Correlation between Superfluid Density and T_C of Underdoped YBa_2Cu_3O_6+x Near the Superconductor-Insulator Transition
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Publisher:American Physical Society
Citation:Yuri Zuev, Mun Seog Kim, Thomas R. Lemberger, "Correlation between Superfluid Density and T_C of Underdoped YBa_2Cu_3O_6+x Near the Superconductor-Insulator Transition," Physical Review Letters 95, no. 13 (2005), doi:10.1103/PhysRevLett.95.137002
We report measurements of the ab-plane superfluid density n_s (magnetic penetration depth λ) of heavily underdoped films of YBa_2Cu_3O_6+x, with T_C’s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO_2 layers. At the lowest dopings, T_C is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO_2 bilayers. Our main finding is that T_C is not proportional to n_s(0); instead, we find T_C∝n_s^1/2.3±0.4. This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.