For most people, basic science explains that matter can exist in just three simple phases–solid, liquid or gas. But condensed matter physicists are shattering those notions, and a team including Northeastern University scientists is producing research to reveal more complex phase diagrams.

Senior Associate Scientist Bernardo Barbiellini-Amidei and Professor Arun Bansil, both of the Department of Physics in the College of Science at Northeastern University, are using cutting edge quantum mechanical modeling to show that silicon has more than one liquid form with distinct molecular makeup. The breakthrough centers on developing a realistic model of how electrons move through liquid silicon. The results of several years of study by the international team, which support that silicon may exist in different liquid phases, have been published in the February 10 issue of Physical Review Letters.

Silicon has “revolutionized our way of being,” Barbiellini said, as it is a key ingredient in integrated circuits, the basis of modern computers. The idea that liquid silicon may harbor different phases has been suggested recently, but remains highly controversial. . The Northeastern team is taking the existing evidence to the next level by showing that different atomic bonds (covalent and metallic) coexist in the liquid phase.

“This is a very fundamental question,” Barbiellini said. “Putting together atoms can give you very different properties depending on which phase they are in.”
The work is a great example of the College of Science’s global collaborations, pulling together a team from around the world.  The present study involved experimental groups from Spring-8 synchrotron facility in Japan and a theoretical group at Princeton University.

The X-rays were sent into a silicon sample that had been heated to about 2,000 degrees Kelvin to melt the sample. Researchers then watched for Compton scattering signal—a change in the scattered X-ray energy resulting from a collision of the X-ray photon with the electrons in the sample. In this way, one can see the motion of the electron in a material. While this technique has been used extensively for analyzing materials in solid state, the present work opens new possibilities by using the technique to investigate the liquid state.

The data from the Compton scattering experiments was analyzed by the Boston and Princeton teams using sophisticated quantum mechanical models to adduce the persistence of a significant number of covalent bonds in liquid silicon.

“The conventional wisdom says that the bonds in a solid element are broken when the element changes to its liquid form,” Barbiellini-Amidei said. “It is highly significant that our study shows clearly that silicon retains covalent bonds even in its liquid form, hovering somewhere between a liquid and a solid on a molecular level.”

“This work has the potential to lead to improved and cost-effective semiconductors, computers and other technology in the future,” Barbiellini-Amidei said: “The experimental and theoretical techniques that we have developed in connection with this work may also impact advanced characterization of Li-battery and other technologically interesting problems using high energy X-rays as a probe.”

-Written by Sarah MacDonald