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

Abstract

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=31T. 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∗ =0.23, consistent with a drop in carrier density n from n=1+p above p∗ to n=p below. Under a pressure of 2.0 GPa, we observe that the large S/T value just below p∗ is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect, which found that pressure lowers p∗, 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∼1/8, which reveals a weakening of this phase under pressure.