Quantum Oddity: Gradual Disappearance of Electrons During Chilling

Comprised of stationary and roaming electrons, which are disrupted by an extremely quick light pulse. Courtesy of the University of Bonn.

A quantum world effect has been observed by scientists, an effect that doesn’t exist in larger scales.

In a joint study by the University of Bonn and ETH Zurich, they explored the intriguing phase transitions in certain metals. The study enhances our understanding of quantum physics and could propel progress in the field of quantum information technology.

Numerous substances alter their properties when cooled below a specific threshold temperature. An example is the phase transition that happens when water freezes. But some metals undergo phase transitions that aren’t found in the macroscopic world. These occur due to the exceptional laws of quantum mechanics that govern the universe’s smallest elements. The traditional view of electrons as the carriers of quantized electric charge is believed to cease to apply near these unique phase transitions.

Researchers have now discovered a direct method to validate this. This gives fresh insights into the enigmatic world of quantum physics. The study conducted by researchers from the University of Bonn and ETH Zurich has been published in Nature Physics journal.

Understanding Phase Transitions

When you cool water below zero degrees Celsius (32 degrees Fahrenheit), it turns into ice. This transition comes with a sudden change in its properties. For instance, as ice, it’s less dense than its liquid state, allowing ice cubes and icebergs to float. This is what physics terms a phase transition.

There are also phase transitions that induce a gradual change in the characteristic attributes of a substance. For instance, if you heat an iron magnet up to 760 degrees Celsius (1,400 degrees Fahrenheit), it loses its magnetic pull to other metals – transitioning from being ferromagnetic to paramagnetic. This transition is not sudden, but continuous. The iron atoms act like tiny magnets.

At colder temperatures, these atoms are parallelly aligned. When heated, they start fluctuating more around this rest position until they are randomly oriented, resulting in complete loss of the material’s magnetism. Therefore, as the metal is heated, it can be partially ferromagnetic and partially paramagnetic.

Prof. Dr. Hans Kroha with students. Courtesy of Bernadett Yehdou/University of Bonn

Matter Particles Are Indestructible

This phase transition is a gradual process until all the iron finally becomes paramagnetic. The transition slows down progressively, a common trait of all continuous phase transitions.

Prof. Dr. Hans Kroha of the Bethe Center for Theoretical Physics at the University of Bonn, explains this as ‘critical slowing down’. This is because the energy levels of the two phases progressively become closer in continuous transitions.

This can be likened to placing a ball on a slope: It rolls down, but the lesser the height difference, the slower it rolls. As iron is heated, the energy gap between the phases shrinks, partly due to the gradual disappearance of magnetization during the transition.

This ‘slowing down’ is typical for phase transitions that involve the excitation of bosons, particles that drive interactions like magnetism. Matter, however, consists of fermions, not bosons, with electrons being a part of the fermions.

Phase transitions happen because particles (or the phenomena they trigger) vanish. This means that as fewer atoms align in parallel, the magnetism in iron reduces. “Fermions, due to fundamental natural laws, cannot be destroyed and hence cannot vanish,” Kroha clarifies. “This is why they usually don’t partake in phase transitions.”

Electrons Evolve Into Quasiparticles

Electrons can be bound within atoms, where they have a fixed position they can’t abandon. Conversely, some electrons in metals are freely mobile – enabling these metals to conduct electricity. In certain exotic quantum materials, both types of electrons can form a superposition state, creating what’s known as quasiparticles. They possess the quantum world’s unique feature of being immobile and mobile at the same time.

Unlike regular electrons, these quasiparticles can vanish during a phase transition. This means the properties of a continuous phase transition, particularly, critical slowing down, can also be observed there.

Previously, this effect could only be observed indirectly. A team of researchers led by theoretical physicist Hans Kroha and experimental group under Manfred Fiebig at ETH Zurich have now developed a new technique that allows direct detection of quasiparticle collapse during a phase transition, particularly the associated critical slowing down.

“Direct evidence that such a slowdown can also occur in fermions has been provided for the first time through this method,” says Kroha, who is also part of the Transdisciplinary Research Area “Matter” at the University of Bonn and the Cluster of Excellence “Matter and Light for Quantum Computing” of the German Research Foundation. This result contributes to a better understanding of quantum world phase transitions. In the long term, these findings might also be valuable for quantum information technology applications.

Reference: “Critical slowing down near a magnetic quantum phase transition with fermionic breakdown” by Chia-Jung Yang, Kristin Kliemt, Cornelius Krellner, Johann Kroha, Manfred Fiebig and Shovon Pal, 31 July 2023, Nature Physics.
DOI: 10.1038/s41567-023-02156-7

This research was a collaborative effort between ETH Zurich and the University of Bonn and received funding from the Swiss National Science Foundation (SNF) and the German Research Foundation (DFG).

Frequently Asked Questions (FAQs) about Quantum Phase Transitions

More about Quantum Phase Transitions

• University of Bonn
• ETH Zurich
• Nature Physics Journal
• Swiss National Science Foundation (SNF)
• German Research Foundation (DFG)
• Bethe Center for Theoretical Physics
• Introduction to Quantum Mechanics
• Understanding Phase Transitions