A Blink of the Eye from the Insulator to the Metal

The clever combination of novel technologies allows us to examine promising materials for the electronics of tomorrow.

High harmonic spectroscopy of the light-induced phase transition. The vertical red line shows where the electric field of the laser (yellow oscillating curve) crosses the threshold field, destroying the insulating phase of the material. The upper graph shows the average number of doubloon-hole pairs per square (blue) and the decay of the insulating, field-free ground state (red). Source: MBI Berlin

In recent decades, computers have become faster and faster, while hard drives and memory chips have reached huge capacities. However, the development can not always go on like this: Even today, physical limits are becoming apparent, which make further drastic acceleration of silicon-based computer technology impossible. In the search for novel materials and technologies for information processing, researchers hope that the combination of electrical and optical circuits will give them a whole new impetus. With the help of short laser pulses, a research team led by Misha Ivanov from the Max Born Institute in Berlin, together with scientists from the Russian Quantum Center near Moscow, succeeded in shedding light on the extremely fast processes in such novel materials.

Of particular interest to modern materials research in solid state physics are so-called “highly correlated systems” in which the electrons interact in the material. An example of this are magnets: Here, the electrons in the material align in a preferred direction of rotation and thereby generate a magnetic field. However, completely different organizational structures are also conceivable. In so-called Mott insulators, which are currently being intensively researched, the electrons should actually be able to flow freely and the material should be electrically conductive like a metal. Due to the mutual interactions in this highly correlated material, however, they hinder each other and the material becomes an insulator.

If one disturbs this order by a strong laser pulse, the physical properties also change dramatically. This is known from the transition from solid to liquid: When ice melts, the rigid ice crystal transforms into freely moving water molecules. Similarly, the electrons in highly correlated materials gain mobility when their order undergoes a phase transition through external laser pulses. Therefore, such phase transitions open the possibility to develop completely new switching elements for modern electronics, which are faster and presumably more energy efficient than today’s transistors. In principle, computers could become around 1000 times faster thanks to the combination of electrical components with light pulses.

The problem with analyzing such phase transitions: they are extremely fast and therefore difficult to study. Previously, scientists could only determine the state of the material before and after such a phase transition. However, the researchers Rui EF Silva, Olga Smirnova and Misha Ivanov from the Max Born Institute have come up with a method to throw light on these processes in the truest sense of the word: According to their theory, these materials can be irradiated with extremely short, tailor-made laser pulses that are only now available in this quality. Thus, as a reaction of the material to these pulses, it can be observed how the electrons in the material are excited to move, emitting harmonics with certain frequencies like a bell, as harmonics of the incident light.

“When we analyze this high harmonic spectrum, we can observe for the first time the change in the ordering structure in these strongly correlated materials,” says first author Rui Silva. Only recently have there been laser sources that are able to trigger these transitions specifically. On the one hand, the laser pulses must be strong enough – and on the other hand extremely short and in the femtosecond range (millionth of a billionth of a second).

In part, a single light oscillation is sufficient to spin the order of the electrons in the material and turn a insulator into a metal-like conductor. The scientists at the Max Born Institute are among the leading experts worldwide in this field of ultrashort laser pulses.

“If we want to control the properties of the electrons in the material with light, we need to understand exactly how the electrons react to light pulses,” explains Ivanov. Thanks to the novel laser sources, which can even completely control individual oscillations of the electromagnetic field, the now published method offers deep insights into the materials of the future.

Source : Forschungsverbund Berlin e. V.