arXiv subjects

Licun Fu

Publications and source records attributed to Licun Fu.

3 recordsLinked to original sources

arXiv2026

On the ratio between longitudinal and transverse Ioffe-Regel frequencies in glasses

The temperature dependence of thermal conductivity in glasses differs characteristically from that in crystals, and is largely controlled by how sound waves are damped. The Ioffe-Regel (IR) frequency sets the high-frequency limit of well-defined sound waves. Although the transverse IR frequency has been established to coincide with the boson peak frequency and reported to be far below the longitudinal IR frequency, the quantitative relation between longitudinal and transverse IR frequencies has remained unknown. Here we examine this relation in two- (2D) and three-dimensional (3D) model glasses with vastly different stability. We observe that, in 3D glasses, the longitudinal-to-transverse IR frequency ratio is approximately equal to the ratio of the low-frequency transverse to longitudinal sound attenuation coefficient, whereas this correspondence is not observed in 2D glasses. Moreover, we find that the glass stability strongly controls the longitudinal-to-transverse IR frequency ratio in both 2D and 3D glasses: the ratio decreases significantly with increasing glass stability and approaches unity for the most stable glasses under study. Accordingly, in sufficiently stable glasses, the longitudinal sound attenuation should be comparable to the transverse attenuation, and therefore cannot be treated as negligible, challenging common assumptions adopted especially in theoretical studies. Our work is another demonstration of the glass stability as a key parameter for investigating glass properties, and cautions against simply extrapolating observations from poorly annealed glasses to stable glasses.

cond-mat.soft↗

arXiv2025

Correlation between the boson peak frequency and transverse Ioffe-Regel limit in four-dimensional structural glasses

The emergence of excess vibrational modes over the Debye prediction, typically manifested as the well-known boson peak in the plot of vibrational density of states scaled by the Debye prediction, has become a hallmark of various amorphous solids. The origin of the boson peak has been attracting considerable attention but is still under debate. A popular view is that the position of the boson peak coincides well with that of the Ioffe-Regel limit for transverse modes in both two- and three-dimensional glasses, which is primarily derived from simulation studies of model structural glasses. However, it remains unknown whether the proposed coincidence could be generalized to higher spatial dimensions, and addressing this could contribute to the advancement of relevant phenomenological theories. Here, we find that the transverse Ioffe-Regel limit frequency is higher than and not proportional to the boson peak frequency in our studied four-dimensional glasses. Our findings therefore suggest that the proposed coincidence between the boson peak frequency and the transverse Ioffe-Regel limit depends on spatial dimensions, which was not anticipated previously.

cond-mat.soft↗

arXiv2022

Density of States below the First Sound Mode in 3D Glasses

Glasses feature universally low-frequency excess vibrational modes beyond Debye prediction, which could help rationalize, e.g., the glasses' unusual temperature dependence of thermal properties compared to crystalline solids. The way the density of states of these low-frequency excess modes $D(ω)$ depends on the frequency $ω$ has been debated for decades. Recent simulation studies of 3D glasses suggest that $D(ω)$ scales universally with $ω^4$ in a low-frequency regime below the first sound mode. However, no simulation study has ever probed as low frequencies as possible to test directly whether this quartic law could work all the way to extremely low frequencies. Here, we calculated $D(ω)$ below the first sound mode in 3D glasses over a wide range of frequencies. We find $D(ω)$ scales with $ω^β$ with $β<4.0$ at very low frequencies examined, while the $ω^4$ law works only in a limited intermediate-frequency regime in some glasses. Moreover, our further analysis suggests our observation does not depend on glass models or glass stabilities examined. The $ω^4$ law of $D(ω)$ below the first sound mode is dominant in current simulation studies of 3D glasses, and our direct observation of the breakdown of the quartic law at very low frequencies thus leaves an open but important question that may attract more future numerical and theoretical studies.

cond-mat.soft↗