Under a tangled mess of pipes, tubes, gauges, metal plates, pumps and duct tape lies a place colder than outer space. Working in this environment of organized chaos, the Quantum Matter team at the University of Cambridge’s Cavendish Laboratory is beginning to unveil the exotic quantum properties of supercold materials in a fridge the size of an SUV.
Because humans can easily perceive the difference between hot and cold, temperature is a feature of science that people have a fairly intuitive grasp on. But what people actually experience when they distinguish hot from cold is the amount of thermal energy that a system contains—an ice cream cone, for instance, contains less thermal energy than a bowl of hot soup. And because this energy comes from the movement of atoms and molecules within a substance, that means the soup molecules are moving more than the ones in the ice cream.
The team at the University of Cambridge, however, monitors energy on a more extreme level as they try to approach absolute zero—the coldest temperature allowed by the laws of thermodynamics. Absolute zero, technically known as zero kelvins, equals −273.15 degrees Celsius, or -459.67 Fahrenheit, and marks the spot on the thermometer where a system reaches its lowest possible energy, or thermal motion.
There’s a catch, though: absolute zero is impossible to reach. The reason has to do with the amount of work necessary to remove heat from a substance, which increases substantially the colder you try to go. To reach zero kelvins, you would require an infinite amount of work. And even if you could get there, quantum mechanics dictates that the atoms and molecules would still have some irreducible motion.
Quantum mechanics also means that the closer these researchers creep toward absolute zero, the weirder a substance’s properties become. At low enough temperatures, liquid helium, for example, morphs into a superfluid—a liquid that flows without the resistance of friction. As a result, it can spontaneously flow upwards and out of a container; seep through molecule-thin cracks; remain perfectly still while spinning at high speeds; and—most surprising to physicists—coalesce into one “super-atom,” known as a Bose-Einstein condensate. Working at around just 1 to 10 millikelvins, or thousandths of a kelvin, the Cavendish team is in the process of surveying a variety of other materials that also show funky quantum behavior. And the technology that the group uses to reach such frigid temperatures is almost as complicated as the behavior it’s trying to induce.
The journey towards absolute zero began in the early 1700s when Guillaume Amontons contended that if temperature is the measure of heat in a system, then there must be a lowest possible temperature. Yet it wasn’t until two centuries later that Amontons’ theory would find its place in experimentation. At Leiden University, Heike Kamerlingh Onnes and his colleagues raced against others around the world to develop techniques to liquify helium. After many failed attempts, they succeeded, and says Dirk van Delft, director of Museum Boerhaave, the Dutch National Museum for the History of Science and Medicine, “Leiden briefly became the coldest place on Earth.”
Onnes’ success eventually came thanks to one of the earliest forms of high-powered refrigeration. Like everyday refrigerators, the cooling system in Onnes’ lab and now labs around the world works in a cycle. The cooling process itself is similar to what happens when you blow on hot cup of coffee to cool it down. As the person blows, the more chaotic, faster-moving coffee molecules are encouraged to evaporate and, therefore, move away from the cup. The molecules left behind are on average moving slower—consequently making the coffee a more drinkable temperature. Unlike everyday refrigerators that use vapor from inside the fridge, however, Onnes used helium in the gas state and hydrogen and oxygen in the liquid state to achieve low temperatures.
By cycling gaseous helium through a chamber bathed in cold liquid hydrogen and air, Onnes’ group successfully reached a temperature where a small teacup’s worth of Helium could liquify. In doing so, the excess heat from the gaseous state dissipated and the system achieved a temperature merely six
kelvins above absolute zero—the closest attempt of its time. This research won Onnes the Nobel Prize in 1913. He also accidentally discovered superconductivity, the ability of a substance to carry electric current with no resistance. This property makes the powerful superconducting magnets used in today’s MRI detectors and giant particle accelerators possible, amongst other things.
The best refrigeration systems in the world today are based on Onnes’ original work, but they can now reach a few millikelvins, utilizing two different isotopes of helium. Unlike most liquids, which freeze and turn into a solid at some temperature point, helium remains liquid all the way down to absolute zero. Because its atoms are so light at these temperatures, helium is weakly drawn to other helium atoms such that they become locked in a persistent jiggle, known as zero-point motion, a quantum mechanical effect defined by the Heisenberg uncertainty principle.
Operating in what is essentially a closed loop, helium acts almost exactly like those disordered coffee molecules in your mug and dissipates excess heat to the environment as it circulates. When the helium-3 isotope migrates towards the helium-4 isotope as a result of attraction and pressure differences caused by the fridge apparatus, it absorbs heat and cools the entire system down to the millikelvin level.
The Cambridge lab uses this kind of refrigerator to inspect many different types of materials and material properties. Perhaps the most surprising of them is iron germanide, YFe2Ge2. At low temperatures, this iron-based material contorts into a superconductor. “The most startling discovery is really YFe2Ge2’s existence as a superconductor at all,” says Keiron Murphy, a PhD student in the Cambridge Quantum Matter group.
Iron, he explains, typically destroys any superconducting properties in a material, regardless of temperature, due to the magnetic nature of iron. Superconductivity has many applications in science, medicine and computing, and each new superconductor can help foster novel technology. Because of this lab’s work, YFe2Ge2 is now considered a reference material for investigating superconductivity in compounds with a similar iron structure.
Unfortunately, says Murphy, quantum states are “inherently fragile,” and a substantial portion of the interesting properties that naturally arise in some materials are “overwhelmed by vibrations at higher temperatures.” Operating at just around 1 to 10 millikelvins, the Quantum Matter group can perform measurements at these temperatures for several months. But the group is currently in the process of developing another more efficient fridge that can sustain these low temperatures for longer.
With this new fridge, the team will look at other iron based materials at low temperatures for sustained periods of time and also continue working with materials known as topological semimetals, such as ZrSiS. The low-temperature magnetic behavior of topological semimetals is in large part a mystery, for their properties are dominated by their topology (or the arrangement of its parts), not their constituent elements. And the Cambridge team is ready to unearth their enigmas once the new refrigerator is up and running.
Strange physical properties thrive under the extremes of low temperature, and the implications of these bizarre qualities are seemingly boundless. Supercooling techniques such as the ones used in dilution refrigeration are critical for a wide range of disciplines: gravitational wave research, superconductivity, spintronics, quantum computing and other up-and-coming technologies. Alleviating high temperature strains, work at absolute zero is crucial in understanding and uncovering a lot of unknowns in both quantum mechanics and physics in general.
“At these temperatures, we gain access to a world of exotic phenomena, and the materials that were once ordinary become extraordinary,” says Murphy.
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