Monthly Archives: February 2021

The high-pressure magnetic phase observed in iron trithiohypophosphate, a 2D material that changes from an insulator to a metal when compressed, likely forms a precursor to superconductivity.

Iron trithiohypophosphate (FePS₃), or magnetic graphene, belongs to a family of materials called van der Waals materials.

First synthesized in the 1960s, this 2D material is similar to graphene and can be “exfoliated” in ultra-thin layers. Unlike graphene however, FePS₃ is magnetic.

In 2018, Dr Siddharth Saxena of the Cavendish Laboratory and his colleagues established than FePS₃ becomes a metal at high pressure, and describes how the crystal structure and arrangement of atoms in the layers of this material changes during the transition.

In the new study, the researchers used new techniques to measure magnetic structure up to record pressures, using diamond anvils and specially designed neutrons to act as the magnetism probe.

They were then able to follow the evolution of magnetism to the metallic state.

“To our surprise, we found that the magnetism survives and is in some ways enhanced,” Dr Saxena said.

“This is unexpected, because the newly roving electrons in a newly conductive material can no longer be locked to their parent iron atoms, generating magnetic moments there, unless the conduction comes from an unexpected source.”

In their previous study, the scientists showed that these electrons were “frozen” in a sense. But when they made them sink or move, they started to interact more and more.

Magnetism survives, but changes into new forms, giving rise to new quantum properties in a new type of magnetic metal.

The behavior of a material, whether a conductor or an insulator, is primarily based on the way electrons, or charge, move. However, the “spin” of electrons has been shown to be the source of magnetism. The spinning causes the electrons to behave like tiny magnetic bars and point in a certain way.

“The combination of the two, charge and spin, is the key to the behavior of this material,” said Dr. David Jarvis, researcher at the Laue-Langevin Institute.

“Finding this kind of quantum multifunctionality is another leap forward in the study of these materials. “

“We don’t know exactly what’s going on at the quantum level, but at the same time, we can manipulate it,” Dr Saxena said.

“It’s like those famous ‘unknown unknowns’: we’ve opened a new door to the properties of quantum information, but we don’t yet know what those properties might be.”

“Now that we have an idea of ​​what happens to this material at high pressure, we can make predictions about what might happen if we try to adjust its properties by adding free electrons by compressing it further,” said the Dr Matthew Coak, researcher. at the Cavendish Laboratory and the University of Warwick.

“What we are looking for is superconductivity,” added Dr Saxena.

“If we can find a magnetism-related type of superconductivity in a 2D material, it could give us a chance to solve a problem that dates back decades.”

The results were published on February 5, 2021 in the journal Physical examination X.

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Matthew J. Coak et al. 2021. Emerging magnetic phases in the pressure-regulated van der Waals FePS antiferromagnetic₃. Phys. Rev. X 11 (1): 011024; doi: 10.1103 / PhysRevX.11.011024

From childhood, we are taught that the world exists in three physical dimensions. That’s true, for the most part, but it jumps on to something quite fascinating: the weird two-dimensional world of nanoscale materials, like the “wonderful material” of graphene.

Graphene and its engineering, single layer counterparts actually exist in three dimensions, though barely – sitting just on the fringes, atomically speaking. That’s because these so-called 2D materials are only one atom thick, embodying incredible structural finesse that gives them all kinds of weird powers.

We see it in the formidable strength of graphene and in the way it approaches superconductivity.

Things get even stranger when graphene makes friends: stack sheets of this two-dimensional material in a three-layered, three-atom-tall sandwich, and a rare form of magnetism is revealed.

Now in a new study Led by physicists from the University of Cambridge, the scientists achieved the same kind of magnetic feat with a different two-dimensional material called iron phosphorus trisulfide (FePS₃).

(University of Cambridge)

Above: Illustration of the magnetic structure of iron phosphorus trisulfide (FePS₃), a two-dimensional material that undergoes a transition from an insulator to a metal when compressed.

FePS₃ is not the same as graphene – which is made up of a single layer of carbon atoms – but it is often referred to as “magnetic graphene”, due to its mysterious capabilities at ultra-thin, layered dimensions .

In a previous study by some of the same researchers, the team found that when crushed FePS layers₃ were subjected to high pressure levels, the material went from an insulator, impeding the flow of electrons, to a metallic state where it became a conductor.

But researchers still haven’t fully understood what underlies the magnetic behavior of this pressurized “magnetic graphene”, as FePS was expected to be₃ would cease to be magnetic when it entered the metallic state.

“The missing piece remained however, the magnetism”, said quantum physicist Matthew Coak.

“In the absence of experimental techniques capable of probing the signatures of magnetism in this material at such high pressures, our international team had to develop and test our own new techniques to make this possible.”

According to the new research, FePS₃ retains its magnetism under extremely high pressure due to a newly discovered type of magnetism that still exists during the metallic phase.

“To our surprise, we found that the magnetism survives and is in some ways enhanced”, Explain Principal Investigator and Physicist Siddharth Saxena, Group Leader at the Cavendish Laboratory, Cambridge.

“This is unexpected, because the newly roving electrons in a newly conductive material can no longer be locked to their parent iron atoms, generating magnetic moments there – unless the conduction comes from an unexpected source.”

While we do not yet have all the answers to what is going on here, during compression, the “spin” of electrons in the material appears to be a source of magnetism – and the phenomenon can be adjusted depending on the pressure. FePS.₃ is subject to.

While the results contradict previous observations on how this material should behave, the surprises found here suggest that we may be able to tweak magnetic graphene and its ilk even further – potentially finding materials that support superconductivity due to of those exotic forms of magnetism that we do not yet know. fully understand.

“We don’t know exactly what’s going on at the quantum level, but at the same time, we can manipulate it”, Saxena says.

“It’s like those famous ‘unknown unknowns’: we’ve opened a new door to the properties of quantum information, but we don’t yet know what those properties might be.”

The results are reported in Physical examination X.

The researchers, led by the University of Cambridge, were able to monitor the conductivity and magnetism of iron thiophosphate (FePS3), a two-dimensional material that undergoes a transition from an insulator to a metal when compressed. This class of magnetic materials offers new avenues for understanding the physics of new magnetic states and superconductivity.

Using new high-pressure techniques, researchers have shown what happens to magnetic graphene during the transition from insulation to conductor and in its unconventional metallic state, achieved only under conditions of very high pressure. . When the material becomes metallic, it remains magnetic, which is contrary to the previous results and gives indications on the functioning of the electrical conduction in the metallic phase. The newly discovered high-pressure magnetic phase likely forms a precursor to superconductivity, so understanding its mechanisms is essential.

Their results, published in the journal Physical examination X, also suggest a way in which new materials could be designed to have combined conduction and magnetic properties, which could be useful in the development of new technologies such as spintronics, which could transform the way computers process information. .

The properties of matter can change dramatically with the change in dimensionality. For example, graphene, carbon nanotubes, graphite, and diamond all consist of carbon atoms, but have very different properties due to their different structure and dimensionality.

“But imagine if you could also change all of these properties by adding magnetism,” said first author Dr Matthew Coak, who is jointly based at the Cavendish Lab in Cambridge and the University of Warwick. “A material that could be mechanically flexible and form a new kind of circuit for storing information and performing calculations. This is why these materials are so interesting, and because they drastically change their properties when put under pressure. so that we can control their behavior. “

In a previous study by Sebastian Haines of the Cavendish Laboratory in Cambridge and the Department of Earth Sciences, researchers established that the material becomes a metal at high pressure and described how the crystal structure and arrangement of atoms in the layers of this 2D material change through the transition.

“The missing piece remained however, the magnetism,” Coak said. “In the absence of experimental techniques capable of probing the signatures of magnetism in this material at such high pressures, our international team had to develop and test our own new techniques to make this possible.”

The researchers used new techniques to measure the magnetic structure up to record pressures, using diamond anvils and specially designed neutrons to act as a probe for magnetism. They were then able to follow the evolution of magnetism to the metallic state.

“To our surprise, we discovered that magnetism survives and is in some ways enhanced,” co-author Dr Siddharth Saxena, group leader at the Cavendish Laboratory. “This is unexpected, because the newly roving electrons in a newly conductive material can no longer be locked to their parent iron atoms, generating magnetic moments there, unless the conduction comes from an unexpected source.”

In their previous article, the researchers showed that these electrons were “frozen” in a sense. But when they made them sink or move, they started to interact more and more. Magnetism survives, but changes into new forms, giving rise to new quantum properties in a new type of magnetic metal.

The behavior of a material, whether a conductor or an insulator, is primarily based on the way the electrons, or charge, move. However, the “spin” of electrons has been shown to be the source of magnetism. The spinning causes the electrons to behave like tiny magnetic bars and point in a certain way. Magnetism from the arrangement of electron spins is used in most memory devices: it is important to harness and control it to develop new technologies such as spintronics, which could transform the way computers process information.

“The combination of the two, charge and spin, is the key to the behavior of this material,” said co-author Dr David Jarvis of the Laue-Langevin Institute, France, who carried out this work as a basis. of his doctoral studies. at the Cavendish Laboratory. “Finding this kind of quantum multifunctionality is another leap forward in the study of these materials.”

“We don’t know exactly what’s going on at the quantum level, but at the same time, we can manipulate it,” Saxena said. “It’s like those famous ‘unknown unknowns’: we’ve opened a new door to the properties of quantum information, but we don’t yet know what those properties might be.”

There are more potential chemical compounds to be synthesized than could ever be fully explored and characterized. But by carefully selecting and adjusting materials with special properties, it is possible to pave the way for the creation of compounds and systems, but without having to apply enormous pressures.

In addition, gaining a fundamental understanding of phenomena such as low-dimensional magnetism and superconductivity enables researchers to take the next steps in materials science and engineering, with particular potential for efficiency. energy, production and storage.

As for the case of magnetic graphene, the researchers then plan to continue research into superconductivity within this unique material. “Now that we have an idea of ​​what happens to this material at high pressure, we can make predictions about what might happen if we try to adjust its properties by adding free electrons by compressing it further,” Coak said. .

“What we’re looking for is superconductivity,” Saxena said. “If we can find a magnetism-related type of superconductivity in a two-dimensional material, it could give us a chance to solve a problem that dates back decades.”