Discovery of a new quantum state analogous to water that does not freeze

Discovery of a new quantum state analogous to water that does not freeze

An exotic game of electrons

Cryostat used to reach temperatures up to 20 millikelvin. Credit: HZDR/Jürgen Jeibmann

Water that simply won’t freeze no matter how cold it gets – a research group involving the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has discovered a quantum state that could be described in this way.

Experts from the Institute for Solid State Physics at the University of Tokyo in Japan, Johns Hopkins University in the United States and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) in Dresden, in Germany, succeeded in cooling a material to a temperature close to absolute zero.

They discovered that a central property of atoms – their alignment – did not “freeze”, as usual, but remained in a “liquid” state. The new quantum material could serve as a model system for developing new, highly sensitive quantum sensors. The team presented their findings in the review Natural Physics.

At first glance, quantum materials look no different from normal substances, but they are certainly doing their own thing: Inside, electrons interact with unusual intensity, both with each other and with atoms in the crystal lattice. This intimate interaction results in powerful quantum effects that act not only at the microscopic scale, but also at the macroscopic scale.

Thanks to these effects, quantum materials exhibit remarkable properties. For example, they can conduct electricity without any loss at low temperatures. Often, even slight changes in temperature, pressure or electrical voltage are enough to drastically alter the behavior of the material.

In principle, magnets can also be considered as quantum materials; after all, magnetism is based on the intrinsic spin of electrons in the material. “In some ways, these spins can behave like a liquid,” explains Professor Jochen Wosnitza from the High Field Magnetic Laboratory (HLD) in Dresden at HZDR. “When temperatures drop, these messy rotations can then freeze, much like water freezes in ice.”

For example, some types of magnets, called ferromagnets, are non-magnetic above their “freezing point”, or more precisely their order point. Only when they sink below can they become permanent magnets.

High purity material

The international team intended to create a quantum state in which the atomic alignment associated with spins did not order itself even at ultra-cold temperatures, similar to a liquid that would not solidify even in extreme cold. . To achieve this state, the research group used a special material – a compound of the elements, praseodymium, zirconium and oxygen. They speculated that in this material, the properties of the crystal lattice would allow the spins of the electrons to interact with their orbitals around the atoms in a particular way.

“The prerequisite, however, was to have crystals of extreme purity and quality,” says Professor Satoru Nakatsuji from the University of Tokyo. It took several attempts, but the team finally succeeded in producing sufficiently pure crystals for their experiment: in a cryostat, a kind of super thermos, the experts gradually cooled their sample down to 20 millikelvin, or one fiftieth of degree above the absolute temperature. zero.

To see how the sample reacted to this cooling process and inside the magnetic field, they measured how much it changed in length. In another experiment, the group recorded how the crystal reacted to ultrasonic waves sent directly through it.

An intimate game

The result: “If the spins had been ordered, it should have caused an abrupt change in the behavior of the crystal, such as a sudden change in length,” describes HLD ultrasound expert Dr. Sergei Zherlitsyn. “Yet, as we observed, nothing happened! There was no sudden change in its length or its response to the ultrasound waves.”

The conclusion: The pronounced interaction of spins and orbitals had prevented order, which is why the atoms remained in their liquid quantum state – the first time such a quantum state had been observed. Further investigations in magnetic fields have confirmed this hypothesis.

This fundamental research result could also one day have practical implications: “At some point, we might be able to use the new quantum state to develop very sensitive quantum sensors”, speculates Jochen Wosnitza. “To do this, however, we still need to figure out how to consistently generate excitations in this state.”

Quantum sensing is considered a promising technology of the future. Because their quantum nature makes them extremely sensitive to external stimuli, quantum sensors can register magnetic fields or temperatures with far greater precision than conventional sensors.

More information:
Satoru Nakatsuji, Spin-orbital liquid state and liquid-gas metamagnetic transition on a pyrochlore lattice, Natural Physics (2022). DOI: 10.1038/s41567-022-01816-4

Provided by the Helmholtz Association of German Research Centers

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