David Benjamin

Microscopic theory of resonant soft-x-ray scattering in materials with charge order: the example of charge stripes in high-temperature cuprate superconductors.

We present a microscopic theory of resonant soft-x-ray scattering that accounts for the delocalized character of valence electrons. Unlike past approaches based on local form factors, our functional determinant method treats realistic band structures. This method builds upon earlier theoretical work in mesoscopic physics and accounts for excitonic effects as well as the orthogonality catastrophe arising from interaction between the core hole and the valence band electrons. We show that the two-peak structure observed near the O K edge of stripe-ordered La1.875Ba0.125CuO4 is due to dynamical nesting within the canonical cuprate band structure. Our results provide evidence for reasonably well-defined, high-energy quasiparticles in cuprates and establish resonant soft-x-ray scattering as a bulk-sensitive probe of the electron quasiparticles.

Exploring quasiparticles in high-Tc cuprates through photoemission, tunneling, and x-ray scattering experiments

One of the key challenges in the field of high-temperature superconductivity is understanding the nature of fermionic quasiparticles. Experiments consistently demonstrate the existence of a second energy scale, distinct from the d-wave superconducting gap, that persists above the transition temperature into the ‘pseudogap’ phase. One common class of models relates this energy scale to the quasiparticle gap due to a competing order, such as the incommensurate ‘checkerboard’ order observed in scanning tunneling microscopy (STM) and resonant elastic x-ray scattering (REXS). We develop a minimal phenomenological model that allows us to quantitatively describe STM and REXS experiments and discuss their relation with photoemission spectroscopy. Experimental signatures of the incommensurate order are explained in terms of scattering of short-lived quasiparticles from local impurities. We identify the unknown second energy scale with the inverse lifetime of the quasiparticles, refocusing questions about the nature of the pseudogap phase to the study of the origin of inelastic scattering.

Friedel oscillations as a probe of fermionic quasiparticles

When immersed in a see of cold electrons, local impurities give rise to density modulations known as Friedel oscillations. In spite of the generality of this phenomenon, the exact shape of these modulations is usually computed only for non-interacting electrons with a quadratic dispersion relation. In actual materials, one needs to take into account several additional factors, such as (i) the details of the band structure, (ii) the lifetime of quasiparticles, (iii) in superconductors, the presence of a pairing gap. Studying how these effects influence Friedel oscillations is a viable way to access the properties of fermionic excitations in strongly-correlated materials. In this work we analyze the signatures of Friedel oscillations in STM and X-ray scattering experiments, focusing on the concrete example of cuprates superconductors. A detailed comparison with recent experiments reveals a rich interplay between local modulation of the chemical potential and of the pairing gap.