Terahertz charge dynamics unveil fundamental transport anisotropy in charge-ordered Pr_(0.5)Eu_(0.5)NiO_3 nickelate thin films

摘要

Electrons condensed in a collective mode and behaving as free charge carriers are two important facets induced by the complex intertwining of charge, spin, orbital, and lattice degrees of freedom. Existence of these two electronic ground states in the same structural/chemical phase is as much desired as it is elusive in complex oxides. Using some unique attributes of terahertz (THz) spectroscopy, we show existence of two fundamentally different electronic phenomena in Pr_(0.5)Eu_(0.5)NiO_3 nickelate in the same structural phase but along two orthogonal in-plane axes [001] and [110] of thin films. While a collective response of charge via charge-density-wave type excitation manifests along the [001] direction, an entirely different Drude-Smith type free carrier response manifests along the other in-plane orthogonal axis. This anisotropy, on one hand, unveils the strain engineered crystallographic preferences of the underlying charge-ordering phenomenon; on the other hand, the different conduction channels open up possibilities of application of nickelates such as in THz transmission modulators.