Puzzling quantum phenomenon: Electrons slowly disappear during cooling
Puzzling quantum phenomenon: Electrons slowly disappear during cooling
Puzzling quantum phenomenon: Electrons slowly disappear during cooling.
Many substances change their properties when cooled below a certain critical temperature.
This phase transition occurs, for example, when water freezes. In some metals, however, there are phase transitions that do not exist in the macroscopic world. T
his phase transition occurs because the special laws of quantum mechanics apply to the realm of nature’s smallest building blocks.
It was thought that the concept of electrons as quantized charge carriers no longer applied to these exotic phase transitions.
Consists of localized and mobile electrons, here shattered by ultrashort light pulses. Image Source: University of Bonn
Scientists have now found a way to directly prove this.
Their discovery sheds new light on the bizarre world of quantum physics. Researchers from the University of Bonn and ETH Zurich published the paper in the journal Nature Physics.
Understanding Phase Transitions
If you lower the temperature of water below zero degrees Celsius (32 degrees Fahrenheit), it will freeze into ice.
During this process, the properties of the water change suddenly. For example, as ice, it is much less dense than in a liquid state.
This is why ice cubes and icebergs float. In physics, this is called a phase transition.
But there are also phase transitions in which the properties of a substance change gradually.
For example, if you heat an iron magnet to 760 degrees Celsius (1,400 degrees Fahrenheit), it loses its attraction to other pieces of metal — at which point it’s no longer ferromagnetic, but paramagnetic.
However, this did not happen suddenly, but continuously: the iron atoms behaved like tiny magnets.
At low temperatures, they run parallel to each other.
When heated, they fluctuate more and more around this resting position until they are completely randomly arranged and the material loses its magnetism altogether.
Therefore, when a metal is heated, it can be both somewhat ferromagnetic and somewhat paramagnetic.
Prof. Dr. Hans-Kroha with students. image source: Bernadett Yehdou/University of Bonn
Matter particles cannot be destroyed
It can be said that the phase transition occurs gradually until finally all iron is paramagnetic.
During this process, the phase transition occurs at a slower and slower rate. This behavior is characteristic of all continuous phase transitions.
“We call this ‘critical slowing’,” explains Prof. Dr. Hans-Kroha from the Beite Center for Theoretical Physics at the University of Bonn. “The reason is that as successive transitions occur, the two phases get closer and closer energetically.”
It’s like putting a ball on a slope: the ball will then roll downhill, but the smaller the difference in height, the slower the roll.
As the iron is heated, the energy difference between the two phases becomes smaller and smaller, partly because the magnetization fades away during the transition.
This “slowing down” is typical of phase transitions based on bosonic excitations. Bosons are particles that “create” interactions (for example, magnetism is based on such interactions).
Matter, on the other hand, is not made of bosons, but fermions. For example, electrons are fermions.
Phase transitions are based on the disappearance of particles (or phenomena induced by particles).
This means that iron becomes less and less magnetic as the number of atoms aligned in parallel decreases.
“However, fermions cannot be destroyed according to fundamental laws of nature and therefore cannot disappear,” Kroha explained. “That’s why, normally, they never participate in phase transitions.”
Electrons become quasiparticles
Electrons can be bound in atoms; this way they have a fixed position and 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 electrons can form a superposition state.
These are so-called quasiparticles. In a sense, they are both immobile and movable at the same time — something that 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, especially the critical deceleration, are also observed here.
So far, this effect has only been observed indirectly in experiments.
Researchers led by theoretical physicist Hans Kroha and the experimental group of Manfred Fiebig at ETH Zurich have now developed a new method to directly identify Quasiparticle collapse at phase transitions, in particular the associated critical deceleration.
Kroha is also a member of the interdisciplinary research area “Matter” at the University of Bonn and the German Research Foundation “Matter and Light for Quantum Computing” cluster of excellence. This achievement contributes to a better understanding of phase transitions in the quantum world.
In the long run, these findings may also contribute to the application of quantum information technology.
