Spin Zone: physicists get first glimpse of strange quantum magnetism


Using super-cooled atoms, physicists for the first time observed a strange phenomenon called quantum magnetism, which describes the behavior of single atoms when they act like tiny magnetic bars.

Quantum magnetism is a little different from classical magnetism, the kind you see when you stick a magnet on a fridge because individual atoms have a quality called spin, which is quantized, or in discrete states (usually called high or low. ). However, it was difficult to see the behavior of individual atoms, as it required cooling the atoms to extremely cold temperatures and finding a way to ‘trap’ them.

The new discovery, detailed in the May 24 issue of the journal Science, also opens the door to a better understanding of physical phenomena, such as superconductivity, which appears to be related to the collective quantum properties of certain materials. [Twisted Physics: 7 Mind-Blowing Findings]

Spin science

The ETH Zurich research team focused on the spin of atoms, because that’s what makes magnets magnetic: all the spins of atoms in a magnetic bar are oriented in the same way.

To get a clear picture of the spin behaviors of the atoms, the researchers had to cool the potassium atoms to a level close to absolute zero. That way, the random thermal “noise” – essentially background radiation and heat – didn’t spoil the view by jostling the potassium atoms.

Scientists then created an “optical network” – a collection of intersecting laser beams. The beams interfere with each other and create regions of high and low potential energy. Neutral atoms with no charge will tend to sit in lattice “sinks”, which are regions of low energy.

Once the lattice is built, atoms sometimes randomly “tunnel” through the sides of the wells, as the quantum nature of the particles allows them to be in multiple places at the same time, or to have varying amounts of energy. . [Quantum Physics: The Coolest Little Particles in Nature]

Another factor that determines where atoms are in the optical lattice is their upward or downward spin. Two atoms cannot be in the same well if their spins are the same. This means that the atoms will tend to tunnel in wells with others that have opposite spins. After a while, a line of atoms should organize spontaneously, spins in a non-random pattern. This type of behavior is different from the materials of the macroscopic world, whose orientations can have a wide range of intermediate values; this behavior is also the reason why most things are not magnets – the spins of electrons in atoms are randomly oriented and cancel each other out.

And that’s exactly what the researchers found. The spins of the atoms organize themselves, at least on the scale examined by experience.

“The question is, what are the magnetic properties of these one-dimensional chains?” said Tilman Esslinger, a physics professor at ETH whose lab carried out the experiments. “Do I have materials with these properties? How can these properties be useful? “

Quantum magnetism

This experiment opens up possibilities to increase the number of atoms in a lattice, and even to create two-dimensional, grid-shaped arrangements of atoms, and possibly triangular lattices.

A debate among experts is whether, on a larger scale, the spontaneous order of atoms would occur in the same way. A random pattern would mean that in a block of iron atoms, for example, one is just as likely to see an atom spin up or down in any direction. The spin states are in what is called a “spin liquid” – a mishmash of states. But it could be that atoms arrange themselves spontaneously on larger scales.

“They laid the groundwork for various theoretical questions,” said Jong Han, professor of condensed matter physics theory at the State University of New York at Buffalo, who was not involved in the research. “They weren’t really establishing long-range order, rather they wanted to establish that they had observed a local magnetic order.”

Whether the order scientists find spans larger scales is an important question, as the magnetism itself comes from the spins of atoms when they all line up. Usually these rotations are randomly aligned. But at very low temperatures and on a small scale, that changes, and such quantum magnets behave differently.

Han noted that such networks, especially configurations where potential sinks connect to three others, rather than two or four, would be of particular interest. Esslinger’s lab showed that atoms tend to jump towards potential wells where the spins are opposite; but if the wells are arranged so that the atom can jump to two other atoms, it cannot “choose” which well to go because one of the two atoms will always be in the same spin state.

Esslinger said his lab wanted to try to build two-dimensional networks and explore this question. “What happens to magnetism if I change the geometry?” It is no longer clear whether the rotations should be up or down. “

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