How Light Directs Electrons In Metals

ETH researchers have measured how electrons in transition metals are redistributed in fractions of an optical oscillation cycle. The electrons concentrate in less than a femtosecond around the metal atoms. This redistribution could influence important macroscopic properties of compounds with transition metals, such as conductivity, magnetization, or optical characteristics, which could thus be controlled on the shortest time scales.

The distribution of the electrons in the transition metals, which make up a large part of the periodic table of the chemical elements, is responsible for many of their properties of interest to applications. The magnetic properties of some members of this material group are used for data storage, for example, while others are characterized by their excellent electrical conductivity. Transition metals also play a crucial role in materials with more exotic properties, which are based on strong interactions between the electrons and are promising for a range of future applications.

In their experiment, the results today in the journal Nature Physics published, Mikhail Volkov and have fellow colleagues in the group for short-term laser physics by Professor Ursula Keller thin titanium and Zirconiumfolien exposed to a short laser pulse and the redistribution of electrons in these transition metalsobserved over the resulting changes in optical properties in extreme ultraviolet (XUV). In order to be able to follow the induced changes with sufficiently fine time resolution, they used XUV pulses with a duration of a few hundred attoseconds (10 ^{-18 s}) for the measurement . By comparison with theoretical models, which were contributed by the group of Prof. Angel Rubio of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, it could be shown that in less than one femtosecond (10 ^-15s) is due to a localization of the electrons around the metal atoms. The theory also predicts that in transition metals with more filled outermost atomic shells, an opposite movement - that is, a delocalization of the electrons - is to be expected.

Ultrafast control of metal properties
The electron distribution determines the microscopic electric fields in a material, which not only hold it together, but also determine a large part of its macroscopic properties. Changing the distribution of the electrons also influences the properties of the material. The experiment by Volkov et al.has shown that this is possible within time scales that are much shorter than the cycle of visible light oscillation (by the two femtoseconds). Probably more important is the fact that the time scales are much shorter than the so-called thermalization time, within which the electrons would collapse by impacting each other and with the crystal lattice any effect of such an external control of the electron distribution.

Initial surprise
That the laser pulse in titanium and zirconium leads to increased localization of electrons, was initially surprising to the researchers. A general trend in nature is that when you supply bound electrons with more energy, they are less localized. The theoretical analysis supporting the observations from the experiments showed that the increased localization of the electron density is a net effect, which results from the stronger filling of the d- orbitals of the metal atoms , which are characteristic of the transition metals and only partially filled . For transition metals, which over already more than half filled d-Orbitals - these can be found further to the right in the periodic table - the net effect, however, is a delocalization of the electron density.

Towards faster electronic components
While the now published result is of a fundamental nature, the experiments show the possibility of a very rapid modification of material properties. Such modulations are used in electronics and optoelectronics for the processing of electronic signals or the transmission of data. While signal currents with frequencies in the gigahertz range (10 ⁹ Hz) are modulated in current components , the results of Volkov et al. to the possibility of signal processing in the Petahertz area (10 ¹⁵Hz). The very basic findings can thus influence the development of the next generation of ever faster components and thus indirectly find their way into our daily lives.