Unusual sound waves discovered in quantum liquids

Ordinary sound waves—small oscillations of density—can propagate through all fluids, causing the molecules in the fluid to compress at regular intervals. Now physicists have theoretically shown that in one-dimensional ...

New clues emerge in 30-year-old superconductor mystery

One of the greatest mysteries of experimental physics is how so-called high-temperature superconducting materials work. Despite their name, high-temperature superconductors—materials that carry electrical current with no ...

X-ray laser probes tiny quantum tornadoes in superfluid droplets

An experiment at the Department of Energy's SLAC National Accelerator Laboratory revealed a well-organized 3-D grid of quantum "tornadoes" inside microscopic droplets of supercooled liquid helium – the first time this formation ...

Superfluids: Observation of 'second sound' in a quantum gas

Second sound is a quantum mechanical phenomenon, which has been observed only in superfluid helium. Physicists from the University of Innsbruck, Austria, in collaboration with colleagues from the University of Trento, Italy, ...

Upgrade of LHC underway paving way for new discoveries

(Phys.org) —The Large Hadron Collider (LHC) has been shut down so that it can be upgraded, a process that is expected to take at least two years. Researchers on the project hope the upgrade will allow the facility to reach ...

Quantum refrigerator offers extreme cooling and convenience

Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a solid-state refrigerator that uses quantum physics in micro- and nanostructures to cool a much larger object to extremely low temperatures.

New experiment corrects prediction in quantum theory

An international team of scientists is rewriting a page from the quantum physics rulebook using a University of Florida laboratory once dubbed the coldest spot in the universe.

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Liquid helium

Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium at its boiling point and 1 atm is approximately 0.125 g/mL

Helium-4 was first liquefied on 10 July 1908 by Dutch physicist Heike Kamerlingh Onnes. Liquid helium-4 is used as a cryogenic refrigerant; it is produced commercially for use in superconducting magnets such as those used in MRI or NMR. It is liquefied using the Hampson-Linde cycle.[citation needed]

The temperatures required to liquefy helium are low because of the weakness of the attraction between helium atoms. The interatomic forces are weak in the first place because helium is a noble gas, but the interatomic attraction is reduced even further by quantum effects, which are important in helium because of its low atomic mass. The zero point energy of the liquid is less if the atoms are less confined by their neighbors; thus the liquid can lower its ground state energy by increasing the interatomic distance. But at this greater distance, the effect of interatomic forces is even weaker.[citation needed]

Because of the weak interatomic forces, helium remains liquid down to absolute zero; helium solidifies only under great pressure. At sufficiently low temperature, both helium-3 and helium-4 undergo a transition to a superfluid phase (see table below).[citation needed]

Liquid helium-3 and helium-4 are not completely miscible below 0.9 K at the saturated vapor pressure. Below this temperature a mixture of the two isotopes undergoes phase separation into a lighter normal fluid that is mostly helium-3, and a denser superfluid that is mostly helium-4. (This occurs because the system can lower its enthalpy by separating.) At low temperatures, the helium-4 rich phase may contain up to 6% of helium-3 in solution, which makes possible the existence of the dilution refrigerator, capable of reaching temperatures of a few mK above absolute zero.[citation needed]

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