Tunneling Current Update

So I constructed a feedback circuit to make it easier to achieve a tunneling current, and after a lot of testing I think that I finally did it!

Feedback control

The feedback loop was built on the same circuit board as the transimpedance amplifier. I will probably make several changes to the final version as this one has a few flaws in its performance.

It starts by inverting the amplified tunnel signal. This now positive voltage is summed with a negative set voltage to produce an error signal. The error signal is then fed into an op amp integrator. The integrator takes the integral of the error signal, meaning that it rises and falls when the error signal is above or below zero. The resulting output voltage Is then used as a Z signal to control the height of the tip.

Tip attachment

In order for the feedback loop to do anything at all the tip needs to be attached to a piezo element. The piezo element is what does the actual movement, and needs to hold the tip firmly while still keeping it electrically isolated. I achieved this with a small piece of glass between the piezo and the tip. I also used a small piece of prototyping board and a M2 nut to make it more rigid. Epoxi glue was used to hold everything together. If I need to change tip I simply solder it in the prototyping board.

What I got

When I tested this setup, I simply turned the adjustment screw to get the tip and sample as close as possible while keeping an eye on the outputs through my pc oscilloscope. I had it set up so that the green trace showed the height signal going to the piezo and the red trace showed the amplified current signal going from the tip. Each division in the vertical axis represents 3 volts and each division in the horizontal axis represents one second. When I got what I was looking for I took this screenshot:

This looked interesting to me because the signals usually looked different. When the tip and sample were shorted, the current signal would be steady at about -9V, and the height signal would be at about +9V, trying to raise the tip as much as possible. When they were completely disconnected the current signal would stay at zero, while the green line would be at about -9V, trying to lower the tip. During both of these cases the signals would be quite steady. Before I used any feedback loop, I could only get signals that looked like the tip and sample were either shorted, or completely disconnected. I was only able to get the effect shown above when using the new setup. This effect could be explained by quantum tunneling. The current is held at a non-zero voltage, which means that it could be caused by a small tunneling current, while the height signal is going up and down trying to compensate for any changes in tip-sample distance. The extra noise could also be explained by the fact that the system is very sensitive while tunneling. Any tiny changes in tip-sample distance will create a big difference in tunneling current.

Of course this is no proof that I actually got a tunneling current. Some other unforeseen imperfection in the system could have caused this effect. In order to get more evidence for quantum tunneling, I tried to rule out other causes. The signal seemed to be sensitive to mechanical forces, which would suggest that the cause is not purely electrical. I also tried cutting the wire going to the piezo disc, to see if the piezo disc had any effect on the signals, or if it just acted as a capacitor to ground. The results seem to show that it did have an effect.

These small experiments are still not enough to prove anything about quantum tunneling. Maybe the effect is caused by a bad solder joint somewhere, maybe the signal above just changed because I hit the setup when trying to cut the wire. While these screenshots are far from a proof of quantum tunneling it does give me confidence. I could not get anything like this before, and now I can replicate it. The electronics I bought have arrived and the pcbs should arrive soon. I am looking forward to doing more research with a more precise system.