Chinese physicists from Peking University have discovered a new phase in superconductors that precisely coincides with the boundary where the material suddenly begins to conduct current without resistance on the "left flank" of the temperature-pressure diagram.
The researchers Yu-Cheng Zhu, Jia-Xi Zeng and Xin-Zheng Li have shown that this phase — lattice quantum disorder — arises from quantum fluctuations of atomic nuclei. Unlike the usual models, in which the main role is assigned to electrons, here the behavior of the crystal lattice itself turns out to be key.
The scientists applied the path-integral method in molecular dynamics in order to take into account the quantum nature of the nuclei. The calculations revealed a triangular region on the pressure-temperature phase diagram where the lattice remains disordered owing to quantum effects. The left boundary of this region coincides with the temperature at which superconductivity begins.
The effect manifests especially vividly in nickel-containing materials. When hydrogen is replaced by deuterium, the boundary of the quantum phase shifts, and the maximum superconductivity temperature falls from 220 to 160 kelvins. This confirms a direct link between nuclear fluctuations and the mechanism of electron pairing.
The discovery changes the understanding of high-temperature superconductivity: it must now be regarded as a joint quantum state of electrons and the lattice. Previously, structural diagrams were constructed without taking into account the many-body nature of the nuclei, which led to inaccurate predictions.
The new phase provides a guide for the search for materials with higher superconductivity temperatures and explains why superconductivity breaks off sharply on the "left flank" of the dome.
