Shining a light on disordered and fractal systems

Date:
September 15, 2020
Source:
University of Tsukuba
Summary:
A research team has investigated the acoustic properties of
disordered lysozyme proteins by using terahertz spectroscopy. They
found that the fractal nature of the proteins is responsible for
its unusually large vibrations at low frequencies, which may lead
to a better theory for disordered materials.



FULL STORY ========================================================================== Researchers led by the University of Tsukuba studied the vibrational modes
of an intrinsically disordered protein to understand its anomalously
strong response at low frequencies. This work may lead to improvements
in our knowledge of materials that lack long-range order, which may
influence industrial glass manufacturing.


========================================================================== Glassy materials have many surprising properties. Not quite a solid or a liquid, glasses are made of atoms that are frozen in a disordered, non- crystalline state. Over a century ago, physicist Peter Debye proposed a
formula for understanding the possible vibrational modes of solids. While mostly successful, this theory does not explain the surprisingly universal vibrations that can be excited in disordered materials -- like glass --
by electromagnetic radiation in the terahertz range. This deviation has
been seen often enough to gets its own name, the "boson peak," but its
origin remains unclear.

Now, researchers at the University of Tsukuba have conducted a series
of experiments to investigate the physics behind the boson peak using
the protein lysozyme. "This protein has an intrinsically disordered
and fractal structure," first author of the study Professor Tatsuya
Mori says. "We believe that it makes sense to consider the entire
system as a single supramolecule." Fractals, which are mathematical
structures that exhibit self-similarity over a wide range of scales,
are common in nature. Think of trees: they appear similar whether you
zoom out to look at the branches, as well as when you come close to
inspect the twigs. Fractals have the surprising ability to be described
by a non-integer number of dimensions. That is, an object with a
fractal dimension of 1.5 is halfway between a two-dimensional and a three-dimensional object, which means that its mass increases with its
size to the 1.5 power.

On the basis of the results of terahertz spectroscopy, the mass fractal dimension of the lysozyme molecules was found to be around 2.75. This
value was also determined to be related to the absorption coefficient
of the material.

"The findings suggest that the fractal properties originate from the self- similarity of the structure of the amino acids of the lysozyme proteins," Professor Mori says. "This research may hold the key to resolving a long- standing puzzle regarding disordered and fractal materials, which can
lead to more efficient production of glass or fractal structures."

========================================================================== Story Source: Materials provided by University_of_Tsukuba. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Tatsuya Mori, Yue Jiang, Yasuhiro Fujii, Suguru Kitani, Hideyuki
Mizuno,
Akitoshi Koreeda, Leona Motoji, Hiroko Tokoro, Kentaro
Shiraki, Yohei Yamamoto, Seiji Kojima. Detection of boson
peak and fractal dynamics of disordered systems using
terahertz spectroscopy. Physical Review E, 2020; 102 (2) DOI:
10.1103/PhysRevE.102.022502 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200915090121.htm

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