Rice University scientists conducted a study on superconductivity. Physicists Pengcheng Dai and Andriy Nevidimoskyy, with some of their colleagues, used simulations and neutron scattering experiments that show that the atomic structure of materials to reveal tiny distortions of the crystal lattice in a so-called iron pnictide compound of sodium, iron, nickel, and arsenic.
Usually, these distortions are observed among the symmetrical atomic order in the material at ultracold (134°K) temperatures near the point of optimal superconductivity. However, the researchers found out that there is a possibility of some wiggle room as they work to increase the temperature at which iron pnictides become superconductors.
The key to the material’s superconductivity seems to lie within a subtle property that is unique to iron pnictides: a structural transition in its crystal lattice, the ordered arrangement of its atoms, from tetragonal(stretched cubes) to orthorhombic(brick-like).
When the Rice team was studying sodium-iron-arsenic pnictide crystals dope with nickel, they noticed that the material changed its structure at some regions well above the conventional structural transition temperatures.
“In the tetragonal phase, the (square) A and B directions of the lattice are absolutely equal,” said Dai, who carried out neutron scattering experiments to characterize the material at Oak Ridge National Laboratory, the National Institute of Standards and Technology Center for Neutron Research and the Research Neutron Source at the Heinz Maier-Leibnitz Center.
“The whole paper suggests there are local distortions that appear at a temperature at which the system, in principle, should be tetragonal,” Dai said. This suggests the nematic ( the physics of liquid crystals that align in reaction to an outside force) fluctuation may also help superconductivity because it changes temperature dependence around optimum doping.
Being able to manipulate that point of optimum doping may give researchers better ability to design materials with novel and predictable properties.