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 [Formula: see text]Sr x Cu 3 [Formula: see text]Se x with [Formula: see text], 0.75 and 1; and [Formula: see text] 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 ([Formula: see text][Formula: see text]wt.%) being [Formula: see text][Formula: see text]Cu 3 O 6 [Formula: see text]. The synthesized superconductor demonstrates unique properties, including (i) two superconducting transitions with [Formula: see text][Formula: see text]K and [Formula: see text][Formula: see text]K yielding the Wohlleben effect; (ii) strong-coupling d-wave superconductivity (with [Formula: see text] 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 ([Formula: see text][Formula: see text]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 [Formula: see text][Formula: see text]Cu 3 O 6 [Formula: see text] is analyzed using machine learning methods.