Electron Correlations

Nobuo Tsuda, Keiichirō Nasu, Atsushi Fujimori +1 more

Thomas Hansen, Simon Vendelbo Bylling Jensen, Lars Bojer Madsen

Using the Hubbard model we study how the process of high-order harmonic generation (HHG) is modified by electron-electron correlation for both finite and bulk systems. A finite-size enhancement of the HHG signal is found and attributed to electrons backscattering off the lattice edges. Additionally, with the increasing strength of the electron-electron correlation an enhancement of the high-frequency regime of the HHG spectrum is found. This is attributed to the on-site Coulomb repulsion between electrons giving rise to a localized correlation-induced quiver motion of the electrons. The finite-lattice enhancement dominates the HHG spectra from a few harmonic orders until a threshold from which the correlational enhancement dominates. This threshold is determined by the degree of correlation and decreases into the low-frequency regime for increasing electron-electron correlation. This infers that as the electron-electron correlation increases, the finite-size effect on the electron dynamics decreases.

U. Fano

Jincheng Zhang

Electron correlation is a cornerstone of modern condensed matter physics, traditionally quantified by the magnitude of the two‑particle correlation function. In this work we introduce the concept of the Electron Correlation Phase Spectrum (ECPS), which records not only the amplitude but also the phase of the correlation function. The phase of a correlation function contains information about the relative timing and spatial ordering of electronic excitations, and its spectral decomposition provides a new observable for quantum materials spectroscopy. We formulate a rigorous definition of the ECPS, derive its properties from many‑body theory, and demonstrate how it can be computed from first‑principles wavefunctions. The ECPS is shown to be sensitive to hidden orders, fractionalization, and topological invariants that are invisible to conventional amplitude‑only probes. This work lays the theoretical foundation for future experimental realizations of phase‑sensitive correlation spectroscopy, with potential applications to high‑temperature superconductors, quantum spin liquids, and engineered quantum devices.

Jenő Sólyom