Light-Induced Superconductivity: A New Frontier in Quantum Physics

Mid-infrared laser pulses coherently drive atomic modes in YBa2Cu3O6.48 and stabilize superconducting fluctuations at high temperature. This quantum coherence leads to the ultrafast expulsion of a static magnetic field. Credit: S. Fava / J. Harms, MPSD

Researchers have developed methods to explore and utilize superconductivity in non-equilibrium states, such as those induced by laser pulses, at temperatures much higher than traditional superconductors operate.

This light-induced superconductivity has been shown to replicate crucial features like zero electrical resistance and expulsion of magnetic fields, suggesting potential applications in high-speed devices and extending superconductivity to ambient temperatures.

Superconductivity is a remarkable phenomenon that enables a material to carry an electrical current with zero loss. This collective quantum behavior is unique to certain conductors and only occurs at temperatures significantly below room level.

A number of modern studies have investigated this behavior in so-called non-equilibrium states, that is in situations in which the material is pushed away from thermal equilibrium. In these conditions, it appears that at least some of the features of superconductivity can be recreated even at ambient temperatures. Such non-equilibrium high temperature superconductivity, shown to exist under irradiation with a laser pulse, may be useful for applications different from the ones envisaged for the stationary version of superconductivity, as for example in high-speed devices controlled by laser pulses.

Light-Induced Superconductivity

This phenomenon has been termed “light-induced superconductivity,” signaling an analogy with its equilibrium counterpart.

An important frontier in the past decade has been to characterize the properties of one such light-induced superconducting state and understand how far this phase reproduces the known properties of a conventional superconductor.

Besides being capable of transporting electrical currents without loss, superconductors are also known to expel magnetic fields from their interior. This phenomenon, known in equilibrium conditions as the Meissner effect, is a direct consequence of the mutual coherence of the charge carriers and of their tendency to march in lockstep. However, measuring the expulsion of magnetic fields for light-induced superconductivity has been challenging, because the effect only persists for a few picoseconds (one trillionth of a second), making it impossible to measure magnetic field changes with precision.

Breakthroughs in Magnetic Field Measurements

A team of researchers at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, led by Andrea Cavalleri, has developed a new experiment capable of monitoring the magnetic properties of superconductors at very fast speeds. They have worked on laser-irradiated YBa₂Cu₃O_6+x, a compound for which static superconductivity is only seen down to about −200 degrees Celsius. “We have discovered that photo-excited YBa₂Cu₃O_6.48, in addition to featuring near zero resistance, also expels a static magnetic field from its interior,” says Sebastian Fava, author of the article now published in Nature.

Insights Into Material Behavior Under Light

This experiment was made possible by placing a spectator crystal in close vicinity of the sample under investigation and using it to measure the local magnetic field strength. The crystal reflects changes in the magnetic field into changes in the polarization state of a femtosecond laser pulse. “Due to the short duration of the probe pulse, we can reconstruct the time evolution of the magnetic field surrounding the YBa₂Cu₃O_6.48sample with sub-picosecond resolution and unprecedented sensitivity,” says Giovanni de Vecchi, one of the co-authors.

Implications for Superconducting Materials

“The photo-induced magnetic field expulsion we observed is comparable in size to the one measured when YBa₂Cu₃O_6+x is made superconducting at equilibrium by cooling,” adds fellow author Michele Buzzi. “This suggests that driving the material may even be an effective route to bring its superconducting properties closer to ambient conditions,” continues co-author Gregor Jotzu, now a faculty member at EPFL and head of the Dynamic Quantum Materials Laboratory. As a consensus on the microscopic origin of light-induced superconductivity in YBa₂Cu₃O_6.48 is still missing, these results are an important benchmark for current theories.

Potential of Photo-Excited Superconductivity

In YBa₂Cu₃O_6+x, the superconducting order does not completely disappear above the equilibrium superconducting transition temperature and some local fluctuating superconducting order remains, somewhat similar to a disordered state. These groundbreaking discoveries suggest that photo-excitation of YBa₂Cu₃O_6+xwith tailored light pulses can be used to synchronize this fluctuating state and restore the superconducting order at temperatures much higher than the ones at which the material becomes superconducting at equilibrium, all the way to room temperature.

Reference: “Magnetic field expulsion in optically driven YBa₂Cu₃O_6.48” by S. Fava, G. De Vecchi, G. Jotzu, M. Buzzi, T. Gebert, Y. Liu, B. Keimer and A. Cavalleri, 10 July 2024, Nature.
DOI: 10.1038/s41586-024-07635-2

The research at the MPSD received financial support from the Deutsche Forschungsgemeinschaft via the Cluster of Excellence CUI: Advanced Imaging of Matter. The MPSD is a member of the Center for Free-Electron Laser Science (CFEL), a joint enterprise with DESY and the University of Hamburg. The research was carried out in close collaboration with scientists at the Max Planck Institute for Solid State Research (MPI-FKF).