BaAl₂O₄中退化软模与选择性凝结的机制：通过非弹性X射线散射
Degenerate Soft Modes and Selective Condensation in BaAl$_2$O$_4$ via Inelastic X-ray Scattering
AI总结：
BaAl₂O₄中通过非弹性X射线散射观测到两个退化软模，揭示其在结构相变中的作用及选择性凝结机制。
摘要：
BaAl₂O₄是一种表现出通过化学成分调节的结构量子临界性的铁电材料。尽管理论计算和若干衍射实验已暗示其铁电结构相变中涉及软模，但直接实验证实仍缺乏。在本研究中，我们成功利用X射线非弹性散射观测到BaAl₂O₄的两个软模，提供了其在结构相变中作用的直接实验证据。此外，我们揭示了M点和K点的软模在能量上几乎退化，表明其中任一模态都有可能冻结。K点模态在接近相变温度（T_C）时以与M点模态几乎相同的方式变软。然而，相变仅在M点凝结，M点模态在低温结构中稳定为声学模态，而K点模态随着温度降低而变硬。
英文摘要：
BaAl$_2$O$_4$ is a ferroelectric material that exhibits structural quantum criticality through chemical composition tuning. Although theoretical calculations and several diffraction experiments have suggested the involvement of a soft mode in its ferroelectric structural phase transition, direct experimental verification is still lacking. In this study, we successfully observed two soft modes of BaAl$_2$O$_4$ using x-ray inelastic scattering, providing direct experimental evidence for their role in the structural phase transition. Furthermore, we reveal that the soft modes at the M and K points are nearly degenerate in energy, indicating a delicate balance in which either mode could potentially freeze. The K-point mode simultaneously softens toward the transition temperature ($T_{\rm C}$) in a manner nearly identical to the M-point mode. However, the phase transition condenses only at the M point, with the M-point mode stabilizing as an acoustic mode in the low-temperature structure and the K-point mode hardening as temperature decreases.
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