Influence of the c-axis transport crossover on the charge dynamics and plasmons in Sr2RuO4

The intriguing properties of Sr2RuO4, including its unconventional superconductivity and strong Hund's interactions, have been the focus of significant attention over the past three decades. Here, we investigate another one of its unusual properties, namely the crossover from coherent-to-incoherent c-axis transport and its impact on the charge dynamics and surface plasmons using a combination of angle-resolved photoemission spectroscopy (ARPES), high-resolution, momentum-resolved, reflection electron energy loss spectroscopy (EELS), and molecular beam epitaxy. For these experiments, we synthesized thin films with the a-axis oriented out of the substrate surface. This specific orientation enables the use of ARPES and EELS to investigate the in-plane versus out-of-plane anisotropy of the electronic structure and plasmon excitations. Using EELS, we observed that the surface plasmon exhibits an anisotropic negative dispersion along both the b- and c- directions. More remarkably, we found that the plasmon energy exhibits an anomalously large softening around the c-axis crossover temperature of 130 K, in contrast to conventional metals or other strongly correlated electron systems.