Characterization of Superconducting LiNbO₂ Crystals and Thin Films synthesized through CaH₂ Reduction

We have investigated CaH₂ reduction on ferroelectric LiNbO₃ single crystals and thin films. CaH₂ reduction of the parent LiNbO₃ phase facilitates a structural transformation to a layered LiNbO₂ phase. The resulting LiNbO₂ phase consists of  interconnected 2D sheets of NbO₂, separated by planes of Li. In addition, superconductivity emerges in the LiNbO₂ crystals and thin films when doped through delithiation or hydrogenation. A Li⁺_1-x or H^-_x content yields a Nb^(3+x)+ valence, with an electron configuration of [Kr] 4d_2-x. No additional doping is required after the CaH₂ reduction as both the films and crystals become superconducting after this treatment. Our single crystals reduced for 20 days achieve a superconducting transition with an onset at 14.2 K, and our thin films show the superconducting transition with an onset of 6.4 K.

We have carried out the structural, elemental and low temperature transport analysis in detail for both the crystals and thin films. Magnetic susceptibilities of thin films and crystals were measured using a tunnel-diode resonator. We confirm the bulk nature of superconductivity in single crystals, which is consistent with transport measurements. The high-resolution temperature variation of the London penetration depth and its possible connection to the superconducting gap structure will be discussed.