International research team discovers novel quantum state – ScienceDaily

Water that just doesn’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 of Solid State Physics at the University of Tokyo in Japan, Johns Hopkins University in the USA and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) in Dresden succeeded in cooling a special material to a temperature near absolute zero. They found that a central property of atoms – their orientation – did not “freeze” as usual, but remained in a “liquid” state. The new quantum material could serve as a model system to develop novel, highly sensitive quantum sensors. The team presented their results in the journal natural physics.

At first glance, quantum materials do not look any different from normal substances – but they do their own thing: Inside, the electrons interact with unusual intensity, both with each other and with the atoms of the crystal lattice. This close interaction leads to strong quantum effects that act not only on a microscopic but also on a macroscopic scale. Thanks to these effects, quantum materials exhibit remarkable properties. For example, they can conduct electricity at low temperatures without any loss. Small changes in temperature, pressure or electrical voltage are often enough to drastically change the behavior of the material.

In principle, magnets can also be regarded as quantum materials; after all, magnetism is based on the intrinsic spin of the electrons in the material. “In a way, these spins can behave like a liquid,” explains Prof. Jochen Wosnitza from the Dresden High Magnetic Field Laboratory (HLD) at the HZDR. “As temperatures drop, these disordered spins can then freeze, much like water freezes into ice.” For example, certain types of magnets, called ferromagnets, are non-magnetic above their “freezing point,” or more specifically, their ordering point. Only when they fall below that do they become permanent magnets.

High purity material

The international team intended to create a quantum state in which the atomic alignment associated with the spins is not ordered even at ultra-cold temperatures – similar to a liquid that does not solidify even at extreme cold. To achieve this state, the research group used a special material – a combination of the elements praseodymium, zirconium and oxygen. They assumed that in this material the properties of the crystal lattice would allow the electron spins to have a special interaction with their orbitals around the atoms.

“The prerequisite, however, was crystals of extreme purity and quality,” explains Prof. Satoru Nakatsuji from the University of Tokyo. It took several attempts, but finally the team managed to produce crystals that were pure enough for their experiment: In a cryostat, a kind of super thermos flask, the experts gradually cooled their sample down to 20 millikelvin – just a fiftieth of a degree above that in absolute terms zero point. To see how the sample responded to this cooling process and within the magnetic field, they measured how much it changed in length. In another experiment, the group recorded how the crystal responded to ultrasonic waves sent directly through it.

An intimate interaction

The result: “If the spins had been ordered, there would have been an abrupt change in the behavior of the crystal, such as a sudden change in length,” says Dr. Sergei Zherlitsyn, HLD’s ultrasound scan expert. “But as we observed, nothing happened! There were no sudden changes in length or in its response to ultrasonic waves.” The conclusion: the pronounced interplay of spins and orbitals had prevented order, which is why the atoms remained in their liquid quantum state – such a quantum state had been observed for the first time. Further investigations in magnetic fields confirmed this assumption.

This basic research result could one day also have practical implications: “At some point we will be able to use the new quantum state to develop highly sensitive quantum sensors,” speculates Jochen Wosnitza. “However, we still have to find out how we can generate specific excitations in this state.” Quantum sensor technology is considered a promising technology of the future. Being extremely sensitive to external stimuli due to their quantum nature, quantum sensors can detect magnetic fields or temperatures far more accurately than conventional sensors.

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