Thermopower across the phase diagram of the cuprate La${}_{1.6-x}$Nd${}_{0.4}$Sr${}_x$CuO${}_4$ : signatures of the pseudogap and charge-density-wave phases

Abstract

The Seebeck coefficient (thermopower) $S$ of the cuprate superconductor La${}_{1.6-x}$Nd${}_{0.4}$Sr${}_x$CuO${}_4$ was measured across its doping phase diagram (from $p=0.12$ to $p=0.25$), at various temperatures down to $T\simeq 2$ K, in the normal state accessed by suppressing superconductivity with a magnetic field up to $H=37.5$ T. The magnitude of $S/T$ in the $T=0$ limit is found to suddenly increase, by a factor $\simeq 5$, when the doping is reduced below $p^{\star }=0.23$, the critical doping for the onset of the pseudogap phase. This confirms that the pseudogap phase causes a large reduction of the carrier density $n$, consistent with a drop from $n=1+p$ above $p^{\star }$ to $n=p$ below $p^{\star }$, as previously inferred from measurements of the Hall coefficient, resistivity and thermal conductivity. When the doping is reduced below $p=0.19$, a qualitative change is observed whereby $S/T$ decreases as $T\to 0$, eventually to reach negative values at $T=0$. In prior work on other cuprates, negative values of $S/T$ at $T\to 0$ were shown to result from a reconstruction of the Fermi surface caused by charge-density-wave (CDW) order. We therefore identify $p_{\mathrm{CDW}}\simeq 0.19$ as the critical doping beyond which there is no CDW-induced Fermi surface reconstruction. The fact that $p_{\mathrm{CDW}}$ is well separated from $p^{\star }$ reveals that there is a doping range below $p^{\star }$ where the transport signatures of the pseudogap phase are unaffected by CDW correlations, as previously found in YBa${}_2$Cu${}_3$O${}_y$ and La${}_{2-x}$Sr${}_x$CuO${}_4$.

Present version includes a comparison with new x-ray data on Nd-LSCO by Gupta et al., arXiv:2012.08450. 13 pages, 12 figures, includes SM file