Check-in before christmas

This is just a small check-in to show that I haven't abandoned the project. I am still waiting for those PCBs that I ordered. For a moment I was afraid that they had gotten stuck somewhere, but according to their tracking they have “arrived in country”. Hopefully “country” means Sweden. Since it’s the holidays they they will not arrive before christmas, but it’s still nice to have some sort of confirmation that they are not forgotten in some box in China.

Next time I post will be after christmas so I wish you happy holidays!

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.

While I'm waiting for new parts

While waiting for the new materials to arrive I’ve been doing some testing with the components I already have access to. As mentioned in my last post I borrowed a few op amps from school. I used one of these to create a transimpedance amplifier circuit. While the components are not as nice as the ones I’ll use later this circuit will hopefully work good enough to measure a tunneling current.

Trying to Achieve Tunneling

In my first tests I used a multimeter to measure the transimpedance amplifiers output while bringing the tip close to a metal sample. The tip in this case was just a copper wire which was isolated from the scan head by two glass microscope slides. The tip was connected to the circuits current input. The sample was a piece of a circuit board with a gold plated pad. This was connected to a resistor divider which produced a bias voltage of about 0,8VDC. When the tip crashed into the sample and they shorted the multimeter showed a value of about -1,1V. When they were not connected the multimeter showed a value of -8 or -7 mV. The lower value stayed pretty consistent with sample distance so it could not have been a tunneling current. After a lot of touching and adjusting I was able to get an output that fluctuated between -500 and -300 mV. However, I got this by slightly tilting the top half of the scanhead. With how slowly the multimeter updates and how unstable the entire setup was I think that It was most likely due to some play between the tip and the sample.

In my later tests I also had an oscilloscope connected to get a higher update rate, but I was never able to recreate the effect. I tried to make the setup more rigid by adding a rubber band to keep the two halves of the scan head together. This was still not giving me any major results. I knew that I was going to have to think of noise and vibrations later on in the project, so I turned to that. I hanged the setup underneath a tripod to eliminate vibrations, and I even tried to get rid of some noise by adding a saucepan connected to ground.

        

While this setup is pretty funny, I realized that this was not really helping me. All the extra stuff made it really hard to spot problems or to do adjustments. Also, I haven’t studied shielding very deeply but I suspect that a big opening at the top might not be optimal. While they did not yield the results I was hoping for these experiments might have given me some useful experience for later on when focusing on stuff like vibration isolation and noise reduction. For now I have another idea for how to approach this problem.

Feedback Loop

In my first overview post I wrote that I was not planning on focusing on the feedback loop until after I’ve gotten images. However, I now realize that It might be a good idea to reschedule. A working control circuit connected to a piezo crystal is a much better adjuster than my human fingers on a screw. Instead of having to get the tip within a nanometer range of the sample I’d just have to get it within the piezo crystals travel range. A simpler version of this would be to simply have a potentiometer connected to the piezo so that it acts like a very fine, but still manual, adjustmer. I currently have no way of attaching a tip to the piezo crystal while keeping it isolated, which means more tunneling testing might have to wait until monday when I have access to my schools workshop, (and epoxy glue). If that's the case, I should still be able to do some electronics work tomorrow.