Quantum critical behaviour in the superfluid density of strongly underdoped ultrathin copper oxide films

Abstract

A central challenge in the physics of high-temperature superconductors is to understand superconductivity within a single copper oxide layer or bilayer, the fundamental structural unit, and how superconductivity is lost with underdoping of charge carriers. A seminal property of crystals and thick films is that when mobile holes are removed from optimally doped CuO"2 planes, the transition temperature, Tc, and superfluid density, ns(0), decrease in a surprisingly correlated fashion. We elucidate the essential physics of strongly underdoped bilayers by studying two-dimensional (2D) samples near the critical doping level where superconductivity disappears. We report measurements of ns(T) in films of Y1xCaxBa2Cu3O7 as thin as two copper oxide bilayers with Tc values as low as 3K. In addition to seeing the 2D Kosterlitz-Thouless-Berezinski transition at Tc, we observe a remarkable scaling of Tc with ns(0), which indicates that the disappearance of superconductivity with underdoping is due to quantum fluctuations near a 2D quantum critical point. © 2007 Nature Publishing Group.