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The behavior of so-called “strange metals” has long puzzled scientists, but a group of researchers at the University of Toronto may be on the verge of understanding these materials.
Electrons are discrete subatomic particles that flow through wires like water molecules flowing through a pipe. The flow is known as electricity, and it’s harnessed to power and control everything from light bulbs to the Large Hadron Collider.
In quantum matter, on the other hand, electrons do not behave as they do in normal materials. They are much more powerful, and the four fundamental properties of electrons – charge, spin, orbit and lattice – intertwine, resulting in complex states of matter.
“In quantum matter, electrons lose their character as particles and exhibit strange collective behavior,” says condensed matter physicist Arun Paramekanti, a professor in the University of Toronto’s Department of Physics in the Faculty of Arts and Science. . “These materials are known as non-Fermi liquids, in which the simple rules break down.”
Now, three researchers from the university’s physics department and the Quantum Information and Quantum Control Center (CQIQC) have developed a theoretical model describing the interactions between subatomic particles in non-Fermi liquids. The framework expands existing models and will help researchers understand the behavior of these “strange metals.”
Their research was published in the journal Proceedings of the National Academy of Sciences (PNAS). The main author is Ph.D. in physics. student Andrew Hardy, with co-authors Paramekanti and post-doctoral researcher Arijit Haldar.
“We know that the flow of a complex fluid like blood through arteries is much harder to understand than water through pipes,” says Paramekanti. “Similarly, the flow of electrons in non-Fermi liquids is much more difficult to study than that of simple metals.”
Hardy adds: “What we have done is build a model, a tool, to study the behavior of non-Fermi liquids. And more specifically, to deal with what happens when there is symmetry breaking, when there is a phase transition to a new type of system.”
“Symmetry breaking” is the term used to describe a fundamental process that occurs throughout nature. Symmetry breaks when a system – whether it is a drop of water or the entire universe – loses its symmetry and homogeneity and becomes more complex.
For example, a water drop is symmetrical regardless of its orientation – rotate it in any direction and it will look the same. But its symmetry is broken when it undergoes a phase transition and freezes into an ice crystal. As a snowflake, it is still symmetrical but only in six different directions.
The same thing happened with all subatomic particles and forces after the Big Bang. With the explosive birth of the cosmos, all particles and forces were the same, but the breaking of symmetry immediately transformed them into the multiple particles and forces we see in the cosmos today.
“Symmetry breaking in non-Fermi liquids is much more complicated to study because there is no comprehensive framework for working with non-Fermi liquids,” says Hardy. “Describing how this symmetry breaking occurs is difficult to do.”
In a non-Fermi liquid, the interactions between electrons become much stronger when the particles are on the verge of symmetry breaking. As with a ball sitting on top of a hill, a very gentle nudge in either direction will send it flying in opposite directions.
The new research provides insight into these transitions in non-Fermi liquids and could lead to new ways to tune and control the properties of quantum materials. Although it remains a serious challenge for physicists, the work is important for new quantum materials that could shape the next generation of quantum technology.
These technologies include high-temperature superconductors that achieve zero resistance at temperatures much closer to room temperature, making them much more practical and useful. There are also graphene devices, technologies based on thick layers of single-atom carbon atoms that have a myriad of electronic applications.
“Quantum materials exhibit both unusual electron flow and complex types of symmetry breaking that can be controlled and tuned,” says Hardy. “It’s exciting for us to be able to make theoretical predictions for such systems that can be tested in new laboratory experiments.”
Andrew Hardy et al, Nematic Phases and Elastoresistivity of a Non-Fermi Multiorbital Liquid, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2207903120
Proceedings of the National Academy of Sciences
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