A new revolution in the field of quantum materials

Materials, whether they are magnets or superconductors, are known for their diverse qualities. Moreover, under harsh environments, these qualities may suddenly change. Scientists from Dresden Technical University (TUD) and the Technical University of Munich (TUM) have identified an entirely new type of phase transition.

Image Credit: Technical University

The authors demonstrate the phenomenon of quantum entanglement involving many atoms, which was previously observed only in the world of a few atoms. The results have just been published in the prestigious scientific journal temper nature.

New quantum cat fur

In physics, Schrödinger’s cat is an analogy to two of the most amazing effects of quantum mechanics: entanglement and superposition.

Scholars from Dresden and Munich have now witnessed these characteristics on a much larger scale.

So far, it is known that materials with features such as magnetism have so-called domains – islands in which the features of matter are uniform of one or a distinct type (imagine that they are either black or white, for example).

Use of lithium holmium fluoride (LiHoF .).4) as a model, the researchers identified a very new transitional phase in which surprise domains exhibit quantum mechanical capabilities, culminating in the entanglement of their features (being black and white at the same time).

Our quantum cat now has new fur because we discovered a new quantum phase transition in LiHoF4 which were not previously known to exist.

Matthias Voeta, Chair, Theoretical Solid State Physics, Technical University of Dresden

Phase transitions and entanglement

When people look at water, one can see how its properties spontaneously change: at 100 ° C, it turns into gas, and at zero ° C, it freezes into ice. In both cases, these new states of matter appear due to a phase shift where the water molecules rearrange themselves, changing the properties of the material.

Electrons undergoing phase changes in crystals provide properties such as magnetism and superconductivity. Quantum mechanical effects such as entanglement come into play during phase changes at temperatures close to absolute zero at −273.15 °C, and are referred to as quantum phase transitions.

Although there has been more than 30 years of extensive research devoted to phase transitions in quantum materials, we previously hypothesized that entanglement played a role only on a microscopic scale, involving only a few atoms at a time..

Christian Pflederer is Professor of Topology of Interconnected Systems, Technical University of Munich

Quantum entanglement is one of the most amazing physical phenomena, in which entangled quantum particles reside in a common superposition, allowing naturally incompatible qualities (for example, black and white) to occur concurrently. The laws of quantum mechanics generally apply only to small particles.

Research groups in Munich and Dresden have now observed the effects of quantum entanglement on a much larger scale, the effect of thousands of atoms. The researchers chose to work with the well-known chemical LiHoF4 For this.

Spherical samples enable accurate measurements

LiHoF4 behaves like a ferromagnet at extremely low temperatures, as all magnetic moments automatically point in the same direction. When a magnetic field is applied exactly perpendicular to the desired magnetic direction, the magnetic moments will change direction, causing variations. The higher the magnetic field, the greater the fluctuations until the ferromagnetism disappears at the quantum phase transition. This causes the surrounding magnetic moments to become entangled.

If you carry LiHoF4 Sample to a very strong magnet, suddenly it stops being magnetic automatically. This has been known for 25 years‘, summarizes Vojta.

What’s new is what happens when the direction of the magnetic field changes.

We discovered that a quantum phase transition continues to occur, while it was previously thought that even the smallest tilt of the magnetic field would dampen it immediately.Pfleiderer explains.

However, under these conditions, magnetic fields – large magnetic spots – rather than individual magnetic moments experience quantum phase transitions. Domains are islands of magnetic moments, all oriented in the same direction.

We used spherical samples for our exact measurements. This enables us to study precisely the behavior when small changes in the direction of the magnetic field occurAdds Andreas Wendel, who led the experiments as part of his doctoral thesis.

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 microcosm of a few atoms.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, i.e. creating a quantum pattern, before completely decomposing. “

The recently created theoretical model satisfactorily explains the experimental data.

For our analysis, we generalized the existing microscopic models and also considered the reactions of large magnetic fields on the microscopic propertiesExplains Heike Eisenlohr, who performed the calculations as part of her Ph.D. hypothesis.

New applications and the basis for the study of quantum processes in materials are made possible by the discovery of new quantum phase transitions.

Quantum entanglement is applied and used in technologies such as quantum sensors and quantum computers, among others‘ says Vojta.

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 manner.. “

magazine reference

Wendell, A.; and others. (2022) Appearance of mesoscale quantum phase transitions in ferromagnets. temper nature. doi.org/10.1038/s41586-022-04995-5.