Revealing a distortive polar order buried in the Fermi sea

Jiaojian Shi; Wenjing You; Xian Li; Frank Y Gao; Xinyue Peng; Shangjie Zhang; Jiangxu Li; Yang Zhang; Liang Fu; Patrick J Taylor; Keith A Nelson; Edoardo Baldini

doi:10.1126/sciadv.adn0929

Revealing a distortive polar order buried in the Fermi sea

Sci Adv. 2024 Jul 12;10(28):eadn0929. doi: 10.1126/sciadv.adn0929. Epub 2024 Jul 12.

Authors

Jiaojian Shi¹, Wenjing You², Xian Li³, Frank Y Gao², Xinyue Peng², Shangjie Zhang², Jiangxu Li^{4

5}, Yang Zhang^{4

5

6}, Liang Fu⁶, Patrick J Taylor⁷, Keith A Nelson³, Edoardo Baldini²

Affiliations

¹ Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA.
² Department of Physics and Center for Complex Quantum Systems, The University of Texas at Austin, Austin, TX 78712, USA.
³ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
⁴ Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA.
⁵ Min H. Kao Department of Electrical Engineering and Computer Science, University of Tennessee, Knoxville, TN 37996, USA.
⁶ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
⁷ DEVCOM Army Research Laboratory, Adelphi, MD 20783, USA.

Abstract

Polar metals are challenging to identify spectroscopically because the fingerprints of electric polarization are often obscured by the presence of screening charges. Here, we unravel unambiguous signatures of a distortive polar order buried in the Fermi sea by probing the nonlinear optical response of materials driven by tailored terahertz fields. We apply this strategy to investigate the topological crystalline insulator Pb_1-xSn_xTe, tracking its soft phonon mode in the time domain and observing the occurrence of inversion symmetry breaking as a function of temperature. By combining measurements across the material's phase diagram with ab initio calculations, we demonstrate the generality of our approach. These results highlight the potential of terahertz driving fields to reveal polar orders coexisting with itinerant electrons, thus opening additional avenues for material discovery.