Delocalized fermions in underdoped cuprate superconductors.

Low-temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly doped cuprate YBa2Cu3O6:33. Samples were measured with doping concentrations on either side of the superconducting phase boundary. We report the observation of delocalized fermionic excitations at zero energy in the nonsuperconducting state, which shows that the ground state of underdoped cuprates is a thermal metal. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e., nodal. The insulating ground state observed in underdoped La2� xSrxCuO4 is attributed to the competing spin-density-wave order. Electrons in cuprates adopt a remarkable sequence of ground states as one varies the density of charge carriers. When the electron density in the CuO2 planes of a cuprate material is exactly 1.0 per Cu atom in the plane, the material is a Mott insulator with static long-range antiferromagnetic order, and the electrons are localized on their sites by strong Coulomb repulsion. This electronic gridlock can be relaxed by removing electrons from the planes, a charge-transfer process induced by chemical substitution away from the planes. This doping process adds p holes per Cu atom in the planes, and at high carrier density yields a normal metal with the basic signatures of a Fermi liquid. At intermediate density, it is a superconductor with d-wave symmetry, but the nature of the underdoped phase that lies between the insulator and the superconductor is one of the central puzzles of the field. It is known to be characterized by a pseudogap, and is thought to be an exotic state of matter [1– 4]. As one moves into this enigmatic underdoped phase by adding carriers to the Mott insulator, a key question remains: does the onset of superconductivity coincide with the onset of hole mobility? In V2O3 [5] and 2D organic conductors [6], pressure studies have answered this question in the affirmative: the electron system goes directly from insulator to superconductor, with no intermediate phase.