Superfluidity

superfluidity designates the condition of a liquid in physics, with which it loses each internal friction. Superfluidity became 1937 of Pyotr Leonidovich Kapitsa, John F. All and Don Misener discovers. The subsection of physics, which concerns itself with superfluidity, is quantum hydrodynamics.

Table of contents

description

helium and lithium 6 are the only elements, with which this phenomenon became observable so far. They change into the superfluids condition, if its temperature that Lambdapunkt falls below. About 3 He is this critical temperature with approximately 2.6 mK, with 4 He with 2,17 K. In superfluids the phase one can observe unusual phenomena:

  • Almost ideal heat conductivity of the liquid, since the atoms in superfluids the condition cannot transport entropy S.The finite value of the heat conductivity is to be attributed to normal fluids the remainder portion with> T 0, which can transport entropy and thus heat energy.
  • As put on conditions with a turn of the liquid form (similarly magn. River eddies in the superconductor or eddies in the bath tub) mechanical eddies quantized. These arrangeitself with sufficiently high eddy density in a regular hexagonal lattice on.
  • He-levels place themselves in neighbour containers due to the so-called. Roll in film (film-creep) on same height (Onnes effect).

Superfluid 4 He also helium II, contrary to the normal fluids (liquid) helium i is called.

Explanation beginnings

superfluidity cannot completely be explained so far theoretically. There are however different beginnings, which describe the characteristics of superfluidem helium at least qualitatively.

two-fluid model

the two-fluid model for the explanation of superfluidity goes back on Lew Dawidowitsch Landau. There in the temperature range of1K up to the Lambda point helium both superfluids and viscose rayon characteristics shows, assumes one that the total density of the liquid consists of a normal portion, which become increasingly smaller with sink-sinking that temperature and superfluids a portion. However also suggestions in the portion leave themselves to superfluidsproduce, which look like a viscosity from superfluidem helium. One e.g. pulls. over superfluid helium, then this does not feel a swimming body up to a certain critical speed (the so-called land outer criterion) any friction. Above this speed however Rotonen can and at still higher speeds phonons livelybecome, which affects the body such as friction. Computationally here a critical speed of approx. results. 60 cm/s. Indeed one states however that the critical speed is clearly below 1 cm/s. The cause is the suggestion of quantized eddies in the superliquid, so-called Vortices. This phenomenon is comparable with the suggestion of quantized circle stream in superconductors. The Vortices may not be confounded thereby with the Rotonen, since represent a latter macroscopic suggestion of the superliquid.

quantum-mechanical beginning

superfluidity can be explained in the model of the Bose Einstein condensation. Afterthis model occupies a macroscopic portion of all bosons same quantum state. Thus all He-particles, which are condensed into this initial state, can be described by only one wave function. The superfluids phase is just like the lasers and the superconducting phase a macroscopic quantum state. Ascritical temperature for the phase transition to superfluidem helium one receives thereby 3.1 K, which is somewhat higher than the measured 2.2 K. It is to be noted that the Bose Einstein condensation does not contradict the two-fluid model. The portion of the particles, which condenses in the initial stateis, depends on the temperature. Below a critical temperature (Lambda point with 4 He) occupy ever more particles the initial state, the lower the temperature is. The condensed portion can be regarded as superfluid helium, with the remaining particles concerns it normalliquid helium.

Contrary to the bosonischen 4 He - atoms concerns it with the atoms in nature of the rarely occurring 3 He fermions. To these applies not the Bose Einstein statistics, but the Fermi Dirac statistics. For the 3 He-atoms can thereforethe model of the Bose Einstein condensation not to be used. One observes He superfluidity nevertheless also with 3. This is however no contradiction, if one proceeds with superfluidity with 3 He not with isolated atoms, but from the coupling of two atoms, so that one similar to Cooper pair - the educationwith the superconduction a bosonisches pair with spin 1 receives. Two 3 He-atoms can take here an energetically lower (and therefore more probable) condition, if their magnetic core moments (nuclear spins) rectify themselves (magn. Conditions) or opposite direct (nichtmagn. Condition).

technical applications

inphysics and chemistry superfluid 4 He in the spectroscopy one uses. The technology becomes as superfluid helium Droplet Spectroscopy (SHeDS) and/or. Helium nano Droplet isolation (HeNDI) spectroscopy designates. The helium droplets used for it are produced in an adiabatic expansion by helium in vacuum equipment andpossess a temperature of only 370 mK. Molecules or cluster, which are solved in superfluidem 4 He, can rotate in fact freely, as if they would be in the vacuum.

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