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.

2021, Meisenheimer, P.B., Steinhardt, R.A., Sung, S.H., Williams, L.D., Zhuang, S., Nowakowski, M.E., Novakov, S., Torunbalci, M.M., Prasad, B., Zollner, C. J., Wang, Z., Dawley, N.M., Schubert, J., Hunter, A.H., Manipatruni, S., Nikonov, D.E., Young, I.A., Chen, L.Q., Bokor, J., Bhave, S.A., Ramesh, R., Hu, J.-M., Kioupakis, E., Hovden, R., Schlom, D.G., Heron, J.T.

Magnetostrictive materials transduce magnetic and mechanical energies and when combined with piezoelectric elements, evoke magnetoelectric transduction for high-sensitivity magnetic field sensors and energy-efficient beyond-CMOS technologies. The dearth of ductile, rare-earth-free materials with high magnetostrictive coefficients motivates the discovery of superior materials. Fe1−xGax alloys are amongst the highest performing rare-earth-free magnetostrictive materials; however, magnetostriction becomes sharply suppressed beyond x = 19% due to the formation of a parasitic ordered intermetallic phase. Here, we harness epitaxy to extend the stability of the BCC Fe1−xGax alloy to gallium compositions as high as x = 30% and in so doing dramatically boost the magnetostriction by as much as 10x relative to the bulk and 2x larger than canonical rare-earth based magnetostrictors. A Fe1−xGax − [Pb(Mg1/3Nb2/3)O3]0.7−[PbTiO3]0.3 (PMN-PT) composite magnetoelectric shows robust 90° electrical switching of magnetic anisotropy and a converse magnetoelectric coefficient of 2.0 × 10−5 s m−1. When optimally scaled, this high coefficient implies stable switching at ~80 aJ per bit.