Pressure-induced enhancement of polar distortions in a metal and implications for the Rashba spin splitting

Polar metals are an intriguing class of materials that feature a polar crystal structure while also exhibiting metallic conductivity. The unique properties of polar metals challenge expectations, making way for the exploration of exotic phenomena such as unconventional magnetism, hyperferroelectric multiferroicity, and the development of multifunctional devices that can leverage both the material's polar structure and its asymmetry in the spin conductivity, that arises due to the Rashba effect. Here, via a high-pressure single-crystal diffraction study, we report the pressure-induced enhancement of polar distortions in such a metal, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>Ca</a:mi><a:mn>3</a:mn></a:msub><a:msub><a:mi>Ru</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mi mathvariant="normal">O</a:mi><a:mn>7</a:mn></a:msub></a:mrow></a:math>. Our density functional theory calculations highlight that naive assumptions about the linear dependency between polar distortion amplitudes and the magnitude of the Rashba spin splitting may not be generally valid.