Quantum Critical Polar Metals

Quantum critical point of a polar (inversion breaking) transition
Polar fluctuations couple linearly to interband transitions
Nonlinear coupling (upper right) leads to superconductivity at low density and dome-like Tc(n), in agreement with experiments on SrTiO3 (crosses)

Polar metals are intriguing materials where an inversion-symmetry breaking transition (structurally similar to a ferroelectric one) occurs in a conducting system. The polar order fluctuations being equivalent to a transverse optical phonon, it is yet an open question whether novel many-body phenomena may occur in such a system.

I have shown (Phys. Rev. Lett. 124, 237601 (2020)), that a number of novel behaviors including non-Fermi liquid can be realized in topological semimetals (with nodal points or lines) at a polar quantum critical point.

More recently [Nature Communications,13, 4599 (2022)], I have demonstrated that nonlinear coupling to the critical fluctuations can drive superconductivity at extremely low densities, possibly giving a key to the puzzle of superconductivity in doped SrTiO3. This mechanism also provides to formation of light bipolarons at strong coupling, opening a potential way for higher Tc’s.