Strain-modified thermopower of ultrathin LaNiO$_3$ films*

The influence of epitaxial strain on electronic transport in the correlated metal LaNiO$_3$ is investigated through measurements of thermopower (TEP) in the temperature range $5{\rm K}\leq {\rm T}\leq 330 {\rm K}$ on a series of fully-strained, 10-unit-cell-thick films grown by pulsed-laser deposition on (100)-oriented YAlO$_3$, LaAlO$_3$, SrTiO$_3$, and GaScO$_3$ substrates. The TEP exhibits an electron-like, linear-$T$ contribution for $T\geq 150$~K with a slope approximately independent of strain, but a magnitude that varies systematically with strain. A peak in the TEP at $T\approx 25$~K also correlates with strain and is unaffected by a 9-T magnetic field. The implications of these results for strain-modified charge-carrier diffusion and phonon drag contributions to the TEP will be discussed.