A laser pulse hits the La2CuO4 (LCO, lanthanum copper oxide)
crystal, which is a layered structure of two-dimensional planes of
octahedra. The illumination temporarily turns the insulating LCO into a
coherent three-dimensional metal. During this metallization process the
crystal lattice is vibrating, as indicated by the motion of ions.
The condensed-matter physics world was shaken up when high-temperature superconductivity was reported in a copper oxide material in 1986 by Alex MÃ¼ller and Georg Bednorz. Their discovery not only led to an immediate Nobel Prize in 1987, it also created the â€žWoodstock of Physicsâ€œ at the following American Physical Societyâ€™s March Meeting, which in turn made it onto the New York Times front page. This was the start of the â€œfever of high Tcsâ€, a world-wide scientific undertaking towards achieving room temperature superconductivity. Some thirty years later and counting, the copper oxides keep posing puzzles to physicists. A universally agreed-upon explanation of their electronic properties has not yet been reached.
Conventional superconductors, like mercury or aluminum, are well understood. In those, electrons that repel each other because they carry the same negative electric charges, are bound together by deformations of the crystal lattice, formed by positively charged ions. An intuitive way to understand why this happens is the soft-mattress analogy: two people sharing the same bed inadvertently end up close by one another because their weight deforms the mattress. In conventional materials, such binding of electrons can convert a normal metal with ohmic resistance into a superconductor in which all electrons form one quantum soup that zips through a wire without any losses. However, this superconducting state can only occur if the materials are cooled with liquid helium to near absolute zero temperature, a cost-expensive process that limits the scope of consumer applications.
In high-temperature superconductors, almost everything is different. Those with the highest superconducting transition temperatures require only liquid nitrogen to be sufficiently cold to superconduct. But when they are not that cold, they are poor conductors rather than good metals. And in the parent state, they are even good insulators with magnetic behavior - things that traditional wisdom told scientists would be bad for superconductivity. On top of that, the crystal lattice was long thought to not be involved in the superconducting mechanism. Rather, strong Coulomb repulsion leading to electronic localization - think morning traffic jams at rush hour - in Mott insulators was thought to play the key role.
The new results by the team involving research groups at the Ã‰cole Polytechnique FÃ©dÃ©rale de Lausanne (EPFL), the Max Planck Institute for the Structure and Dynamics of Matter (MPSD Hamburg), and Kingâ€™s College London (KCL), paint a slightly different picture. In LCO, one of the insulating parent compounds to high-temperature superconductors, certain lattice vibrations not only accompany the metallic behavior induced by laser pulses â€“ the vibrations may even help in the metallization process.
The studyâ€™s first author, experimentalist Edoardo Baldini, who conducted the experiment as a PhD student in Fabrizio Carboneâ€™s group at the EPFL and is now a postdoctoral researcher at the Massachusetts Institute of Technology, explains: â€žWe were exploring how to induce the insulator-to-metal transition in LCO with ultrashort pulses of laser radiation. At some point we found an extremely fast metallization along all the crystallographic axes of LCO, and the material suddenly reflected light at colors which were initially not reflected.â€œ A more careful analysis of the ultrafast snapshots then brought a surprise: â€žWe saw interesting oscillations in the signal,â€œ adds Baldini. â€žThe crystal lattice was shaking at characteristic vibrational frequencies as LCO transformed into a metal and recovered back to its insulating state.â€œ
Intrigued by their discovery, the experimentalists shared these findings with theorists in Hamburg. Thomas Brumme, then a postdoc in Hamburg and now at the University of Leipzig, together with MPSD Theory Director Ãngel Rubio, and Emmy Noether research group leader Michael Sentef, decided to first compute the relevant crystal lattice vibrations. With the experimental data and the computed vibrational modes, Sentef contacted the group at KCL, who are experts in computing electronic and optical spectra of complex materials. â€žWe wanted to know what happens when the LCO crystal is deformed along the same vibrational coordinates for which the experimentalists saw the shaking in their moviesâ€œ, says Sentef. â€žThere had to be something special about those lattice
The results of the computational Gedankenexperiment were stunning. â€žWe had expected that the lattice positions would have some influence on the electronic spectraâ€œ, says CÃ©dric Weber, Senior Lecturer at KCL. â€žBut we were surprised by how much the spectra changed when we looked at the displaced structures. In fact, for exactly the same vibrational modes observed in the experiments, LCO becomes metallic. The seemingly robust correlated insulating state is not as robust as researchers have thought for decades.â€œ
These new insights highlight the role of the crystal lattice even in strongly correlated electron systems. They suggest that we may need to revisit our current understanding of these materials, where small deformations of the crystal have long been thought irrelevant. Rubio sees this not as a challenge in a story that is already riddled with complications, but as an opportunity: â€žBeing able to change fundamental properties of exciting materials is a major driver for our researchâ€œ, he says. â€žWe are always looking for efficient ways to control superconductivity and other important features in materials. If we identify the right tuning knobs, they may be used in future quantum technologies.â€œ