Abstract

Superconductivity in cuprates has many mysterious facets, but the most important question is why the critical temperature (Tc ) is so high. Our experiments target this question. We use atomic-layer-by-layer molecular beam epitaxy to synthesize atomically perfect thin films and multilayers of cuprates and other complex oxides. By atomic-layer engineering, we optimize the samples for a particular experiment. [1] I will present the results of a focused and comprehensive study that took twelve years and over two thousand cuprate samples, perhaps without precedence in Condensed Matter Physics. We have measured the key physical parameters of the normal and superconducting states and established their precise dependence on doping, temperature, and external fields. This large data basis contains a wealth of information and constrains tightly the theory. One striking conclusion is that superconducting state cannot be described by the standard Bardeen-Cooper- Schrieffer theory, anywhere in the phase diagram. Next, the rotational symmetry of the electron fluid in the normal metallic state above Tc is always spontaneously broken — the so-called “electronic nematicity” — unlike in standard metals that behave like Fermi Liquids. Finally, the insulating state on the underdoped side is also unusual, with mobile charge clusters formed by localized pairs. All these features are quite exceptional and point to a new picture of high-Tc superconductivity in cuprates. [1] Nature 547, 432 (2017); 536, 309 (2016); 472, 458 (2011); 455, 782 (2008); 422, 873 (2003). Science 359, xxx (2018); 326, 699 (2009); 316, 425 (2007); 297, 581 (2002). Nature Materials 12, 877 (2013); 12, 387 (2013); 12, 1019 (2013); 12, 47 (2013); 11, 850 (2012). Nature Physics 10, 256 (2014); 7, 298 (2011). Nature Nanotechnology 9, 443 (2014); 5, 516 (2010). Nature Communications 2, 272 (2011). Phys. Rev. Letters 106, 237003 (2011); 102, 107004 (2009); 101, 247004 (2008); 93, 157002 (2004); 89, 107001 (2002). Proc. Nat. Acad. Sci. 113, 4284 (2016); 107, 8103 (2010).