Coherent c-axis transport in the underdoped cuprate superconductor YBa2Cu3Oy

The electrical resistivity rho(c) of the underdoped cuprate superconductor YBa2Cu3Oy was measured perpendicular to the CuO2 planes on ultrahigh quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of rho(c) at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature. This crossover coincides with the emergence of the small electron pocket detected in the Fermi surface of YBa2Cu3Oy via quantum oscillations, the Hall and Seebeck coefficients, and with the detection of a unidirectional modulation of the charge density as seen by high-field nuclear magnetic resonance measurements. The low coherence temperature is quantitatively consistent with the small hopping integral t(perpendicular to) inferred from the splitting of the quantum oscillation frequencies. We conclude that the Fermi-surface reconstruction in YBa2Cu3Oy at dopings from p = 0.08 to at least p = 0.15, attributed to stripe order, produces a metallic state with three-dimensional coherence deep in the underdoped regime.