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Sandwich-Making Method Gets More Advanced By Cornell Researchers

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If you thought the ability to perfectly layer ingredients in an ideal order is a skill useful only in the kitchen, you would be wrong.

In order to engineer the best thin films for electronics, one must perfect the art of unconventional sandwich making, according to a Cornell University study.

A team of researchers discovered that assembling atomic layers in the perfect order is the trick to developing ideal thin films of oxides, which are used because of their properties of superconductivity, magnetoresistance and ferromagnetism. In other words, thin films allow for the ability to change electrical resistance when a magnetic field is applied and form permanent magnets, making them usefeul for a wide variety of devices. 

Thin-films expert Darrell Schlom led the study. His lab uses a "sandwich-making" method called molecular beam epitaxy to assemble the films layer-by-layer in a precise order, in a process Schlom says is not unlike spray-painting, but with atoms.

Co-author Lena Kourkoutis examined this method and found that the resulting samples were missing a layer of strontium oxide. This missing layer had been overlooked for years until this recent study found that the missing layer did not occur when using a different method of atomic layering for what is called Ruddlesden-Popper films.

"Imagine laying down two meat patties on a bun, followed by a layer of bread, and another two meat patties, only to find that the resulting sandwich consists of just one meat patty below the layer of bread and three above it," first author Yuefeng Nie equates to the study's findings that the strontium oxide layer was not where it was supposed to be.

It turned out that a layer of titanium atoms seeped down into the strontium oxide layers, so the first layer of strontium oxide ended up on the surface. Therefore the researchers perfected the method by laying down an extra meat patty (strontium oxide layer). This will enable more precise development of thin films for electronic devices.

June Lee, of Argonne National Laboratory, wrote a competing paper published the same week as Schlom's group's study, arriving at the same conclusions.

"Our dream is to control these materials with atomic precision," Schlom says. "We think that controlling interfaces between Ruddlesden-Poppers will lead to exotic and potentially useful, emergent properties."

Using this method, rather than the conventional sandwich-making method, will allow physicists to harness the properties of thin films more effectively. The paper, describing how to perfect the growth of oxide films, was published in Nature Communications.

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