# Noise Correlations in time- and angular-resolved photoemission spectroscopy

## TL;DR

This paper proposes a theoretical framework linking electron momentum correlations in time-resolved photoemission to two-particle correlations in solids, enabling the study of superconducting and charge density fluctuations beyond traditional energy gap measurements.

## Contribution

It introduces a novel method to probe non-equilibrium pairing correlations via angular correlations in photoemission, extending spectroscopy's capabilities beyond energy gap detection.

## Key findings

- Correlations between emitted electron momenta relate to solid's two-particle correlations.
- Ultrafast gap quenching persists with non-equilibrium pairing correlations.
- Angular correlations reveal pairing dynamics independent of the energy gap.

## Abstract

In time-resolved photoemission experiments, more than one electron can be emitted from the solid by a single ultra-short pulse. We theoretically demonstrate how correlations between the momenta of outgoing electrons relate to time-dependent two-particle correlations in the solid. This can extend the scope of time- and angular-resolved photoemission spectroscopy to probe superconducting and charge density fluctuations in systems without long-range order, and to reveal their dynamics independent of the electronic gap and thus unrestricted by the energy-time uncertainty. The proposal is illustrated for superconductivity in a BCS model. An impulsive perturbation can quench the gap on ultrafast timescales, while non-equilibrium pairing correlations persist much longer, even when electron-electron scattering beyond mean-field theory is taken into account. There is thus a clear distinction between a dephasing of the Cooper pairs and the thermalization into the normal state. While a measurement of the gap would be blind to such pairing correlations, they can be revealed by the angular correlations in photoemission.