近日，中国科学院半导体研究所的骆军委及其研究小组与宁波东方理工学院的魏苏淮等人合作并取得一项新进展。经过不懈努力，他们提出在无去极化下通过降低原子间键强度实现光学声子软化的方法。相关研究成果已于2024年10月30日在国际权威学术期刊《自然》上发表。

本文提出一种驱动横向光学（TO）声子软化的新途径。研究人员发现，岩盐结构超宽带隙BeO中异常的软TO声子，主要是由短程键合相互作用的显著减弱引起的，这种减弱是由于围绕一个极小的铍（Be）离子呈八面体排列的两个相邻氧（O）离子之间由电子云重叠诱导的库仑斥力导致Be-O键伸长。

他们还进一步证明，在应变诱导的钙钛矿结构BaZrO_3，以及外延生长在晶格失配的SiO₂/Si衬底上的超薄HfO₂和ZrO₂薄膜中，由于双轴应变诱导的拉伸键的短程键合强度降低，驱动了TO声子的软化，从而产生稳健的铁电性。这些发现为通过利用离子半径差异、应变、掺杂和晶格畸变来定制化学键，从而在没有去极化场的情况下增强超薄薄膜中的铁电性，开发统一理论提供了新思路。

据悉，通常，通过增强长程库仑相互作用以超过短程键合力（例如，通过增加玻恩有效电荷），可以实现横向光学（TO）声子的软化，这可能触发铁电相变。然而，当主体材料的尺寸减小到高密度纳米级电子器件时，由于去极化效应，所诱导的铁电性会受到抑制，因此这一方法面临挑战。

附：英文原文

Title: Softening of the optical phonon by reduced interatomic bonding strength without depolarization

Author: Cao, Ruyue, Yang, Qiao-Lin, Deng, Hui-Xiong, Wei, Su-Huai, Robertson, John, Luo, Jun-Wei

Issue&Volume: 2024-10-30

Abstract: Softening of the transverse optical (TO) phonon, which could trigger ferroelectric phase transition, can usually be achieved by enhancing the long-range Coulomb interaction over the short-range bonding force, for example, by increasing the Born effective charges. However, it suffers from depolarization effects as the induced ferroelectricity is suppressed on size reduction of the host materials towards high-density nanoscale electronics. Here, we present an alternative route to drive the TO phonon softening by showing that the abnormal soft TO phonon in rocksalt-structured ultrawide-bandgap BeO  is mainly induced by a substantial reduction in the short-range bonding interaction due to the Be–O bond stretching caused by an electron cloud-overlap-induced Coulomb repulsion between two adjacent oxygen ions that are arranged octahedrally around an extremely small Be ion. We further demonstrate the emergence of robust ferroelectricity in strain-induced perovskite BaZrO₃ and ultrathin HfO₂ and ZrO₂ films grown epitaxially on lattice-mismatched SiO₂/Si substrate arising from the softening of the TO phonon driven by a reduction in the short-range bonding strength of biaxial strain-induced stretching bonds. These findings shed light on developing a unified theory for ferroelectricity enhancement in ultrathin films free from depolarization fields by tailoring chemical bonds using ionic radius differences, strains, doping and lattice distortions.

DOI: 10.1038/s41586-024-08099-0

Source: https://www.nature.com/articles/s41586-024-08099-0