Disordered Crystals Reveal Soft Quasilocalized Glassy Excitations

Area of Science:

• Condensed Matter Physics
• Materials Science
• Statistical Mechanics

Background:

• Structural glasses exhibit universal low-energy nonphononic excitations following a quartic distribution.
• This universal behavior is typically associated with the absence of long-range order in glasses.

Purpose of the Study:

• To investigate the presence and universality of nonphononic excitations in disordered crystals.
• To compare the density of these excitations in disordered crystals versus structural glasses.

Main Methods:

• Analysis of low-energy excitations in disordered crystalline solids.
• Quantification of mechanical disorder using shear modulus fluctuations.
• Comparison of excitation spectra across different disordered materials.

Main Results:

• The universal quartic law (∼ω⁴) for nonphononic excitations is also observed in disordered crystals with finite long-range order.
• Disordered crystals host a higher density of quasilocalized excitations than structural glasses with similar mechanical disorder.
• The degree of universality of the quartic law extends beyond glasses.

Conclusions:

• The universal quartic law governing nonphononic excitations is not exclusive to glasses but also applies to disordered crystals.
• Disordered crystals present a richer landscape of quasilocalized excitations than previously understood.
• These findings advance the understanding of universal phenomena in disordered solids and their relation to glasslike anomalies.