via time-resolved optical pump-probe spectroscopy*

Despite intensive study, the unconventional superconductivity in the layered oxide perovskite

Sr₂RuO₄ (SRO) still eludes explanation. An understanding of the superconducting state is likely

to require a complete and accurate description of the normal state out of which electron pairing

emerges. Previous angle-resolved transport studies have uncovered an apparent electronic

anisotropy in thin films of SRO which emerges well above the superconducting critical

temperature [Wu et al., PNAS 117, 10654 (2020)]. Here, we use time-resolved optical pump-

probe spectroscopy experiments to study the ultrafast electronic dynamics in SRO thin films.

Remarkably, we find that the in-plane four-fold rotational symmetry of the crystal lattice is

reduced to a two-fold symmetry in the transient response. This finding suggests spontaneously

broken rotational symmetry or a large nematic susceptibility exists in the normal state of SRO.

We discuss the implications of this normal-state nematic symmetry breaking on the much-

debated superconducting order parameter.