AI used to show how hydrogen becomes a metal inside giant planets

Dense metallic hydrogen — a phase of hydrogen which behaves like an electrical conductor — makes up the interior of giant planets, but it is difficult to study and poorly understood. By combining artificial intelligence and quantum mechanics, researchers have found how hydrogen becomes a metal under the extreme pressure conditions of these planets.

The researchers, from the University of Cambridge, IBM Research and EPFL, used machine learning to mimic the interactions between hydrogen atoms in order to overcome the size and timescale limitations of even the most powerful supercomputers. They found that instead of happening as a sudden, or first-order, transition, the hydrogen changes in a smooth and gradual way. The results are reported in the journal Nature.

Hydrogen, consisting of one proton and one electron, is both the simplest and the most abundant element in the Universe. It is the dominant component of the interior of the giant planets in our solar system — Jupiter, Saturn, Uranus, and Neptune — as well as exoplanets orbiting other stars.

At the surfaces of giant planets, hydrogen remains a molecular gas. Moving deeper into the interiors of giant planets however, the pressure exceeds millions of standard atmospheres. Under this extreme compression, hydrogen undergoes a phase transition: the covalent bonds inside hydrogen molecules break, and the gas becomes a metal that conducts electricity.

“The existence of metallic hydrogen was theorised a century ago, but what we haven’t known is how this process occurs, due to the difficulties in recreating the extreme pressure conditions of the interior of a giant planet in a laboratory setting, and the enormous complexities of predicting the behaviour of large hydrogen systems,” said lead author Dr Bingqing Cheng from Cambridge’s Cavendish Laboratory.

Experimentalists have attempted to investigate dense hydrogen using a diamond anvil cell, in which two diamonds apply high pressure to a confined sample. Although diamond is the hardest substance on Earth, the device will fail under extreme pressure and high temperatures, especially when in contact with hydrogen, contrary to the claim that a diamond is forever. This makes the experiments both difficult and expensive.

Theoretical studies are also challenging: although the motion of hydrogen atoms can be solved using equations based on quantum mechanics, the computational power needed to calculate the behaviour of systems with more than a few thousand atoms for longer than a few nanoseconds exceeds the capability of the world’s largest and fastest supercomputers.

It is commonly assumed that the transition of dense hydrogen is first-order, which is accompanied by abrupt changes in all physical properties. A common example of a first-order phase transition is boiling liquid water: once the liquid becomes a vapour, its appearance and behaviour completely change despite the fact that the temperature and the pressure remain the same.

In the current theoretical study, Cheng and her colleagues used machine learning to mimic the interactions between hydrogen atoms, in order to overcome limitations of direct quantum mechanical calculations.

“We reached a surprising conclusion and found evidence for a continuous molecular to atomic transition in the dense hydrogen fluid, instead of a first-order one,” said Cheng, who is also a Junior Research Fellow at Trinity College.

The transition is smooth because the associated ‘critical point’ is hidden. Critical points are ubiquitous in all phase transitions between fluids: all substances that can exist in two phases have critical points. A system with an exposed critical point, such as the one for vapour and liquid water, has clearly distinct phases. However, the dense hydrogen fluid, with the hidden critical point, can transform gradually and continuously between the molecular and the atomic phases. Furthermore, this hidden critical point also induces other unusual phenomena, including density and heat capacity maxima.

The finding about the continuous transition provides a new way of interpreting the contradicting body of experiments on dense hydrogen. It also implies a smooth transition between insulating and metallic layers in giant gas planets. The study would not be possible without combining machine learning, quantum mechanics, and statistical mechanics. Without any doubt, this approach will uncover more physical insights about hydrogen systems in the future. As the next step, the researchers aim to answer the many open questions concerning the solid phase diagram of dense hydrogen.

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Controlling quantumness: Simulations reveal details about how particles interact

In everyday life, matter behaves in a predictable, expected way. If you throw a ball, you assume it will travel in a certain direction and have a predictable recoil. What’s more, forces exerted on one object would not have an impact on another, independent object.

But in quantum mechanics — the physics of the tiny — the rules are completely different. In one, two, and three-particle systems, actions that happen in one spot can strongly influence atoms far away. Scientists don’t yet have a full understanding of this but, by analyzing the behavior of these systems and more complex ones, they are hoping to find insights.

Researchers from the Quantum Systems Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), alongside collaborators at University College Dublin and Durham University, simulated one of these systems, which revealed quantum states — ways that particles arrange themselves in isolated systems — that were unexpected. Their results, published in New Journal of Physics, could have applications for quantum technologies.

“If you throw a stone off a boat, the stone goes one way and the boat goes the other,” explained Professor Thomas Busch, who leads the Unit. “In quantum mechanics, we can have much stronger correlations at much greater distances. It’s like if you put on one red sock and one green sock, then someone in Antarctica, who you’ve never met, would have to do the same. And our work has found new states with these very strong correlations, which can be controlled very well.”

Experimenting with two atoms

When scientists research macroscopic systems, they tend to look at many particles — say 10 to the 23 atoms. Because there are so many, they can’t follow every atom and must make assumptions. To avoid this, the researchers in this study used another option.

“We simulated a system with just two atoms,” said first author Ayaka Usui, a Ph.D. student in the Unit. “This provided a building block of the larger system, but we could control everything and see exactly what was happening. And, to further control this system, we considered super-cold atoms.”

At room temperature, particles move around very quickly. The warmer it is, the faster they move. By using laser cooling, these atoms can be slowed and cooled down until they reach almost zero velocity and are thus super-cold. This made it much easier for Ayaka and colleagues to describe them in their simulations.

In a system like this, the simplest thing the particles can do is collide with each other. This forces them to move around and change direction, but particles also have something called spin. The spin of a particle is either pointing up or down and further influences how it moves — an effect called spin-orbit coupling. When the researchers simulated a system with two super-cold atoms that were spin-orbit coupled, these new states, with their very strong correlations, were revealed.

“We have the systems with two-particles where you get these states and the ones with 10 to the 23 where you don’t,” said Dr. Thomas Fogarty, Postdoctoral Scholar in the Unit. “Somewhere along this long chain of adding particles, these new states go away.”

Engineering further insights

“Alongside the new states, we’ve discovered the formulas that describe this system exactly,” said Ayaka. “So now, we can engineer it.”

By finding these formulas, the researchers have control over the system and they now plan on changing the parameters to look at the system’s dynamics.

“We’re going to split the system, so we have two of them,” said Ayaka. “We can use the strong correlations to help us measure the system. If we find one atom in one of the systems, we know the other one is also in that one, without measuring it, because they are tightly correlated.”

Although this research is just concentrating on a small aspect of what quantum mechanics can do, it has numerous applications, said Professor Busch.

“Quantum technologies need these correlations,” he explained. “These new states have the strongest non-classical correlations that we know, and we can engineer them. With this research, we could build more powerful computers. We could create measurement devices that measure tiny differences in gravity or electric pulses in the brain. There’re so many applications to work towards.”

Alongside Ayaka Usui, Dr. Fogarty, and Professor Busch, this research involved Dr. Steve Campbell from University College Dublin and Professor Simon Gardiner from Durham University.

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The mysterious movement of water molecules

Water is a mysterious substance. Understanding how it behaves at the atomic level is still a challenge for experimental physicists, as light hydrogen and oxygen atoms are difficult to observe using conventional experimental methods. This is especially true for any researcher looking to study the microscopic movements of individual water molecules that run off a surface in a matter of picoseconds. As they report in their paper, entitled ‘Nanoscopic diffusion of water on a topological insulator’, researchers from the Exotic Surfaces working group at TU Graz’s Institute of Experimental Physics joined forces with counterparts from the Cavendish Laboratory at the University of Cambridge , the University of Surrey and Aarhus University. Together, they made significant advances, performing research into the behaviour of water on a material that is currently attracting particular interest: a topological insulator called bismuth telluride. This compound could be used to build quantum computers. Water vapour would be one of the environmental factors to which applications based on bismuth telluride might be exposed during operation.

In the course of their research, the team used a combination of a new experimental method called helium spin-echo spectroscopy and theoretical calculations. Helium spin-echo spectroscopy uses very low-energy helium atoms that allow isolated water molecules to be observed without influencing their motion in the process. The researchers discovered that water molecules behave completely differently on bismuth telluride compared with those on conventional metals. On such metals, attractive interactions between water molecules can be observed, leading to accumulations in the form of films. But the opposite is the case with topological insulators: the water molecules repel one another and remain isolated on the surface.

Bismuth telluride appears to be impervious to water, which is an advantage for applications exposed to typical environmental conditions. Plans are in place for further experiments on similarly structured surfaces, which are intended to clarify whether the movement of water molecules is attributable to specific features of the surface in question.

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Materials provided by Graz University of Technology. Original written by Birgit Baustädter. Note: Content may be edited for style and length.

Journal Reference:

  1. Anton Tamtögl, Marco Sacchi, Nadav Avidor, Irene Calvo-Almazán, Peter S. M. Townsend, Martin Bremholm, Philip Hofmann, John Ellis, William Allison. Nanoscopic diffusion of water on a topological insulator. Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-019-14064-7

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First microscopic look at a tiny phenomenon with big potential implications

Matter behaves differently when it’s tiny. At the nanoscale, electric current cuts through mountains of particles, spinning them into vortexes that can be used intentionally in quantum computing. The particles arrange themselves into a topological map, but the lines blur as electrons merge into indistinguishable quasiparticles with shifting properties. The trick is learning how to control such changeable materials.

For the first time, researchers have taken a microscopic look at this process. The international team has now published their results on July 11, 2019 in Communications Physics, a Nature journal.

In certain conductive materials, such as Manganese Silicon (MnSi), the quasiparticles can accumulate into a magnetic skyrmion with a vortex-like shape and motion. The skyrmion creates a lattice of connection points within the MnSi crystal.

“Magnetic skyrmions have attracted interest due to the potential for spintronics applications,” said Taku Sato, study author and professor at the Institute of Multidisciplinary Research for Advanced Materials at Tohoku University.

Spintronics refer to theoretical electronics that rely not only on the charge state of a current, but also on the characteristics of electrons to transfer and store quantum information.

“The first step to realize such spintronic applications of skyrmions may be electric current control of skyrmion flow,” Sato said. “Once created, the skyrmion can almost never be annihilated. It also strongly couples to electric current flow, meaning it takes very little current to move the system.”

To understand how electric current affects the magnetic skyrmion changes under an electrical current, the researchers used a method called small-angle neutron scattering. They powered a neutron beam through a MnSi crystal, causing the skyrmion particles to react — the neutrons literally scatter against and around the components of the skyrmion system. How they scatter tells the researchers about the system.

In this case, the researchers saw that the lattice structure of the skyrmion was deformed, causing the vortex motion of the skyrmion to change. They also saw that the edges of the skyrmion were significantly disturbed, almost as if it were pushing against itself. Sato attributes this to what he called “pinned edges.” The skyrmion might push against its outermost limits, causing friction.

“Such a friction effect has not been reported to date as far as we are aware of,” Sato said. “It’s fundamental key information for the realistic spintronics device design utilizing magnetic sykrmions.”

Sato and his team plan to further investigate the dynamics of magnetic skyrmions with the eventual goal of developing spintronic devices.

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Materials provided by Tohoku University. Note: Content may be edited for style and length.

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