Electron Pairing Without Superconductivity

Strontium titanate (SrTiO$_3$) exhibits an extremely low carrier density threshold for superconductivity, and possesses a phase diagram similar to high-temperature superconductors—two factors that suggest an unconventional pairing mechanism. We describe transport experiments with nanowire-based quantum dots localized at the interface between SrTiO$_3$ and LaAlO$_3$. Electrostatic gating of the quantum dot reveals a series of two-electron conductance resonances--paired electron states--that bifurcate above a critical magnetic field B$_p$~1-4 Tesla, an order of magnitude larger than the superconducting critical magnetic field. For B$<$B$_p$, these resonances are insensitive to applied magnetic fields; for B$>$B$_p$, the resonances exhibit a linear Zeeman-like energy splitting. Electron pairing is stable at temperatures as high as $T=900$ mK, far above the superconducting transition temperature (T$_c$~300 mK). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by an attractive-U Hubbard model that describes real-space electron pairing as a precursor to superconductivity.