Artist’s impression of electron spins “frustrated” as the magnetic sample is forced into a spin liquid state. (Image Credit: Daniel Haskel)

 

Researchers at the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Argonne National Laboratory, inserted a magnetic crystal between two small flat-head diamonds and slowly squeezed them together to create a magnetic liquid. This study, published in Physics Review Letters, could pave the way to new insights on high-temperature superconductivity and quantum computing.

 

For decades, scientists and engineers have been using superconductive materials, but exactly how high-temperature superconductors conduct electricity without resistance is still a mystery. Loss of resistance and magnetism are usually revealing signs of a superconductor. High-temperature superconductors can function at temperatures that exceed those of liquid nitrogen (-320 degrees Fahrenheit), which makes them useful for lossless transmission lines in power grids and other applications in the energy sector.

 

Daniel Haskel, a physicist in Argonne’s X-ray Science Divison (XSD), lead a team through a number of experiments at APS to force a magnet into a spin liquid state by applying pressure to it. The pressure provides a way to adjust the separation between the electron spins, which causes the magnet to go into a “frustrated” state where magnetism disappears at a certain pressure, and thus, a liquid state is created.

 

Haskel also says the results do not demonstrate the quantum nature of the spin liquid state, wherein the atomic spins would keep moving even at absolute zero temperatures. To confirm this, the team will need to conduct additional experiments.

 

The results show that by applying a slow and steady amount of pressure, some materials can be driven into a liquid-like state. This causes the electron spins to become disordered, and the material loses its magnetism, and at the same time, the crystalline arrangement of the atoms hosting electron spins are preserved. The team is certain they have produced a spin liquid where the electron spins are disordered, but it’s unknown if the spins are entangled, which would suggest that it’s a quantum spin liquid.

 

Haskel says, “Some types of quantum spin liquids can enable error-free quantum computing,” Haskel said. “A quantum spin liquid is a superposition of spin states, fluctuating but entangled. It’s fair to say that this process, should it create a quantum spin liquid with quantum superposition, will have made a qubit, the basic building block of a quantum computer.”

 

The team used two diamond anvils and positioned the flat edges together, placed a strontium-iridium alloy, measuring 100 microns in diameter, between the two and squeezed them together. Each of the team’s experiments, which involved using APS’s intense X-ray imaging to measure the sample’s magnetism, took about a week since they needed to slowly apply pressure to the material. This was done slowly since the researchers didn’t know at which pressure magnetism would disappear, and as a result, they needed to measure it carefully as it slightly increased.

 

“The samples are very small, and if you try to measure magnetism with other techniques in a university lab, you will pick up the magnetic signal from components in the diamond anvil cell,” assistant physicist Gilberto Fabbris said. “The measurements we did are impossible without a light source like the APS. It is uniquely capable of this.”

 

Magnetism eventually disappeared at approximately 20 gigapascals, which is similar to 200,000 atmospheres. The electron spins stayed connected over short distances, much like a liquid. However, it was still disordered even in extremely cold temperates at 1.5 Kelvin (-457 degrees Fahrenheit)

The key to producing a spin liquid state was to preserve both the crystalline order and the symmetry of the atomic arrangement. Otherwise, the random disorder in atomic positions creates an undesirable effect, which leads to a different magnetic state.

 

More experiments will need to be conducted to determine if a quantum spin liquid has been produced. Future experiments will involve investigating the nature of spin dynamics and how it links to the spin liquid state. These results will pave the way to help understand quantum states, which could eventually lead to new-found knowledge on superconductivity and quantum information sciences. Haskel also mentioned the APS Upgrade, which will brighten the instruments up to 1,000 times. This will enable researchers to look deeply into these states of matter.

 

“It’s up to anyone’s imagination which surprising quantum mechanical effects are waiting to be discovered,” he said.

 

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