bounded by the adiabatic condition

The dilute oxide semiconductor SrTiO₃ exhibits superconductivity over a wide range of carrier densities (10¹⁷-10²¹ cm^-3) lying outside of the Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg theories for conventional superconductors. Across this range, the Fermi level traverses a variety of vibrational modes in the system and therefore spans the widest range of adiabatic parameter (ω/E_F) of any superconductor, making it an ideal choice to study the physics of dilute superconductivity. Recently, we have developed an approach using atomically controlled oxide heterostructures to spectroscopically probe the electronic structure of SrTiO₃ using planar tunneling junctions [1, 2]. Using this technique, we have examined the superconductivity of bulk doped SrTiO₃ across the superconducting dome and found that superconducting state is bounded by the adiabatic condition, maintaining the thermodynamic relationship of BCS weak-coupling theory (2Δ/k_BT_c = 3.53).

[1] T. Yajima et al., Nano Lett. 15, 1622 (2015)
[2] A. G. Swartz, H. Inoue et al., Proc. Natl. Acad. Sci. USA 115, 1475 (2018)