Effect of pressure on the pseudogap and charge density wave phases of the cuprate Nd-LSCO probed by thermopower measurements

We report thermopower measurements under hydrostatic pressure on the cuprate superconductor La1.6-xNd0.4SrxCuO4 (Nd-LSCO), at low temperature in the normal state accessed by suppressing superconductivity with a magnetic field up to H = 31 T. Using an 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/T in Nd-LSCO was recently found to suddenly increase below 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 the large S/T value just below p(star) is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect, which found that pressure lowers p(star), 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 La1.8-xEu0.2SrxCuO4 in the charge density wave phase near p similar to 1/8, which reveals a weakening of this phase under pressure.