Measure Tc's

It is of great interest to know what the critical temperature (Tc) is for each of the phonon channel. Thus, it is of even greater importance that our base temperature is below all of the Tc’s (hence the care we take with pump- ing out exchange gas, cleaning the mixture, getting the correct amount of mixture into circulation, etc. – we want the base temperature to be as low as possible). To get these measurements, you will slowly raise the temper- ature of the mixing chamber in order to make the sensors transition from superconducting to normal, and then you will let them cool back down to their superconducting state. You will note the temperature at each of these transition points.

Ensure that the SQUIDs have been tuned and locked. You can refresh your memory in the “Zap SQUIDs and SQUID Tuning” appendix. Ensure that the QET bias is set to 0 on all channels.
Ensure the timing link is set up.
Go to the detector control vi. Set the QET bias to 0 uA. Maximize the Output gain to 7.
Go to the Test Signal tab and select all four channels’ QET bias boxes. Select a 10 mVpp wave at 100 Hz. Then start the pulser.
Pull up the detector readout and switch to the “All Channels” view. Select the “External Trigger” mode. You should now see a triangle wave. For the trace length, you should use either 16.384 ms or 3.2768 ms, with a lookback time of 400 or 100 uS, respectively. 3.2768 ms should be ﬁne, though. You want to see at least one full triangle wave in the window. If you can’t see a complete triangle, then lower the Output gain by 1. If you still cannot see the triangle wave completely, continue to lower the Output gain by 1 until you do. The goal is to maximize this value as much as possible while being able to see the triangle wave clearly. Do this for all of the channels on this dib. Record the Output gain values for each channel.
From the responsivity window, record the peak-to-peak amplitude for each phonon channel.
You are now prepared to conduct the Tc measurements. Measuring the Tc’s of the phonon channels involves heating the sensors until the transition to normal from their superconducting state. You are going to do this via the IGH Front Panel. You will use the mixing chamber heater to raise the temperature in very, very small increments. Make sure that the mixing chamber heater settings are at 0 nW. An indication of transition is found in the shape of the triangle wave. If transition occurs, you will see the triangle wave shape begin to wiggle. Parts of the wave may jump erratically at ﬁrst sign. When transition has fully occurred, the peak-to-peak amplitude of the triangle wave will be much smaller than it was originally (hence the reason for maximizing the Output gain).
With all channels displayed, turn on the mixing chamber heater to 100 nW. Monitor the change in the base temperature. If the temperature rises and settles without inducing a transition in any of the channels, you may increase the heater by 2-5 nW. Be warned that this may cause a signiﬁcant change in your temperature, so it is a risk to make higher leaps in the heater level. If you raise the power too high too soon, then the temperature will rise too quickly for you to get an accurate reading of when transition occurs. Thus, it is recommended that you make variations in steps closer to 2 nW. When transition starts happening, make note of the temperature (keep 3 signiﬁcant ﬁgures). The signal will continue to wiggle crazily as the temperature increases, which is indication that the sensor is undergoing a transition from its superconducting state to its normal state. Eventually the signal will stop wiggling and it will return to a clean triangle signal. Record the temperature when the signal completely stops wiggling. If the temperature transitioned too quickly, you will have to re-do this measurement by lowering the power and waiting for it to cool below the Tc. If you are happy with your Tc measurement, then continue to raise the heater’s power in small increments to continue gathering Tc measurements for the rest of the phonon channels.
Once you have collected Tc measurements for all of the phonon channels (going from superconducting to normal), record the peak-to-peak amplitude value for each phonon channel in its normal state. This will be further conﬁrmation that the channels have indeed transitioned because the new amplitude should be an order of magnitude smaller than the original amplitude. 10. Now you will measure the Tc’s again as the sensors cool. This means that you will slowly lower the power of the mixing chamber heater, and as the signal begins to transition from normal to superconducting, you will record the temperature. When the transition ﬁnishes, record that temperature as well. Again, if you lowered the heater power too quickly, you will have to redo this measurement.
If you are happy with your Tc measurements for all four phonon chan- nels, then you should have 2 sets of temperature measurements, and two peak-to-peak amplitude measurements for each channel. Record them in a table and put it in the run diary. Click here to see an example of what is expected. If you don’t have this, then you are not done.