Strong Peak in $T_c$ of Sr$_2$RuO$_4$ Under Uniaxial Pressure (1604.06669v2)

Abstract: We report a combined experimental and theoretical study of the dependence of the superconductivity of the unconventional superconductor Sr$_2$RuO$_4$ on anisotropic strain. Novel piezoelectric apparatus is used to apply uniaxial pressures of up to $\sim$1~GPa along a $\langle 100 \rangle$ direction ($a$-axis) of the crystal lattice. $T_c$ increases from 1.5~K in unstrained material to 3.4~K at compression by $\approx$0.6\%, then falls steeply. The $c$-axis upper critical field for the strained $T_c$ = 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 $T_c$ 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 $T_c$=3.4~K Sr$_2$RuO$_4$ 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.

• The paper presents experimental evidence showing T₍c₎ enhancement from 1.5 K to 3.4 K under 0.6% lattice compression.
• It employs DFT and weak-coupling calculations to reveal a Lifshitz transition and a potential shift to even-parity superconductivity.
• The findings highlight strain engineering as a promising route to control superconducting phases in oxide thin films.

Insights Into the Superconductivity of Sr$_2$RuO$_4$ Under Uniaxial Pressure

This paper presents a detailed paper on the superconducting behavior of Sr$_2$RuO$_4$ under anisotropic strain, particularly focusing on the material's critical temperature (T$_c$) and how it responds to uniaxial pressure applied along a ⟨100⟩ crystal axis. Sr$_2$RuO$_4$ has been of considerable interest within the superconductor community due to its unconventional characteristics and sensitivity to disorder.

Experimental Observations and Numerical Findings

The experimentation reveals a significant enhancement of T$_c$ with approximately 0.6% lattice compression, increasing from an initial 1.5 K to a peak of 3.4 K. This enhancement is attributed to moving the Fermi level through a Van Hove singularity, a phenomenon confirmed through density functional theory (DFT) calculations. The paper notes a steep decline in T$_c$ beyond this critical strain, suggesting that the peak correlates with a Lifshitz transition.

The paper employs robust computational techniques to model the electronic structure of Sr$_2$RuO$_4$ under strain. The DFT calculations, enhanced by spin-orbit coupling considerations, reveal changes in the Fermi surfaces that support the existence of a Lifshitz transition under the applied conditions. Further, weak-coupling calculations extend beyond these findings to explore potential modifications in the superconducting order parameter, proposing the intriguing possibility of even-parity superconducting states in the highly strained regime.

Implications and Future Directions

The potential for a transition from odd-parity to even-parity order parameters under uniaxial compression offers substantial implications for understanding the pairing mechanisms in unconventional superconductors. This strain-induced transition opens avenues for revisiting interpretations of existing experimental evidence, particularly those advocating for odd-parity, spin-triplet superconductivity in unstrained Sr$_2$RuO$_4$. Furthermore, should the highly strained state exhibit even-parity, it challenges prevalent theories that hinge on the material’s presumed odd-parity nature.

A key practical implication lies in the potential application of these findings to thin-film heterostructures, where strain tuning might afford greater control over superconducting phases, facilitating advances in electronic device technologies. Additionally, extending strain application methodologies demonstrated in this research to other material classes provides a powerful tool for probing electronic phase transitions in complex oxides.

Conclusion

The rigorous experiments combined with theoretical calculations presented in this paper enrich the understanding of the superconducting properties of Sr$_2$RuO$_4$. The observed interplay between electronic topological transitions and superconductivity under strain highlights the nuanced mechanisms governing unconventional cases, serving as a benchmark for further paper. As this field progresses, the continuous development of strain engineering techniques, as evidenced by this work, promises novel insights and breakthroughs in high-T$_c$ superconductivity research.