Use pressure to make liquid magnetism

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Artist’s rendering of frustrated electronic spins when the sample of magnetic material is pressurized into a spin liquid state. Credit: Daniel Haskel

Using two flat-topped diamonds and a lot of pressure, the scientists forced a magnetic crystal into a spinning liquid state, which can lead to information about high-temperature superconductivity and quantum computing.

It sounds like a riddle: what do you get if you take two small diamonds, place a small magnetic crystal between them, and squeeze them together very slowly?

The answer is a magnetic liquid, which seems counterintuitive. Liquids become solid under pressure, but usually not the other way around. But this unusual, crucial discovery, unveiled by a team of researchers working at the Advanced Photon Source (APS), a user facility from the U.S. Department of Energy’s (DOE) office of science at the DOE’s Argonne National Laboratory , could provide scientists with a new insight into high-tech. superconductivity in temperature and quantum computation.

Although scientists and engineers have used superconducting materials for decades, the exact process by which high-temperature superconductors conduct electricity without resistance remains a mystery in quantum mechanics. The telltale signs of a superconductor are loss of resistance and loss of magnetism. High temperature superconductors can operate at higher temperatures than liquid nitrogen (-320 degrees Fahrenheit), which makes them attractive for lossless transmission lines in power grids and other applications in the power sector.

But no one really knows how high temperature superconductors achieve this state. This knowledge is needed to increase the operating temperature of these materials to room temperature, which would be necessary for the large-scale implementation of superconductors in energy efficient power grids.

“A quantum spin liquid is a superposition of spin states, fluctuating but entangled. It’s fair to say that this process, if it creates a quantum spin liquid with quantum superposition, will have made a qubit, the basic building block of a quantum computer. – Daniel Haskel, physicist and group leader, XSD

An idea put forward in 1987 by the late theorist Phil Anderson of Princeton University involves putting materials into a liquid state of quantum spin, which Anderson says could lead to high-temperature superconductivity. The key lies in the spins of the electrons in each of the material’s atoms, which under certain conditions can be pushed into a state where they become “frustrated” and unable to organize themselves into an orderly pattern.

To alleviate this frustration, the directions of the electron spins fluctuate over time, aligning with neighboring spins only for short periods of time, like a liquid. It is these fluctuations that can aid in the formation of electron pairs necessary for high temperature superconductivity.

The pressure provides a way to “tune” the separation between the spins of the electrons and drive a magnet into a frustrated state where the magnetism disappears at a certain pressure and a spin liquid emerges, according to Daniel Haskel, the physicist and group leader. in the Argonne X-ray Science Division (XSD) which led a research team through a series of experiments at APS to do just that. The team included Argonne assistant physicist Gilberto Fabbris and physicists Jong-Woo Kim and Jung Ho Kim, all from XSD.

Haskel is careful to say that his team’s results, recently published in Physical examination letters, do not conclusively demonstrate the quantum nature of the spin liquid state, in which atomic spins would continue to move even at absolute zero temperatures – more experiments would be needed to confirm this.

But they show that by applying slow and constant pressure, certain magnetic materials can be pushed into a liquid-like state, in which the electronic spins become disordered and the magnetism disappears, while preserving the crystalline arrangement of the atoms harboring them. electronic spins. . The researchers are convinced that they have created a spin liquid, in which the spins of the electrons are disordered, but are not sure whether these spins are entangled, which would be the sign of a quantum spin liquid.

If it is a quantum spin liquid, Haskel said, the possibility of creating one by this method would have far-reaching implications.

“Certain 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, if it creates a quantum spin liquid with quantum superposition, will have made a qubit, the basic building block of a quantum computer.

So what did the team do and how did they do it? This brings us back to diamonds, which are part of an experimental setup unique to APS. The researchers used two diamond anvils, cut similar to what you would see in jewelry stores, with a wide base and a narrower flat edge. They placed the smaller flat edges together, inserted a sample of magnetic material (in this case a strontium-iridium alloy) between them, and pushed.

“The idea is that when you pressurize it, it brings the atoms together,” Fabbris said. “And since we can do it slowly, we can do it continuously, and we can measure the properties of the sample as we build up pressure.”

When Fabbris says the pressure was applied slowly, he’s not kidding – each of those experiments took about a week, he said, using a sample about 100 microns in diameter, or the width of ‘a thin sheet of paper. Since the researchers did not know at which pressure the magnetism would disappear, they had to carefully measure with each very slight increase.

And they saw it disappear, at about 20 gigapascals, or the equivalent of 200,000 atmospheres, or about 200 times the pressure that can be found at the bottom of the Mariana Trench in the Pacific Ocean, the deepest pit on Earth. The spins of the electrons have remained correlated over short distances, like a liquid, but have remained disordered even at temperatures as low as 1.5 Kelvin (-457 degrees Fahrenheit).

The trick, Haskel said – and the key to creating a liquid spin state – was to preserve the crystal order and symmetry of the atomic arrangement, as the unwanted effect of the random disorder in atomic positions would have led to a different magnetic state, one without the unique properties of the spin liquid state. Haskel compares electron spins to neighbors a block away – as they get closer, they all want to make each other happy, changing their direction of rotation to match that of their neighbors. The goal is to bring them so close to each other that they cannot satisfy all of their neighbors, thus “frustrating” their rotational interactions, while maintaining the block structure.

The research team used the intense X-ray imaging capabilities of the APS to measure the magnetism of the sample, and according to Haskel and Fabbris, the APS is the only facility in the United States where such an experiment could be achieved. In particular, Fabbris said, the ability to focus on one type of atomignoring everyone else was crucial.

“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 the components of the diamond anvil cell,” Fabbris said. “The measurements we have taken are impossible without a light source like the APS. He is particularly capable of this.

Now that the team has reached a liquid state of rotation, what’s next? More experimentation is needed to see if a quantum spin liquid has been created. Future experiments will be to probe the nature of the dynamics and spin correlations more directly in the spin liquid state. But recent findings, Haskel said, pave the way for the realization of these elusive quantum states, a path that could lead to new knowledge about superconductivity and quantum information science.

Haskel also highlighted the APS upgrade, a massive project that will see the instrument’s brightness increase up to 1,000 times. This, he said, will allow much deeper probes into these fascinating states of matter.

“It is for everyone’s imagination to know what surprising effects of quantum mechanics are waiting to be discovered,” he said.

Reference: “Quantum paramagnetism possible in compressed Sr2IrO4By D. Haskel, G. Fabbris, JH Kim, LSI Veiga, JRL Mardegan, CA Escanhoela, Jr., S. Chikara, V. Struzhkin, T. Senthil, BJ Kim, G. Cao and J.-W. Kim, February 11, 2020, Physical examination letters.
DOI: 10.1103 / PhysRevLett.124.067201


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