Thursday, May 02, 2013

Electrons and Lattice Vibrations: A Strong Team in the Nano World

La electrónica de semiconductores genera, controla y amplifica la corriente eléctrica en dispositivos como los transistores. Los portadores de corriente eléctrica son los electrones , lo cuales se mueven a altas velocidades (10000 a algunos millones de metros por segundo) en las estructuras cristalinas de los semiconductores. Sin embargo al hacer esto, pierden una parte de su energía cinética al chocar con los átomos en dichas estructuras, y como la energía no se pierde, se transmite en forma de vibración a dichos átomos. En el arsenuro de galio por ejemplo dichas vibraciones tienen un periodo de 100fs (1 fs = 10-15 s = 1 billionth part of one millionth of a second).
In the microcosm of electrons and ions such vibrations are quantized. This means that the vibrational energy can only be an integer multiple of a vibrational quantum, also known as a phonon. When an electron interacts with the crystal lattice (the so called electron-phonon interaction), energy is transferred from the electron to the lattice in the form of such vibrational quanta.
Berlin researchers report in the latest issue of the scientific journal Physical Review Letters that the strength of the electron-phonon interaction depends sensitively on the electron size, i.e., on the spatial extent of its charge cloud. Experiments in the time range of the lattice vibration show that reducing the electron size leads to an increase of the interaction by up to a factor of 50. This results in a strong coupling of the movements of electrons and ions. Electron and phonon together form a new quasi particle, a polaron.
To visualize this phenomenon, the researchers used a nanostructure made from gallium arsenide and gallium aluminum arsenide, in which the energies of the movements of electrons and ions were tuned to each other. The coupling of both movements was shown by a new optical technique. Several ultrashort light pulses in the infrared excite the system under study. The subsequent emission of light by the moving charge carriers is measured in real time. In this way two-dimensional nonlinear spectra (see Fig.) are generated, which allow the detailed investigation of coupled transitions and the determination of the electron-phonon coupling strength. From the coupling strength one finds the size of the electron cloud, which is just 3-4 nanometers. Furthermore, this new method shows for the first time the importance of electron-phonon coupling for optical spectra of semiconductors. This is of interest for the development of optoelectronic devices with custom-tailored optical and electric properties.

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