Experimental cell
used for the experiment of electron incompressibility. The cell is open
to show the ring
electrodes. The tungsten filament that provides the electrons and
the input for
liquid helium are also indicated.
(November 2, 2015) Helium
usually reminds people of colorful gas balloons. However, helium is much more
than the filling for these children’s treats.
It also helps quantum physicists to study the most exotic and hidden
properties of matter. An international team led by Denis Konstantinov,
Professor at the Okinawa Institute of Science and Technology Graduate
University (OIST), discovered a condition by which electrons trapped on the
surface of liquid helium become incompressible at very low temperatures and
under microwave radiation. These findings were published in Nature
Communications in collaboration with researchers from the
Universite’ Paris-Sud and the RIKEN Institute. Compressibility measures a
change of volume when a pressure is applied. Normally compressibility is always
positive. But more pressure results in less volume. Imagine a rubber ball: the
more you press it, the more it squeezes. This happens because the distance
between the atoms of the rubber ball decreases when you apply pressure. The
same is true even for solid objects, like a table. Although you cannot perceive
it, if you press on a table, you actually force the table to become just a
little bit thinner. On the other hand, in a condition of zero compressibility,
even if you increase the applied pressure, you cannot change the volume.
Konstantinov and his collaborators are interested in studying these curious
states and found that under specific conditions, electrons’ compressibility
becomes zero.
EXperimental cell
used for the experiment of electron incompressibility. The cell is open
to show the ring
electrodes. The tungsten filament that provides the electrons and
the input for liquid
helium are also indicated.
The team is studying electrons pulsed out from a tungsten
filament and trapped on the surface of liquid helium at extremely low
temperatures. Helium was chosen because it is the only known quantum liquid: it
becomes liquid at low temperatures and it remains liquid even at zero Kelvin.
Moreover, it is free of impurities and it can form a very smooth surface with
only tiny ripples of less than 1 Angstrom, which is comparable to the size of a
helium atom. In a 3D space, usually particles can move in every direction, but
at low temperatures the system of trapped electrons can be simplified to a 2D
configuration and the scientists can concentrate on the 2D movement of the
electrons on the surface of the helium layer.
Above and below the helium 2D layer there are two
ring-shaped electrodes with an oscillating electric potential. Applying an
oscillating potential is equivalent to applying a force, therefore pressure, to
the electrons. The team measured the density of the electrons, which is linked
to the distance between the electrons under specific magnetic field conditions
and microwave frequencies, and found that the distance between the electrons
does not change, meaning that their compressibility is zero.