Enhancement of the superconducting transition temperature in cuprate   heterostructures
  Is it possible to increase Tc by constructing cuprate heterostructures, which combine the high pairing energy of underdoped layers with the large carrier density of proximate overdoped layers? We investigate this question within a model bilayer system using an effective theory of the doped Mott insulator. Interestingly, the question hinges on the fundamental nature of the superconducting state in the underdoped regime. Within a plain slave boson mean field theory, there is absolutely no enhancement of Tc. However, we do get a substantial enhancement for moderate inter-layer tunneling when we use an effective low energy theory of the bilayer in which the effective quasiparticle charge in the underdoped regime is taken as an independent phenomenological parameter. We study the Tc enhancement as a function of the doping level and the inter-layer tunneling, and discuss possible connections to recent experiments by Yuli et al. [Phys. Rev. Lett. 101, 057005 (2008)]. Finally, we predict a unique paramagnetic reduction of the zero temperature phase stiffness of coupled layers, which depends on the difference in the current carried by quasiparticles on the two types of layers as (J1-J2)^2. 
Lilach Goren and Ehud Altman