New ways to tune electrical conductivity revealed by electron interaction

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The difference between anelectrical conductorand an insulator is thatelectronsin the latter cannot move freely through the crystal. This is because insulators have a gap in their energy spectrum that electrons cannot overcome. Hiroshi Watanabe, Tomonori Shirakawa and Seiji Yunoki from the RIKEN Advanced Science Institute in Wako and the Japan Science and Technology Agency have now uncovered how the electronic gap in Sr2IrO4arises. Other RIKEN scientists had shown previously that the compound is aninsulator.

Sr₂IrO₄is a member of the oxygen-containing compounds based on transition metals that have high atomic numbers. In these transition metals, the electrons of elements such as nickel, copper or cobalt strongly interact with each other, which results in effects such assuperconductivityor magnetism.

In compounds made from the heavier transition metals, the outermost electrons circle the atoms in the so-called‘5d electron shell’, which is relatively distant from the core. For electrons that occupy this shell there is an unusually strong interaction between their magnetic property, called spin, and the orbital motion around the atomic nucleus. The energy of this spin–orbit interaction is as large as the electron’s energy of motion or the energy arising from the electrostatic interaction between the electrons. This has dramatic consequences on their electronic properties, according to Yunoki, who led the research team.“Literally anything can happen in 5d electron systems because of the subtle balance of those three fundamental energy scales.”

How this energetic interplay modifies the electron conducting behavior in Sr2IrO4 became evident from the researchers’ calculations. The strong spin–orbit interaction in Sr₂IrO₄shifts some of the electronic states to higher energies, which is sufficiently strong to create an energy gap in the electronic states.

Furthermore, the calculations reveal an intriguing connection to the family of high-temperature superconductors that have a similar gap in their electronic states. In these compounds, superconductivity is achieved through a small addition of atoms introducing an electron surplus. The researchers are now investigating the possibility that this could also be the case here.“It would have an enormous impact if one can make Sr₂IrO₄superconducting,” says Yunoki.“We hope that our theoretical calculations will be of help to experimentalists.”

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