Theory of high energy spectroscopy in strongly correlated electron systems

Norikazu Tomita, Masahiro Yamazaki, Keiichirō Nasu

Roman O. Kuzian

The discovery of high-Tc superconductivity in cuprates in 1986 moved strongly correlated systems from exotic worlds interesting only for pure theorists to the focus of solid-state research. In recent decades, the majority of hot topics in condensed matter physics (high-Tc superconductivity, colossal magnetoresistance, multiferroicity, ferromagnetism in diluted magnetic semiconductors, etc.) have been related to strongly correlated transition metal compounds. The highly successful electronic structure calculations based on density functional theory lose their predictive power when applied to such compounds. It is necessary to go beyond the mean field approximation and use the many-body theory. The methods and models that were developed for the description of strongly correlated systems are reviewed together with the examples of response function calculations that are needed for the interpretation of experimental information (inelastic neutron scattering, optical conductivity, resonant inelastic X-ray scattering, electron energy loss spectroscopy, angle-resolved photoemission, electron spin resonance, and magnetic and magnetoelectric properties). The peculiarities of (quasi-) 0-, 1-, 2-, and 3- dimensional systems are discussed.

Pavel V. Shevchenko

Hong Yao

L. Craco, M. S. Laad

We study the electron energy-loss spectra of strongly correlated electronic systems doped away from half-filling using dynamical mean-field theory ( d = ∞ ). The formalism can be used to study the loss spectra in the optical ( q = 0 ) limit, where it is simply related to the optical response, and hence can be computed in an approximation-free way in d = ∞ dimensions. We apply the general formalism to the one-band Hubbard model away from n = 1, with inclusion of site-diagonal randomness to simulate effects of doping. The interplay between the coherence-induced plasmon feature and the incoherence-induced high-energy continuum is explained in terms of the evolution in the local spectral density upon hole doping. Inclusion of static disorder is shown to result in qualitative changes in the low-energy features, in particular to the overdamping of the plasmon feature, resulting in a completely incoherent response. The calculated lineshapes of electron energy-loss spectra are compared to the lineshapes of experimentally observed spectra for the normal state of the high- T c materials near optimal doping and good qualitative agreement is found.

Annabelle Bohrdt, Eugene Demler, F. Grusdt

Understanding the nature of charge carriers in doped Mott insulators holds the key to unravelling puzzling properties of strongly correlated electron systems, including cuprate superconductors. Several theoretical models suggested that dopants can be understood as bound states of partons, the analogues of quarks in high-energy physics. However, direct signatures of spinon-chargon bound states are lacking, both in experiment and theory. Here we propose a rotational variant of angle-resolved photo-emission spectroscopy (ARPES) and calculate rotational spectra numerically using the density-matrix renormalization group. We identify long-lived rotational resonances for an individual dopant, which we interpret as a direct indicator of the microscopic structure of spinon-chargon bound states. Similar to Regge trajectories reflecting the quark structure of mesons, we establish a linear dependence of the rotational energy on the superexchange coupling. The rotational peaks we find are strongly suppressed in standard ARPES spectra, but we suggest a multiphoton extension of ARPES which allows us to access rotational spectra. Our findings suggest that multiphoton spectroscopy experiments should provide new insights into emergent universal features of strongly correlated electron systems.