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Nov 11th 2019

It’s Absurd, It’s Insane, It’s … Superfluid!

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Quantum mechanics is quantitatively strange, describing probability-based phenomena across a host of atomic processes. This atypical action is typically odd at small scales, but there’s an exception: superfluid. This macro-scale manifestation comes with unique properties and massive scientific potential.

The Fundamentals

The concept of superfluidity was first explored by Pyotr Leonidovich Kapitsa, John F. Allen and Don Misener in two 1938 papers. As noted by Nature, the trio discovered that when liquid helium-4 was cooled below its “lambda point” of 2.17 kelvins (K), the typically colorless liquid known as He-I became He-II — a liquid with no friction, no viscosity and the ability to “creep” up the sides of any holding container.

What was going on? The explanation rests with Bose-Einstein condensate, which Scientific American defines as a process “in which individual particles overlap until they behave like one big particle.” Under the right conditions fundamental particles known as bosons can “condense” and begin acting in unison rather than exhibiting the typical randomness associated with particulate probabilities. Helium-4 atoms are composite bosons made up of six fermions — two protons, two neutrons and two electrons — and subject to this condensed state under super-cooled conditions.

Worth noting? The absent friction is analogous to superconductivity; as noted by Nobel Prize-winning physics professor Anthony Leggett, “according to our modern understanding, superconductivity is nothing more than superfluidity occurring in an electrically charged system.” Just as superfluid liquid can flow seemingly forever without friction, an electric current can do the same in superconductor systems, potentially continuing “for a time much longer than the age of the universe.”

Second Sounds and Strange Stars

Frictionless fluidity across a non-viscous volume is only the first of condensed helium’s superpowers. Along with the ability to creep upward along any surface and escape through microscopic cracks, this ultra-cold element also displays:

  • Quantum Critical Velocity: As noted by the New World Encyclopedia, if superfluid is placed in a rotating chamber, it remains perfectly stable until reaching critical angular velocity. It then begins to rotate in quantized vortices — unlike normal liquids, it doesn’t rotate uniformly and can only spin at specific rates. If more speed is added, more vortices will form.
  • Mega Thermal Conductivity: No other substance can currently match He-II’s thermal conductivity, which is 1 million times greater than He-1. Why? Second sound. This phenomena occurs when heat moves like a wave — similar to the propagation of sound through air — rather than via diffusion.

Although helium-4 is the most common superfluid, it’s possible to coax helium-3 into the same state. New research also suggests that other elements could be super-cooled to create a fermionic condensate with superfluidic properties. With the search for new chemical elements and the so-called “island of stability,” there’s plenty of room an entire league of super-powered subatomic standouts.

There’s also strong evidence that pulsars — rapidly-spinning neutron stars — are made of spinning superfluids surrounded by two crustal layers. Repeated detection of rotational anomalies in these pulsars suggests that some of the superfluid core may occasionally move outward and strike the outer crust, causing a temporary speed increase. Other inner-core superfluid then catches this outlier and reduces rotation to normal values.

Turning Up the Heat

Beyond their unique properties, there is also offer some practical potential.

In chemistry, helium-4 is being used as a quantum solvent for Superfluid Helium Droplet Spectroscopy (SHeDS). Single molecules placed in He-II solvents exhibit their gas-phase characteristics, allowing more precise observation and measurement of molecular properties.

Researchers from Cornell University are also working on a long-held scientific goal: high-temperature superfluidity. The term “high” is relative to the starting state superfluids, many of which hover around 1 K (or -457.87 F). By using an exciton — an electron paired with an electron “hole,” which is the absence of an electron where one could exist in atomic lattice, causing a net positive charge — the Cornell-led team was able to raise minimum condensation temperature from just above absolute zero to 100 K, or -279.67 F.

While this is a far cry from room-temperature superfluidity, postdoctoral researcher Zefang Wang notes that “the realization of an exciton condensate at much higher temperature than earlier studies provides an exciting opportunity to explore this quantum phase of matter at significantly less stringent experimental conditions.” In practice, more stable quantum states comprised of condensed excitons could be used to create new lighting technologies that are both brighter and more efficient than current LEDs.

Smooth Operator

At super-low temperatures, helium-4 isotopes exhibit unique fluidic properties. Lacking friction and viscosity, these smooth superfluids have changed our understanding of superconductor mechanics, paved the way for improved stellar science and are expanding the potential for room-temperature quantum technologies.

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