Good vibrations key for superconductors

The interaction between an electron carrying charge in a superconductor and the conductor’s lattice is sensitively linked to the size of the electron, or more specifically the electron cloud say researchers in Germany. Researchers from Forschungverbund Berlin have used a new optical technique to study the nanostructure of gallium arsenide and gallium aluminium arsenide in which the energies of the movement of electrons and ions were tuned to each other. They revealed that electrons in a semiconductor are best described as a cloud, and that the size of the cloud is determined by the interaction of the electrons with vibrations in the crystal lattice.

Mobile electrons move at high velocity through the crystal lattice of a semiconductor – in the process they lose kinetic energy causing atoms in the lattice to vibrate. This vibrational energy is known as a phonon and the strength of the electron-phonon interaction depends on the size of the electron, specifically the electron cloud. The new spectroscopic technique used several ultrashort pulses of infrared light to excite the semiconductors being studied. The subsequent emission of light by the moving charge carriers can be measured in real time, allowing detailed investigation of the coupled transitions and the determination of the electron-phonon coupling strength. Researchers can then find the size of the electron cloud – usually between 3-4 nanometres. They also showed that reducing the electron size can increase the electron-phonon interaction by up to a factor of 50. This results in a strong coupling of the movements of electrons and ions. A new quasi particle is formed from the electron and phonon – a polaron. This new method has highlighted the importance of the electron-phonon coupling for optical spectra of semiconductors, which may have implications for the development of optoelectronic devices.