Evolution of Fermi-liquid interactions in Sr2RuO4 under pressure

The superconductivity of Sr2RuO4 under c-axis uniaxial stress

Applying in-plane uniaxial pressure to strongly correlated low-dimensional systems has been shown to tune the electronic structure dramatically. For example, the unconventional superconductor Sr₂RuO₄ can be tuned through a single Van Hove point, resulting in strong enhancement of both T_c and H_c2. Out-of-plane (c axis) uniaxial pressure is expected to tune the quasi-two-dimensional structure even more strongly, by pushing it towards two Van Hove points simultaneously. Here, we achieve a record uniaxial stress of 3.2 GPa along the c axis of Sr₂RuO₄. H_c2 increases, as expected for increasing density of states, but unexpectedly T_c falls. As a first attempt to explain this result, we present three-dimensional calculations in the weak interaction limit. We find that within the weak-coupling framework there is no single order parameter that can account for the contrasting effects of in-plane versus c-axis uniaxial stress, which makes this new result a strong constraint on theories of the superconductivity of Sr₂RuO₄.

In the superconductor Sr2RuO4, in-plane strain is known to enhance both the superconducting transition temperature Tc and upper critical field Hc2, but the effect of out-of-plane strain has not been studied. Here, the authors find that Hc2 is enhanced under out-of-plane strain, but Tc unexpectedly decreases.

Unsplit superconducting and time reversal symmetry breaking transitions in Sr2RuO4 under hydrostatic pressure and disorder

There is considerable evidence that the superconducting state of Sr2RuO4 breaks time reversal symmetry. In the experiments showing time reversal symmetry breaking, its onset temperature, TTRSB, is generally found to match the critical temperature, Tc, within resolution. In combination with evidence for even parity, this result has led to consideration of a dxz ± idyz order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding TTRSB = Tc, but it has a hard-to-explain horizontal line node at kz = 0. Therefore, s ± id and d ± ig order parameters are also under consideration. These avoid the horizontal line node, but require tuning to obtain TTRSB ≈ Tc. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr2RuO4 under hydrostatic pressure, and Sr1.98La0.02RuO4 at zero pressure. Both hydrostatic pressure and La substitution alter Tc without lifting the tetragonal lattice symmetry, so if the degeneracy is symmetry-protected, TTRSB should track changes in Tc, while if it is accidental, these transition temperatures should generally separate. We observe TTRSB to track Tc, supporting the hypothesis of dxz ± idyz order.

Strain Control of Fermiology and Many-Body Interactions in Two-Dimensional Ruthenates

Here we demonstrate how the Fermi surface topology and quantum many-body interactions can be manipulated via epitaxial strain in the spin-triplet superconductor Sr_{2}RuO_{4} and its isoelectronic counterpart Ba_{2}RuO_{4} using oxide molecular beam epitaxy, in situ angle-resolved photoemission spectroscopy, and transport measurements. Near the topological transition of the γ Fermi surface sheet, we observe clear signatures of critical fluctuations, while the quasiparticle mass enhancement is found to increase rapidly and monotonically with increasing Ru-O bond distance. Our work demonstrates the possibilities for using epitaxial strain as a disorder-free means of manipulating emergent properties, many-body interactions, and potentially the superconductivity in correlated materials.

Enhanced spin-triplet superconductivity near dislocations in Sr₂RuO₄

Superconductors with a chiral p-wave pairing are of great interest because they could support Majorana modes that could enable the development of topological quantum computing technologies that are robust against decoherence. Sr₂RuO₄ is widely believed to be a chiral p-wave superconductor. Yet, the mechanism by which superconductivity emerges in this, and indeed most other unconventional superconductors, remains unclear. Here we show that the local superconducting transition temperature in the vicinity of lattice dislocations in Sr₂RuO₄ can be up to twice that of its bulk. This is all the more surprising for the fact that disorder is known to easily quench superconductivity in this material. With the help of a phenomenological theory that takes into account the crystalline symmetry near a dislocation and the pairing symmetry of Sr₂RuO₄, we predict that a similar enhancement should emerge as a consequence of symmetry reduction in any superconductor with a two-component order parameter.

Strong spin-orbit coupling effects on the Fermi surface of Sr2RuO4 and Sr2RhO4

We present a first-principles study of spin-orbit coupling effects on the Fermi surface of Sr2RuO4 and Sr2RhO4. For nearly degenerate bands, spin-orbit coupling leads to a dramatic change of the Fermi surface with respect to nonrelativistic calculations; as evidenced by the comparison with experiments on Sr2RhO4, it cannot be disregarded. For Sr2RuO4, the Fermi surface modifications are more subtle but equally dramatic in the detail: Spin-orbit coupling induces a strong momentum dependence, normal to the RuO2 planes, for both orbital and spin character of the low-energy electronic states. These findings have profound implications for the understanding of unconventional superconductivity in Sr2RuO4.