Nodal
Metallic Phase in Underdoped Cuprates
Electrons
in cuprates adopt a remarkable sequence of ground states as one varies the
density of charge carriers. At zero
hole density, the material is a Mott insulator with static long-range
antiferromagnetic order. At high density,
it is a normal metal with the basic signatures of a Fermi liquid. At intermediate density, it is a
superconductor with d-wave symmetry. A
central outstanding question is: what is the nature of the underdoped phase
that lies between the insulator and the superconductor? Recent measurements of low temperature
thermal transport offer new insight into this question. We track the evolution of the residual
electronic contribution to the thermal conductivity, κ0/T,
across the cuprate phase diagram. In the extreme underdoped limit, we observe
delocalized fermionic excitations at zero energy in the non-superconducting
state of strongly underdoped YBa2Cu3Oy. This
reveals that the ground state of clean underdoped cuprates is metallic, and we
argue that this metallic phase has a nodal spectrum akin to the superconductor
and is therefore distinct from the metallic phase in the overdoped regime. This contrasts with the insulating ground
state observed in underdoped La2-xSrxCuO4,
likely caused by the spin-density-wave order present in that system.