Liquid helium

Liquid helium is a unique physical state of the chemical element helium. It occurs only at extremely low temperatures when measured at standard atmospheric pressures. This substance is vital to modern science because it can exhibit superfluidity. A superfluid is a liquid that flows in a very unusual way.

Because helium atoms have very weak attractions to one another, the element remains liquid all the way down to absolute zero at standard pressure. This makes liquid helium one of the coldest substances used in scientific research today.

The behavior of liquid helium is driven by the tiny particles called atoms. Helium is a noble gas, which means its atoms do not pull on each other very strongly. These interatomic forces are even weaker due to the effects of quantum mechanics. This is significant because helium has a very low atomic mass of about four daltons. At these low temperatures, the atoms try to find a state with less ground state energy. They do this by naturally increasing the average distance between the atoms. Because these forces are so weak, the helium stays liquid rather than freezing into a solid. To make helium a solid, scientists must apply very high pressures and very low temperatures.

There are two stable isotopes of helium that scientists study: helium-4 and helium-3. Helium-4 is the common isotope found in nature. Helium-3 is a much rarer isotope. These two types of helium behave differently when they are mixed together. Below 0.9 kelvin at their saturated vapor pressure, a mixture of these isotopes undergoes phase separation. This means they split into two distinct layers. The denser superfluid layer, which is mostly helium-4, sinks to the bottom. A normal fluid layer, which is mostly helium-3, floats on top.

This separation happens because the overall mass can reduce its thermodynamic enthalpy by splitting.

Each isotope also has its own unique way of becoming a superfluid. In helium-3, the atoms are fermions. At very low temperatures, these atoms form pairs called Cooper pairs. These pairs act as bosons, which allows them to condense into a superfluid. These Cooper pairs are actually much larger than the distance between the atoms themselves. Helium-4 also undergoes a transition to a superfluid state when it reaches its specific transition temperature. These different behaviors allow scientists to manipulate the liquid for various high-tech experiments.

Humans first achieved the liquefaction of helium on July 10, 1908. A Dutch physicist named Heike Kamerlingh Onnes succeeded in this at the University of Leiden in the Netherlands. At that time, the isotope helium-3 was still unknown because the mass spectrometer had not been invented yet. In 1923, Kamerlingh Onnes advised Canadian physicist John Cunningham McLennan on how to produce liquid helium almost on demand. Later, Soviet physicist Lev Landau and American physicist Richard Feynman performed important work on the characteristics of the liquid. In 1961, researchers Vignos and Fairbank discovered a specific phase of solid helium-4 called the gamma-phase. This phase only exists within a narrow pressure range between 1.45 and 1.78 K.

Liquid helium is used today as a cryogenic refrigerant, which is a substance used to create extreme cold. It is essential for operating superconducting magnets. These magnets are used in many medical and scientific tools. For example, they are used in magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). They are also used in magnetoencephalography (MEG) and low temperature Mössbauer spectroscopy. The scale of its use can be massive. The Large Hadron Collider uses 120 tonnes of liquid helium to cool its superconducting magnets.

Because of its unique properties, liquid helium is a bridge to understanding advanced physics. Scientists use a device called a dilution refrigerator to reach incredibly low temperatures. This device works by using the way helium-3 and helium-4 interact. At extremely low temperatures, the helium-4 rich superfluid phase can hold up to 6% helium-3 in solution. This allows researchers to reach temperatures of just a few millikelvins. By studying these tiny, cold states, scientists can explore the fundamental rules of the universe and how matter behaves at the very edge of coldness.