AI 中文总结
本文研究范德华磁体 Nb₃Cl₈₋ₓBrₓ，发现无序可使一阶相变圆滑化，保留滞后却无宏观相共存，该体系为相关研究提供了有前景平台。
AI 中文摘要
淬火无序可深刻改变相变，理论预测低维系统中即使弱淬火无序也能使一阶相变相关的热力学不连续性圆滑化。本文采用时域太赫兹光谱研究准二维三聚化 kagome 范德华磁体系列 Nb$_3$Cl$_{8-x}$Br$_x$（$x=0$、1 和 8）。观测到 Br 取代时出现额外声子支，其谱权重随 Br 浓度增加而增大、频率随浓度增加而软化。声子频率的温度演化显示，Nb$_3$Cl$_8$ 中存在以宏观相共存和热滞后为特征的清晰一阶相变；相比之下，取代无序化合物 Nb$_3$Cl$_7$Br 的相变保留了滞后，同时呈现显著展宽的相变且无可分辨的宏观相共存。这些观测表明，无序诱导相变破碎为局域优域，而非由稳定相界分隔的明确定义的体相。该行为与 Imry-Wortis 和 Aizenman-Wehr 关于低维系统淬火无序效应的情景一致，二者可使宏观相共存失稳并圆滑化一阶相变相关的热力学不连续性。尽管无序化合物中无可分辨的共存，热滞后仍持续存在，表明常被视为一阶特征单一标志的这两个特征实际是不同的，可被无序分离。此外，本文结果确立 Nb$_3$Cl$_{8-x}$Br$_x$ 为研究低维系统中无序对一阶相变效应的有前景平台。
英文摘要
Quenched disorder can profoundly modify phase transitions. In low-dimensional systems, theory predicts that even weak quenched disorder can round the thermodynamic discontinuities associated with a first-order phase transition. Here, we employ time-domain terahertz spectroscopy to investigate the quasi-two-dimensional trimerized kagome van der Waals magnet family Nb$_3$Cl$_{8-x}$Br$_x$ ($x=0$, 1 and 8). We observe the emergence of an additional phonon branch upon Br substitution, whose spectral weight increases and frequency softens with increasing Br concentration. The temperature evolution of the phonon frequencies reveals a clean first-order transition in Nb$_3$Cl$_8$ characterized by macroscopic phase coexistence and thermal hysteresis. In contrast, the transition in the substitutionally disordered compound Nb$_3$Cl$_7$Br retains its hysteresis while exhibiting a substantially broadened transition with no resolvable macroscopic phase coexistence. These observations reveal disorder-induced fragmentation of the transition into locally favored domains instead of well-defined bulk phases separated by stable phase boundaries. The behavior is consistent with the Imry-Wortis and the Aizenman-Wehr scenarios for the effect of quenched disorder in low-dimensional systems, which destabilizes macroscopic phase coexistence and rounds the thermodynamic discontinuities associated with first-order transitions. Thermal hysteresis persists in the disordered compound despite the lack of resolvable coexistence, indicating that the two features often treated as a single hallmark of first-order character arise distinctly and can be separated by disorder. Moreover, our results establish Nb$_3$Cl$_{8-x}$Br$_x$ as a promising platform for investigating the effects of disorder on first-order transitions in low-dimensional systems.
