2021, Ruf, J.P., Paik, H., Schreiber, N.J., Nair, H.P., Miao, L., Kawasaki, J.K., Nelson, J.N., Faeth, B.D., Lee, Y., Goodge, B.H., Pamuk, B., Fennie, C.J., Kourkoutis, L.F., Schlom, D.G., Shen, K.M.

Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.

2013, Allan, M.P., Tamai, A., Rozbicki, E., Fischer, M.H., Voss, J., King, P.D.C., Meevasana, W., Thirupathaiah, S., Rienks, E., Fink, J., Tennant, D.A ., Perry, R.S., Mercure, J.F., Wang, M.A., Lee, Jinho, Fennie, C.J., Kim, E.A., Lawler, M.J., Shen, K.M., Mackenzie, A.P., Shen, Z.X., Baumberger, F.

The phase diagram of Sr3Ru2O7 shows hallmarks of strong electron correlations despite the modest Coulomb interaction in the Ru 4d shell. We use angle-resolved photoelectron spectroscopy measurements to provide microscopic insight into the formation of the strongly renormalized heavy d-electron liquid that controls the physics of Sr3Ru2O7. Our data reveal itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of <6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from a combination of back-folding due to a structural distortion and the hybridization of light and strongly renormalized, heavy quasiparticle bands. The resulting heavy Fermi liquid has a marked k-dependence of the renormalization which we relate to orbital mixing along individual Fermi surface sheets.