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Ultrafast terahertz spectroscopy reveals new knowledge on ultrafast electron motion in graphene and magnetic materials

Scientists have explored how graphene and ferromagnetic metals conduct electricity on ultrafast timescales, relevant for the next generation high-speed electronics, in a series of innovative experiments.
Ultrafast terahertz spectroscopy reveals new knowledge on ultrafast electron motion in graphene and magnetic materials
Work by the German-led LIGHTER project exploring the interaction of light and matter on ultrafast timescales has produced findings which are highly relevant for physicists and engineers alike.

The project, supported by a Marie Curie Career Integration Grant for researcher Dmitry Turchinovich, has harnessed advanced terahertz spectroscopy to explore some fundamental aspects of electronics.

‘We wanted to understand how the electrons in technologically relevant materials — ferromagnetic metals, graphene and others — interact with electric fields on the ultrafast timescale of picoseconds or faster,’ says Professor Turchinovich, who at the time of the project led a research group at the Max Planck Institute for Polymer Research in Mainz, Germany. ‘These are fundamental timescales for elementary dynamics in solids.’

Mott’s theory finally demonstrated

Modern magnetic memories, such as high-capacity computer hard drives, store information in the form of tiny magnetic bits, interrogated using nanoscale magnetic sensors called spin valves. The spin valves operate based on the idea of electrical conduction in ferromagnetic metals put forward by UK physicist Nevill Mott back in 1936. According to Mott’s theory, the electric currents in these metals are carried by two types of electrons with opposite spins, which experience different resistance when travelling within a metal

‘We found a way to experimentally disentangle these currents by spin-up and spin-down electrons,’ says Prof Turchinovich. Using ultrafast terahertz spectroscopy the researchers managed to count the number of spin-up and spin-down electrons and to measure how fast they slow down within the metal.

This, the first direct observation of Mott’s fundamental theory, also allowed the researchers to gain better understanding of modern magnetic memory technology. ‘We found that the traditional measurements on slower timescales significantly underestimate the spin asymmetry which is responsible for magnetic sensor operation,’ says Prof Turchinovich.

This method, published in Nature Physics contributes to an entirely new field — terahertz spintronics — and provides a way for scientists to easily screen many different magnetic compounds to find those most suitable for efficient devices.

Thermodynamics rules

A second line of research among many was exploring how graphene conducts electricity at very high frequencies, relevant for next generation ultrafast electronics.

The LIGHTER team used solely optical means to create transistor-like conditions of strong high-frequency electric fields within graphene and to directly measure the electronic response. ‘When you drive an electric current through graphene, its free electron population, usually described as a sort of a liquid, “vaporises” and becomes more like a gas, which significantly changes the conductive properties of material,’ says Prof Turchinovich. ‘In the end, the way graphene conducts electricity simply depends on the temperature of this electron gas. This makes the description of graphene electronics very simple — essentially you need to apply some basic conservation laws,’ he adds.

This discovery was published in Nature Communications on 16 July 2015. Prof Turchinovich believes this new insight will help advance product development. ‘Engineers can use our simple thermodynamic model to predict the performance of their graphene transistors or photodetectors and optimise them,’ he concludes.

New knowledge from LIGHTER has led to 27 published papers during the four-year project. Towards the end of the project, its success contributed to Profe Turchinovich’s appointment as professor of physics at the University of Duisburg-Essen, Germany.

Subjects

Life Sciences

Keywords

LIGHTER, conductivity, ferromagnetic metals, graphene, thermodynamics, ultrafast, terahertz spectroscopy, spintronics
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