Fingerprints of preformed pairs in two-electron angle-resolved photoemission spectroscopy

TL;DR

This paper uses variational exact diagonalization to identify spectral fingerprints of electron pairs in two-electron ARPES, providing a method to confirm pair formation and coherence in various models.

Contribution

It introduces a novel spectral analysis approach to detect and distinguish electron pairs and their symmetry, applicable across different models and conditions.

Findings

Spectral weight segregates at lower binding energy for pairs from the same electron pair.

Characteristic momentum dependence with distinct symmetry signals pair formation.

Fingerprints are universal for models with electron-boson coupling leading to pairing.

Abstract

We use variational exact diagonalization (VED) to calculate the two-electron removal spectral weight for the Hubbard-Holstein model, starting from the ground-state with two electrons on a one-dimensional chain. We argue that this spectral weight provides a valuable proxy for the intensity of 2eARPES processes. Our results show that when contrasted to the presumably larger signal due to two electrons ejected from two different pairs, the presumably weaker signal due to two electrons ejected from the same pair (i) is segregated in energy, appearing at a lower binding energy, and (ii) has a very characteristic momentum dependence, with a different symmetry than that of the signal corresponding to two electrons emitted from two different pairs. We verify that these fingerprints appear for pairs with different symmetries, and prove that they arise as a direct consequence of momentum and energy conservation, therefore they are generic for any model with electron-boson coupling that can lead to formation of electron pairs. Experimental observation of these fingerprints will confirm the existence of pairs. Moreover, the momentum dependence map allows one to distinguish whether the pairs are coherent (superconducting) or not. Finally, we argue that these considerations generalize to finite but low electron concentrations, finite temperatures and higher dimensions.