On Local Pairs vs. BCS: Quo Vadis High- T c Superconductivity

Since the discovery of high-temperature superconductivity in cuprates, proposals have been made that pairing may be local, in particular in underdoped samples. Here, we briefly review evidence for local pairs from our experiments on thin films of La 2−x Sr x CuO 4, synthesized by atomic layer-by-layer molecular beam epitaxy (ALL-MBE).