Infinite chains of hydrogen atoms have amazing properties including a metal phase

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Around a series of hydrogen atoms where electrons are most likely to be found. Bright colors indicate higher possibilities. At this gap between atoms, electrons try to add pairs of proximal atoms as dihydrogen molecules. Because the protons are fixed in place, these molecules cannot form. Instead, each electron is bent towards a neighboring atom. Credit: m. Motta et al. /Physical review x 2020

An infinite series of hydrogen atoms is about the simplest bulk material imaginable – a never ending single-file line of protons surrounded by electrons. Yet a new computational study combining four state-of-the-art methods shows that the nominal material boasts spectacular and surprising quantum properties.

By calculating the results of varying the spacing between atoms, an international team of researchers from the Flatiron Institute and the Simons collaboration on the multiple electron problem found that the properties of hydrogen chains can vary in unpredictable and drastic ways. It involves a series of changes from a magnetic insulator to a metal, researchers reported on 14 September Physical review x.

The computational methods used in the study present an important step with demanding properties of custom-designing materials, such as the possibility of high-temperature superconductivity in which electrons flow freely through a material without losing energy , Says the study’s senior author, Xian Zhang. Zhang is a senior research scientist at the Center for Computational Quantum Physics (CCQ) at the Simons Foundation’s Flatiron Institute in New York City.

“The main objective was to apply our devices in a realistic situation,” Zhang says. “Almost as a side product, we discovered all of this interesting physics of the hydrogen chain. We didn’t think it would be as rich as it turned out.”

Zhang, who is also a chancellor professor of physics at William and Mary College, co-led the research with Mario Quanta of IBM Quantum. Motta works with Claudio Genovese of the International School for Advanced Studies (SISSA) in Italy, Fengjie Ma of Beijing Normal University, Zhi-Hao Cui of the California Institute of Technology and Randy Svea of ​​the University of California as the first authors of the paper. , Irwin. Additional co-authors include CCQ co-director Andrew Millis, CCQ Flatiron Research Fellow Hao Xi and CCQ research scientist Miles Stodenmaier.

The paper’s long author list — 17 in total — is unusual for the co-author field, Zhang says. Methods are often developed within different research groups. The new study brings together several methods and research groups to combine forces and tackle a particularly thorny problem. “The next step in the field is to move towards more realistic problems,” Zhang says, “and there is no shortage of people who cooperate in these problems.”

Although traditional methods can explain the properties of some materials, other materials, such as infinite hydrogen chains, pose a more challenging computational barrier. This is because the behavior of electrons in those materials is greatly affected by the interaction between electrons. As electrons join together, they become entangled quantum-mechanically. Once entangled, electrons can no longer be treated individually, even if they are physically isolated.

The sheer number of electrons in a bulk material – about 100 billion trillion per gram – means that even traditional brute force methods cannot come close to providing a solution. The number of electrons is so large that it is practically infinite when thinking on a quantum scale.

Thankfully, quantum physicists have developed clever ways of dealing with this multiple-electron problem. The new study combines four such methods: variable Monte Carlo, lattice-regularized diffusion Monte Carlo, tributary-field quantum Monte Carlo, and standard and truncated base density-matrix recombination groups. Each of these cutting-edge methods has its strengths and weaknesses. Zhang says that using them in parallel and in concert provides a fuller picture.

Researchers, including the authors of the new study, previously used methods in 2017 to calculate the amount of energy of each atom in a hydrogen chain as a function of the spacing of the chain. This computation, known as the equation of state, does not provide a complete picture of the properties of the series. Respecting their methods, the researchers did the same.

In large partitions, the researchers found that electrons remain confined to their respective protons. Even at such a large distance, electrons still ‘know’ about each other and get entangled. Because electrons cannot easily hop from atom to atom, the chain acts as an electrical insulator.

As the atoms get closer together, the electrons try to form molecules of two hydrogen atoms. Because the protons are fixed in place, these molecules cannot form. Instead, electrons ‘wave’ to each other, as Zhang puts it. The electrons will be inclined towards the adjacent atom. In this step, if you find an electron bending towards one of its neighbors, you will find that the neighboring electron is reacting in turn. This pattern of pairs of electrons tilting towards each other will continue in both directions.

Moving the hydrogen atoms closer together, the researchers found that the hydrogen chain changes from an insulator to a metal, moving freely between electrons with atoms. Under a simple model of interacting particles known as the one-dimensional Hubbard model, this transition should not occur, as electrons must repel each other sufficiently to restrict movement. In the 1960s, British physicist Neville Mott predicted the existence of a metal transition from an insulator based on a mechanism involving so-called extrons, each consisting of an electron trying to break free from its atom. Tha and the hole leaves it behind. The MOT proposed a sudden transition induced by the breakdown of these excites – something not seen in new hydrogen chain studies.

Instead, the researchers discovered a more granular insulator-to-metal transition. As the atoms get closer together, the electrons are slowly peeled off the tightly bound inner core around the proton line and form a thin `vapor ‘that is only bound to the line and provides interesting magnetic structures. It displays.

The eternal hydrogen chain will be an important benchmark in the development of computational methods in the future, Zhang says. Scientists can model the series using their own methods and check their results for accuracy and efficiency against new studies.

The researchers say the new work is a leap forward in the discovery of using computational methods to model realistic materials. In the 1960s, the British physicist Neil Ashcroft proposed that metal hydrogen, for example, could be a high-temperature superstar. Although the one-dimensional hydrogen chain does not exist in nature (it will crumble into a three-dimensional structure), the researchers say that the lessons they learned are an important step in the development of methods and the physical understanding required to deal with them. is. Even more realistic content.

Scientists address the multiple-electron problem by modeling an infinite series of hydrogen atoms

more information:
Mario Motta et al., Ground-State Properties of Hydrogen Chains: Refinement, Insulator-to-Metal Transition and Magnetic Phase, Physical review x (2020). DOI: 10.1103 / PhysRevX.10.031058

Provided by Simmons Foundation

Quotes: Infinite chains of hydrogen atoms have surprising properties, ranging from on a metallic phase (2020, 14 September). September 2020 is recaptured.

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