Universal responses in nonmagnetic polar metals

TL;DR

This paper reveals that the kinetic magneto-electric and non-linear Hall effects are universal in polar metals, serving as tools to characterize their polarity and underlying electronic structure, demonstrated through theoretical calculations on electron-doped PbTiO3.

Contribution

It establishes the universality of these effects in polar metals and links their microscopic origin to inversion-symmetry-breaking-induced inter-orbital hoppings and odd-parity charge multipoles.

Findings

Effects indicate the polar axis direction.

Response coefficients change sign with polarization reversal.

Effects vanish in non-polar phases.

Abstract

We demonstrate that two phenomena, the kinetic magneto-electric effect and the non-linear Hall effect, are universal to polar metals, as a consequence of their coexisting and contraindicated polarization and metallicity. We show that measurement of the effects provides a complete characterization of the nature of the polar metal, in that the non-zero response components indicate the direction of the polar axis, and the coefficients change sign on polarization reversal and become zero in the non-polar phase. We illustrate our findings for the case of electron-doped PbTiO$_3$ using a combination of density functional theory and model Hamiltonian-based calculations. Our model Hamiltonian analysis provides crucial insight into the microscopic origin of the effects, showing that they originate from inversion-symmetry-breaking-induced inter-orbital hoppings, which cause an asymmetric charge density quantified by odd-parity charge multipoles. Our work both heightens the relevance of the kinetic magneto-electric and non-linear Hall effects, and broadens the platform for investigating and detecting odd-parity charge multipoles in metals.