In this section of the experiment, we are trying to find the transition (to superconducting state) temperature(\(Tc\)) of a high \(Tc\) superconductor, \(Bi_2 Sr_2 Ca_2 Cu_3 O_{10}\) (Bi2223).
We placed our superconductor in a jug, and filled the jug with small glass beads to cover the Bi2223 superconductor. This will allow the temperature to change slowly so we can obtain a smoother graph of \(V[\textrm{V}]\) vs. \(T[\textrm{K}]\) to determine the transition temperature.
We used a commercial superconductor where all the leads for current input, voltage and type T thermo-coupled are readily attached. The current input was an AC signal of \(10\textrm{mA}\) at \(f = 101.19\textrm{Hz}\), and connected the voltage leads to a lock-in amplifier (sensitivity=\(200\mu V\)) and then to a DMM (Digital Multimeter) and thermo-couple leads to another DMM. We recorded an offset of \(10\textrm{K}\) in room temperature. The phase of the signal were recorded before the sample was cool. When the sample becomes superconducting, we expect the phase to shift by \(180\) degrees, since resistance in the sample and thus the voltage will go to \(0\).
Then, when all the connections are made, we added liquid nitrogen to the jug, little by little, until the temperature of the superconductor read about \(30\textrm{K}\) below the expected transition temperature of \(108\textrm{K}\). We did, indeed, observe a \(180\) degrees shift in the phase of the voltage. As the sample starts to warm back up, we started collecting data for \(V\) and \(T\), until the sample is warmed back to almost room temperature, and that the sample has shifted back to a non-superconducting state.
The result of \(V\) is plotted against \(T\) as shown in Figure \ref{fig:TransTemp}.