How does helium behave at room temperature?
Next:Fermions and bosons Up:introduction Previous:introduction
Helium-4 superconducting liquidAlthough chemically completely identical, 4He behaves completely differently from 3He at low temperatures. In particular, there is no comparable Bose Einstein condensation with 3He. Of all gases, helium has the lowest boiling point and was the last gas that could be liquefied. In fact, the boiling point of He is around 4 Kelvin. Another remarkable feature of liquid helium is that it never crystallizes at even the lowest temperatures. This is due to the considerable zero-point energy (Heisenberg's uncertainty principle). At around 2 Kelvin, there is also an abrupt change in the fluid. Helium suddenly stops boiling and becomes completely calm. The characteristic temperature curve for the specific heat - the thermal energy that is required to heat one gram of helium by one Kelvin - shows a curve similar to the Greek letter lambda. The lambda curve shows that helium suddenly reduces its viscosity by a factor of 106 below a temperature of approx. 2 Kelvin (lambda point). This leads to the stranger phenomenon that liquid helium literally creeps up the walls of a vessel below the lambda point. This film transfer phenomenon was first discovered by Kurt Mendelssohn. This phenomenon gives helium, as the only known element, the property of automatically flowing into a beaker, which is placed in a tub full of liquid helium, until the liquid level between the tub and the beaker is equalized. If you slowly pull the cup out of the tub, the liquid level will also equalize. If the beaker is completely pulled out of the tub, the helium migrates over the edge of the inside walls of the beaker to collect as droplets on the underside and drip back into the tub. The strange properties of helium result from the condensation of many helium atoms in a single energy state. The atoms, which suddenly find themselves in large numbers in a state of lowest energy, can cooperate with each other and thereby form a quantum mechanical superfluid.
Next:Fermions and bosons Up:introduction Previous:introductionEberhard Hilf
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