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Triplet supercurrents in clean and disordered half-metallic ferromagnets


Interfaces between materials with differently ordered phases present unique opportunities to study fundamental problems in physics. One example is the interface between a singlet superconductor and a half-metallic ferromagnet, where Cooper pairing occurs between electrons with opposite spin on the superconducting side, whereas the other exhibits 100% spin polarization. The recent surprising observation of a supercurrent through half-metallic CrO2 therefore requires a mechanism for conversion between unpolarized and completely spin-polarized supercurrents. Here, we suggest a conversion mechanism based on electron spin precession together with triplet-pair rotation at interfaces with broken spin-rotation symmetry. In the diffusive limit (short mean free path), the triplet supercurrent is dominated by inter-related odd-frequency s-wave and even-frequency p-wave pairs. In the crossover to the ballistic limit, further symmetry components become relevant. The interface region exhibits a superconducting state of mixed-spin pairs with highly unusual symmetry properties that open up new perspectives for exotic Josephson devices.

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Figure 1: Conversion between singlet and triplet supercurrents.
Figure 2: Spin mixing and broken spin-rotation symmetry around M.
Figure 3: Proximity amplitudes induced at superconductor–ferromagnet interfaces.
Figure 4: Critical Josephson current Jc.
Figure 5: Non-monotonic temperature dependence.
Figure 6: Peak position Tpeak in Jc(T).

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The authors would like to thank J. Kopu and G. Schön for important contributions, A. Posazhennikova for comments on the manuscript, U. Rüdiger for discussions on interface properties of ferromagnets, and R. Keizer and T. Klapwijk for communications in relation to ref. 2. T.L. acknowledges financial support from the Alexander von Humboldt Foundation. M.E. acknowledges the hospitality of the Aspen Center for Physics.

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M.E. and T.L. shared equal responsibility for all aspects of this project.

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Correspondence to Matthias Eschrig or Tomas Löfwander.

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Eschrig, M., Löfwander, T. Triplet supercurrents in clean and disordered half-metallic ferromagnets. Nature Phys 4, 138–143 (2008).

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