# Ultrafast quasiparticle dynamics in correlated semimetal Ca$_3$Ru$_2$O$_7$

## TL;DR

This study uses ultrafast optical spectroscopy to investigate quasiparticle dynamics in Ca$_3$Ru$_2$O$_7$, revealing how the pseudogap influences electron relaxation and clarifying the nature of the crossover at T*.

## Contribution

It introduces a gap-modified two-temperature model and Rothwarf-Taylor modeling to explain quasiparticle relaxation and pseudogap behavior in a correlated semimetal.

## Key findings

- Identification of electron-phonon relaxation and phonon bottleneck processes.
- Correlation of pseudogap onset with quasiparticle dynamics.
- T* is due to carrier gapping, not a separate transition.

## Abstract

The correlated polar semimetal Ca$_3$Ru$_2$O$_7$ exhibits a rich phase diagram including two magnetic transitions ($T_N$=56 K and $T_C$=48 K) with the appearance of an insulating-like pseudogap (at $T_C$). In addition, there is a crossover back to metallic behavior at $T^*$=30 K, the origin of which is still under debate. We utilized ultrafast optical pump optical probe spectroscopy to investigate quasiparticle dynamics as a function of temperature in this enigmatic quantum material. We identify two dynamical processes, both of which are influenced by the onset of the pseudogap. This includes electron-phonon relaxation and, below $T_C$, the onset of a phonon bottleneck hindering the relaxation of quasiparticles across the pseudogap. We introduce a gap-modified two-temperature model to describe the temperature dependence of electron-phonon thermalization, and use the Rothwarf-Taylor to model the phonon bottleneck. In conjunction with density functional theory, our experimental results synergistically reveal the origin of the $T$-dependent pseudogap. Further, our data and analysis indicate that $T^*$ emerges as a natural consequence of $T$-dependent gapping out of carriers, and does not correspond to a separate electronic transition. Our results highlight the value of low fluence ultrafast optics as a sensitive probe of low energy electronic structure, thermodynamic parameters, and transport properties of Ruddlesden-Popper ruthenates.