YBCO’s temperature-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 [1]. Increasing the oxygen doping changes the Fermi surface (FS) from small pockets to a large FS beyond the superconducting dome. This change in FS are critical to our understanding of the cuprates and the universal phase diagram.
Instead of chemical doping, we have used pressure to tune the FS of YBCO, and measured it by looking at the quantum oscillations (QO) in high magnetic fields via the Tunnel Diode Oscillator method. The small coil that makes up the inductor of this LC circuit resides in the high pressure volume of the DAC senses changes in sample resistivity due to variations in temperature, pressure or magnetic field Our group performed high pressure Shubnikov-de Haas studies of YBCO6.5 (p=0.1) at He-3 temperatures in pulsed fields to 70 T and 7 GPa at EMFL-HLD and DC fields of 45 T and pressures of 25 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. .
Our high pressure studies show an enhancement of the superconducting critical field from 24 to 42T between ambient pressure and 6 GPa, which limits the observation of QO to 5 GPa in the 45T Hybrid. Our Fermiology studies clearly show a strongly diverging effective mass at 4.5 GPa along with a local maximum in frequency and superconducting critical temperature, attributed to the effect of various charge orders present in this material. For pressures greater than 15 GPa we are able to measure a critical field of the order of 30T and to measure again QOs. We find that the orbital frequency has increased from 550 T at ambient pressure to 690 T at 15 GP and above. Assuming that the samples are driven by pressure to the overdoped state, those results do not match the reported frequency of 18kT observed for the overdoped analog Tl-2201. This indicates that pressure and doping are not playing an equivalent role on the CDWs and the superconducting state as also pointed in other studies [2][3], and shows that pressure is a new axis in the YBCO phase diagram which can help understand the interplay between CDWs and superconductivity in this material.
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. We acknowledge the support of the HLD-member of the European Magnetic Field Laboratory (EMFL).
[1] S. Badoux, et al., Nature, 531, 210 (2016).
[2] S.M. Souliou, Phys. Rev. B, 2018 97, 020503.
[3] H.-H. Kim et al., Science 2018, 362, 1040