Xavier Roy Synthesizes New Materials and Works With Other Scientists to Explore Them

Xavier Roy, a synthetic chemist, grew up in Montreal and studied at Polytechnique Montréal, the city’s French-speaking engineering school, before making the switch to chemistry and working in English for his doctoral work, which he completed at the University of British Columbia. He came to Columbia as a postdoctoral fellow in 2011 and joined the Chemistry faculty in 2013. Roy synthesizes new materials in his lab, and then works with Columbia scientists from several disciplines to study those materials’ properties. His lab has created, among other things, a magnetic semiconductor in which researchers can see spin waves, the world’s fastest semiconductor, and a new form of ultra-thin carbon.

In a paper out this week in Nature, Roy and his colleagues detail the properties and behaviors of the latest material they created in their lab, cerium silicon iodide (CeSiI). Columbia News caught up with Roy to discuss the findings, and the field of two-dimensional materials research.

Can you describe this new material that your lab synthesized?

The material is called cerium silicon iodide, CeSiI. It’s the first two-dimensional heavy fermion material. Heavy fermion materials require two main ingredients: magnetic atoms (in our case cerium) and electrons that can move freely through the material. Combining these two components in just the right way can lead to a host of different quantum phenomena, including magnetism, superconductivity, and complex combinations of the two. Heavy fermion materials are usually bulky, three-dimensional structures, but this is the first time we’ve been able to make such a material in two dimensions, just a few atoms thick.

Our new paper in Nature looks at how we synthesized CeSiI and proves that its heavy fermion properties are really confined to two dimensions. This means that the material is intrinsically 2D, even in the bulk scale. Its heavy electrons are always traveling in 2D. More than that, we can take crystals of this compound and exfoliate it to create flakes that are just a few atoms thick.

Another paper, still under peer review, will look at some of the interesting behaviors CeSiI exhibits because of this low dimensionality. As one example, it shows this weird behavior where if you shoot electrons across it in certain directions, the electrons will be very, very heavy. But if you change the angle by just a few degrees, suddenly the electron appears very light. It just depends on which direction the electron is traveling.

Don’t electrons always have the same mass?

The mass of a stationary electron is always the same, but what we’re measuring here is the mass as it travels across the material, which we call an effective mass. In typical metals, like gold or copper, this effective mass is similar to the value of a free electron. In other words, the movement of electrons isn’t strongly affected by the metal atoms. CeSiI, by contrast, contains atoms with “local” magnetic moments, which behave as atomic-scale bar magnets that are locked into specific positions in the crystal.  When traveling through this special thicket of tiny magnets, the electrons are slowed down and appear to be heavy.

A good analogy is making a syrup from sugar and water. In solid sugar, the molecules are locked in place, whereas when you dissolve sugar in water, molecules can flow freely, like electrons. When we combine sugar and water in the right ratio, we get a viscous, slow-flowing syrup because of the interactions between water and sugar. That’s similar to what we see in CeSiI. The electrons become heavier (flow more slowly) because of their interaction with the magnetic moments.

Many quantum mechanics and synthetic chemistry research papers focus on two dimensional materials. Why is there so much excitement around 2D?

One reason is because things start changing when you get down to two dimensions. Think of it like this: If we had flying cars that could travel in three-dimensional space, we would be able to reduce most of the traffic in New York. But since our current cars can only travel in two-dimensions, we end up with huge traffic jams in Times Square. The same thing happens for electrons when we move from 3D to 2D, but in our case, “traffic” between electrons is beneficial! As these electron-electron interactions become stronger, we can completely change the properties of a material. For example, as the thickness of 3D heavy fermion materials is reduced (i.e. as they become more 2D), they can transition from being magnetic to superconducting, which motivates us to study these materials at the 2D limit.

These confinement effects, when electrons are confined to two dimensions instead of three, are an extension of the quantum phenomena observed in quantum dots, which are nanocrystals that are just a few nanometers in size. (Professor Emeritus Louis Brus was awarded the Nobel Prize this year for discovering quantum dots.) While the 2D materials we’re synthesizing now are still only a few nanometers thick, they’re big enough in the other dimensions that we can see them with our own eyes using a simple microscope! This allows us to attach wires and directly measure the resistance of electrons moving through a single 2D flake. Not only does this allow us to study new physics that emerge from dimensional confinement, it also means that we can turn these materials into electrical components for next-generation computing technologies, or even for quantum computing.

What are your favorite things to do when you’re not synthesizing new materials?

I love to run. I often run up to the Wien Stadium, Columbia’s athletics facility in Inwood, in Upper Manhattan. Sometimes I run across the George Washington Bridge and go to the Palisades in New Jersey. And sometimes I run in Riverside Park. Central Park has too many people; I like to be by myself.

And for food?

In the neighborhood, my wife and I like to go to Pisticci with our son, especially in the summer when you can sit outside.

That’s a Columbia classic. I’ve never been.

You have to go.