扭曲双层SnSe中的宽带声子速度抑制与有限各向异性 crossover
Broadband phonon-velocity suppression and a finite anisotropic crossover in twisted bilayer SnSe
AI总结：
本研究针对低对称层状材料双层SnSe，结合DFT等方法发现其扭转后声子速度呈宽带抑制，且存在有限各向异性 crossover，拓展了声子 twistronics 的应用。
AI中文摘要：
莫尔超晶格在不改变化学成分的前提下重塑晶格动力学，但晶体各向异性如何改变这种调控作用仍不清楚。我们结合密度泛函理论（DFT）校准的晶格动力学计算与角度匹配的未扭曲对照组，研究7个 commensurate 扭转角（3.18°--8.77°）下的褶皱双层SnSe。在300K时，扭转将带路径热容加权均方群速度抑制至对照组的2.6%--8.4%；该抑制覆盖宽频率范围，而非仅少数软支。速度响应在4.78°与3.82°之间 crossover 至弛豫堆叠纹理与频率分辨速度剖面自相似的 regime，归一化均方速度比在最小三个角度间仅占其均值的11.1%——这是有限各向异性 crossover，而非单角度条件。DFT与MACE力常数的直接一致性（r=0.996）、均匀4×4×1稳定性扫描及声学求和规则与路径密度测试均支持该趋势；排除1个弛豫敏感案例后，平衡趋势由6个结构定义。这些结果将声子 twistronics 拓展至低对称层状材料，并确定晶体各向异性是有限角声子 crossover 行为的关键决定因素。
英文摘要：
Moiré superlattices reshape lattice dynamics without altering chemical composition, yet how crystal anisotropy modifies this control remains unclear. We combine density-functional-theory (DFT)-calibrated lattice-dynamical calculations with angle-matched untwisted controls to study puckered bilayer SnSe across seven commensurate twist angles ($3.18^\circ$--$8.77^\circ$). At 300 K, twisting suppresses the band-path heat-capacity-weighted mean-square group velocity to 2.6--8.4\% of the control values; the suppression spans a broad frequency range rather than a few soft branches. The velocity response crosses over between $4.78^\circ$ and $3.82^\circ$ into a regime where the relaxed stacking textures and frequency-resolved velocity profiles become self-similar, with the normalized mean-square velocity ratio spanning only 11.1\% of its mean across the three smallest angles---a finite anisotropic crossover, not a singular-angle condition. Direct DFT--MACE force-constant agreement ($r=0.996$), uniform $4\times4\times1$ stability scans, and acoustic-sum-rule and path-density tests support the trend. The equilibrium trend is defined by six structures after excluding one relaxation-sensitive case. These results extend phonon twistronics to low-symmetry layered materials and identify crystal anisotropy as a key determinant of finite-angle phonon crossover behavior.
