Here’s a measurement showing the response of a Josephson Junction as it switches from the zero voltage (superconducting) state to the voltage state, corresponding to escape of the phase from a potential well in the junction’s ‘washboard’ (energy) potential. The switching is a probabilistic process, so many measurements are compiled into a histogram at each temperature to get an average, stochastic response. The width of this histogram is then monitored as a function of temperature:
Classically, the phase gets excited out of the metastable minimum due to thermal fluctuations (the state gets a ‘kick’ out of the well from the thermal energy available in the system). As the temperature is decreased, this is less likely to happen, and so the state stays in the well for longer, and the histogram gets narrower. If the junction is small enough it may be possible to see escape due to quantum tunneling of the phase, a competing escape mechanism. This is a temperature independent process and the width should saturate at low temperatures if quantum tunneling occurs. Unfortunately, the prescence of external interference gives a very similar effect. So here we measure a large junction, which should behave classically to a very low temperature. Any saturation of the width would demonstrate noise limitation.
This is a textbook response: The straight line demonstrates that the system follows the thermal activation theory, and furthermore is not noise limited. So in future measurements of junctions, any saturation observed must be either due to quantum effects, or noise sources intrinsic to the junction itself.
So I can now believe that I’m seeing real quantum processes in the junctions.
This makes me happy.
The CF cryostat is a lovely piece of kit for quickly measuring samples down to 4.2K. Here is a picture:
The dewar containing the liquid Helium can be seen on the left. The setup is rather simple, you have a gas pump, a thin transfer tube (the silver tube joining the dewar to the cryostat) and the sample space (inside the brass-coloured cryostat body). When the pump is turned on, it draws helium from the dewar through the thin tube and the sample space and returns it to a pipe on the wall (part of the overall in-house helium gas system installed in all the labs). Because the transfer tube is thin, the liquid Helium comes through quite slowly (you can adjust the flow rate) and a single dewar of liquid Helium (which holds ~45L) can therefore go a long way.
The electronics to control the experiment can be seen on the rack of equipment to the right in the photograph. This particular setup allows current-voltage (IV) characteristics and differential conductance (dI/dV) measurements to be performed on Josephson Junctions, with a PC controlled DAQ (data acquisition) system. A patch panel on top of the experimental insert allows up to 20 DC lines to be chosen for the measurement, so that many junctions to be tested in the same run.
Controlling the temperature on this type of cryostat is notoriously tricky. The generally accepted method is to run the pump to cool the system, whilst simultaneously heating the sample until the temperature stabilises at the required value. Which is a bit wasteful, it’s like running the heater and the air-con in your car at the same time. It can be done automatically with a PID temperature controller. Another way to obtain a measurement as a function of temperature is just to record data whilst the system cools down to 4.2K whilst the pump is running, or warms back up to 300K after the pump is switched off. Doing this on the warming cycle is slightly more stable. Additionally you don’t have the additional electrical noise of the pump if you are trying to conduct a low-noise experiment. This is a good way of obtaining Resistance-versus-Temperature (RT) measurements, which allow you to see the point at which your sample/junction goes superconducting. From this data you can also obtain the Residual Resistivity Ratio (RRR) which gives a measure of the quality/purity of the material being used.
A couple of thermometers are usually placed near the sample to give a good idea of the temperature gradient inside the cryostat. If there is a high temperature gradient near the sample, the temperature you read on your thermometer (which is generally a few cm away) might not be the exact sample temperature.
One of the best features of the CF is that if you run the pump for long enough, once the temperature inside the cryostat gets down to 4.2K, liquid Helium starts to collect at the bottom. Once you have collected some, you can turn off the pump and the system will stay at 4.2K until the collected liquid has all boiled off. This is a nice way to ensure that your measurement stays at a definite temperature for about an hour. Of course we don’t turn the heater on when we have liquid Helium in the bottom of the cryostat, or else the top of your experiment blows out and hits the ceiling as the Helium gas tries to occupy 700 times its liquified volume 🙂
With Niobium based Josephson junctions (Tc~9K), 4.2K is a good temperature to take measurements of their superconducting IV curves.
Why would you want to measure IV curves? Well, that’s another post 🙂
I’m currently reading Distress by Greg Egan. The story is set at a Physics conference, so I’m actually really enjoying it (although I think overall I preferred Permutation City). It is however most unlike any Physics conference I’ve ever attended. With murder, mystery, intrigue, fanatical religious cults, shadowy biotech corporations, kidnapping, deadly bioweapons and potentially the end of the multiverse as we know it, maybe I should be frequenting TOE conferences instead of LT ones 🙂
However there are some descriptions which aren’t so far fetched, such as the conference venue being a picturesque tropical coral-reef island. That one does happen occasionally.