Correlated Electrons in Polar Metals

Polar metals are materials in which itinerant electrons coexist with an inversion-symmetry breaking order, structurally identical to ferroelectric one in insulators. In my talk I will discuss the potential of electrons in polar metals to realize interaction-driven behaviors such as strange metallicity [1], dilute superconductivity [2] or collective modes [3]. The focus will be on systems close to quantum critical points of the polar order, where strong fluctuations of the soft mode – an optical phonon are present. A well-known example of such a system is doped SrTiO3, where the mechanism of superconductivity has remained an open question for more than 50 years.

It will be demonstrated that unconventional electron-phonon coupling mechanisms can lead to strong interaction effects in critical polar metals. For nodal semimetals like graphene, this leads to a potential non-Fermi liquid behavior [1]. In dilute 3D metals, the coupling to the energy fluctuations of the polar phonon results in s-wave superconductivity at extremely low densities [2], where standard BCS theory is not applicable. This offers a potential explanation for the superconductivity of doped SrTiO3. Finally, the emergence of unconventional collective modes [3] in spin-orbit coupled polar metals and predictions of their experimental signatures will be discussed.



[1] P. A. Volkov and P. Chandra, PRL 124, 237601 (2020)

[2] P. A. Volkov, P. Chandra, P. Coleman, Nat. Comm., 13 4599 (2022)

[3] A. Kumar, P. Chandra, P. A. Volkov, PRB 105, 125142 (2022); 108, 075162 (2023)