Most are aware that electrons are negatively charged particles that surround the nucleus of atoms and whose behaviour governs chemical interactions.
Most are aware that electrons are negatively charged particles that surround the nucleus of atoms and whose behaviour governs chemical interactions. However, it is less commonly known that electrons come in two distinct kinds: spin-up and spin-down. And the tendency for pairing between up and down spin electrons, forming "dance partners" with one another, is one of the most important behaviours affecting the electron clouds that control the chemistry of nature. Under pressures like those deep inside the Earth, the orbits in which the electrons move are squeezed, the "dance floor" changes. Electron pairs are sometimes forced to change their dance pattern and the way that they partner with one another, giving rise to what is termed an "electron spin-pairing crossover" ("spin crossover" is often used as a shorthand expression).
Such a spin-crossover has long been predicted to occur at elevated pressures of the middle mantle (~1500 km deep) in a mineral called "ferro-periclase" that is thought to be the second-most abundant material in Earth's rocky mantle. Such predictions for a ferropericlase spin-crossover have been broadly confirmed, both by high-pressure laboratory experiments as well as computational models based on quantum mechanics. However, the predicted effects of this spin-crossover escaped seismological detection, leaving deep-Earth researchers to wonder if the predictions were flawed or if conditions in the mantle suppress the seismic expression.
A new research paper published in Nature Communications by an international research team including Earth-Life Science Institute (ELSI) Professor John W. Hernlund (Tokyo Institute of Technology) and ELSI Specially Appointed Assistant Professor Christine Houser proposes a unique seismological signature of this spin crossover in ferropericlase. The team's detection method is based on the varying behaviour of the spin crossover for P-waves and S-waves, two distinct kinds of seismic waves that propagate through the Earth. Seismologists use both of these waves (generated by earthquakes and recorded at global seismographic stations) to produce tomographic images of the mantle in a procedure that is roughly analogous to a medical CT scan. The images reveal material that propagates these two kinds of seismic waves faster or slower than the average.
Read more at Tokyo Institute of Technology
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