Mostly I confine my current research to single-CuO2-layer cuprates such as LSCO or NCCO, although it is also my desire to pay attention to multi-layer ones at the same time. I feel the most mysterious physics should already exist in a single isolated layer, whereas inter-layer correlations are only of secondary effects. A recent publication [1] showed an interesting aspect of inter-layer interrelations. This was discovered in Bi2Sr2Ca2Cu3O101d (Bi2223), which boasts of three intimately related CuO2 layers within a unit cell. They found that: Tc increases to a maximum and the goes down to a trough, but eventually rises to a new peak surpassing the earlier one, as the pressure is varied monotonically. They interpreted this finding in terms of two energy scales: the paring energy and the phase coherence energy, together with the observation that hole concentration is enhanced by pressure. All this is summarized in the FIG.
But it should be useful to consider a different picture: suppose at low concentration, all three layers are underdoped SC; as the concentration increases, the holes may transfer from the inner layer to the two outer layers so that the inner layer is still underdoped while the outer ones may be in optimum doping (the first maximum is reached); as the concentration increases further, the extra holes can hardly be deployed to increase Tc [2], until the point where the extra holes going to the inner is favorable; now Tc rises again. This picture could also explain the observed Tc trend. However, a quantitative study is desired. The simplest model consists of BCS-type Hamiltonian for each layer and inter-layer tunneling.
[1]Vol 466| 19 August 2010| doi:10.1038/nature09293;
[2] If the same amount of holes is added to underdoped or the optimum doped materials, in which case the energy will be reduced more ?
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