Physicists tangled up in quantum behavior of ferromagnets

Physicists, including researchers at Rutgers University, have unraveled a mystery behind the strange behavior of electrons in a ferromagnet, a finding that could eventually help develop novel high temperature superconductors. The physicists report their findings in a paper in Nature.

The Rutgers Center for Materials Theory, a world leader in the field, studies ‘quantum phase transitions’. Normal phase transitions, such as when ice melts, require heat to jiggle atoms and melt ice crystals. Quantum phase transitions, on the other hand, are driven by the jiggling of atoms and electrons from quantum fluctuations that never cease, even at low temperatures.

A quantum phase transition can be achieved by tuning a material to enhance quantum fluctuations, either by applying a magnetic field or exposing the material to intense pressure when the temperature is near absolute zero. In certain quantum phase transitions, the quantum fluctuations become infinitely intense, forming a ‘quantum critical point’.

These unusual states of matter are of great interest because of their propensity for forming superconductors. They are like an electronic ‘stem cell’, a form of matter that can transform itself in many ways.

Meanwhile, in the weird world of quantum mechanics, ‘entanglement’ allows something to be in two different states or places at the same time. Inside materials with electrons moving through them, entanglement often involves the spin of electrons, which can be simultaneously up and down.

Typically, only electrons near each other are entangled in quantum materials, but at a quantum critical point, the entanglement patterns can change abruptly, spreading out across the material and transforming it. Electrons, even distant ones, become entangled.

Ferromagnets are an unlikely setting for studying quantum entanglement because the electrons moving through them align in one direction instead of spinning up and down. But the physicists found that the ferromagnetism in a material known as ‘Cerge’ (CeRh₆Ge₄) must have a large amount of entanglement, with electrons that spin up and down and are connected with each other. That had never been seen before in ferromagnets.

"We believe our work connecting entanglement with the strange metal and ferromagnets provides important clues for our efforts to understand superconductors that work at room temperature," said co-author Piers Coleman, a professor in the Department of Physics and Astronomy at Rutgers University-New Brunswick. "As we learn to understand how nature controls entanglement in matter, we hope we'll develop the skills to control quantum entanglement inside quantum computers and to design and develop new kinds of quantum matter useful for technology."

Rutgers scientists have already used some of their findings to propose a new theory for a family of iron-based superconductors that were discovered about 10 years ago. "If we are right, these systems, like ferromagnets, are driven by forces that like to align electrons," Coleman said.

This story is adapted from material from Rutgers University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.