YBCO’s temperature-oxygen doping phase diagram exhibits a small antiferromagnetic region at lowest doping and charge and spin orders around p=0.1 that compete with or induce superconductivity, as well as a pseudogap region and a QCP under the SC dome [1]. Over this range of doping, the Fermi surface changes from small pockets to arcs and finally a large pocket beyond the superconducting dome. Both the QCP and this change in FS are critical to our understanding of the cuprates and the universal phase diagram. Ramshaw, et al. [2] have found a divergence of the effective mass in the region of the CDW that hints at a QCP around p=0.19. Ideally, strong fields could also be used to suppress Hc2, allowing for the observation of quantum oscillations (QOs) in the region around the QCP, but this would require fields of approximately 150 T, well above the 100 T limit currently available. Instead doping has been used to suppress the dome to about 30 K [3], but doping at this level precludes the observation of QOs. Our group performed high pressure SdH studies of YBCO6.5 (p=0.1) at He-3 temperatures in pulsed fields to 70 T and 7 GPa at HLD and dc fields of 45 T and pressures of 24.7 GPa at NHMFL using plastic and metal diamond anvil cells (DACs), respectively, that are coupled with an LC tank circuit based on a tunnel diode oscillator. The small coil that makes up the inductor of this LC circuit and resides in the high pressure volume of the DAC senses changes in sample resistivity due to variations in temperature, pressure or magnetic field. Our Fermiology studies clearly show a strongly diverging effective mass at 4.5 GPa that is associated with a local maximum in frequency and Tc. The high Hc2 in this material limits our study in the low pressure range to pressures below 7 GPa. However,, at P> 24.7 GPa we are able to once again see quantum oscillations and find that the orbital frequency has increased from 550 T at ambient pressure to 690 T monotonically. Pulsed field high pressure studies are currently planned to shed light on the region between 7 and 24 GPa. This now allows us to use pressure to develop a B-P-T phase diagram that will permit a more complete picture of HTS to be pursued, perhaps answering how CDWs and the pseudogap play a role in superconductivity and allowing for the investigation of the QCP.

Acknowledgments : The National High Magnetic Field Laboratory is supported by National Science Foundation through NSF/DMR-1157490 and DMR-1644779 and the State of Florida.

[1] S. Badoux, et al., Nature, 531, 210 (2016).

[2] B. Ramshaw, et al., Science, 348, 317 (2015).

[3] J.L. Tallon, et al., Phys. Rev. Lett., 79, 5294 (1997).

[4] S. Sadewasser, et al., Phys. Rev. B, 56, 14168 (1997).

Contact : anne-laure.barra@lncmi.cnrs.fr