# Presentation: 512461

## Making a Movie of a Single Electron 

- An electron cannot be seen in an ordinary microscope because the optical scattering cross-section is much too small.
- We have been able to make a movie of electrons that are immersed in liquid helium.
- Electrons introduced into liquid helium form bubbles of diameter one sixth of a millionth of an inch (= 4 nm).

- Making a Movie of a Single Electron

-  By applying a negative pressure for one millionth of a second we make the bubbles much larger, just long enough to be photographed by a camera with a strong flash.   J. Low Temp. Phys., 148, 199 (2007).

**Notes:**
At low temperatures liquid helium becomes a superfluid. This means that unlike all other liquids, helium has no viscosity. If a cup of coffee is stirred with a spoon, after the spoon is removed the liquid continues to flow around for a while and then comes to rest. Superfluid helium, in contrast, will continue to flow for ever. Because of the absence of viscosity, superfluid helium can flow without resistance through very small holes, even holes that are hard to see under a microscope. 

Starting in the 1970’s physicists began to wonder whether solid  helium could also become superfluid. It was anticipated that if this did happen it would only occur at very low temperatures. What would a superfluid solid be like?  It was expected that a superfluid solid would have what is called a “non-classical moment of inertia”. If you pick up a piece of wood and try to spin it, you have to provide a force to make this happen. The bigger the piece of wood, the more force that you need. The amount of force that is required is determined by the moment of inertia of the object and this is a relatively simple thing to calculate. A “non-classical moment of inertia” means that the moment of inertia is less  than expected and so the force needed to spin the object is also less. Evidence of this effect in solid helium was found in a pioneering experiment by Kim and Chan in 2004. 

The reduced moment of inertia arises because when the object is rotated by grasping it at its surface, some part of the mass of the object does not rotate but instead stays at rest. This implies that some of the mass must be able to move through the object. This means, for example, that if a section of a pipe was blocked by a piece of superfluid solid it would still be possible for mass to flow from one side of the “block” to the other. This is the experiment that we have performed. We find that when a tube is blocked off with a slab of solid helium, liquid helium can still flow from one side to the other. However, our experiments suggest that the liquid is flowing along boundaries where two crystals of solid helium meet and not through the individual crystals. This means that superfluidity is not a property tha is possessed by a perfect crystal of solid helium but is only present when the crystal has defects. 

## EDUCATION and OUTREACHThe movie was reproduced in many science magazines and websites. We have answered many e-mails from high school and undergraduate students asking for explanations of how the experiment works.  

- We hope to use this method to make detailed studies of quantized vortices in superfluids.

- EDUCATION and OUTREACHThe movie was reproduced in many science magazines and websites. We have answered many e-mails from high school and undergraduate students asking for explanations of how the experiment works.

- Each of the little white spots moving across the screen is an electron. The electrons moving along snake-like paths are sliding along a quantized vortex!

**Humphrey J. Maris, Brown University, DMR 0605355**