Micron-scale measurements of low anisotropic strain response of local Tc in Sr2RuO4

Abstract

Strontium ruthenate (Sr 2 RuO 4 ) is a multiband superconductor that displays evidence of topological superconductivity, although a model of the order parameter that is consistent with all experiments remains elusive. We integrated a piezoelectric-based strain apparatus with a scanning superconducting quantum interference device (SQUID) microscope to map the diamagnetic response of single-crystal Sr 2 RuO 4 as a function of temperature, uniaxial pressure, and position with micron-scale spatial resolution. We thereby obtained local measurements of the superconducting transition temperature T c vs. anisotropic strain with sufficient sensitivity for comparison to theoretical models that assume a uniform p x ± ip y order parameter. We found that T c varies with position and that the locally measured T c vs. curves are quadratic (T c ∝ 2 ), as allowed by the C 4 symmetry of the crystal lattice. We did not observe the low-strain linear cusp (T c ∝ | |) that would be expected for a two-component order parameter such as p x ± ip y . These results provide new input for models of the order parameter of Sr 2 RuO 4 .

Effect of strain inhomogeneity on a chiral p -wave superconductor

“…Experimental observations of local strain variations have been reported recently in Ref. 8, and constrain theories of unconventional superconductivity in this system.…”

“…In particular, we focus on the effect of strain inhomogeneity, which are always present in any real experiment (see, for instance Ref. 8, which provides quantitative estimates of such strain inhomogeneity). Our key result is simply that local variations in strain qualitatively alters the behavior of T c , causing it to vary quadratically as a function of weak average strain.…”

Constraints on the superconducting order parameter in Sr2RuO4 from oxygen-17 nuclear magnetic resonance

“…The behavior was interpreted as a consequence of tuning the Fermi energy E F of the quasi-2D band through a van Hove singularity (vHs), and concomitant singularity in the density of states (DOS) at E F 18 . These observations motivated a study of 17 O nuclear magnetic resonance (NMR) in uniaxially pressurized samples; indeed, evidence for the DOS maximum and accompanying enhanced Stoner factor came from normal-state 17 O NMR experiments 19 . The results have bearing on the chiral p-wave state hypothesis: on the one hand, an odd-parity order parameter vanishes at the location of the vHs, thereby reducing the impact of the DOS enhancement;…”

“…For instance, due to TRSB, there is the generic expectation of measurable chiral edge currents (which propagate within a coherence length of the edge, but which are screened over a somewhat larger penetration depth scale). However, such currents have not been detected despite several attempts with progressively improved sensitivity [15][16][17] .…”

A Personal Perspective on the Unconventional Superconductivity of Sr2RuO4

“…The original motivation for the uniaxial pressure project that emerged from the conversation with Jim Sauls was to search for one of the expectations for a time reversal symmetry breaking order parameter, namely a cusp in T c at zero uniaxial pressure. As is so often the case with experiments designed to search for some specific phenomenon, we did not observe it, either in the original experiment [32] or a more sophisticated follow-up in collaboration with the group of Ted's colleague Kathryn Moler at Stanford [33]. However, the experiment reaped rewards that we had not thought of when we conceived it.…”