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Superconducting transition of YBa2Cu3O7
Navin Deendyal
Physics Department, Purdue University, West Lafayette, IN 47907-1396
Final submission submitted Friday, May 5, 2000
Abstract
Measurements are made on superconducting YBa2Cu3O7 to observe the transition
temperature.
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Introduction
In 1933, two German physicists, Meissner and Ochsenfeld, observed that a
superconductor expels magnetic flux completely, a phenomenon known as the Meissner
effect1. In a series of experiments on superconducting cylinders, they demonstrated that,
as the temperature is lowered to Tc, the flux is suddenly and completely expelled as the
specimens become superconducting, as shown in figure 1. The flux expulsion continues
for all T < Tc. They also demonstrated that the effect is reversible: when the temperature
is raised from below Tc, the flux suddenly penetrates the specimen after it reaches Tc, and
the substance is in the normal state.
This experiment is designed to observe this effect in superconducting
YBa2Cu3O7, which normally has a Tc ≈ 91 K.
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Fabrication of the sample
The fabrication of the superconducting 123 copper oxide wafer was created from
a 10 g pellet of YBa2Cu3O7. The pellet was prepared from mixing powders of Y2O3
(99.9% purity), CuO (99.7%), and BaCO3 (99.9%) which were ground, pressed, and
annealed in oxygen at 9500C for a total of 50 hours. The pellet is ground, pressed, and
annealed twice to ensure proper homogeneity of the sample. The pellet becomes an actual
wafer of radius 0.8 cm and thickness of 0.5 cm after running it through a press. The wafer
looks like a black quarter but is somewhat fragile and can easily be crumpled with little
force.
Measurement
To determine the superconducting transition range, the specimen was mounted
onto the cold stage of a He closed-cycle refrigerator, with wires attached for a standard
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four-point resistance measurement. The wafer was sandwiched between two Helmholtz
coils using the principle that an AC current will produce an EMF. The driving coil has
100 turns and is composed of copper. Having similar composition, the pickup coil has 50
turns as shown in figure 2. The cryostat system is monitored by a dual trace scope to
observe the transition at Tc, when a normal sinusoidal wave is distorted as the
temperature slowly crosses the threshold temperature. A five-ohm resistor is used in
series with the power supply to reduce noise and keep too high of a current from passing
through the sample, as shown in figure 3. The driving frequency was 36 kHz and the
cryostat system was pumped down to 15 µATM.
Analysis and Conclusions
As can be seen by the plot of resistance vs. temperature (figure 4), there is a
transition peak around Tc ∼ 85 K. For our sample the transition range was between 82-88
K. These values may be off a little due to the fact that the temperature of the wafer was
determined by the copper slab which it rested on which had a temperature gauge hooked
up to it. There was most likely a temperature gradient across the slab making the actual
temperature of the sample one or two degrees off. However, the main thing to notice is
that the two curves on the resistance vs. temperature plot show a distinguished peak
around Tc and not a smooth linear graph as would be expected for T > Tc. In essence the
experiment displays the Meissner effect and beautifully illustrates the superconducting
transition temperature of YBa2Cu3O7.
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Acknowledgements
Thanks to Kristl Adams, Ricardo Vasquez, and Dorjderem Nyamjav for their
many dedicated hours of hard work trying to get the experiment to work. We are indebted
to Eric Dedrick for his helpful tips and useful insights.
References
1 M. Ali Omar, 1993, Elementary Solid State Physics, Reading, Mass.: Addison-Wesley
2 S. M. Durbin et al., Phyics C 324 (1999) 1331.
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