Brendan Faeth
An artist’s rendering of strontium iridate, an oxide whose
properties can be controlled by applying spin-orbit interactions or changing
molecular bond angles.
Designing or exploring new materials is all about controlling their
properties. In a new study, Cornell scientists offer insight on how different
“knobs” can change material properties in ways that were previously unexplored
or misunderstood.
“The ultimate goal is to control electronic and magnetic properties of new
materials using various knobs,” said Kyle Shen, associate professor of physics,
who led the study published in Physical
Review Letters in January. “What you want is to turn one knob, change some
parameter, and turn a material from this to that.”
The bread-and-butter methods of tuning materials’ properties include
introducing impurities, such as chemical dopants, or modifying their atomic
structures. Here, the researchers took a different approach, by employing an
effect known as the spin-orbit interaction. This is the phenomenon an electron
experiences when moving past another charged object, such as the atomic nucleus,
and it is particularly pronounced in heavier elements near the bottom of the
periodic table.
Typical complex electronic materials are “transition metal oxides,” where the
metal is usually a lighter element such as copper, manganese, titanium or
nickel. In this study, Shen and his group replaced the lighter transition metal
with a much heavier element – iridium, a very rare element often found in
meteorites. This replacement enhanced the effect of the spin-orbit interactions
in the compound strontium iridate (SrIrO3).
In the absence of spin-orbit interactions, theoretical calculations predicted
that strontium iridate would be a conventional metal. However, the researchers
discovered that the strong spin-orbit interactions caused strontium iridate to
teeter on the brink of being either a metal or a semiconductor – a
“semimetal.”
Small disturbances to the crystal structure would cause the material to flip
from being insulating to metallic, suggesting that the spin-orbit interaction
could be a new way of controlling the electronic properties of complex
materials.
The lead authors on the paper, “Interplay of Spin-Orbit Interactions,
Dimensionality and Octahedral Rotations in Semimetallic SrIrO3” were
former postdoctoral scholars Yuefeng Nie (now at Nanjing University), and Phil
King (a Kavli Postdoctoral Fellow, now at St. Andrews' University). The team
included Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial
Engineering in the Department of Materials Science and Engineering, and Craig
Fennie, associate professor of applied and engineering physics. Jacob Ruff, a
staff scientist at the Cornell High Energy Synchrotron Source, helped with
crucial X-ray measurements of atomic structures.
The research was supported by the Kavli Institute at Cornell for Nanoscale
Science, the Cornell Center for Materials Research, the National Science
Foundation, and the Air Force Office of Scientific Research.
By Anne Ju