New ways to tune electrical conductivity revealed by electron interaction

January 7, 2011
The energetic states of Sr2IrO4. The left panel shows the spectrum of all the computed electronic states. A gap in energy (y-axis) is clearly visible for electrons moving in all crystallographic directions (x-axis). The letters on the x-axis denote specific crystallographic directions. The right panel shows an isolation of the gap arising from states that form as a result of the spin-orbit interaction. Credit: Reproduced, with permission, from Ref. 1 2010 The American Physical Society

Researchers in Japan have demonstrated why the material Sr2IrO4, -- a transition metal oxide -- that was expected to be an electrical conductor is actually an insulator. Harnessing this material’s unusual conducting properties could form the basis for novel electronic devices or superconductors.

The difference between an and an insulator is that in 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 Sr2IrO4 arises. Other RIKEN scientists had shown previously that the compound is an .

Sr2IrO4 is 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 as or 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 Sr2IrO4 shifts 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 Sr2IrO4 superconducting,” says Yunoki. “We hope that our theoretical calculations will be of help to experimentalists.”

Explore further: Illuminating a Second 'Kink' in High-Tc Superconductors

More information:
-- Watanabe, H., et al. Microscopic study of a spin-orbit-induced Mott insulator in Ir oxides. Physical Review Letters 105, 216410 (2010).
-- Kim, B., et al. Phase-sensitive observation of a spin-orbital Mott state in Sr2IrO4. Science 323, 1329–1332 (2009).

Related Stories

Illuminating a Second 'Kink' in High-Tc Superconductors

July 6, 2007

There’s another kink in the mystery of high-temperature (Tc) superconductors – literally. Using photoemission studies at the NSLS, a group of researchers has revealed a new anomaly, or “kink,” in the energy spectrum ...

Closing the 'Pseudogap' on Superconductivity

March 13, 2008

One of the biggest mysteries in studying high-temperature (Tc) superconductors - materials that conduct electrical current with no resistance below a certain transition temperature - is the origin of a gap in the energy level ...

An electronic dance of spins and orbits

December 13, 2010

Because of their potential application in spintronic devices such as next-generation spin-based transistors, the quest for new materials with significant spin-orbit interactions in the electronic ground state is an area of ...

Recommended for you

Fusion reactors 'economically viable' say experts

October 2, 2015

Fusion reactors could become an economically viable means of generating electricity within a few decades, and policy makers should start planning to build them as a replacement for conventional nuclear power stations, according ...

Iron-gallium alloy shows promise as a power-generation device

September 29, 2015

An alloy first made nearly two decades ago by the U. S. Navy could provide an efficient new way to produce electricity. The material, dubbed Galfenol, consists of iron doped with the metal gallium. In new experiments, researchers ...

Extending a battery's lifetime with heat

October 1, 2015

Don't go sticking your electronic devices in a toaster oven just yet, but for a longer-lasting battery, you might someday heat them up when not in use. Over time, the electrodes inside a rechargeable battery cell can grow ...

Invisibility cloak might enhance efficiency of solar cells

September 30, 2015

Success of the energy turnaround will depend decisively on the extended use of renewable energy sources. However, their efficiency partly is much smaller than that of conventional energy sources. The efficiency of commercially ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jan 07, 2011
...It would have an enormous impact if one can make Sr2IrO4 superconducting..
Iridium is one of most expensive metals, comparable to palladium.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.