Low Temperature Physics: The What, the How, the Why

The Why: Quantum Effects and Freaky Fermions

Many of the physicists using the lab's Millikelvin Facility are studying the fractional quantum Hall effect. This is a still-mysterious variant of the conventional (integer) Hall effect, in which a voltage is produced across some materials when a magnetic field is applied. In the fractional quantum Hall effect, special resistance-free electrically conducting states occur for ultra-thin sheets of semiconductors when a specific magnetic field is applied. These states, observed in two-dimensional structures, correspond to new states of matter. Some fractional quantum Hall states may one day be used as qubits in future quantum computers.

Others physicists, like the MagLab's Tim Murphy, are interested in heavy fermions, such as CeCoIn₅. In these intriguing materials, electrons appear to be much heavier than they actually are due to the interactions between the electrons and the crystal structure of the material, resulting in unusual behaviors.

"Usually, magnetism destroys superconductivity," explains Murphy, who directs the Millikelvin Facility. "But in some heavy fermions, magnetism not only coexists with superconductivity, it actually enhances the superconductivity. That flies in the face of our traditional understanding of superconductivity, and reminds us we don't understand as much as we think we do."

The High B/T Facility attracts scientists who study special phase transitions. These are not the run-of-the-mill transitions we're familiar with, such as water freezing to ice on a cold day, or hot tea vaporizing. Rather, these are exotic phase transitions that happen either at really low temperatures, or at the combination of very high fields and very low temperatures. These include Bose-Einstein condensates, superfluids (fluids that flow without any friction) and supersolids, a phase that has been theorized but not yet fully demonstrated.

Physicist Moses Chan of Pennsylvania State University, for example, has been studying pure helium-4 at High B/T, where he has observed behavior that suggests it is, at very low temperatures, a supersolid. Further experiments are underway, using innovative nuclear magnetic resonance techniques and specialized equipment, that may confirm whether this supersolid actually exists.

By no means are such experiments for the faint of heart.

"They're large, they're complicated and they can take a long time," says Sullivan, the High B/T director. "They are very costly experiments."

One experiment can last for months, as scientists painstakingly map out data points at different temperatures and magnetic fields. Because rapid changes in magnetic field create heat in the system, adjustments must be made very, very slowly.

"It's just the nature of the physics," says Murphy. "It has to be slow, there's just no other way. As soon as you go fast – oops – there's your temperature."

As little as an extra nanowatt of heat (remember that push-up pumping mosquito?) can undo a costly experiment many months in the planning. For that reason, all the electronics associated with an experiment – the wires and coils that measure what's going on -- are meticulously designed and built to generate next to no heat.

Working at the cutting edge of science and technology always entails risks.

"Many experiments fail," says Sullivan. "When you cool things down, there's lots of stress and strain, and sometimes things break or don't work well, or the sample itself is faulty." Many High B/T experiments are first tested under less demanding conditions to see if they are likely to hold up to the extremes of high fields and low temperatures.

Electronics are not the only nemesis of low-temperature experiments. Vibrations can also create heat that sensitive experiments can't tolerate. Both the Millikelvin and High B/T facilities were designed with this in mind. At High B/T, for example, a 35-foot concrete tripod supports the magnet system, each foot encased in 10 tons of concrete. Atop this tripod sits a shock mount, a kind of pneumatic isolation pad that further isolates the system from ground vibrations.

In addition, both low-temperature facilities are surrounded by shields that prevent external electromagnetic radiation, such as radio waves, from disrupting sensitive equipments.

So next time you're hot and bothered by summer weather, remember the ultra-cold microclimates being created inside magnets at two laboratories in Florida – one of the hottest states. Thinking of experiments that cool will make you feel positively nippy!

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Thanks to the Magnet Academy's scientific advisers on this article, Tim Murphy, director of the MagLab's Millikelvin Facility, and Neil Sullivan, director of the MagLab's High B/T Program.