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Joint TDLI/ICMP/WQC Quantum Seminar

Is Sr2RuO4 a chiral superconductor?

by Dr Vadim Grinenko (TU Dresden)

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Abstract

The origin of the superconductivity in one famous compound, Sr2RuO4, remains a mystery despite 27 years of intense research [1]. For most of its history, the superconductivity of Sr2RuO4 has been understood in terms of an odd-parity chiral order parameter with equal spin pairing in the RuO2 planes: px ± ipy [2]. This conclusion was based on evidence for a broken time-reversal symmetry (BTRS) state at a superconducting transition temperature (Tc) from the observation of a magnetic response in zero-field muon spin rotation (ZF-μSR) data [3] and for spin-triplet pairing from the absence of suppression in Knight shift below Tc in nuclear magnetic resonance (NMR) data [4]. Further evidence for chirality came from the polar-Kerr effect [5] and the phenomenology of junctions between Sr2RuO4 and conventional superconductors, which shows evidence for domains in the superconducting state [6]. However, evidence from the strain dependence of Hc2 [7] and a recent revision of NMR data [8] point to even parity, spin-singlet pairing, as in the vast majority of known superconductors. This result has led to the consideration of a chiral two-component dxz ± idyz order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding naturally that BTRS transition temperature TBTRS = Tc, but it has a hard-to-explain horizontal line node at kz = 0. Therefore, s ± id and d ± ig order parameters with accidentally degenerated components are also under consideration [9]. These avoid the horizontal line node, but require fine-tuning to obtain TBTRS ≈ Tc.

To distinguish between chiral and accidentally degenerated order parameters, we studied the system under uniaxial strain [10], hydrostatic pressure and chemical disorder[11]. Based on general theoretical arguments and simulations, the uniaxial pressure applied along (100) or (110) crystallographic directions (in a tetragonal lattice) splits TBTRS and Tc for all possible types of the BTRS order parameters. However, hydrostatic pressure and chemical disorder lead to Tc = TBTRS for the chiral phase but would separate them for other types of two-component order parameters [12]. Our systematic experimental study demonstrates that the transition splits under uniaxial pressure. In contrast, no splitting results from hydrostatic pressure and disorder with a nearly 50% reduced superconducting critical temperature than an impurity-free compound. These results, combined with theoretical analysis, strongly suggest a chiral dxz ± idyz superconductivity in Sr2RuO4. This superconductivity implies interlayer pairing, which is highly unusual in such a strongly anisotropic layered system as Sr2RuO4 and, therefore, may require a new type of pairing mechanism. In this talk, I will also discuss what we can learn from μSR experiments regarding the nature of spontaneous currents in the BTRS state [13].

 

  1. Y. Maeno, et al., Nature 372, 532 (1994).
  2. A. P. Mackenzie, et al., npj Quantum Materials 2, 40 (2017).
  3. G. Luke et al., Nature 394, 558-561 (1998).
  4. K. Ishida, et al., Nature 396, 658 (1998).
  5. J. Xia, et al., Phys. Rev. Lett. 97, 167002 (2006).
  6. F. Kidwingira, et al., Science 314, 1267 (2006).
  7. A. Steppke, et al., Science 355, eaaf9398 (2017).
  8. A. Pustogow et al., Nature 574, 72-75 (2019).
  9. A. T. Rømer et al., Phys. Rev. Lett. 123, 247001 (2019); A. T. Rømer et al., Phys. Rev. B 102, 054506 (2020); S. A. Kivelson et al., npj Quantum Mat. 5, 43 (2020); R. Willa, Phys. Rev. B 102, 180503(R) (2020).
  10. V. Grinenko et al., Nat. Phys. (2021) https://doi.org/10.1038/s41567-021-01182-7.
  11. V. Grinenko et al., https://arxiv.org/pdf/2103.03600.pdf.
  12. B. Zinkl and M. Sigrist, Phys. Rev. Research 3, L012004 (2021).
  13. V. Grinenko et al., Nat. Phys. 16, 789 - 794 (2020).
Biography

Dr. Vadim Grinenko graduated National Research Nuclear University (MEPhI), Moscow, Russia in 2004, with the specialization "Superconductivity and Nanotechnology". In 2008 he got a PhD degree in the Institute of Superconductivity and Solid State Physics, the part of National Research Center “Kurchatov Institute”, Russia. On results of the PhD work, he got a Russian Science Support Foundation prize in the nomination of "The best young doctors of the Russian Academy of Sciences”. At the end of 2008, he moved to Germany and joined IFW Dresden, Germany. He was working with applying the high temperatures superconductors in electrical motor and generators in collaboration with Toyota company. In 2012 he moved for one year to the Institute for Theoretical Solid State Physics, IFW-Dresden, Germany to work in the field of unconventional superconductivity. In 2013 Dr. Grinenko moved to the Institute for Metallic Materials, IFW-Dresden, Germany. He was studying thin-film grows of iron-based superconductors. At the end of 2015 - present: he moved to the Institute of Solid State and Materials Physics, TU Dresden, Germany as principle investigator of DFG-project devoted to studying multiband superconductors that break time-reversal symmetry. This project is performed in strong collaboration with the Paul Scherrer Institut (PSI), Switzerland, where a big part of the research was performed. In 2016 Dr. Grinenko spent one month at Nagoya University (Graduate School of Engineering) as Associate Professor.

Division
Condensed Matter
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