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Spin-Selective Point-Contact Spectroscopy of Leggett Modes in Superconducting Proximity-Coupled MgB2/La0.67Sr0.33MnO3 Nanocomposites

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Abstract

Leggett mode is a collective excitation of the phase difference of superconducting condensates whose energy is determined by the strength of Josephson coupling between them. Since the Leggett mode does not perturb the symmetry of Cooper pairs, it should be sensitive to the spin symmetry of the condensate order parameters. Using the point-contact (Andreev) spectroscopy, we have revealed their presence in a three-gap superconducting state realized in MgB2/La0.67Sr0.33MnO3 nanocomposite. Two of them, Δπ and Δσ-enh, are directly connected to intrinsic gaps in MgB2, with a slight increase in the larger σ-gap probably due to a spin-triplet pairing contribution. The third spin-triplet gap Δtr is intrinsic to the La0.67Sr0.33MnO3 compound while manifests itself only when it is proximitized to a superconductor. In such nanocomposite, the appearance of periodic Leggett peaks of two symmetry types is expected to be associated with the spin state of superconducting condensates. The idea of our experiments was to use two types of counter-electrodes, an ordinary normal metal and a spin-polarized conductor. In the first case, the Leggett mode associated with two spin-singlet gaps would be dominant while, due to spin selectivity, excitations of the relative phase for two triplet-spin order parameters are expected to prevail in the second case. In the work, we confirmed this assumption by discovering that the period of Leggett peaks in contacts of the nanocomposite with an Ag tip turned out to be twice as large as those for contacts with La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3 counter-electrodes.

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Acknowledgements

We thank Dr. I. Popova for very helpful discussions on the microanalyses of our samples.

Funding

M.B. acknowledges the EU NextGenerationEU financial support through the Recovery and Resilience Plan for Slovakia under project no. 09I03-03-V01-00139.

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Authors and Affiliations

  1. O.O. Galkin Donetsk Institute for Physics and Engineering, NAS of Ukraine, Nauki Ave. 46, Kyiv, 03028, Ukraine

    V. N. Krivoruchko & V. Yu. Tarenkov

  2. Centre for Nanotechnology and Advanced Materials, Faculty of Mathematics, Physics and Informatics, Comenius University Bratislava, Mlynská dolina, Bratislava, 84248, Slovak Republic

    M. Belogolovskii

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  1. V. N. Krivoruchko
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V.K. and M.B. wrote the main manuscript text and figures. V.T. prepared samples and made measurements. All authors reviewed manuscript.

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Correspondence to V. N. Krivoruchko.

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Krivoruchko, V.N., Tarenkov, V.Y. & Belogolovskii, M. Spin-Selective Point-Contact Spectroscopy of Leggett Modes in Superconducting Proximity-Coupled MgB2/La0.67Sr0.33MnO3 Nanocomposites. J Supercond Nov Magn (2024). https://doi.org/10.1007/s10948-024-06756-z

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