We explore the combined pressure and finite-size effects on the in-plane penetration depth \lambda_{ ab} in YBa_2Cu_4O_8. Even though this cuprate is stoichiometric the finite-size scaling analysis of \lambda_{ ab}^{ -2}(T) uncovers the granular nature and reveals domains with nanoscale size L_{ c} along the c-axis. L_{ c} ranges from 33.2 Angstrom to 28.9 Angstrom at pressures from 0.5 to 11.5 kbar. These observations raise serious doubts on the existence of a phase coherent macroscopic superconducting state in cuprate superconductors.
Categories: cond-mat.supr-con