From insulator to conductor in a flash

2 mins read

The demand for more data capacity has been a constant industry venture and the capacities current hard disks and storage chips have managed to attain are impressive. But, with the physical limitations of silicon-based computer technology, it’s becoming harder and harder to accomplish impressive speed gains.

Some researchers have expressed optimism over the next era of technological advancements, believing that they will start with the development of novel information-processing materials and technologies that combine electrical circuits with optical ones.

Using short laser pulses, a research team believes it has shed light on the rapid processes taking place within these novel materials.

The team, made up of researchers from Max Born Institute in Berlin and scientists from the Russian Quantum Centre, has pointed to ‘strongly correlated systems’, which have strong interactions between the electrons in these materials, as a particular interesting material in solid state physics research.

Magnets are a good example these systems. The electrons in magnets align themselves in a preferred direction of spin inside the material, and it is this that produces the magnetic field.

However, there are other different structural orders that also deserve attention, the research team adds. In so-called Mott insulators for example, the electrons ought to flow freely and the materials should, therefore, be able to conduct electricity as well as metals. But, the mutual interaction between electrons in these strongly correlated materials impedes their flow and so the materials behave as insulators instead.

By disrupting this order with a strong laser pulse, the physical properties can be made to change dramatically, the research team explains. This can be likened to a phase transition from solid to liquid. As ice melts, for example, rigid ice crystals transform into free-flowing water molecules. Very similarly, the electrons in a strongly correlated material become free to flow when an external laser pulse forces a phase transition in their structural order. Such phase transitions should allow the industry to develop entirely new switching elements for next-generation electronics that are faster and potentially more energy efficient than present-day transistors. In theory, the researchers believe computers could be made around a thousand times faster by ‘turbo-charging’ their electrical components with light pulses.

The problem with studying these phase transitions is that they are extremely fast, the team counters, so it’s difficult to “catch them in the act". So far, scientists have had to content themselves with characterising the state of a material before and after a phase transition of this kind.

But the researchers from Max Born Institute in Berlin and scientists from the Russian Quantum Centresay they have devised a new theory. This involves firing extremely short, tailored laser pulses at a material - pulses that can only recently be produced in the appropriate quality given the latest developments in lasers. One then observes the material's reaction to these pulses to see how the electrons in the material are ‘excited’ into motion and, like a bell, emit resonant vibrations at specific frequencies, as harmonics of the incident light.

"By analysing this high harmonic spectrum, we can observe the change in the structural order in these strongly correlated materials 'live' for the first time," says first author of the paper Rui Silva of the Max Born Institute. Laser sources capable of triggering these transitions have only been available since very recently. The laser pulses namely have to be amply strong and extremely short - on the order of femtoseconds in duration (millionths of a billionth of a second).

In some cases, it takes only a single oscillation of light to disrupt the electronic order of a material and turn an insulator into a metal-like conductor.

"If we want to use light to control the properties of electrons in a material, then we need to know exactly how the electrons will react to light pulses," says the team’s leader, Misha Ivanov. With the latest-generation laser sources, which allow full control over the electromagnetic field even down to a single oscillation, the team believes this new method will allow deep insights into the materials of the future.

The vertical red line on the image above shows when the laser electric field (yellow oscillating curve) crosses the threshold field, destroying the insulating phase of the material. The top panel shows the average number of doublon-hole pairs per site (blue) and the decay of the insulating field-free ground state (red).