Phonons in stringlet-land and the boson peak

Area of Science:

• Condensed Matter Physics
• Materials Science
• Statistical Mechanics

Background:

• Solids deviating from harmonic crystal models show anomalies in specific heat and vibrational density of states (VDOS).
• The boson peak (BP), a VDOS excess over Debye's law, is a key anomaly whose origin remains debated.
• Recent simulations suggest localized 1D string-like excitations (stringlets) may cause the BP.

Purpose of the Study:

• To theoretically investigate the dynamics of acoustic phonons interacting with vibrating stringlets.
• To determine if stringlet dynamics can explain the boson peak anomaly and associated sound properties.
• To validate recent simulation findings on the microscopic origin of the BP.

Main Methods:

• Developing a theoretical model of acoustic phonons interacting with a bath of 1D stringlets with exponential size distribution.
• Analyzing the renormalization of the phonon propagator due to stringlet interactions.
• Calculating the VDOS, sound attenuation, and speed of sound within the model.

Main Results:

• Stringlets strongly renormalize the phonon propagator, inducing a boson peak anomaly in the VDOS.
• A dispersionless BP flat mode emerges due to phonon-stringlet interactions.
• Phonon-stringlet interactions lead to enhanced sound attenuation and a dip in sound speed near the BP frequency.
• The model predicts qualitative trends for BP frequency and intensity consistent with observations.

Conclusions:

• The theoretical model provides strong support for stringlet dynamics as the microscopic origin of the boson peak.
• The findings reconcile theoretical predictions with recent simulation and experimental data.
• This work offers a simplified theoretical framework for understanding anomalies in disordered solids.