Nano-antennas, breakdown of Debye approximation and dynamics of glass formation

A few papers in the recent issue of PNAS.

[1] Connecting the dots: Reinventing optics for nanoscale dimensions

N J Halas

While optics is one of our oldest scientific tools, enabling some of the earliest advances in astronomy and biology, it is also currently one of the most dynamic and exciting areas of applied science. Developments of the past two decades in nanoscale device fabrication, nanomaterials synthesis and patterning, and advanced computational modeling capabilities have converged to fuel a revolution in optical science, leading to an entirely new tool set for optics at nanometer length scales. The work described in a recent issue of PNAS (1) illustrates the innovative use of nanoparticles as sensitive optical tools that provide a new way to measure the properties of light at nanometer-scale dimensions.

[2] Breakdown of the Debye approximation for the acoustic modes with nanometric wavelengths in glasses

G Monaco and V M Giordano

On the macroscopic scale, the wavelengths of sound waves in glasses are large enough that the details of the disordered microscopic structure are usually irrelevant, and the medium can be considered as a continuum. On decreasing the wavelength this approximation must of course fail at one point. We show here that this takes place unexpectedly on the mesoscopic scale characteristic of the medium range order of glasses, where it still works well for the corresponding crystalline phases. Specifically, we find that the acoustic excitations with nanometric wavelengths show the clear signature of being strongly scattered, indicating the existence of a cross-over between well-defined acoustic modes for larger wavelengths and ill-defined ones for smaller wavelengths. This cross-over region is accompanied by a softening of the sound velocity that quantitatively accounts for the excess observed in the vibrational density of states of glasses over the Debye level at energies of a few milli-electronvolts. These findings thus highlight the acoustic contribution to the well-known universal low-temperature anomalies found in the specific heat of glasses.

[3] Growing length and time scales in glass-forming liquids

S Kamarkar, C Dasgupta and S Sastry

The glass transition, whereby liquids transform into amorphous solids at low temperatures, is a subject of intense research despite decades of investigation. Explaining the enormous increase in relaxation times of a liquid upon supercooling is essential for understanding the glass transition. Although many theories, such as the Adam–Gibbs theory, have sought to relate growing relaxation times to length scales associated with spatial correlations in liquid structure or motion of molecules, the role of length scales in glassy dynamics is not well established. Recent studies of spatially correlated rearrangements of molecules leading to structural relaxation, termed “spatially heterogeneous dynamics,” provide fresh impetus in this direction. A powerful approach to extract length scales in critical phenomena is finite-size scaling, wherein a system is studied for sizes traversing the length scales of interest. We perform finite-size scaling for a realistic glass-former, using computer simulations, to evaluate the length scale associated with spatially heterogeneous dynamics, which grows as temperature decreases. However, relaxation times that also grow with decreasing temperature do not exhibit standard finite-size scaling with this length. We show that relaxation times are instead determined, for all studied system sizes and temperatures, by configurational entropy, in accordance with the Adam–Gibbs relation, but in disagreement with theoretical expectations based on spin-glass models that configurational entropy is not relevant at temperatures substantially above the critical temperature of mode-coupling theory. Our results provide new insights into the dynamics of glass-forming liquids and pose serious challenges to existing theoretical descriptions.

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