Kyle Shen, Physics, likens his work to the surprise of receiving a holiday present. Only, instead of getting a shiny gadget or fashionable tie, the gifts are new insights into quantum materials.
Shen creates and investigates artificial and unconventional materials with unusual electronic and magnetic properties. Think superconductors, which transport electricity with zero losses, and very thin materials just two or three atoms thick, which could be incorporated into transistors.
The novelty of such materials makes it nearly impossible to anticipate everything that they can do. A researcher can make educated guesses about various properties, but end up seeing something entirely different. For Shen, this sense of unpredictable discovery is one of the most exciting aspects of his field.
“My students will phone me in the middle of the night, and I’ll head over to the lab or log into our system to see data on a new material coming in for the first time. Usually there’s really cool data that I don’t expect,” he says. “I love these surprises.”
The Search Is On
Shen spends the bulk of his time on transition metal oxides (TMOs), a class of materials that share a common crystal structure known as the perovskite structure. TMOs consist of oxygen and a transition metal element, such as copper, iron, manganese, cobalt, or titanium.
The objective is to preserve the perovskite structure while swapping out different transition metal elements to create materials with a range of electronic and magnetic properties. This includes superconductivity, ferromagnetism (the phenomena in which materials are permanently magnetic or attracted to magnets), and large thermopower. They are useful toward a range of applications, from electronics to power transmission to refrigeration.
Shen’s lab most recently discovered that a particular TMO, known as lanthanum nickelate (LaNiO3), switches from a metal to an insulator when its thickness is reduced by a single layer of atoms. The material is a metal at a thickness of three nickel atoms, but when shaved down to two unit cells, its conductivity abruptly switches off.
Understanding lanthanum nickelate’s ability to transition from metal to insulator at an atomic level paves the way for figuring out how to manipulate its properties for practical applications. For example, researchers can potentially use it to make very thin switches to make smaller devices, Shen says, like smartphones or wearable electronics. Shen published the results in the June 2014 issue of Nature Nanotechnology.
Collaborating to Make a New Machine
Like many first-time faculty, when Shen arrived at Cornell in 2007 he wanted to establish a niche in his field. He had worked with top-notch researchers in condensed matter physics while getting his PhD in Applied Physics at Stanford University and doing a post-doc at the University of British Columbia. Instead of staying with the same research as his former advisers, he aimed to find a new direction.
At Cornell he met Darrell Schlom, Materials Science and Engineering. Schlom is one of the world’s leading experts in molecular beam epitaxy (MBE), a method by which researchers can spray paint elements one atomic layer at a time to create artificial materials with atomic precision. Shen’s expertise is in Angle-Resolved Photoemission Spectroscopy (ARPES), a measurement technique that uses x-rays to create maps of the directions and speeds at which electrons travel inside a material.
Both techniques are ideal for making and studying TMOs. Shen and Schlom decided to combine the two techniques and construct an instrument that nobody had made before. By 2009, they had built the world’s first MBE-ARPES system in Duffield Hall. The machine, which fills an entire room, can make materials at high temperatures (for example, 1,000 degrees Celsius), and then perform measurements just a few degrees above absolute zero, all while maintaining a vacuum comparable to outer space.
The system has proven to be very successful and productive, and now numerous labs at other universities and national laboratories around the world are developing similar systems, Shen says. Cornell’s machine has over 20 users, primarily graduate students, postdoctorates, as well as visiting scholars from other institutions and countries. Its high demand means that it’s in use 24 hours a day, 365 days a year.
Their work is an important part of a larger, campus-wide effort on electronic materials research. Shen and Schlom co-lead an Interdisciplinary Research Group on Controlling Complex Electronic Materials—part of the NSF-funded Cornell Center for Materials Research—which consists of more than 20 members, including faculty, graduate students, and postdoctorates in Physics, Materials Science, and Applied and Engineering Physics.
Given the first machine’s success, Shen and Schlom sought to build a new and improved version 2.0. At the end of 2013, they received $4.13 million in funding from the Gordon and Betty Moore Foundation to develop the Moore Creation and Observation of Novel Quantum Electronic Structures (CONQUEST) facility, which includes not only improved MBE and ARPES capabilities, but also a scanning tunneling microscope (STM)—a specialty of J. C. Séamus Davis, Physics—which measures a material’s electron density and atomic structure with subatomic precision.
CONQUEST will increase the speed at which Shen, Schlom, and Davis can explore, discover, and create quantum materials. Unlike the first version of the machine, the CONQUEST facility’s STM capabilities will be able to provide crucial information on how atoms and electrons are arranged in a material at the nanoscale. The new system will be housed in the sub-basement of the Physical Sciences Building, and is slated for completion by 2018.
Shen says that collaborations with Schlom and others are some of the highlights of his time at Cornell. “People at Cornell like to think about how we can work together to achieve something totally new and different. If you look at a lot of the most successful research groups around here, a lot of them are really collaborative. Other people see that and realize it’s a good way of doing things. They follow their lead, which is basically what I do,” he says with a laugh.
A Materials Holy Grail
Since coming to Cornell, Shen has received the Presidential Early Career Award for Scientists and Engineers, the Office of Naval Research Young Investigator Award, the Air Force Office of Scientific Research Young Investigator Award, the Research Corporation Cottrell Scholars Award, the National Academy of Sciences Kavli Frontiers Fellowship, and the National Science Foundation CAREER Award. Each has facilitated Shen’s work in discovering materials that could some day have significant applications in our lives.
Shen is lighthearted when he talks about his ultimate dream of creating a room-temperature superconductor. “This is a very, very far fetched, ambitious goal. But I have to have goals,” he says, laughing. He jokes about how he could retire happy after discovering this holy grail of materials physics. But when it comes down to the work and the science, it’s clear that Shen is serious.
A high-temperature superconductor … could be used for zero-loss power transmission, levitating trains, low-power consumption electronics, and “all sorts of crazy, revolutionary ideas that no one has thought of yet.”
Using the MBE-ARPES system, Shen and Schlom are stacking different TMOs (like LEGO blocks, since they all share the same structure) in order to see how various TMOs affect the properties of the materials. They are particularly interested in the interface where the two TMOs meet. They’ve discovered that the interface often acts as an entirely new material, distinct from either of the individual components. Their hope is to exploit this interfacial phenomenon as one avenue to create higher temperature superconductors.
“We know that some empirical rules exist. If we change the structure a little bit so it correlates well with higher transition temperatures, we might be able to engineer the material to be an even higher temperature superconductor,” he says. “Researchers couldn’t optimize these properties when making bulk crystals,” so by stacking materials and measuring them with MBE-ARPES system, Shen and Schlom are a step closer.
A high-temperature superconductor could have worlds of applications. It could be used for zero-loss power transmission, levitating trains, low-power consumption electronics, and “all sorts of crazy, revolutionary ideas that no one has thought of yet,” Shen says. Anything that requires electricity could potentially become dramatically more efficient.
Shen is quick to add that he has modest goals as well. For example, he’d like to make a material that has an atomically thin layer of high-density electrons that can be easily switched on and off. Such a material could play an important role in future electronic applications, such as next-generation transistors.
No matter what goals or aspects of his work he speaks of, Shen returns to embracing the surprises.
“In our field discoveries are made all the time,” Shen says. “A lot of people will think something is impossible, and three years later, somebody proves the contrary. You can never count things out. There are always lots of surprises.”