What pairs electrons in doped SrTiO_3 at Fermi energies below phonons? · revision history

Revision 2

Accepted · core change · record

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Title · changed
Revision 1: What pairs electrons in doped SrTiO3 at Fermi energies below phonons
Revision 2: What pairs electrons in doped $\mathrm{SrTiO_3}$ at Fermi energies below phonons?
Plain
Strontium titanate with very few added electrons superconducts below about 0.4 K. Its electrons have less kinetic energy than the lattice vibrations that should pair them, so standard theory does not apply.
Precise · changed · core
Revision 1: Electron-doped SrTiO3 (Nb, La or oxygen vacancies) superconducts from $n\ \text{below}\ 1\mathrm{e}18\,\mathrm{cm}^{-3}\ \text{to about}\ 1\mathrm{e}21\,\mathrm{cm}^{-3}$ with maximal $T_c$ of approximately $0.4\,\mathrm{K}$ near $n \sim 1\mathrm{e}20\,\mathrm{cm}^{-3}$; at the lowest densities $E_F \sim 1\,\mathrm{meV}$ is far below the longitudinal optical phonon energies (up to about $100\,\mathrm{meV}$). Identify the pairing interaction (soft transverse-optical mode via two-phonon or Rashba-type coupling, dynamically screened longitudinal-optical phonons and plasmons, or other) that reproduces $T_c(n)$ quantitatively over the full dome without adjustable couplings. An answer is a mechanism with computed $T_c(n)$ within a factor of about 2.
Revision 2: Electron-doped $\mathrm{SrTiO_3}$ (Nb, La or oxygen vacancies) superconducts from $n\ \text{below}\ 1\mathrm{e}18\,\mathrm{cm}^{-3}\ \text{to about}\ 1\mathrm{e}21\,\mathrm{cm}^{-3}$ with maximal $T_c$ of approximately $0.4\,\mathrm{K}$ near $n \sim 1\mathrm{e}20\,\mathrm{cm}^{-3}$; at the lowest densities $E_F \sim 1\,\mathrm{meV}$ is far below the longitudinal optical phonon energies (up to about $100\,\mathrm{meV}$). Identify the pairing interaction (soft transverse-optical mode via two-phonon or Rashba-type coupling, dynamically screened longitudinal-optical phonons and plasmons, or other) that reproduces $T_c(n)$ quantitatively over the full dome without adjustable couplings. An answer is a mechanism with computed $T_c(n)$ within a factor of about 2.
Settled by
A first-principles strong-coupling calculation reproducing $T_{c}(n)$, the isotope effect and the gap-to-$T_{c}$ ratio across the dome.
Kind
phenomenon
Answer type
mechanism
References
(empty)
Literature status
open
Status note
Strong-coupling theories with ferroelectric fluctuations (e.g. Saha et al., npj Quantum Materials 2025) reproduce trends, but no mechanism has quantitative consensus.
Posed since
1964

Decisions