Geometric lattice engineering provides a way to build new topological materials
Topological insulators, such as layered metal oxides, are atomically thin sheets with electronic or magnetic properties that differ from those in the bulk materials. The interior of these materials behaves as an insulator, while the outside is conductive. Also, electrons carry no heat as they speed along the surface of these materials. Partly because of that property, topological materials are thought to be important for quantum computing and a new generation of electronics.
Naturally occurring topological materials are hard to find. However, a method called geometric lattice engineering allows researchers to build artificial topological materials and test them for new and interesting electronic or magnetic properties, including a magnetic material with a graphene-like crystal structure and materials that may exhibit a new quantum state of matter.
In geometric lattice engineering, researchers use a crystal with a specific structure to template the growth of atomic layers of other materials with the same crystallographic orientation. The template can be prepared to grow cubic, tetrahedral, hexagonal, triangular, or star-shaped atomic lattices, and the thickness and composition of the new atomic layers can impact the electronics or magnetics of the new material. The combination of one material superimposed upon another is called a superlattice or heterostructure.
Typically, layered oxide materials are grown in the horizontal plane of a crystal’s unit cell, the (001) direction. In a prospective article published recently in MRS Communications, Jak Chakhalian, at Rutgers University, and his colleagues describe various topological materials synthesized using crystal templates from the (111) face of the original crystal, the diagonal of its unit cell. Depending on the structure of the original crystal, the (111) face can have two different geometries.
The (111) face of perovskite crystals reveals a buckled hexagonal lattice. Chakhalian and colleagues grew two atomic layers of neodymium nickelate (NdNiO3) on a lanthanum aluminate (111) substrate to create a new magnetic material with atoms arranged in a hexagonal pattern like graphene (read the abstract in Annu. Rev. Mater. Res.). Perovskites are a common structure, Chakhalian says, so there are a variety of potential templates in this class of materials. In the future, he hopes geometric lattice engineering could be used to grow hexagonal lattices using metals from platinum group elements such as iridium or rhodium, as materials like this are predicted to have properties different from currently known bulk topological materials.
For crystals with a spinel structure, the (111) face presents either a triangular or a star-shaped lattice called Kagomé. Kagomé lattices are found in materials that may exhibit a new state of matter called quantum spin liquid. In this state, the electron spins in the material entangle and appear to be in a liquid-like state. Magnetism in these materials is seemingly disordered, compared to the fixed orientation of spins in a typical magnet.
In 2015, Yusuke Kozuka, at the University of Tokyo, and colleagues grew superlattices of zinc ferrite (ZnFe2O4) and zinc chromite (ZnCr2O4) on the (111) face of MgAl2O4, a material with a spinel structure (read the abstract in J. Appl. Phys). The researchers varied the number of layers of each zinc-containing material from one to 10 layers. Below 28 K, the material showed a spin glass state, where the spins were frozen in fixed orientations. But when the superlattice contained four layers or less of each material, the transition temperature for the spin glass state decreases, and it behaves more like spin liquid, Kozuka says.
To build superlattices, researchers need to match the electrical charges and structural symmetry between the crystal template and the material they want to grow. Models that can predict these matches are important to help researchers design these crystals, Chakhalian says. “I want people to embrace this field, because if you understand the growth, you may produce something unusual.”