How material defects influence the melting process

Date:
June 15, 2020
Source:
Brown University
Summary:
A new study helps to reconcile a Nobel Prize-winning theory with
experiments on how solids actually melt.



FULL STORY ==========================================================================
In 1972, physicists J. Michael Kosterlitz and David Thouless published a groundbreaking theory of how phase changes could occur in two-dimensional materials. Experiments soon showed that the theory correctly captured
the process of a helium film transitioning from a superfluid to a normal
fluid, helping to usher in a new era of research on ultra-thin materials,
not to mention earning Kosterlitz, a professor at Brown University,
and Thouless shares of the 2016 Nobel Prize in Physics.


==========================================================================
But the Kosterlitz-Thouless (K-T) theory aimed to explain more than the superfluid transition. The pair also hoped it might explain how a two- dimensional solid could melt into a liquid, but experiments so far have
failed to clearly validate the theory in that case. Now, new research
by another group of Brown physicists could help explain the mismatch
between theory and experiment.

The research, published in Proceedings of the National Academy of
Sciences, shows how impurities -- "extra" atoms in the crystalline
structure of a material -- can disrupt the order of a system and
cause melting to begin before the K-T theory predicts it should. The
findings are a step toward a more complete physical theory of melting,
the researchers say.

"The solid-liquid transition is something we're all familiar with,
yet it's a profound failure of modern physics that we still don't
understand exactly how it happens," said Xinsheng Ling, a professor of
physics at Brown and senior author of the new paper. "What we showed is
that impurities -- which are not included in K-T theory but are always
found in real materials -- play a major role in the melting process."
While the details remain a major mystery, scientists have a basic
understanding of how solids melt. As temperature increases, atoms in the crystalline lattice of a solid start to jiggle around. If the jiggling
becomes too violent for the lattice to hold together, the solid melts
into a liquid. But how exactly the melting process starts and why it
starts in certain places in a solid instead of others aren't known.

For this new study, the researchers used tiny polystyrene particles
suspended in highly deionized water. Electrical forces between the
charged particles cause them to arrange themselves in a crystal-like
lattice similar to the way atoms are arranged in a solid material. Using
a laser beam to move individual particles, the researchers can see how
lattice defects affect the order of the lattice.



========================================================================== Defects can come in two general forms -- vacancies, where particles
are missing, and interstitials, where there are more particles than
there should be. This new study looked in particular at the effect of interstitials, which no previous studies had investigated.

The research found that while one interstitial in a given region made
little difference in the behavior of the lattice, two interstitials made
a big difference.

"What we found was that two interstitial defects break the symmetry of the structure in a way that single defects don't," Ling said. "That symmetry- breaking leads to local melting before K-T predicts." That's because
the K-T theory deals with defects that arise from thermal fluctuations,
and not defects that may have already existed in the lattice.

"Real materials are messy," Ling said. "There are always impurities. Put simply, the system cannot distinguish which are impurities and which are defects created by thermal agitation, which leads to melting before
what would be predicted." The technique used for the study could
be useful elsewhere, the researchers say. For example, it could be
useful in studying the transition of hard glass to a viscous liquid,
a phenomenon related to the solid-liquid transition that also lacks a
complete explanation.

"We think we have accidentally discovered a new way to uncover symmetry- breaking mechanisms in materials physics," Ling said. "The method
itself may end up being the most significant thing about this paper in
addition to the findings." Co-authors of the paper were former Brown
Ph.D. students Sung-Cheol Kim, Lichao Yu and Alexandros Pertsinidis, who
all completed their Ph.D. theses in the Ling Lab at Brown. The research
was supported by the National Science Foundation (DMR- 1005705).


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


========================================================================== Journal Reference:
1. Sung-Cheol Kim, Lichao Yu, Alexandros Pertsinidis, Xinsheng
Sean Ling.

Dynamical processes of interstitial diffusion in a two-dimensional
colloidal crystal. Proceedings of the National Academy of Sciences,
2020; 201918097 DOI: 10.1073/pnas.1918097117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200615115722.htm

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