Penetrating But Not Yet Definitive Probe of Strontium Ruthenate

by Tommy on 9/03/2017
Strontium Ruthenate Under Strain

Strontium Ruthenate Under Strain

If you have been paying attention, strontium ruthenate, Sr2RuO4, is still controversial.

It’s as controversial as the cuprates were back in 1994 when it was first discovered to be superconducting. Here it is demonstrated that there is a one dimensional sub-character reminiscent of bismuth iodide, Bi4I4. And unixial stress, which has not yet been applied to bismuth iodide, elevated the critical transition temperature. Bismuth iodide is up next for this.

Quasiparticle Interference and Strong Electron-Mode Coupling in the Quasi-One-Dimensional Bands of Sr2RuO4, Zhenyu Wang, Daniel Walkup, Philip Derry, Thomas Scaffidi, Melinda Rak, Sean Vig, Anshul Kogar, Ilija Zeljkovic, Ali Husain, Luiz H. Santos, Yuxuan Wang, Andrea Damascelli, Yoshiteru Maeno, Peter Abbamonte, Eduardo Fradkin and Vidya Madhavan
(10 January 2017)

The single-layered ruthenate Sr2RuO4 has attracted a great deal of interest as a spin-triplet superconductor with an order parameter that may potentially break time reversal invariance and host half-quantized vortices with Majorana zero modes. While the actual nature of the superconducting state is still a matter of controversy, it has long been believed that it condenses from a metallic state that is well described by a conventional Fermi liquid. In this work we use a combination of Fourier transform scanning tunneling spectroscopy (FT-STS) and momentum resolved electron energy loss spectroscopy (M-EELS) to probe interaction effects in the normal state of Sr2RuO4. Our high-resolution FT-STS data show signatures of the \beta-band with a distinctly quasi-one-dimensional (1D) character. The band dispersion reveals surprisingly strong interaction effects that dramatically renormalize the Fermi velocity, suggesting that the normal state of Sr2RuO4 is that of a ‘correlated metal’ where correlations are strengthened by the quasi 1D nature of the bands. In addition, kinks at energies of approximately 10 meV, 38 meV and 70 meV are observed. By comparing STM and M-EELS data we show that the two higher energy features arise from coupling with collective modes. The strong correlation effects and the kinks in the quasi 1D bands may provide important information for understanding the superconducting state. This work opens up a unique approach to revealing the superconducting order parameter in this compound.

See also:

Strong peak in Tc of Sr2RuO4 under uniaxial pressure, Alexander Steppke, Lishan Zhao, Mark E. Barber, Thomas Scaffidi, Fabian Jerzembeck, Helge Rosner, Alexandra S. Gibbs, Yoshiteru Maeno, Steven H. Simon and Andrew P. Mackenzie, Clifford W. Hicks, Science, 355, eaaf9398 (13 January 2017), doi:0.1126/science.aaf9398

We report a combined experimental and theoretical study of the dependence of the superconductivity of the unconventional superconductor Sr2RuO4 on anisotropic strain. Novel piezoelectric apparatus is used to apply uniaxial pressures of up to ∼1 GPa along a ⟨100⟩ direction (a-axis) of the crystal lattice. Tc increases from 1.5 K in unstrained material to 3.4 K at compression by ≈ 0.6%, then falls steeply. The c-axis upper critical field for the strained Tc = 3.4 K material is a factor of twenty larger than that of the unstrained crystal, whereas the in-plane (a-axis) critical field increases by only a factor of three. First-principles electronic structure calculations give evidence that the observed maximum Tc occurs at or near a Lifshitz transition when the Fermi level passes through a van Hove singularity. Finally, we perform order parameter analyses using three-band renormalization group calculations. These, combined with the unexpectedly low in-plane critical field, open the possibility that the highly strained Tc = 3.4 K Sr2RuO4 has an even- rather than an odd-parity order parameter. Potential implications such as a transition at nonzero strain between odd- and even-parity order parameters are discussed.

Now this is getting really interesting. And don’t forget … axions!

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