The vortex-Nernst effect in a superconducting infinite-layer nickelate

We report measurements of the Nernst and Seebeck effects in Nd${}_{1-x}$Sr${}_x$NiO${}_2$ thin films near the superconducting transition temperature, $T$ = 6.5 - 15 K. Our main result is the observation of a vortex-Nernst signal $S_{yx}(T,H)$ with a maximum at ${\mu }_{0}H$ = 5 T and a tail that extends to ${\mu }_{0}H$ $\approx$ 15 T, which we identify as the upper-critical field $H_{c2}$. At $T>T_{\mathrm{c}}=$ 6.1 K, $H_{c2}$ remains large (15 T), up to the highest temperature we can resolve from $S_{yx}$ (11 K). These results indicate the existence of a vortex-liquid state over a wide range of finite-resistance temperatures, as in the high-$T_{\mathrm{c}}$ cuprates.