Model evidence of a superconducting state with a full energy gap in small cuprate islands

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We investigate subdominant order parameters stabilizing at low temperatures in nano-scale high-Tc cuprate islands, motivated by the recent observation of a fully gapped state in nanosized YBa₂Cu₃O₇₋δ [D. Gustafsson et al, Nature Nanotech. 8, 25 (2013)]. Using complementary quasi-classical and tight-binding Bogoliubov-de Gennes methods, we show on distinctly different properties dependent on the symmetry being dₓ₂₋y₂+i s or dₓ₂₋y₂+i dxy. We find that a surface-induced dₓ₂₋y₂+i s phase creates a global spectroscopic gap which increases with applied magnetic field, consistent with experimental observation.