Helium-4 Superfluid. Hassan Mason. Phy 335

Transcription

1 Helium-4 Superfluid Hassan Mason Phy 335

2 When some types of liquids are cooled to extremely low temperatures, they may condense to a very interesting state. The small thermal energy of each atom means that it becomes energetically favorable for individual atoms within the fluid to enter the same quantum state. Molecules in this state are named Bose-Einstein condensates. 1 One particular Bose-Einstein condensate has attracted a significant amount of attention: superfluid helium-4. Helium enters this state when cooled to below 2 Kelvin. Initially, only a small fraction of the liquid is in the superfluid state. As the 4 He is cooled to temperatures that approach absolute zero, the proportion of atoms in the superfluid state compared to the total number of atoms gradually increases. There are many fantastic properties of superfluid 4 He that have only been observed within the past few decades. 2 Superfluid 4 He has zero viscosity, which is responsible for a couple of its phenomena. The first is its ability to flow through extremely small holes in its container. When a pressure is applied to the superfluid across a porous membrane, the fluid will flow through the pores at the maximum flow rate (20 cm/s) defined for the fluid. This effect has been observed for pores smaller than 1 nm, which is only a few times larger than the classically calculated size of the 4 He atom. 3 Because regular liquid 4 He cannot flow through similar sized pores, a fountain can be constructed that spurts the fluid out of the top. The heater within the tube creates the pressure gradient that forces the flow of superfluid 4 He through the porus material that plugs the bottom. When the fluid arrives inside of the tube, it's

3 temperature is raised to a level above what is required for superfluidity. The trapped 4 He then increases in pressure until it spurts out of the top of the tube. 4 An additional effect due to its zero viscosity is the creep flow effect. Superfluid 4 He will form a film over its containing vessel and slowly siphon itself out of the container. Unlike other fluids that exhibit this effect due to the distinct shapes of the individual molecules, the superfluid 4 He shows this reverse flow for a different reason. The atoms flowing over the container experience zero friction, and so even an extremely small pressure is enough to overcome gravity and send it over the edge. 2 By Design: Aarchiba; SVG rendering: Júlio Reis - Redrawn after Aarchiba's Helium-II-creep.png, CC BY-SA 3.0, curid=

4 Another property of superfluid 4 He is that the fluid cannot rotate rigidly, or as a unit. Regular fluids, like water, will begin to rotate when its container is rotated, due to the friction between the container and the fluid. Superfluid 4 He, however, will not rotate, provided the angular velocity of the container is sufficiently small. When 4 He with a relatively small, non-zero angular velocity is cooled to below 2 K, the superfluid portion of the fluid will begin to lose its angular momentum, until the entire liquid is still. When the angular velocity of the container is increased to above a critical value, the superfluid responds with the creation of many quantized vortices, which are vortices that have a quantized angular momentum determined through quantum mechanics. The zero viscosity allows the vortices to coexist, and they form a sort of lattice throughout the entire fluid. 5

5 1. Emspak, Jesse. States of Matter: Bose-Einstein Condensate. LiveScience, Purch, 6 May 2016, 2. Leggett, Anthony James. Superfluidity. Encyclopædia Britannica, Encyclopædia Britannica, Inc., 6 Nov. 2016, 3. Ohba, Tomonori. Limited Quantum Helium Transportation through Nano-Channels by Quantum Fluctuation. Scientific Reports, vol. 6, no. 1, 1 July 2016, doi: /srep Kimball, Mark. Introduction to Liquid Helium. NASA, NASA, 11 Sept. 2014, cryo.gsfc.nasa.gov/introduction/liquid_helium.html. 5. Yamamoto, Yoshihisa. Bose-Einstein Condensation and Matter-Wave Lasers. Quantum Information Processing Project, National Institute of Informatics,