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Stars draw atoms closer together

Previously unknown bonding mechanism predicted in magnetic fields of white dwarfs.

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Magnetism may be the secret to a strong marriage between atoms in the atmospheres of stars. Computer simulations show that a previously unknown type of powerful chemical bond should be induced by the stars’ ferocious magnetic fields. If the effect can be harnessed in the lab, ‘magnetized matter’ could be exploited for quantum computing.

Chemists identify two classes of strong molecular bonds: ionic bonds, in which electrons from one atom hop over to another, and covalent bonds, in which electrons are shared between atoms. But Trygve Helgaker, a quantum chemist at the University of Oslo, and his colleagues accidentally discovered a third bonding mechanism when they simulated how atoms should behave under magnetic fields of about 105 tesla — 10,000 times the biggest fields that can be generated on Earth. Their results are published in Science today1.

NASA/ESA/H. Bond (STScI)/M. Barstow (Univ. Leicester)

White-dwarf stars have huge magnetic fields that could force molecular bonds into powerful new modes.

The team first examined how the lowest energy state, or ground state, of a two-atom hydrogen molecule was distorted by the magnetic field. The dumb-bell-shaped molecule oriented itself parallel to the direction of the field and the bond became shorter and more stable, says Helgaker. When one of the electrons was boosted to an energy level that would normally break the bond, the molecule simply flipped so that it was perpendicular to the field and stayed together.

“We always teach students that when an electron is excited like this, the molecule falls apart,” says Helgaker. “But here we see a new type of bond keeps the atoms hanging together.” The team also reports that a similar effect should occur between helium atoms, which normally don't bond at all.

The atoms are held together by the way their electrons dance around the magnetic-field lines, explains Helgaker. “The way electrons move relative to the field, and their kinetic energy, can become as important for chemical bonding as the electrostatic attraction between the electrons and the nuclei,” he says. Depending on their geometry, molecules will turn to allow electrons to rotate around the direction of the magnetic field. 

Star field

If the new states remain bound at very high temperatures, they could well exist in the atmospheres of some white dwarfs and neutron stars, where the magnetic fields are similar to those simulated by the team. But it will be difficult to spot them, says Dong Lai, an astrophysicist at Cornell University in Ithaca, New York. The team will need to extend its model to see whether the unusual bonding states would modify the spectra of light coming from the stars in a way that can be detected, he says. The simulation of the states “is an important step, but several more are needed to see how relevant this is in astrophysics”.

Closer to home, it is virtually impossible to generate such high magnetic fields, because they are accompanied by drastic changes in the chemistry of everything affected by them. The bond length between atoms can shrink by around 25% under such high fields, says Helgaker. “The experimental apparatus would cease to be an apparatus in these extreme conditions!”

Nevertheless, the findings boost hopes that ‘magnetized matter’ in the lab could have properties that may be exploited.

In 2009, physicists created a weakly bound state called a Rydberg molecule2, which some people have suggested could be used to carry information in a quantum computer. Rydberg molecules are highly sensitive to magnetic effects, says Chris Greene, an atomic physicist at the University of Colorado Boulder, who was one of the first people to posit the molecules' existence3. “That means we could use magnetic fields as a knob to tightly control the strength of the binding, to manipulate them to store and erase quantum memory as needed.”

Journal name:
Nature
DOI:
doi:10.1038/nature.2012.11045

References

  1. Lange, K. K., Tellgren, E. I., Hoffmann, M. R. & Helgaker, T. Science 337, 327331 (2012).

  2. Bendkowsky, V. et al. Nature 458, 10051008 (2009).

  3. Greene, C. H., Dickinson, A. S. & Sadeghpour, H. R. Phys. Rev. Lett. 85, 24582461 (2000).

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