-ranged dynamically screened Coulomb interaction. In addition, the calculations strongly suggest the ‘fractionalization’ paradigm is not compatible with these materials due to the indiscrimination of the orbital and spin degree of freedom of the Coulomb interaction, in which the collective modes do not simply separate into charge-only, spin-only, and ‘orbital’-only degrees of freedom.