Superfluid density in highly underdoped YBa₂Cu₃O_6+x

David Broun

Department of Physics, Simon Fraser University, Burnaby

The two most prominent states of matter in the phase diagram of the high temperature superconductors are the Mott insulator and d-wave superconductor. Important clues for connecting the two states can be obtained by studying how the fundamental excitations evolve with doping. Using a breakthrough in the preparation of high purity YBa2Cu3O6+x crystals, we can tune into a regime of the underdoped cuprate phase diagram in which the transition temperature can be varied continuously between Tc = 25 K and zero, without changing cation disorder. A combination of microwave spectroscopy and measurements of the lower critical field Hc1 has been used to map out the temperature-dependent superfluid density as a function of doping, with surprising results. The superfluid density becomes anomalously small in this doping range, but shows no sign of a Kosterlitz-Thouless transition, indicating that nodal quasiparticles, not phase fluctuations, control its temperature dependence. In addition, the temperature slope of the superfluid density is roughly proportional to Tc in this doping range, suggesting a strong doping dependence of the quasiparticle charge renormalization factor. In contrast to this, electron currents between the CuO2 layers are not renormalized, implying that the in-plane quasiparticles are fundamentally different from the physical electrons that tunnel between layers.