Helium-3
Helium-3
Helium-3 is stable isotope of helium with a nucleus comprised of 2 protons and 1 neutron. Some molecules are much more difficult to cool into super fluids than others are and helium-3 is one of them. It has to be cooled to 2 mK (0.002 K) before it exhibits any super fluid properties. That's a thousand times colder than Helium-4's lambda point, the temperature at which helium-4 changes from liquid to super fluid.
The helium-3 nucleus is a fermion rather than a boson. That means it contains an odd number of spin ½ particles. Fermions are often associated with matter and include elementary particles such as electrons and quarks (quarks are what neutrons and protons are made of). Composite particles such as protons and neutrons, with half-integer spins are also called fermions. Unlike bosons, only one fermion can occupy a particular quantum state at a time.
Fermions such as helium-3 follow Fermi-Dirac statistics and should not actually be condensable in the lowest energy state. For this reason superfluidity should not be possible in helium-3 which, like helium-4, can be liquidised at a temperature of some degrees above absolute zero. But fermions can in fact be condensed, but in a more complicated manner. This was proposed in the BCS theory which is based on the fact that electrons are fermions (they consist of one particle only, an odd number) and therefore follow Fermi-Dirac statistics just as helium-3 atoms do. But electrons in greatly cooled metals can combine in twos to form what are termed Cooper pairs and then behave as bosons. These pairs can undergo Bose-Einstein condensation to form a Bose-Einstein condensate. It was expected that the fermions in liquid helium-3 should be capable of forming boson pairs and that superfluidity should be obtainable in very cold samples of the isotope helium-3. Although many research groups had worked with the problem for years, particularly during the 1960s, none had succeeded and many considered that it would never be possible to achieve superfluidity in helium-3.
At last ,superfluidity in Helium-3 was discovered by a group of scientists in 1996 for which they received a noble prize,after which numerous super fluid experiments were started around the globe. Before then, experiments were few and far between, because there wasn't much more that scientists could learn from Helium4. Helium3 opened up a whole new perspective on super fluidity.
The helium-3 nucleus is a fermion rather than a boson. That means it contains an odd number of spin ½ particles. Fermions are often associated with matter and include elementary particles such as electrons and quarks (quarks are what neutrons and protons are made of). Composite particles such as protons and neutrons, with half-integer spins are also called fermions. Unlike bosons, only one fermion can occupy a particular quantum state at a time.
Fermions such as helium-3 follow Fermi-Dirac statistics and should not actually be condensable in the lowest energy state. For this reason superfluidity should not be possible in helium-3 which, like helium-4, can be liquidised at a temperature of some degrees above absolute zero. But fermions can in fact be condensed, but in a more complicated manner. This was proposed in the BCS theory which is based on the fact that electrons are fermions (they consist of one particle only, an odd number) and therefore follow Fermi-Dirac statistics just as helium-3 atoms do. But electrons in greatly cooled metals can combine in twos to form what are termed Cooper pairs and then behave as bosons. These pairs can undergo Bose-Einstein condensation to form a Bose-Einstein condensate. It was expected that the fermions in liquid helium-3 should be capable of forming boson pairs and that superfluidity should be obtainable in very cold samples of the isotope helium-3. Although many research groups had worked with the problem for years, particularly during the 1960s, none had succeeded and many considered that it would never be possible to achieve superfluidity in helium-3.
At last ,superfluidity in Helium-3 was discovered by a group of scientists in 1996 for which they received a noble prize,after which numerous super fluid experiments were started around the globe. Before then, experiments were few and far between, because there wasn't much more that scientists could learn from Helium4. Helium3 opened up a whole new perspective on super fluidity.