Physicists have measured for the first time how long it takes electrostatic interactions to emerge between particles in a semiconductor. Alfred Leitenstorfer and colleagues of the Technical University of Münich in Germany found that such interactions take several femtoseconds to take effect - confirming predictions made by quantum theory. The discovery has far-reaching implications for a wide range of condensed matter systems, including superconductors, organic semiconductors and biological molecules (R Huber et al 2001 Nature 414 286).
The force between two charged particles in a vacuum – whether attractive or repulsive – is governed by the Coulomb potential, and it gets stronger as the particles move closer together. But when many particles are present, this simple relationship gets complicated: a positively charged particle attracts a layer of negatively charged particles – and vice versa – and this shields it from interactions with more distant particles. This effect is known as screening.
Previous experiments appeared to show that this screening effect arose instantaneously, but this is because they could not detect changes on very short timescales. Now Leitenstorfer’s team have used a technique known as ultrafast laser spectroscopy, which is based on pulses of light just femtoseconds – 10-15 seconds – long.
The team fired a pulse of red laser light at a slice of gallium arsenide to create a plasma of electrons and positive holes. Femtoseconds later, an infrared pulse probes the “complex dielectric function” of the plasma, which is a measure of how easily the electrons can be separated by an applied electric field. Leitenstorfer and colleagues repeated this process several times, with different intervals between the first and second pulses. This allowed them to build up a picture of how the dielectric function changes on very short timescales, and showed that the screening effect took around 70 femtoseconds to emerge.
“Our example is the build-up of Coulomb screening in an electron-hole plasma, but the phenomenon is very general”, Leitenstorfer told PhysicsWeb. “It could be important for future semiconductor devices, ultrafast photochemistry, nuclear collisions, superconductors and biological complexes”.
The discovery also shows that quantum effects play an important – but usually neglected – role in the dynamics of many-body systems. Conventional – or “semi-classical” – theories of condensed matter treat particles as perfect spheres that interact with no loss of energy, and these models work well in many cases. “But wave features like interference are ignored by semi-classical models,” explains Leitenstorfer, “and we have shown that these are important on short timescales”.
