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<html> <head><script type="text/javascript" src="/_static/js/bundle-playback.js?v=HxkREWBo" charset="utf-8"></script> <script type="text/javascript" src="/_static/js/wombat.js?v=txqj7nKC" charset="utf-8"></script> <script>window.RufflePlayer=window.RufflePlayer||{};window.RufflePlayer.config={"autoplay":"on","unmuteOverlay":"hidden"};</script> <script type="text/javascript" src="/_static/js/ruffle/ruffle.js"></script> <script type="text/javascript"> __wm.init("https://web.archive.org/web"); __wm.wombat("http://boojum.hut.fi:80/research/theory/helium.html","20050412070034","https://web.archive.org/","web","/_static/", "1113289234"); </script> <link rel="stylesheet" type="text/css" href="/_static/css/banner-styles.css?v=S1zqJCYt" /> <link rel="stylesheet" type="text/css" href="/_static/css/iconochive.css?v=3PDvdIFv" />  <title>Helium</title> </head> <meta name="keywords" content="helium, helium 3, helium 4, He 3, He 4, absolute zero of temperature"> <body> <h1>Helium</h1> Helium is the second lightest element after hydrogen. It is known as a light gas that is used to float balloons. It becomes liquid when it is cooled to a very low temperature. Helium is the only substance that remains liquid at absolute zero of temperature, 0 K (zero Kelvin), or -273.15 Celsius. All other substances solidify at temperatures higher than 10 K, see <a href="typicalpt.html">a typical phase diagram</a>.</p> <p>Helium has two stable isotopes <sup>4</sup>He and <sup>3</sup>He. Usually different isotopes of the same substance differ only in their mass. However, the helium isotopes behave very differently when they are cooled to temperatures below a few Kelvin. <a href="mixture.html">A mixture</a> of the two isotopes separates spontaneously at temperatures below 0.8 K. The liquids of both isotopes become <a href="vortex.html"> superfluids</a> at low temperatures, <sup>4</sup>He below 2.17 K, and <sup>3</sup>He below 0.0025 K.</p> <p><img src="/web/20050412070034im_/http://boojum.hut.fi/research/theory/He4PD.gif" align="LEFT" width="310" height="256"> <sup>4</sup>He is the more common isotope of helium. The figure shows the phase diagram of <sup>4</sup>He at low temperatures. <sup>4</sup>He remains liquid at zero temperature if the pressure is below 2.5 MPa (approximately 25 atmospheres). The liquid has a phase transition to a superfluid phase, also known as He-II, at the temperature of 2.17 K (at vapor pressure). The solid phase has either hexagonal close packed (hcp) or body centered cubic (bcc) symmetry. <br clear="LEFT"> <p><img src="/web/20050412070034im_/http://boojum.hut.fi/research/theory/Phasehe3log.gif" align="LEFT" width="439" height="251"> The phase diagram of <sup>3</sup>He is shown in the figure. Note the logarithmic temperature scale. The dot in the lower right hand corner denotes room temperature and pressure. There are two <a href="he3.html"> superfluid phases of <sup>3</sup>He</a>, A and B. The line within the solid phase indicates a transition between spin-ordered and spin disordered structures (at low and high temperatures, respectively). </p> <br clear="LEFT"> <p>The reason for the different behavior of <sup>4</sup>He and <sup>3</sup>He is quantum mechanics. <sup>4</sup>He is a boson. The appearance of the superfluid phase in <sup>4</sup>He is related to Bose condensation, where a macroscopic fraction of the atoms is in the lowest-energy one-particle state. <sup>3</sup>He is a fermion (like electron) and it is forbidden by the Pauli exclusion principle that more than one fermion is in the same one-particle state. The superfluidity arises from formation of weakly bound pairs of fermions, so called Cooper pairs. The pairs behave as bosons. In the superfluid state there is a macroscopic occupation of a single Cooper pair state.</p> <h2>Researh on helium in LTL/Helsinki University of Technology</h2> <ul> <li><a href="index.html">Superfluid phases of <sup>3</sup>He, theory</a> <li><a href="/web/20050412070034/http://boojum.hut.fi/research/applied/rotating3he.html">Superfluid phases of <sup>3</sup>He, experiment</a> <li><a href="/web/20050412070034/http://boojum.hut.fi/research/optical/index.html">Optical studies of liquid-solid interfaces</a> </ul> <hr> <address>19.8.2003, Erkki Thuneberg, <a href="email.html">Email</a>, <a href="heliums.html">Suomeksi</a> </address> </body> </html>