What did we do?


Today, we started by a visit at the STM (Scanning Tunneling Microscope) lab. The researchers were preparing the bakeout of the apparatus. There are turbo-molecular pumps which make the vacuum, nearly the interstellar vacuum. We need it because the STM scans only the surface and we have to get rid of the contaminants which can be on the surface.


The mass spectrometer enabled us to control the quality of the vacuum: there is for instance hydrogen, water and oxygen, which pollute the vacuum.


To get rid of the water, the chamber has to be heated (until the set point 150°C): thermocouples are used in order to control the heating by measuring temperature and stopping the heating when it is more than the set point.


Once there is a good vacuum (10^-11 mb) it is possible to study a clean surface during one day.



How does the STM work?


The STM is constituted of a metallic tip, which scans the surface. When the distance between the tip and the surface is of the order of half a nanometer, quantum mechanics enables a tunneling current when applying a voltage to the tip: electrons behave like a wave and a part of the wave is transmitted through the vacuum to the surface. The intensity decreases exponentially, depending on the distance between the tip and the surface.


The tip is fixed to a piezoelectric tube. The tip can be moved thanks to the piezoelectric materials, whose size depends on the voltage. When operating, the intensity of the tunneling current stays the same (there is a set point for the current). In order to keep the current constant the size of the piezoelectric material is changed. So we can measure the distance between the tip and the atoms in order to find the topology of the surface.


That's how surfaces can be mapped.




Then we went to the cryogenics lab, which recycles and stocks the helium, which is the second component of the Universe. Nevertheless, the helium is very expensive because it is a limited resource on Earth, so we can't waste it so much. There are a lot of machines (liquefactor, compressor, big balloons!) in order to compress, and stock it as a liquid because one 1L of liquid helium corresponds to 700L of helium gas. Helium is used in order to cool down the samples (around to 4K). At a low temperature, the resolution is better, and electronic phase transitions can happen.