Scientists from Leipzig University and Nanyang Technological University showed that, contrary to popular scientific belief, light can generate electrical currents in a material even if the material is transparent to the frequency of the light that is shinned on it. The team’s discovery could lead to new approaches to manipulating electronic behavior for multiple applications. “This opens new paradigms for constructing optoelectronic and photovoltaic devices, such as light amplifiers, sensors, and solar cells,” said Inti Sodemann Villadiego, a professor at Leipzig University. Sodemann Villadiego and his colleagues investigated what are known as “Floquet Fermi liquid” states. A Fermi liquid is a special state of many quantum mechanical particles with properties that can be very different from those of ordinary classical liquids such as water at ambient temperature. An abstract depiction of the Floquet Fermi Liquid. Courtesy of Li-Kun Shi. Fermi liquids can arise in a wide variety of situations, from common materials such as the electrical fluid of electrons in metals like gold or silver, to more exotic situations such as the fluid of Helium-3 atoms at low temperatures. They can display “spectacular properties”, such as becoming superconductors of electricity at low temperatures. The Floquet Fermi liquid is a variant of this state realized when the particles of the fluid are periodically shaken, like what happens to electrons in metals when they are illuminated by ideally periodic light. “In our publication, we explain several properties of these fluid states,” said Sodemann Villadiego. “To study them, we had to develop detailed theoretical models of complex states of electrons shaken by light, which is far from easy.” The researchers identified properties of these fluid states, including their quantum oscillations under magnetic fields, which feature slow beating patterns of their amplitude reflecting the different areas of the Floquet Fermi surfaces. The observations, the researchers said, are consistent with those observed in microwave-induced resistance oscillation experiments. The team also investigated the specific heat and thermodynamic density of these states and demonstrated that by controlling properties of the drive, such as its frequency, one can tune some of the Floquet Fermi surfaces toward nonequilibrium Van Hove singularities without changing the electron density. “It is possible to drive electric currents by light even when the material has a vanishingly small absorption of such light. This is an important new insight,” said postdoctoral researcher Li-kun Shi. The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.132.146402).