Polar metals: Principles and Prospects

Abstract

We review the class of materials known as polar metals, in which polarity and metallicity coexist in the same phase. While the notion of polar metals was first invoked more than 50 years ago, their practical realization has proved challenging, since the itinerant carriers required for metallicity tend to screen any polarization. Huge progress has been made in the last decade, with many mechanisms for combining polarity and metallicity proposed, and the first examples, LiOsO 3 and WTe 2 , identified experimentally. The availability of polar metallic samples has opened a new paradigm in polar metal research, with implications in the fields of topology, ferroelectricity, magnetoelectricity, spintronics, and superconductivity. Here, we review the principles and techniques that have been developed to design and engineer polar metals and describe some of their interesting properties, with a focus on the most promising directions for future work.

Single crystal growths and magnetic properties of hexagonal polar semimetals RAuGe (R = Y, Gd-Tm, and Lu)

“…Magnetic anisotropy is systematically determined by the CEF effect of R ions, and low-temperature transitions are dominated by the lowest CEF levels. Continuous structural evolution of the polarity and its potential interplay with nontrivial magnetism would be viewed as a promising platform for studying metallic multiferroics [62,63]. Findings in the present study provide basic understanding of single crystalline properties of a family of polar semimetal RAuGe.…”

Polar Perturbations in Functional Oxide Heterostructures

“…[166] A large variety of attractive physical properties in polar metals have been not only theoretically predicted, but also experimentally realized for potential applications to a new concept of functional devices. [334,353,354,364] Note that the physical properties of materials are very susceptible to spatial inversion/time reversal/translation symmetry breaking and hence, exotic physical phenomena, which do not exist in the parent materials, can appear in the symmetry-lifted materials with phase transitions. [364] In non-centrosymmetric polar metals, it is inevitable that a diverse range of structural, electrical, magnetic, optical, and transport properties become spatially inhomogeneous due to the presence of a polar axis along which local electric dipoles are ordered with dipolar interaction.…”

“…[364] In non-centrosymmetric polar metals, it is inevitable that a diverse range of structural, electrical, magnetic, optical, and transport properties become spatially inhomogeneous due to the presence of a polar axis along which local electric dipoles are ordered with dipolar interaction. [334,353,354,364] For example, Puggioni and Rondinelli theoretically predicted that the thermoelectric behaviors of non-centrosymmetric ruthenate metals distinctly deviated from those of non-polar metallic ruthenates because of the anisotropic electronic band structures depending on the inversion-center-lifted polar axis, which opened a pathway to manipulate the transport properties in metals through symmetry engineering. [330] Thanks to the forefront study of the rational design of symmetryengineered functionalities in polar metals, considerable efforts have been made to experimentally realize the non-equilibrium non-centrosymmetric metallic phase in complex oxide thinfilm/superlattice heterostructures [225,329,332,333,336,339,347] and to systematically explore emergent physical phenomena [333] Copyright 2016, Nature Publishing Group, a division of Macmillan Publishers Limited.…”