Jiang Cunyuan, Baggioli Matteo
School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China.
Wilczek Quantum Center, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China.
J Phys Condens Matter. 2024 Sep 17;36(50). doi: 10.1088/1361-648X/ad789c.
Solid materials that deviate from the harmonic crystal paradigm exhibit characteristic anomalies in the specific heat and vibrational density of states (VDOS) with respect to Debye's theory predictions. The boson peak (BP), a low-frequency excess in the VDOS over Debye lawg(ω)∝ω2, is certainly the most famous among them; nevertheless, its origin is still subject of fierce debate. Recent simulation works provided strong evidence that localized one-dimensional string-like excitations (stringlets) might be the microscopic origin of the BP. In this work, we study the dynamics of acoustic phonons interacting with a bath of vibrating 1D stringlets with exponentially distributed size, as observed in simulations. We show that stringlets strongly renormalize the phonon propagator and naturally induce a BP anomaly in the VDOS, corresponding to the emergence of a dispersionless BP flat mode. Additionally, phonon-stringlet interactions produce a strong enhancement of sound attenuation and a dip in the speed of sound near the BP frequency, consistent with experimental and simulation data. The qualitative trends of the BP frequency and intensity are predicted within the model and shown to be in good agreement with previous observations. In summary, our results substantiate with a simple theoretical model the recent simulation results by Hu and Tanaka claiming the origin of the BP from stringlet dynamics.
偏离谐波晶体范式的固体材料在比热和态密度（VDOS）振动方面相对于德拜理论预测表现出特征异常。玻色子峰（BP），即VDOS中相对于德拜定律g(ω)∝ω²的低频过剩，无疑是其中最著名的；然而，其起源仍然是激烈争论的主题。最近的模拟工作提供了有力证据，表明局域一维弦状激发（弦线）可能是BP的微观起源。在这项工作中，我们研究了声子与模拟中观察到的具有指数分布大小的振动一维弦线浴相互作用的动力学。我们表明，弦线强烈重整化声子传播子，并自然地在VDOS中诱导出BP异常，对应于无色散BP平坦模式的出现。此外，声子 - 弦线相互作用导致声衰减强烈增强，并且在BP频率附近声速出现下降，这与实验和模拟数据一致。BP频率和强度的定性趋势在模型中得到预测，并显示与先前的观察结果高度一致。总之，我们的结果用一个简单的理论模型证实了胡和田中最近的模拟结果，即声称BP起源于弦线动力学。
