The question of how thin cuprate layers can be while still retaining superconductivity has been challenging to address, in part because experimental studies require the synthesis of near perfect unltrthin HTS layers and ways to profile the SC properties with atomic solution. In this work, the authors addressed this issue.
The idea may be described as follows. As is known from the phase diagram of p-type cuprate superconductors, SC exists in between two limiting doping levels, say x1 and x2. Below x1, the compound is insulating (I). Beyond x2, it is a bad metal (M). By epitaxy technique, it is easy to grow a couple of I layers above a couple of M layers. Now that M layers have too many holes while I layers are wanting holes, holes shall flow from M to I layers and eventually, all layers become SC. At point, one selects a particular layer, which shall be marked by partial substitution of Zn for Cu. As we know, such substitution will dramatically suppress SC within this layer. Now on using STM, it is possible to yield a complete profile of all the layers in terms of carrier concentration and critical temperature. The result shows that, Zn exchange only affects the SC properties of the as-marked layer, which means SC occurs within a single layer.
This finding therefore confirms the assumption that, High Tc is not essentially a 3D phenomenon. This is crucial to many current High Tc SC models.
(1)doi: 10.1126/science.1178863
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