Speaker
Description
Rare-earth borides provide a particularly interesting platform for investigating superconductivity in the presence of magnetic order. The coexistence and competition of these two collective states may lead to behavior that goes beyond the conventional spin-singlet BCS scenario and, under appropriate conditions, may favor unconventional pairing mechanisms. Such systems are of fundamental interest for investigating magnetically mediated superconductivity and possible spin-triplet pairing, with potential relevance to spintronics and topological quantum technologies, particularly in the context of Majorana-state realization.
In the RRh4B4 family, where R denotes a rare-earth element, the interplay between magnetism and superconductivity can be effectively controlled through chemical substitution. In the Dy1−xYxRh4B4 series, replacing magnetic Dy with non-magnetic Y causes a pronounced reduction of the magnetic ordering temperature, whereas the superconducting transition temperature increases. Heat-capacity measurements indicate the occurrence of an additional magnetic transition below Tc, demonstrating that magnetic ordering can develop inside the superconducting state without immediately suppressing it. These compounds also exhibit a number of unusual superconducting properties, including the paramagnetic Meissner (Wohlleben) effect [1-2] and non-monotonic temperature dependences of both the upper critical field and the superconducting energy gap [3]. The observed features collectively indicate a complex interplay between magnetic order and superconductivity and point to a possible unconventional superconducting pairing mechanism, quite possibly of the spin-triplet type.
Additional evidence for unconventional behavior is obtained in Ru-substituted rare-earth rhodium borides. In the ferromagnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4, superconductivity persists in the presence of ferromagnetic order [4], while point-contact Andreev-reflection measurements yield 2Δ/kBTc > 4, exceeding the weak-coupling BCS value [5]. Furthermore, the upper critical field Hc2(T) of Dy0.8Er0.2Rh3.8Ru0.2B4 exhibits a characteristic inflection near 3 kOe [6]. This feature may be associated with low-temperature magnetic ordering. However, a more unconventional scenario involving a change in the superconducting pairing state, including a possible crossover from singlet to triplet pairing, cannot be excluded.
The combination of ferromagnetism and superconductivity, anomalous Hc2(T) behavior, enhanced superconducting gap values, and other non-BCS features makes these rare-earth borides promising systems for further investigation of unconventional superconductivity. Although the available results do not provide direct proof of spin-triplet pairing, they identify a set of experimental signatures that justify further searches for a possible triplet superconducting state in this class of materials.
