Frontiers in Synthetic Moiré Quantum Matter Proceedings of a Workshop (2022) / Chapter Skim
Currently Skimming:
5 Correlated Oxides
5 Correlated Oxides
Pages 50-60
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 50... ...
FCGT: A POLAR MAGNETIC METAL Before speaking about correlated oxides, though, Ramesh began with what he called an "exotic example" of a van der Waals–bonded transition metal dichalcogenide (TMD) : the compound Fe5–xCoxGeTe2, or FCGT for short.
|
From page 51... ...
that revealed the positions of the different atoms, he pointed out that the flipped orientation of the lines of iron/cobalt atoms from one layer to the next in the AA' structure ends up producing a chevron pattern, with the iron/cobalt lines zigzagging from one layer to the next, while in the AA structure the atomic lines all pointed at the same angle from layer to layer. Then he pointed out a more subtle difference between the two phases: Counting the number of iron/cobalt atoms stacked between each of the lines of tellurium atoms, one discovers that while there are seven stacked atoms in the AA phase, there are only six in the AA' phase.
|
From page 52... ...
They are polar magnetic metals, and there are many possibilities for moiré structures. If one examines a sample of the AA' structure, which in its ideal form has a perfect chevron design with the iron-cobalt stacks in alternating layers zigging one way and then zagging the other way in the next layer, it becomes apparent that a typical sample does not have this ideal pattern.
|
From page 53... ...
Oxides present a broad spectrum of crystal structures and physical phenomena and are a very heavily studied class of materials, he said, showing the Hamiltonian equation that describes the various interactions, and "one can start to play games with this, both with the spin–orbit coupling and, of course, the charge–charge interaction between the electrons, leading to some very exotic phases." In these systems, he continued, there is very strong coupling among the lattice, charge, orbital, and spin degrees of freedom. A good example is lanthanum manga nese oxide.
|
From page 54... ...
To learn useful techniques, the complex oxide community turned to the semiconductor community, which has been using epitaxy techniques, such as molecular beam epitaxy (MBE) to create thin layers of semiconductors such as gallium arsenide for decades.
|
From page 55... ...
To illustrate what is possible with this approach, Ramesh then described some examples of complex oxide superlattices that have been studied to date. His first example dealt with creating polar textures using the epitaxy approach.
|
From page 56... ...
Numerical simulations run on the vortices reproduce the observed values of the polarization and the electric field quite well, and they also indicate regions of local negative permittivity right at the center of the vortex where the polarization goes to zero. Switching gears, Ramesh then described another interesting thing that hap pens in these materials, this one in the optical domain.
|
From page 57... ...
Ramesh then describes several tools that can be used to probe the chirality in these materials. For example, resonant soft X-ray diffraction can be used with circularly polarized light to prove the existence of circular dichroism in these
|
From page 58... ...
This type of skyrmion, he said, is formally called the "hopfion." Switching back to the topic of negative permittivity in these superlattices, Ra mesh said that the layered materials have a much higher permittivity than either lead titanate or strontium titanate on their own. The reason, he said, is that the surfaces of the skyrmions are regions of negative permittivity.
|
From page 59... ...
"Once we see that," he said, "I think we can start to do the full twisting operation to get the various moiré structures." The next question concerned the FCGT material: Given that the scanning transmission electron microscope images seemed to show that symmetry was breaking locally, why were there skyrmions everywhere? Ramesh answered that in the AA' phase with 50 percent cobalt there is a long-range ordered twisted structure, so the symmetry is long-range.
|
From page 60... ...
Ramesh answered that the length scales are such that only phase field calculations will work; ab initio calculations cannot get to these kinds of length scales. Some researchers are trying to use phase field calculations to examine, for example, chiral domain boundaries.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.