Spin-Selective Point-Contact Spectroscopy of Leggett Modes in Superconducting Proximity-Coupled MgB₂/La_0.67Sr_0.33MnO₃ Nanocomposites

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/La_0.67Sr_0.33MnO₃ nanocomposite. Two of them, Δ_π and Δ_σ-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 Δ_tr is intrinsic to the La_0.67Sr_0.33MnO₃ 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 La_0.67Sr_0.33MnO₃ and La_0.67Ca_0.33MnO₃ counter-electrodes.