Superfluids Essays - Condensed Matter Physics, Phases Of Matter

Superfluids "As we shall see, it is generally believed that the phenomenon of superfluidity is directly connected with the fact that the atoms of helium-4 obey Bose statistics, and that the lambda-transition is due to the onset of the peculiar phenomenon called Bose condensation." (Leggett, 1989) BOSE-EINSTEIN CONDENSATION This is the phenomenon wherein the bosons (a type of particle) making up a substance merge into the lowest energy level, into a shared quantum state. In general, it refers to the tendancy of bosons to occupy the same state. This state, formed when a gas undergoes Bose-Einstein condensation, is called a "Bose-Einstein condensate." The distinguishing feature of Bose-Einstein condensates is that the many parts that make up the ordered system not only behave as a whole, they become whole. Their identities merge or overlap in such a way that they lose their individuality entirely. A good analogy would be the many voices of a choir, merging to become 'one voice' at certain levels of harmony. HISTORY The phenomenon of superfluidity was discovered in 1937 by a Russian physicist, Peter Kapitza, and then studied independently in 1938 by John Frank Allen, a British physicist, and his coworkers. It wasn't until the 1970's however, that the useful properties of superfluids were discovered. Thanks to the work of David Lee, Douglas Osheroff and Robert Richardson at Cornell University, we have gained valuable information on the effects and uses of superfluids. These three scientists jointly received a Nobel Prize in Physics in 1996 for their discovery of superfluidity in helium-3. It took a while, however, before they actually figured out what this phase in helium was. Superfluidity in helium-3 first manifested itself as small anomalies in the melting curve of solid helium-3 (small structures in the curve of pressure vs. time). Normally, small deviations, like this one, are usually considered to be peculiarities of the equipment, but the three physicists were convinced that there was a real effect. They weren't looking for superfluidity in particular, but rather an antiferromagnetic phase in solid helium-3. According to their predictions, this phase appeared to occur at a temperature below 2mK. In their first publication in 1972, they interpreted this effect as a phase transition. They did not completely agree with this hypothesis, but by further developing their technique they could, just a few months later, pinpoint the effect. They found there were actually two phase transitions in the liquid phase, one at 2.7mK and the second at 1.8mK. This discovery became the starting point of intense activity among low temperature physicists. The experimental and theoretical developments went hand-in-hand in an unusually fruitful way. The field was rapidly mapped out, but fundamental discoveries are still being made. SUPERFLUID HELIUM Superfluidity is a state of matter characterized by the complete absence of viscosity, or resistance to flow. This term is used primarily when involving liquid helium at very low temperatures. It was found that liquid helium (4He), when cooled below 2.17K (-271O C or -456 O F, could flow with no difficulty through extremely small holes, which liquid helium at a higher temperature cannot do. It was also noted that the walls of its container were somehow coated with a thin film of helium (approximately 100 atoms thick). This film flowed against gravity up and over the rim of the container This temperature of 2.17K is called the lambda ( ) point because the graph of the specific heat of liquid helium exhibits a lamda-shaped maximum at that temperature. Under normal pressure, helium will liquefy at a temperature of 4.2K. As the temperature is still lowered, helium behaves as a normal liquid until it reaches the lamda point. Before reaching the lamda point, it can be called helium I. Helium II refers to the liquid state of helium below the lamda point. Superfluidity is found in helium II but it has limited uses. When the temperature is dropped still lower, it was found that the stable isotope helium-3 is formed. This liquid exhibits superfluid characteristics, but only at temperatures lower than 0.0025 K. Nuclei of helium-3 contain two protons and one neutron, rather than the two protons and two neutrons found in the more common isotope, helium-4. Superfluid helium-4 forms at approximately 2.17 K. This superfluid moves without friction, squeezes through impossibly small holes, and it can even flow uphill. Superfluid helium-3 can do all these things, however not so spectacularly. The weird thing about helium-3 is that it can have different properties in different directions, similar to the well-defined grain in a piece of wood. The difference between helium-3 and helium-4 is