Materials, be magnets or superconductors, are known for their diverse qualities. Furthermore, under harsh environments, these qualities may change abruptly. Scientists from Technische Universität Dresden (TUD) and Technische Universität München (TUM) have identified a completely new type of phase transition.
The authors demonstrate the phenomena of quantum entanglement involving numerous atoms, which was previously observed only in the realm of a few atoms. The findings were just published in the prestigious scientific journal Nature.
New Fur for the Quantum Cat
In physics, Schroedinger’s cat is an analogy for two of quantum mechanics’ most awe-inspiring effects: entanglement and superposition.
Dresden and Munich scientists have now witnessed these characteristics on a much wider scale.
To date, materials with features such as magnetism have been known to have so-called domains—islands in which the material’s attributes are uniform of one or a distinct kind (imagine them being either black or white, for example).
Using lithium holmium fluoride (LiHoF4) as a model, researchers have identified a very new phase transition in which the domains’ surprise show quantum mechanical capabilities, culminating in their attributes becoming entangled (being black and white at the same time).
Our quantum cat now has a new fur because we’ve discovered a new quantum phase transition in LiHoF4 which has not previously been known to exist.
Matthias Vojta, Chair, Theoretical Solid State Physics, Technische Universität Dresden
Phase Transitions and Entanglement
When people look at water, one can see how its qualities change spontaneously: at 100 °C, it dissipates into a gas, and at zero degrees Celsius, it freezes into ice. In both situations, these new states of matter emerge because of a phase transformation in which the water molecules rearrange themselves, altering the properties of the matter.
Electrons experiencing phase changes in crystals provide properties such as magnetism and superconductivity. Quantum mechanical effects like entanglement come into play during phase changes at temperatures near absolute zero at −273.15 °C, and they are referred to as quantum phase transitions.
Even though there are more than 30 years of extensive research dedicated to phase transitions in quantum materials, we had previously assumed that the phenomenon of entanglement played a role only on a microscopic scale, where it involves only a few atoms at a time.
Christian Pfleiderer, Professor, Topology of Correlated Systems, Technische Universität München
Quantum entanglement is one of physics’ most amazing phenomena, in which entangled quantum particles dwell in a shared superposition state, allowing normally mutually incompatible qualities (e.g., black and white) to occur concurrently. The laws of quantum mechanics, in general, only apply to minuscule particles.
The Munich and Dresden research groups have now observed the impacts of quantum entanglement on a considerably bigger scale, that of thousands of atoms. Researchers chose to work with the well-known chemical LiHoF4 for this.
Spherical Samples Enable Precision Measurements
LiHoF4 behaves as a ferromagnet at very low temperatures, with all magnetic moments spontaneously pointing in the same direction. When a magnetic field is applied exactly vertically to the desired magnetic orientation, the magnetic moments will shift direction, causing variations. The larger the magnetic field, the greater the fluctuations until ferromagnetism vanishes at a quantum phase transition. This causes surrounding magnetic moments to become entangled.
“If you hold up a LiHoF4 sample to a very strong magnet, it suddenly ceases to be spontaneously magnetic. This has been known for 25 years,” summarizes Vojta.
What is novel is what occurs when the magnetic field’s direction is changed.
“We discovered that the quantum phase transition continues to occur, whereas it had previously been believed that even the smallest tilt of the magnetic field would immediately suppress it,” clarifies Pfleiderer.
However, under these conditions, ferromagnetic domains—large magnetic patches—rather than individual magnetic moments experience quantum phase transitions. The domains are islands of magnetic moments, all oriented in the same direction.
“We have used spherical samples for our precision measurements. That is what enabled us to precisely study the behavior upon small changes in the direction of the magnetic field,” adds Andreas Wendl, who led the experiments as part of his doctoral dissertation.
From Fundamental Physics to Applications
“We have discovered an entirely new type of quantum phase transitions where entanglement takes place on the scale of many thousands of atoms instead of just in the microcosm of only a few,” explains Vojta.
“If you imagine the magnetic domains as a black-and-white pattern, the new phase transition leads to either the white or the black areas becoming infinitesimally small, i.e., creating a quantum pattern, bevor dissolving completely.”
A recently established theoretical model satisfactorily explains the experimental data.
“For our analysis, we generalized existing microscopic models and also took into account the feedback of the large ferromagnetic domains to the microscopic properties,” explains Heike Eisenlohr, who accomplished the calculations as part of her Ph.D. thesis.
New applications and a foundation for studying quantum processes in materials have both been made possible by the discovery of novel quantum phase transitions.
“Quantum entanglement is applied and used in technologies like quantum sensors and quantum computers, amongst other things,” says Vojta.
Pfleiderer adds, “Our work is in the area of fundamental research, which, however, can have a direct impact on the development of practical applications, if you use the materials properties in a controlled way.”
Wendl, A., et al. (2022) Emergence of mesoscale quantum phase transitions in a ferromagnet. Nature. doi.org/10.1038/s41586-022-04995-5.