Whether they are magnets or superconductors, materials are known for their various properties. However, these properties may change spontaneously under extreme conditions. Researchers at Technische Universität Dresden (TUD) and Technische Universität München (TUM) have discovered an entirely new type of these phase transitions. It exhibits the phenomenon of quantum entanglement involving many atoms, previously observed only in the world of a few atoms. The results were recently published in the scientific journal temper nature.
New quantum cat fur
In physics, Schrödinger’s cat is an allegory of two of the most amazing effects of quantum mechanics: entanglement and superposition. Researchers from Dresden and Munich observed these behaviors on a much larger scale than those of the smallest particles. So far, it is known that materials that display properties, such as magnetism, have so-called fields – islands in which the properties of materials are homogeneous either of one type or of a different type (imagine that they are either black or white, for example).
Consider lithium holmium fluoride (LiHoF .).4), physicists have now discovered an entirely new phase shift in which domains surprisingly exhibit quantum mechanics features, causing their properties to become entangled (being black and white at the same time). “Our quantum cat now has new fur because we discovered a new quantum phase transition in LiHoF.”4 “It was previously unknown that they existed,” says Matthias Voeta, head of theoretical solid-state physics at TUD.
Phase transitions and entanglement
We can easily notice the spontaneously changing properties of a substance if we look at water – at 100 ° C it evaporates into gas, at 0 ° C it freezes into ice. In both cases, these new states of matter are formed as a result of a phase transition where water molecules rearrange themselves, thus changing the properties of the material. Properties such as magnetism or superconductivity appear as a result of transition electrons in crystals. For phase transitions at temperatures approaching absolute zero at −273.15 °C, quantum mechanical effects such as entanglement and quantum phase transitions come into play.
“Although there has been more than 30 years of extensive research devoted to phase transitions in quantum materials, we previously hypothesized that the phenomenon of entanglement played a role only on a microscopic scale, involving only a few atoms at a time,” explains Christian Pflederer, Professor of Topology. TUM’s Interconnected Systems.
Quantum entanglement is a state in which entangled quantum particles coexist in a common superposition that typically allows for mutually exclusive properties (eg, black and white) at the same time. As a rule, the laws of quantum mechanics apply only to microscopic particles. The research team from Munich and Dresden has now succeeded in observing the effects of quantum entanglement on a much larger scale, that is, the effect of thousands of atoms. For this, they chose to work with the well-known compound LiHoF4.
Spherical samples allow accurate measurements
At very low temperatures, LiHoF4 It works as a ferromagnet as all magnetic moments automatically point in the same direction. If you then apply a magnetic field exactly perpendicular to the preferred magnetic direction, the magnetic moments will change their direction, which is known as fluctuations. The stronger the magnetic field, the stronger these fluctuations become, until the ferromagnetism completely disappears in a quantum transition. This leads to the entanglement of adjacent magnetic moments. “If you raise LiHoF4 Sample to a very strong magnet, suddenly it stops being magnetic automatically. This has been known for 25 years,” Vojta says.
What’s new is what happens when the direction of the magnetic field changes. “We discover that a quantum phase transition continues to occur, while it was previously thought that even the smallest tilt in the magnetic field would dampen it immediately,” explains Pfleiderer. However, under these conditions, it is not individual magnetic moments, but rather large-scale magnetic regions, the so-called ferromagnetic fields, that undergo these quantum phase transitions. The spheres form whole islands of magnetic moments that point in the same direction.
Andreas Wendel, who conducted the experiments as part of his doctoral thesis, adds: “We used spherical samples for our precise measurements. This enabled us to precisely study the behavior of small changes in the direction of the magnetic field.”
From basic physics to applications
“We have discovered an entirely new type of quantum phase transition where entanglement occurs on the scale of several thousand atoms rather than just a small world for just a few of them,” Vojta explains. “If you imagine the magnetic fields as a black and white pattern, the new phase transition causes either the white or the black regions to become very small, that is, to create a quantum pattern, before completely melting.” The newly developed theoretical model successfully explains the data obtained from the experiments.
“For our analysis, we generalized the existing microscopic models and also took into account the reactions from large ferromagnetic fields to the microscopic properties,” says Heike Eisenlohr, who performed the calculations as part of her PhD. hypothesis.
The discovery of new quantum phase transitions is important as a basis and general frame of reference for research into quantum phenomena in materials, as well as for new applications. “Quantum entanglement is being applied and used in technologies such as quantum sensors and quantum computers, among other things,” Vojta says. Pfleiderer adds, “Our work is in basic research, which, however, can have a direct impact on the development of practical applications, if you use the properties of materials in a controlled way.”
The velocity limits of quantum phenomena are extended to objects of total size
Andreas Wendel et al., The emergence of mesoscale quantum phase transitions in ferromagnets, temper nature (2022). DOI: 10.1038 / s41586-022-04995-5
Presented by Dresden University of Technology
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