Quantum phase transition detected on a global scale deep inside the
Earth

Date:
October 12, 2021
Source:
Columbia University School of Engineering and Applied Science
Summary:
A multidisciplinary team of materials physicists and geophysicists
combine theoretical predictions, simulations, and seismic tomography
to find spin transition in the Earth's mantle. Their findings will
improve understanding of the Earth's interior, and help elucidate
the impact of this phenomenon on tectonic events including volcanic
eruptions and earthquakes.



FULL STORY ==========================================================================
The interior of the Earth is a mystery, especially at greater depths (>
660 km). Researchers only have seismic tomographic images of this region
and, to interpret them, they need to calculate seismic (acoustic)
velocities in minerals at high pressures and temperatures. With
those calculations, they can create 3D velocity maps and figure out
the mineralogy and temperature of the observed regions. When a phase
transition occurs in a mineral, such as a crystal structure change under pressure, scientists observe a velocity change, usually a sharp seismic velocity discontinuity.


==========================================================================
In 2003, scientists observed in a lab a novel type of phase change in
minerals -- a spin change in iron in ferropericlase, the second most
abundant component of the Earth's lower mantle. A spin change, or spin crossover, can happen in minerals like ferropericlase under an external stimulus, such as pressure or temperature. Over the next few years, experimental and theoretical groups confirmed this phase change in both ferropericlase and bridgmanite, the most abundant phase of the lower
mantle. But no one was quite sure why or where this was happening.

In 2006, Columbia Engineering Professor Renata Wentzcovitch published
her first paper on ferropericlase, providing a theory for the spin
crossover in this mineral. Her theory suggested it happened across a
thousand kilometers in the lower mantle. Since then, Wentzcovitch, who is
a professor in the applied physics and applied mathematics department,
earth and environmental sciences, and Lamont-Doherty Earth Observatory
at Columbia University, has published 13 papers with her group on this
topic, investigating velocities in every possible situation of the spin crossover in ferropericlase and bridgmanite, and predicting properties
of these minerals throughout this crossover. In 2014, Wenzcovitch,
whose research focuses on computational quantum mechanical studies
of materials at extreme conditions, in particular planetary materials
predicted how this spin change phenomenon could be detected in seismic tomographic images, but seismologists still could not see it.

Working with a multidisciplinary team from Columbia Engineering,
the University of Oslo, theTokyo Institute of Technology, and Intel
Co., Wenzcovitch's latest paper details how they have now identified
the ferropericlase spin crossover signal, a quantum phase transition
deep within the Earth's lower mantle. This was achieved by looking at
specific regions in the Earth's mantle where ferropericlase is expected
to be abundant. The study was published October 8, 2021, in Nature Communications.

"This exciting finding, which confirms my earlier predictions,
illustrates the importance of materials physicists and geophysicists
working together to learn more about what's going on deep within the
Earth," said Wentzcovitch.

Spin transition is commonly used in materials like those used for magnetic recording. If you stretch or compress just a few nanometer-thick layers
of a magnetic material, you can change the layer's magnetic properties
and improve the medium recording properties. Wentzcovitch's new study
shows that the same phenomenon happens across thousands of kilometers
in the Earth's interior, taking this from the nano- to the macro-scale.

"Moreover, geodynamic simulations have shown that the spin crossover invigorates convection in the Earth's mantle and tectonic plate motion. So
we think that this quantum phenomenon also increases the frequency of
tectonic events such as earthquakes and volcanic eruptions," Wentzcovitch notes.

There are still many regions of the mantle researchers do not understand
and spin state change is critical to understanding velocities,
phase stabilities, etc. Wentzcovitch is continuing to interpret
seismic tomographic maps using seismic velocities predicted by ab initiocalculations based on density functional theory. She is also
developing and applying more accurate materials simulation techniques
to predicting seismic velocities and transport properties, particularly
in regions rich in iron, molten, or at temperatures close to melting.

"What's especially exciting is that our materials simulation
methods are applicable to strongly correlated materials --
multiferroic, ferroelectrics, and materials at high temperatures
in general," Wentzcovitch says. "We'll be able to improve
our analyses of 3D tomographic images of the Earth and learn
more about how the crushing pressures of the Earth's interior
are indirectly affecting our lives above, on the Earth's surface." ========================================================================== Story Source: Materials provided by Columbia_University_School_of_Engineering_and_Applied Science. Original
written by Holly Evarts. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Grace E. Shephard, Christine Houser, John W. Hernlund, Juan
J. Valencia-
Cardona, Reidar G. Tro/nnes, Renata M. Wentzcovitch. Seismological
expression of the iron spin crossover in ferropericlase in the
Earth's lower mantle. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467- 021-26115-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012154734.htm

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