| liquid helium | |
|---|---|
| Name | Liquid helium |
| Other names | Helium II (below lambda point), Helium I (above lambda point) |
| Chemical formula | He |
| Phase | Liquid |
| Discovered | Helium first isolated in 1895; liquefaction achieved 1908–1909 |
| Discovered by | Sir James Dewar (liquefaction efforts), Heike Kamerlingh Onnes (liquefaction of helium) |
| Melting point | ~0.95 K (under pressure) |
| Boiling point | 4.22 K at 1 atm (He-4) |
| Notable properties | Superfluidity, low viscosity, high thermal conductivity |
liquid helium
Liquid helium is the cryogenic liquid phase of the element Helium and is a cornerstone system in experimental and theoretical Quantum Physics. As one of the few substances that remain liquid down to absolute zero at standard pressure, liquid helium provides a macroscopic window onto quantum phenomena such as Bose–Einstein condensation and macroscopic quantum coherence. Research on liquid helium has shaped foundational work by figures and institutions including Pyotr Kapitsa, John F. Allen, Don Misener, Heike Kamerlingh Onnes, Royal Society, University of Leiden, and Kapitza's low temperature laboratory techniques. It remains central to studies at laboratories like CERN, National Institute of Standards and Technology, and national cryogenic facilities.
Liquid helium exists primarily as two isotopic forms: He-4 and He-3. He-4 is a boson (nuclear spin 0) and exhibits the well-known superfluid transition at the lambda point (~2.17 K at saturated vapor pressure). He-3 is a fermion (nuclear spin 1/2) and remains normal down to much lower temperatures, showing superfluid phases only via Cooper-pairing below millikelvin temperatures. Distinctions between the isotopes underpin quantum statistical behavior: He-4 is governed by Bose–Einstein statistics and macroscopic occupation of the ground state, while He-3 obeys Fermi–Dirac statistics and exhibits pairing analogous to BCS theory in unconventional superfluid phases. Precise thermodynamic quantities—specific heat, density, and sound velocities—were measured by experimentalists in institutions like University of Cambridge and Kamerlingh Onnes Laboratory and parameterized in standard cryogenic tables used by National Institute of Standards and Technology.
Superfluidity in He-4 (also called Helium II) manifests as zero viscosity flow, quantized vortices, and second sound (entropy waves). The discovery of superfluid behavior by Pyotr Kapitsa and independent work by John F. Allen and Don Misener established phenomenology that led to two-fluid models developed by Lev Landau and others. Quantized circulation follows from single-valuedness of the superfluid order parameter, linking to concepts in quantum field theory and topological defects. In He-3, the richer order-parameter symmetries produce multiple superfluid phases (A, B, A1) studied extensively by researchers at Cornell University, University of Washington, and Royal Society of London meetings on low temperature physics. Vortex dynamics, mutual friction, and quantum turbulence in helium inform broader quantum hydrodynamics and analogue gravity experiments.
Producing and studying liquid helium requires established cryogenic techniques: helium liquefiers pioneered by Heike Kamerlingh Onnes and smaller-scale closed-cycle refrigerators like the Gifford–McMahon cryocooler and pulse-tube refrigerator. Dilution refrigerators (using He-3/He-4 mixtures) are essential for reaching millikelvin temperatures for He-3 superfluidity and quantum device testing; key developments occurred at Cornell University and Bell Labs. Experimental probes include neutron scattering at facilities such as ISIS Neutron and Muon Source and Institut Laue–Langevin, nuclear magnetic resonance (NMR) pioneered in part at Bell Labs, and torsional oscillators developed by low-temperature groups to detect supersolid-like behavior. Precision thermometry and vacuum systems from National Physical Laboratory (UK) and NIST underpin reliable measurements.
Liquid helium undergirds modern quantum technology and basic research. It cools superconducting magnets for particle accelerators at CERN and magnetic resonance imaging systems produced by companies like Siemens and GE Healthcare. He-3/He-4 dilution refrigeration is standard for operating superconducting qubits in quantum computing platforms developed by firms and labs including IBM, Google Quantum AI, and academic groups at MIT and Harvard University. Liquid helium environments support experiments on quantum fluids, analogues of cosmological phenomena, and precision measurements that test standard model extensions at places like Lawrence Berkeley National Laboratory and Los Alamos National Laboratory. Low-dissipation properties enable ultrasensitive gyroscopes, bolometers, and detectors used in astrophysics and gravitational wave instrumentation.
Theoretical descriptions of liquid helium span quantum statistical mechanics, many-body theory, and effective field theories. Landau's two-fluid model and excitations—phonons and rotons—provide phenomenological spectra matched to neutron-scattering data and microscopically linked to models by Richard Feynman and Lev Landau. Path-integral Monte Carlo and quantum Monte Carlo methods, developed and applied by groups at University of Illinois at Urbana–Champaign and Los Alamos National Laboratory, compute ground-state properties and superfluid fractions. The crossover from microscopic Hamiltonians (realistic interatomic potentials by John Teller and parameterizations from scattering experiments) to macroscopic order parameters illustrates the role of symmetry breaking, spontaneous coherence, and critical phenomena described in works like Landau and Lifshitz and papers in journals such as Physical Review Letters and Journal of Low Temperature Physics. The interplay of Bose–Einstein condensation, vortex quantization, and emergent hydrodynamics in liquid helium continues to inform quantum many-body physics and reinforce stable experimental platforms for national scientific programs.
Category:Helium Category:Quantum fluids Category:Cryogenics