Quantum simulator gives clues about magnetism

May 15, 2014
Pictured is a vacuum system that isolates ultracold atoms from room temperature and pressure. Atoms are suspended in this vacuum with laser beams, and manipulated with magnetic fields. A simple photograph taken with an infrared sensitive camera gives information about the atoms. Credit: Denzil Green / Canadian Institute for Advanced Research

Assembling the puzzles of quantum materials is, in some ways, like dipping a wire hanger into a vat of soapy water, says CIFAR (Canadian Institute for Advanced Research) Fellow Joseph Thywissen (University of Toronto).

Long before mathematical equations could explain the shapes and angles in the soap foams, mathematicians conjectured that soap films naturally found the geometry that minimized surface area, thus solving the problem of minimal surfaces. They could be created simply by blowing soap bubbles.

At the University of Toronto's Ultracold Atoms Lab, Thywissen and his team strive to answer what he calls "soap bubble" questions—deep mysteries of the enigmatic world that simulations can help us solve. Since the electrons within quantum materials, such as superconductors, zoom far too quickly for careful observation, Thywissen's team uses ultracold gases instead, in this way simulating one quantum system with another, more easily studied, quantum system.

"Simulation gives you the answers but not the theory behind them," says Thywissen.

Thywissen's lab has revealed some of these answers in a new paper about the magnetism and diffusion of atoms in ultracold gases, published in the journal Science. The researchers optically trapped a cloud of gas a billion times colder than air in a very low-pressure vacuum.

They oriented the , which behave like microscopic magnets, to make them all point in the same direction in space, then manipulated the spins with an effect that's regularly used in hospitals for MRIs, called a spin echo.

Twisting up the direction into a corkscrew pattern and then untwisting it, they measured the strength of interactions between atoms. They observed that at first the atoms did not interact, but one millisecond later they were strongly interacting and correlated.

This rapid change suggested that something was happening to alter the atoms' magnetism as the process unfolded.

"The Pauli Principle forbids identical ultracold atoms from interacting, so we knew something was scrambling the spins at a microscopic level," Thywissen says.

What was happening, the researchers learned next, was diffusion—the same process that takes place when the smell of perfume fills the air of a room, for example.

"If I open a bottle of perfume in the front of the room, it takes a little while for those particles to diffuse to the back of the room," Thywissen says. "They bump into other particles on the way, but eventually get there. You can imagine that the more particles bump into each other, the slower diffusion occurs."

Cranking up interactions to their maximum allowed level, the Toronto team tried to see how slow diffusion could be. They lowered temperature below a millionth of a degree above absolute zero. You might guess that the speed of diffusion would eventually reach zero, but instead the experiment found a lower limit to diffusion.

"Whereas cars on the freeway need to drive below the speed limit, strongly interacting spins need to diffuse above a quantum speed limit," Thywissen says.

Ultracold atoms are just one of a larger family of strongly interacting materials, that also include superconductors and magnetic materials. Thywissen is a member of the CIFAR Quantum Materials program, which is developing an understanding of these materials' novel properties. Cold offer a promising way to explore the mystery of how electrons self-organize to exhibit unusual and valuable properties, such as superconductivity. Quantum materials contain mysteries that have challenged physicists for decades.

"Our measurements imply a diffusivity bound whose mathematical simplicity is exciting: it hints at a universal principle about spin transport, waiting to be uncovered," he says.

Thywissen says CIFAR's support helped make this successful experiment possible.

"CIFAR enabled me to assemble a world-class team."

Explore further: Ground-breaking insights into quantum chaos in ultracold gas

More information: "Transverse Demagnetization Dynamics of a Unitary Fermi Gas," Science, 2014.

Journal reference: Science search and more info website

Provided by Canadian Institute for Advanced Research

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George_Rajna
not rated yet May 16, 2014
Probably in the superconductivity there is no electric current at all, but a permanent magnetic field as the result of the electron's spin in the same direction in the case of the circular wire on a low temperature.
We think that there is an electric current since we measure a magnetic field. Because of this saying that the superconductivity is a quantum mechanical phenomenon.
This paper explains the magnetic effect of the superconductive current from the observed effects of the accelerating electrons, causing naturally the experienced changes of the electric field potential along the electric wire. The accelerating electrons explain not only the Maxwell Equations and the Special Relativity, but the Heisenberg Uncertainty Relation, the wave particle duality and the electron's spin also, building the bridge between the Classical and Quantum Theories. https://www.acade..._current
Writela
1 / 5 (1) May 16, 2014
speed of diffusion would eventually reach zero, but instead the experiment found a lower limit to diffusion
Such a result is not so surprising, when the quantum tunneling is considered. We can never trap some particle and/or its state at place, because it will tunnel and propagate itself into outside, until the energy barrier is not very high.
Writela
1 / 5 (1) May 16, 2014
There is interesting question, connected to speed of magnetic field propagation inside of ferromagnetic materials. Most of simulations is assuming, that this magnetization will spread with speed of light, but this speed may be actually way lower. When we will magnetize one end of iron rod periodically with rotating magnet, then the standing wave will be established along the rod.

IMO this effect could serve as an analogy of the boson condensates, where the light is propagating very slowly. The ferromagnetic domains do behave like sorta boson condensate at room temperature and they do allow the speed of EM field propagation in limited speed only. Recently this limited speed enabled to form magnetic monopoles inside of boson condensate, which would mean, that the alternating magnetic field could form the monopoles within ferromagnetic material too - even at the room temperature.
Writela
1 / 5 (1) May 16, 2014
One could expect, that the speed of magnetic field propagation in ferromagnetic materials was already thoroughly measured, but nearly nobody of mainstream physics did bother to actually measure it. Nevertheless the slow magnetization of iron was already observed at the end of 19th century as so-called magnetic viscosity with founder of atom nuclei, i.e. Ernst Rutherford. This ancient Tesla patent supposedly utilizes this effect for harnessing of energy.
swordsman
4 / 5 (1) May 16, 2014
More likely defined by Planck's state energy equation. As the temperature drops, the noise power drops. In other words, electron exchange becomes slower. When the temperature approaches absolute zero, the electron movement tend to more orderly, and the magnetic fields of the atoms become aligned.
Whydening Gyre
3 / 5 (2) May 16, 2014
Writela.
What do you suppose is at the EXACT center of a ferromagnetic dipole bar magnet? A "monopole" perhaps?
no fate
1 / 5 (1) May 16, 2014
Writela.
What do you suppose is at the EXACT center of a ferromagnetic dipole bar magnet? A "monopole" perhaps?


Except that if you slice your bar magnet at this point all you get is a shorter bar magnet...repeat process all the way down to 2 atoms. Ferromagnetic materials cannot avoid spin coupling, unless there is another field present.
Whydening Gyre
3 / 5 (2) May 16, 2014
Writela.
What do you suppose is at the EXACT center of a ferromagnetic dipole bar magnet? A "monopole" perhaps?


Except that if you slice your bar magnet at this point all you get is a shorter bar magnet...repeat process all the way down to 2 atoms. Ferromagnetic materials cannot avoid spin coupling, unless there is another field present.

Cut THAT in half and you have a single atom, a single spin - monopolarity.