Nat. Phys. when electrons slowly disappear during cooling ......
Many substances change their properties when they cool below a certain critical temperature: for example, water undergoes a phase transition when it freezes. However, phase transitions in certain metals do not exist in the macroscopic world. Such phase transitions occur because the special laws of quantum mechanics apply to the realm of nature's smallest building blocks.
It is believed that the concept of the electron as a quantized charge carrier no longer applies to these exotic phase transitions. Researchers at the University of Bonn and ETH Zurich have now found a way to prove this directly. Their findings shed new light on the exotic world of quantum physics.
The paper, titled "Critical slowing down near a magnetic quantum phase transition with fermionic breakdown," was published in the journal Nature Physics.
If you drop the temperature of water below zero degrees Celsius, it solidifies into ice. During this process, the properties of water change suddenly. For example, as ice, it is much less dense than when it was liquid: this is why icebergs float. In physics, this is known as a phase change.
But there are also phase transitions in which the characteristics of a substance change gradually. For example, if a ferromagnet is heated to 760 degrees Celsius, it loses its attraction to other pieces of metal: it is then no longer ferromagnetic, but paramagnetic. However, this does not happen suddenly, but continuously; the iron atoms behave like tiny magnets.
At low temperatures, they are parallel to each other. When heated, they fluctuate more and more around this stationary position until they are completely randomly aligned and the material loses its magnetic properties altogether. Thus, when a metal is heated, it may be both somewhat ferromagnetic and somewhat paramagnetic.
The phase change, so to speak, occurs gradually until finally all the iron is paramagnetic. During this process, the phase change becomes slower and slower. This behavior is characteristic of all continuous phase transitions. "We call it 'critical slowing down'." Prof. Dr. Hans Kroha of the Bett Center for Theoretical Physics at the University of Bonn explains, "The reason for this is that the two phases get closer and closer together energetically as successive transitions take place."
It's like putting a ball on a slope, and then the ball rolls down the slope, but the smaller the difference in height, the slower it rolls. When iron is heated, the energy difference between the two phases gets smaller and smaller, partly because the magnetization fades during the transition.
This "slowing down" is typical of phase transitions based on boson excitation. The bosons are the particles that "create" the interactions (e.g., magnetism is based on such interactions). Matter, on the other hand, is not made of bosons, but of fermions; electrons, for example, are fermions.
Phase transitions are based on the disappearance of particles (or phenomena triggered by particles). This means that the magnetism in iron becomes less and less as the number of atoms arranged in parallel decreases. "However, fermions cannot be destroyed according to the fundamental laws of nature and therefore do not disappear." Kroha explains, "This is why fermions are not usually involved in phase transitions."
A quasiparticle consisting of localized and moving electrons, here shattered by an ultrashort pulse of light.
Electrons can be bound in atoms; this gives them a fixed position from which they cannot leave. On the other hand, some electrons in metals are free to move, which is why these metals also conduct electricity. In some exotic quantum materials, these two types of electrons can form a superposition state.
-- which are called quasiparticles.
In a sense, they are both immobile and movable at the same time: this is only possible in the quantum world. Unlike "normal" electrons, these quasiparticles can be destroyed during phase transitions: this means that the properties of continuous phase transitions, and in particular the "critical deceleration", can also be observed here.
Exploring fermionic quantum criticality through time-resolved terahertz reflectivity
Phase diagram of field-tuned YbRh2Si2
Previously, scientists could only infer this effect indirectly through experiments. Researchers led by theoretical physicist Hans Kroha and the experimental group of Manfred Fiebig at ETH Zurich have now developed a new method for directly identifying quasiparticle collapse during phase transitions, and in particular the associated critical deceleration.
Sharing their findings, Kroha said, "This allows us to show directly for the first time that this deceleration may also occur in fermions."
The results contribute to a better understanding of phase transitions in the quantum world. In the long run, the findings may also help in the application of quantum information technology.
Reference link:
[1]https://www.nature.com/articles/s41567-023-02156-7
[2]https://phys.org/news/2023-07-electrons-slowly-cooling-effect-unique.html
[3]https://www.eurekalert.org/news-releases/997212
[4]https://www.spacedaily.com/reports/Unveiling_the_Quantum_Enigma_Phase_Transitions_in_Metals_999.html
