1ft 'J .Ó AP E418 Superconductivity ExperimentS uperconductivity may beome one of the most signifcant inovations of mom times. Ths is a result of very recent discoveries of high temperature superconductorswhich may revolutionize everything from electrcal generation and transmission to high-speed cQmputers to controlled fusion generatig plants.In ths experiment, you will investigate the superconductivity of the new Y -Ba-Cu-O compound system as a function of temperature and magnetic field. The most important question should should answer is "What is the resistance of the superconductor as the temperatu and applied magnetic field ar changed?" Other questions which you wil not be able to answer are related to the determination of curent density which can pass through the material and the mechanical strength and elasticity of the material.Attached to these notes are some articles which describe aspects of superconductivity and the results of recent measurements of the Y -Ba-Cu-O compound made by Prof. M. K. Wu and co-workers. These wil provide background for the experiments that you wil be penormg. Pay paricular attention to Prof. Wu's aricle. Their measurements are identical to those that you can obtan with the equipment supplied in the laboratory. Next, look over the description of tye 1 and j , ty.e 2 superconductors given by Gennes in Superconductivity in Metals and Alloys.o.f\1. , l' -Type 1 superconductors have a "correlation length" between. conduction electrons ((sheri (denoted by the symbol, ÇO) much longer than the magnetic field penetration length ( denoted by Â.). They generally have a very abrupt crtical field, He, below which the P-i1lèt material is superconducting and excludes all magnetic flux. (H e is a function of temperature.) This "penect diamagnetîsm" is charaèteristic of the superconducting state. Type 2 superconductors have Â. ~ Ço and have a more gradual transition to this superconducting state. They are described by thee critical magnetic fields. Below Hci (the lowest critical field), a typ 2 superconductor has no resistace and, like a type 1 superconductor, excludes flux. Between H c1 and H e2, the resistance is very low, but finite, and the superconductor parally excludes flux. Between He2 and He3, the resistance is low, but no flux is excluded. Finally, above the highest of the three crtical fields, He3. the material is no longer superconducting. DescrI ptionThe Y -Ba-Cu-O compound becomes superconducting between 77 and 85 eK.This makes the cryogenic aspects of these experiment very easy compared with experiments using more conventional superconductors that require temperatures below 22 eK. Liquid nitrogen is relatively inexpensive and a styrofoam thermos is al that is required to store it for several hours.-We have built three LN2 containers for you, and these are large enough to hold stainless-steel and copper "sample stands". The sample stands cool the 1 \, W£t.~ i-superconducting sample and provide a stable base to make your measurements. Also, attached to each sample is a calibrated t...

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The intermetallic compounds AAl 2-x Si x , where A = Ca, Sr or Ba, crystallize in the C32 structure, same as the recently discovered MgB 2 with a high superconducting transition temperature of 39 K. For x = 1, superconductivity has been observed in AAlSi with A = Ca and Sr, but not with A = Ba. The transition temperatures are 7.8 and 5.1 K, respectively for CaAlSi and SrAlSi. The CaAl 2-x Si x compound system display a T c -peak at x = 1, a possible x-induced electronic transition at x ~ 0.75 and a possible miscibility gap near x ~ 1.1 which results in a very broad superconducting transition. The Seebeck coefficients of AAlSi indicate that their carriers are predominantly electrons in nature, in contrast to the holes in MgB 2 .

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