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 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=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 La_{1.8−x}Eu_{0.2}Sr_{x}CuO_{4} in the charge density wave phase near p∼1/8, which reveals a weakening of this phase under pressure.