Counter-thermal flow of holes in high-mobility ${\mathrm{LaNiO}}_3$ thin films

Measurements of electronic structure and theoretical models indicate that the Fermi surface of ${\mathrm{LaNiO}}_3$ (LNO) is predominantly holelike, with a small electron pocket. However, measurements of the Hall and Seebeck effects yield nominally opposite signs for the dominant charge carrier type, making charge transport in LNO puzzling. Here, we combine measurements of the Hall, Seebeck, and Nernst coefficients in high-mobility epitaxial LNO thin films, and resolve this puzzle by demonstrating that the negative Seebeck coefficient is generated by the diffusion of holes from cold to hot regions of LNO. We further examine this counter-thermal flow of holes by measuring the evolution of the Nernst coefficient from the diffusive to the ballistic regime, where the suppression of energy-dependent scattering leads to a reversal of the flow of holes.

Figure 1

Electric transport measurements. (a) Temperature dependence of the longitudinal resistivity. (b) Hall voltage measured as a function of magnetic field at different temperatures. The sample has 80 unit cells with a thickness of about 30.5 nm. (c) Temperature dependence of the Hall resistance. (d) Temperature dependence of the Hall angle.

Figure 2

Device structure and measurement geometry, Seebeck and Nernst results, and illustration of charge transport by both holes and electrons. (a) Schematic of device structures. Heater current flowing in the Au wire is indicated by a red arrow. $H$ is the applied magnetic field. Measurements of the Seebeck and Nernst voltage are illustrated in blue and black, respectively. The corresponding signs of the signal are indicated by the plus/minus symbols. (b) Temperature dependence of the measured Seebeck coefficient $S$. (c) Nernst voltage measured as a function of magnetic field at $T=3$ K. (d) Temperature dependence of the measured Nernst coefficient (blue, left axis) and the product of Seebeck coefficient and Hall angle (red, right axis). (e) Four different scenarios for the diffusion of holes (red) and electrons (blue) under a temperature gradient. Only the last two are allowed based on the signs of Nernst and Seebeck measurements.

Figure 3

Out-of-plane Nernst device, illustration of diffusive and ballistic transports, and measurement results. (a) Device structure of the out-of-plane device and measurement geometry. (b) Illustration of the diffusive and ballistic thermal transport of holes in LNO. Thicker arrows represent higher velocity. (c) Temperature dependence of Nernst coefficients measured in the out-of-plane geometry (red) in contrast to that obtained with the temperature gradient in the sample plane (blue). (d) Nernst coefficients measured on samples with different thicknesses and mobilities which are represented by the mean free paths at $T=10$ K.