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 films1,2,3,4 is that when mobile holes are removed from optimally doped CuO2 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 Y1−xCaxBa2Cu3O7−δ as thin as two copper oxide bilayers with Tc values as low as 3 K. In addition to seeing the 2D Kosterlitz–Thouless–Berezinski transition5,6 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.