Be it magnets or
superconductors: materials are known for their various properties. However,
these properties may change spontaneously under extreme conditions. Researchers
at the Technische Universität Dresden (TUD) and the Technische Universität
München (TUM) have discovered an entirely new type of such phase transitions.
They display the phenomenon of quantum entanglement involving many atoms, which
previously has only been observed in the realm of few atoms. The results were
recently published in the scientific journal Nature.
In physics, Schroedinger’s cat is an
allegory for two of the most awe-inspiring effects of quantum mechanics:
entanglement and superposition. Researchers from Dresden and Munich have now
observed these behaviors on a much larger scale than that of the smallest of
particles. Until now, materials that display properties like, e.g., magnetism
have been known to have so-called domains — islands in which the materials
properties are homogeneously either of one or a different kind (imagine them
being either black or white,
for example). Looking at lithium holmium fluoride (LiHoF4), the physicists have now discovered a completely new
phase transition, at which the domains surprisingly exhibit quantum mechanical
features, resulting in their properties 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,” comments Matthias Vojta, Chair of Theoretical
Solid State Physics at TUD.
Phase
transitions and entanglement
We can easily observe the spontaneously
changing properties of a substance if we look at water: at 100 degrees Celsius
it evaporates into a gas, at zero degrees Celsius it freezes into ice. In both
cases, these new states of matter form as a consequence of a phase transition
where the water molecules rearrange themselves, thus changing the
characteristics of the matter. Properties like magnetism or superconductivity
emerge as a result of electrons undergoing phase transitions in crystals. For
phase transitions at temperatures approaching the absolute zero at -273.15
degrees Celsius, quantum mechanical effects such as entanglement come into
play, and one speaks of 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,” explains
Christian Pfleiderer, Professor of Topology of Correlated Systems at the TUM.
Quantum entanglement is one of the most
astonishing phenomena of physics, where the entangled quantum particles exist
in a shared superposition state that allows for usually mutually exclusive
properties (e.g., black and white) to occur simultaneously. As a rule, the laws
of quantum mechanics only apply to microscopic particles. The research teams
from Munich and Dresden have now succeeded in observing effects of quantum
entanglement on a much larger scale, that of thousands of atoms. For this, they
have chosen to work with the well-known compound LiHoF4.
Spherical
samples enable precision measurements
At very low temperatures, LiHoF4 acts as a ferromagnet where all magnetic moments
spontaneously point in the same direction. If you then apply a magnetic field
exactly vertically to the preferred magnetic direction, the magnetic moments
will change direction, which is known as fluctuations. The higher the magnetic
field strength, the stronger these fluctuations become, until, eventually, the
ferromagnetism disappears completely at a quantum phase transition. This leads
to the entanglement of neighboring magnetic moments. “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 new is what happens when you
change the direction of the magnetic field. “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,” explains Pfleiderer. Under these conditions, however, it is not individual
magnetic moments but rather extensive magnetic areas, so-called ferromagnetic
domains, that undergo these quantum phase transitions. The domains constitute
entire islands of magnetic moments pointing 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 conducted 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 newly developed theoretical model successfully
explains the data obtained from the experiments. “For our analysis, we
generalized existing microscopic models and also took into account the feedback
of the large ferromagnetic domains to the microscopic properties,” elaborates
Heike Eisenlohr, who performed the calculations as part of her PhD thesis.
The discovery of the new quantum phase transitions is important as a foundation and general frame of reference for the research of quantum phenomena in materials, as well as for new applications. “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.”
Journal article: https://www.nature.com/articles/s41586-022-04995-5
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