Wohlleben effect and quantum criticality in YBa1.4Sr0.6Cu₃O₆Se0.5 superconductor within a heterophase ceramic: Results for YBCO double-element substitution.

A novel material is obtained by modifying pristine YBCO superconductor via simultaneous partial substitution of Sr for Ba and Se for O. The intended stoichiometry was YBa 2 − x SrxCu3 O 7 − x Sex with x = 0. 5 , 0.75 and 1; and x ≈ 1 yielded the most interesting results considered in detail. We confirmed that Se atoms indeed enter the superconductor lattice cell. The obtained polycrystalline material contains five phases, with the major phase (> 4 4 wt.%) being YBa 1. 4 Sr 0. 6 Cu3O6 Se 0. 5 . The synthesized superconductor demonstrates unique properties, including (i) two superconducting transitions with T c 1 ≈ 3 6 K and T c 2 ≈ 1 3 K yielding the Wohlleben effect; (ii) strong-coupling d-wave superconductivity (with 2 Δ / (k B T c) ≈ 5); (iii) Schottky anomaly in the heat capacity; and, most importantly, (iv) a very unusual anomaly in the heat capacity which can be associated with the quantum criticality typically observable at much higher fields. The fact that the quantum criticality is visible in weaker fields (∼ 7 T) may open opportunities for exploration of the interplay between superconductivity and pair density waves by wide research community particularly when its pure phase is available. To facilitate this, the stability of single-phase YBa 1. 4 Sr 0. 6 Cu3O6 Se 0. 5 is analyzed using machine learning methods. [ABSTRACT FROM AUTHOR]