An infrared laser pulse briefly modifies the structure of a high-temperature superconductor and thus removes its electrical resistance even at room temperature.
Superconductivity is a remarkable phenomenon: superconductors can transport electric current without any resistance and thus without any losses whatsoever. It is already in use in some niche areas, for example as magnets for nuclear spin tomography or particle accelerators. However, the materials must be cooled to very low temperatures for this purpose. But during the past year, an experiment has provided some surprise. With the aid of short infrared laser pulses, researchers have succeeded for the first time in making a ceramic superconducting at room temperature – albeit for only a few millionths of a microsecond. An international team, in which physicists from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg have made crucial contributions, has now been able to present a possible explanation of the effect in the journal Nature: The scientists believe that laser pulses cause individual atoms in the crystal lattice to shift briefly and thus enhance the superconductivity. The findings could assist in the development of materials which become superconducting at significantly higher temperatures and would thus be of interest for new applications.
No resistance at room temperature: The resonant excitation of oxygen oscillations (blurred) between CuO2 double layers (light blue, Cu yellowy orange, O red) with short light pulses leads to the atoms in the crystal lattice briefly shifting away from their equilibrium positions. This shift brings about an increase in the separations of CuO2 layers within a double layer and a simultaneous decrease in the separations between double layers. It is highly probable that this enhances the superconductivity. © Jörg Harms/MPI for the Structure and Dynamics of Matter
The result helps material scientists to develop new superconductors
In 2013, an international team working with Max Planck researcher Andrea Cavalleri discovered that when YBCO is irradiated with infrared laser pulses it briefly becomes superconducting at room temperature. The laser light had apparently modified the coupling between the double layers in the crystal. The precise mechanism remained unclear, however – until the physicists were able to solve the mystery with an experiment at the LCLS in the US, the world’s most powerful X-ray laser. “We started by again sending an infrared pulse into the crystal, and this excited certain atoms to oscillate,” explains Max Planck physicist Roman Mankowsky, lead author of the current Nature study. “A short time later, we followed it with a short X-ray pulse in order to measure the precise crystal structure of the excited crystal.”
The result: The infrared pulse had not only excited the atoms to oscillate, but had also shifted their position in the crystal as well. This briefly made the copper dioxide double layers thicker – by two picometres, or one hundredth of an atomic diameter – and the layer between them became thinner by the same amount. This in turn increased the quantum coupling between the double layers to such an extent that the crystal became superconducting at room temperature for a few picoseconds.
On the one hand, the new result helps to refine the still incomplete theory of high-temperature superconductors. “On the other, it could assist materials scientists to develop new superconductors with higher critical temperatures,” says Mankowsky. “And ultimately to reach the dream of a superconductor that operates at room temperature and needs no cooling at all.” Until now, superconducting magnets, motors and cables must be cooled to temperatures far below zero with liquid nitrogen or helium. If this complex cooling were no longer necessary, it would mean a breakthrough for this technology.
Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures. In complex oxides, this method has been used to melt electronic order drive insulator-to-metal transitions and induce superconductivity8. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in YBa2Cu3O6+x. Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu–O2 intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O–Cu–O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the character of the in-plane electronic structure is likely to favour superconductivity.
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