The LOW TEMPERATURE EXPERIMENT
Introduction
The LOW TEMPERATURE EXPERIMENT deals with the only two macroscopically observable quantum effects namely SUPERCONDUCTIVITY and SUPERFLUIDITY.
SUPERCONDUCTIVITY is a phenomenon which occurs if condensed matter is cooled down below a certain, so called critical temperature, Tc. Then suddenly its electrical conductivity becomes infinite and within certain limitations, current can be transported without losses. Further more a superconductor tries to keep its interior free of any magnetic field, by creating surface currents, an effect called the MEISSNER EFFECT. Materials which fulfill the two requirements R=0 and the MEISSNER EFFECT (B=0) are called superconductors and can be found among metals, alloys and recently also among ceramics.
SUPERFLUIDITY occurs in liquid helium (LHe) below the lambda point, a temperature, where the viscosity becomes zero and the heat conductivity infinite. In that case no local overheating is possible which can be seen in the absence of bubbles. Since the viscosity is zero, superfluid LHe can creep into the smallest openings and pass the thinnest tubes without resistance. Superfluidity is explained by the Bose-Einstein condensation where Bosons condensate into one low-energy macroscopic quantum state, described by one wavefunction. Among atoms only helium remains liquid at such low temperature that the effect can be observed. The case of superconducting electrons is somewhat more diffcult since they are fermions and by Pauli principle not allowed to occupy a single state. They nevertheless do so by coupling into Boson-like pairs as is explained in the BCS-Theorie (Bardeen, Cooper, Schrieffer).
Topics of the experiment:
- Learning how to perform a low temperature experiment. This includes dealing with cryostats, dewars, vacuum pumps, thermometers,liquid nitrogen, liquid helium and the transfer of those liquids.
- Calibration of a low temperature thermometer (carbon glass).
- Measuring the critical temperature of a thin superconducting film.
- Measuring the critical magnetic field of this film as a function of temperature.
- Detection of the Meissner effect on two bulk superconductors at different temperatures.
- Determing the lambda point of LHe by measuring the heat capacity.
- Evaluation of the heat conductivity as a function of temperature.
Demonstration of an Autostable Superconducting
Magnetic Bearing (not content of
experiment).
The experiment is documented in three files:
1) The main file 'lowtemp1': download Word file or view PDF file
2) The description of the computer program 'lowtemp2': download Word file or view PDF file
3) The wiring of the probe 'lowtemp3':
download Word file
or view PDF file
Further Links:
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Instructor |
Room: Kaplun 105 |
Phone: 972-3-640-8895 
