Spin-triplet chiral p-wave superconductor is the primary example of topologically nontrivial superconducting system, which provides the essential testing ground for fascinating physics of Majorana particles in condensed matters having great potential applications to topological quantum computations. On the basis of intensive studies for a quarter century, nowadays, strontium ruthenate superconductor becomes the most promising candidate material for spin-triplet chiral p-wave superconductors. However, the debate has not been settled due to some disagreements between experimental results and theoretical predictions, such as luck of spontaneous supercurrent signature in the scanning SQUID measurements. On the other hand, recently, there are remarkable progress in fabrication techniques for strontium ruthenate thin-film and epitaxial ferromagnet/strontium ruthenate hybrid devices [1], which would open a new avenue toward the identification of superconducting order of strontium ruthenate superconductors.

The goal of this project is to explore unique phenomena that originated from the interplay between spin-triplet superconductivity and ferromagnetism, and to address the settlement of the long-term outstanding problem regarding with the pairing symmetry of strontium ruthenate superconductors. At present, we are focusing on two specific directions:

(i) Characterization of d-vector through the Josephson effect (Fig. 1)

The d-vector is the essential internal degree of freedom for spin-triplet superconducting order. It has been shown that the Josephson current in spin-triplet superconductor/ferromagnet/spin-triplet superconductor junctions is sensitive to the orientation of the d-vectors [2]. D. Manske (MPI-FKF) will develop the theory to embark on the identification of d-vector in strontium ruthenate superconductors. The theoretical proposal will be tested by experiments from M. Kawasaki (U Tokyo) and M. Uchida (U Tokyo).

(ii) Observation of Majorana edge mode through the nonlocal transport (Fig. 2)

The most striking feature of a spin-triplet chiral p-wave superconductor is the appearance of chiral Majorana edge modes. Recently, S. Ikegaya (MPI-FKF) and D. Manske (MPI-FKF) theoretically demonstrated that the presence of chiral Majorana edge modes is manifested in nonlocal transport properties in a device consisting of a spin-triplet chiral p-wave superconductor and two ferromagnetic leads [3]. Together with the experimentalists, M. Kawasaki (U Tokyo) and M. Uchida (U Tokyo), we develop the theoretical proposal to be more practical in experiments, and aim the first conclusive observation of Majorana edge modes in strontium ruthenate superconductors. Furthermore, observing and controlling the domain walls of strontium ruthenate superconductors would be the important subject in the present project.

 

[1] M. S. Anwar, S. R. Lee, R. Ishigro, Y. Sugimoto, Y. Tano, S. J. Kang, Y. J. Shin, S. Yonezawa, D. Manske, H. Takayanagi, T. W. Noh, and Y. Maeno, “Direct penetration of spin-triplet superconductivity” into a ferromagnet in Au/SrRuO3/Sr2RuO4 junctions” Nat. Commum. 7, 13220 (2016).

[2] B. Kastening, D. K. Morr, D. Manske, and K. H. Bennemann, “Novel Josephson effect in triplet superconductor-ferromagnet-triplet superconductor junctions”, Phys. Rev. Lett. 96, 047009 (2006).

[3] S. Ikegaya, Y. Asano, and D. Manske, “Anomalous Nonlocal Conductance as a Fingerprint of Chiral Majorana Edge States” arXiv: 1901.07652 (2019).

 

Principle investigators

D. Manske (MPI-FKF), d.manske@fkf.mpg.de

M. Kawasaki (U Tokyo), kawasaki@ap.t.u-tokyo.ac.jp

Figure1: Schematic picture of a spin-triplet superconductor/ferromagnet/spin-triplet superconductor junction. The Josephson current exhibits a rich dependence on the relative orientation between the magnetic moment and the d-vectors.

Figure2: Proposed setup for observing chiral Majorana edge states. Nonlocal transport between the two ferromagnetic leads is anomalously long-ranged and sensitive to the moving direction of chiral Majorana edge states.

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