Quantised Vortices
vortices at millimeter-scale
One property of the super fluid state is that vortices formed by stirring or disturbing the super fluid are quantized. Moreover, the phase of the order parameter must change by an integer multiple of 2p around the core of the vortex. Previously only single vortices in which the phase changes by 2p have been observed. Now the Helsinki-Leiden-Moscow-Grenoble team has used nuclear magnetic resonance to observe double vortices in which the phase changes by 4p. Furthermore, when the super fluid helium-3 is slowly set into rotation under suitable conditions, only double vortices - which are about 10 microns in diameter - are formed. In addition to its interest to low-temperature physicists, the work could also be relevant to the cosmic strings that have been proposed by some cosmologists.
Super fluids exhibit both quantum mechanical behaviours and classic mechanical behaviours at the same time. For example, super fluids can transmit ordinary sound (pressure) waves, a classical phenomenon. The reason for this duality is that super fluids contain of a small percentage of atoms in ordinary (random and variable) quantum states along with atoms that are all confined to one quantum state. The percentage of ordinary randomly quantized atoms approaches zero as absolute zero is approached (absolute zero has never been experimentally observed and you will learn why shortly). This is called the two-fluid model of super fluids. All super fluids, in practice, have at least some proportion of atoms in an ordinary fluid state.
Super fluids can also quantize vortices. When water goes down a sink drain, it forms a whirlpool, a vortex, with water in the center moving more slowly and water at the edges moving around the fastest. When you spin a super fluid, you get an enormous number of tiny quantized vortices, the number being proportional to h/m where h is Planck's constant and m is the mass of one atom. These quantum vortices "carry" the angular momentum of the super fluid, and they turn out to be theoretically very similar to flux lines in superconductors, where the flux lines "carry" the magnetic field created by electric current traveling through them. A super fluid placed in a rotating container does not spin uniformly with the container. In fact, it will remain perfectly motionless unless the container spins fast enough to reach what is called critical velocity (each kind of atom has its own critical velocity). When this speed is reached, the super fluid instantaneously starts to spin. A super fluid's spin is quantized.
Super fluids exhibit both quantum mechanical behaviours and classic mechanical behaviours at the same time. For example, super fluids can transmit ordinary sound (pressure) waves, a classical phenomenon. The reason for this duality is that super fluids contain of a small percentage of atoms in ordinary (random and variable) quantum states along with atoms that are all confined to one quantum state. The percentage of ordinary randomly quantized atoms approaches zero as absolute zero is approached (absolute zero has never been experimentally observed and you will learn why shortly). This is called the two-fluid model of super fluids. All super fluids, in practice, have at least some proportion of atoms in an ordinary fluid state.
Super fluids can also quantize vortices. When water goes down a sink drain, it forms a whirlpool, a vortex, with water in the center moving more slowly and water at the edges moving around the fastest. When you spin a super fluid, you get an enormous number of tiny quantized vortices, the number being proportional to h/m where h is Planck's constant and m is the mass of one atom. These quantum vortices "carry" the angular momentum of the super fluid, and they turn out to be theoretically very similar to flux lines in superconductors, where the flux lines "carry" the magnetic field created by electric current traveling through them. A super fluid placed in a rotating container does not spin uniformly with the container. In fact, it will remain perfectly motionless unless the container spins fast enough to reach what is called critical velocity (each kind of atom has its own critical velocity). When this speed is reached, the super fluid instantaneously starts to spin. A super fluid's spin is quantized.