A new study finds that “quantum critical points” in exotic electronic materials can act much like polarizing “hot button issues” in an election. On either side of a quantum critical point, electrons fall into line and behave as traditionally expected, but at the critical point itself, traditional physical laws break down.
“The beauty of the quantum critical point is that even though it’s only one point along the zero temperature axis, what happens at that point dictates how electrons will interact in the material under a broad set of physical conditions,” said study co-author Qimiao Si, a theoretical physicist at Rice University. The new study involved “heavy-fermion metals,” magnetic materials with many similarities to high-temperature superconductors.
Flowing electrons power all the lights, computers and gadgets that are plugged into the world’s energy grids, and physicists have spent more than a century describing how these electrons behave. But long-standing theories that describe how electrons interact in traditional metals and semiconductors have yet to explain the strange electronic properties of heavy-fermion metals, human-made composites that contain precise atomic arrangements of transition metals and rare earth elements.
In the new study, Si collaborated with a group of experimental physicists led by Frank Steglich at the Max Planck Institute for Chemical Physics of Solids. The researchers examined several physical properties at extremely cold temperatures — some as much as 10 times colder than any such previous measurements — to show exactly how the standard theory of electron correlations in metals breaks down at the quantum critical point (QCP). That theory, Landau’s Fermi liquid theory, was first introduced in 1956.
“By measuring the ratio of the thermal to electrical transport near the QCP in one of the most-studied heavy-fermion metals — ytterbium dirhodium disilicide — we found a breakdown in the fundamental concepts of Landau-Fermi liquid theory,” said Steglich, the founding director of the Max Planck Institute for Chemical Physics of Solids.
Quantum particles come in two main varieties — bosons and fermions. Bosons are the quantum equivalent of extroverts; they enjoy one another’s company and can occupy the same quantum space. Fermions are the opposite; no two can occupy the same quantum space, and this defines much of their behavior.
Electrons are fermions, and their tendency to seek quantum elbow room affects the way they organize. It’s important for scientists to understand how they behave in concert because even a small electric current in a tiny wire involves billions upon billions of individual electrons.
Landau-Fermi liquid theory is a mathematical system that allows physicists to describe the actions of many billions of electrons with just a handful of variables. Landau’s vehicle for collapsing the actions of so many particles is something he dubbed a “quasiparticle,” a placeholder that acts like an individual but describes the collective fate of many physical particles.