Effect of pressure on the pseudogap and charge-density-wave phases of the cuprate La${}_{1.6-x}$Nd${}_{0.4}$Sr${}_x$CuO${}_4$ probed by thermopower measurements

We report thermopower measurements under hydrostatic pressure on the cuprate superconductor La${}_{1.6-x}$Nd${}_{0.4}$Sr${}_x$CuO${}_4$ (Nd-LSCO), at low-temperature in the normal state accessed by suppressing superconductivity with a magnetic field up to $H=31$ T. Using a newly developed AC thermopower measurement technique suitable for high pressure and high field, we track the pressure evolution of the Seebeck coefficient $S$. At ambient pressure and low temperature, $S\mathrm{/}T$ was recently found to suddenly increase in Nd-LSCO at the pseudogap critical doping $p^{\star }=0.23$, consistent with a drop in carrier density $n$ from $n=1+p$ above $p^{\star }$ to $n=p$ below. Under a pressure of 2.0 GPa, we observe that this jump in $S\mathrm{/}T$ is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect which found $d{p}^{\star }\mathrm{/}dP\simeq -0.01$ holes/GPa, thereby reinforcing the interpretation that this effect is driven by the pressure-induced shift of the van Hove point. It implies that the pseudogap only exists when the Fermi surface is hole-like, which puts strong constraints on theories of the pseudogap phase. We also report thermopower measurements on Nd-LSCO and La${}_{1.8-x}$Eu${}_{0.2}$Sr${}_x$CuO${}_4$ in the charge density-wave phase near $p \sim 1/8$, which reveals a weakening of this phase under pressure.