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Researchers at Columbia for the first time demonstrate the coexistence of superconductivity and ferroelectricity, two properties thought to be incompatible with one another, in bilayer molybdenum ditelluride. Drawing on this discovery, they create a superconducting switch controlled by an external voltage which has potential applications in quantum computing and single-photon detectors. This material is extremely air-sensitive and needs encapsulation in an inert environment to probe its properties in the ambient. Here is an image of the device used in the experiment. Credit: Apoorv Jindal.
When it comes to two-dimensional materials, it’s best to expect the unexpected. Writing in Nature [below], researchers at Columbia find evidence that two competing phenomena—superconductivity and ferroelectricity—can occur within the same material. “This is the first time that a tunable switch between ferroelectricity and superconductivity has ever been seen,” said corresponding author Daniel Rhodes. “We don’t fully understand it just yet, but it’s definitely there.”
Rhodes, now an assistant professor at the University of Wisconsin-Madison, has a particular knack for growing high-quality crystals that can be peeled into atom-thin layers to explore the unusual properties that can arise in two dimensions. He started working on the material in question—MoTe2—as Ph.D. student at The Florida State University. When he joined Columbia’s National Science Foundation-funded Materials Research and Science Engineering Center (MRSEC) as a postdoctoral fellow in 2016, he and colleagues found initial evidence of superconductivity—the resistance-free transport of electrical currents—in single layers of MoTe2.
The effect was there but not as easy to manipulate as theory suggested, so Rhodes and the MRSEC began exploring stacks of the material. They found superconductivity to be much easier to control in a two-layer system, which also comes with a unique structural feature: it’s not symmetric. Atoms between the layers don’t line up, which should inherently create an internal electric field, known as ferroelectricity, explained Apoorv Jindal, first author and Columbia physics Ph.D. student who continued Rhodes’ work on the material after he moved to Madison in 2019.
Whereas superconductivity is typically characteristic of electricity-conducting metals, ferroelectricity was long thought to only occur only within insulating materials. In 2018, however, researchers working with David Cobden at the University of Washington found initial evidence of a ferroelectric metal in WTe2, a tungsten-based material with the same crystal structure as MoTe2. Having already confirmed that superconductivity occurs in MoTe2, Jindal and Rhodes wondered if ferroelectricity might also turn up.
Jindal applied an electric field, and the results blew him away. “I forwarded it to Dan and asked, is this possible? Can a ferroelectric superconductor even exist?” he recalled. The team performed additional measurements across temperatures, magnetic fields, and electrostatic doping and found a manipulatable transition between superconductive and ferroelectric states within MoTe2.
“Finding these two properties together is magical,” said Jindal, especially given the effort it took to build the samples. MoTe2 is an extremely air- and water-sensitive material, so the layers had to be carefully prepared and combined in the confines of a glovebox and then sealed to prevent exposure to the ambient environment as they were transported to different instruments for testing. It took over six months just to perfect a fabrication process that would preserve bilayer MoTe2’s unique properties, he said.
The superconducting state itself is also surprising, particularly in the way it behaves as the number of charge carriers is changed by gating. Theoretical modeling carried out by Ph.D. student Amartya Saha, under the guidance of professors Turan Birol and Rafael Fernandes, all at the University of Minnesota, suggest that superconductivity is not of the conventional type, in which vibrations of the atoms provide the glue that makes electrons form coherent pairs. “The fact that superconductivity requires two types of charge carriers (electron-like and hole-like carriers) to be present is a telltale sign of the importance of electronic interactions,” said Fernandes.
Because superconductivity and ferroelectricity are both intrinsic to the material itself, there could be a number of potential applications, particularly in quantum computing and information. For example, one approach to building quantum computers involves combining superconducting materials with an insulating barrier in between to form what are known as “Josephson junctions”, Jindal explained. The ability to use a single material, like MoTe2, rather than two separate ones could help eliminate interfacial problems that can make these junctions less efficient. The rapid speed at which the material can be switched between the two properties may also hold promise for quantum logic applications.
Those questions, said Rhodes, are for engineers to take over. From here the team, which includes Columbia physicists Cory Dean and Abhay Pasupathy and engineer James Hone as well as theoretical collaborators at the University of Minnesota, plans to explore why these two properties exist and how they can be controlled. “There are fundamental science questions still to ask that to date, we just haven’t had a system in which to explore—now we do,” said Rhodes.
Science paper:
Nature
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In the first half of the 20th century, the first quantum revolution gave us a new way of thinking about the way the world works and brought us technologies such as lasers, MRI machines, and the transistors that underpin all aspects of modern life. Today, the second quantum revolution is underway, and it’s all about control.
The coming generation of quantum technologies will be built on new physical principles and demand new materials, new methods of investigation, and new collaborations. At Columbia, we’re tackling these demands together and training the next generation of quantum scientists and entrepreneurs.
Building on the collaborative culture long fostered at Columbia, the Quantum Initiative is combining interdisciplinary expertise in materials science, photonics, quantum theory, and more, all while taking advantage of our unique position in the global hub that is New York to develop novel quantum technologies that will open new frontiers into how we compute through complex problems, communicate with one another, and sense the world around us.
Columbia University was founded in 1754 as King’s College by royal charter of King George II of England. It is the oldest institution of higher learning in the state of New York and the fifth oldest in the United States.
University Mission Statement
Columbia University is one of the world’s most important centers of research and at the same time a distinctive and distinguished learning environment for undergraduates and graduate students in many scholarly and professional fields. The University recognizes the importance of its location in New York City and seeks to link its research and teaching to the vast resources of a great metropolis. It seeks to attract a diverse and international faculty and student body, to support research and teaching on global issues, and to create academic relationships with many countries and regions. It expects all areas of the University to advance knowledge and learning at the highest level and to convey the products of its efforts to the world.
Columbia University is a private Ivy League research university in New York City. Established in 1754 on the grounds of Trinity Church in Manhattan Columbia is the oldest institution of higher education in New York and the fifth-oldest institution of higher learning in the United States. It is one of nine colonial colleges founded prior to the Declaration of Independence, seven of which belong to the Ivy League. Columbia is ranked among the top universities in the world by major education publications.
Columbia was established as King’s College by royal charter from King George II of Great Britain in reaction to the founding of Princeton College. It was renamed Columbia College in 1784 following the American Revolution, and in 1787 was placed under a private board of trustees headed by former students Alexander Hamilton and John Jay. In 1896, the campus was moved to its current location in Morningside Heights and renamed Columbia University.
Columbia scientists and scholars have played an important role in scientific breakthroughs including brain-computer interface; the laser and maser; nuclear magnetic resonance; the first nuclear pile; the first nuclear fission reaction in the Americas; the first evidence for plate tectonics and continental drift; and much of the initial research and planning for the Manhattan Project during World War II. Columbia is organized into twenty schools, including four undergraduate schools and 15 graduate schools. The university’s research efforts include the Lamont–Doherty Earth Observatory, the Goddard Institute for Space Studies, and accelerator laboratories with major technology firms such as IBM. Columbia is a founding member of the Association of American Universities and was the first school in the United States to grant the M.D. degree. With over 14 million volumes, Columbia University Library is the third largest private research library in the United States.
The university’s endowment stands at $11.26 billion in 2020, among the largest of any academic institution. As of October 2020, Columbia’s alumni, faculty, and staff have included: five Founding Fathers of the United States—among them a co-author of the United States Constitution and a co-author of the Declaration of Independence; three U.S. presidents; 29 foreign heads of state; ten justices of the United States Supreme Court, one of whom currently serves; 96 Nobel laureates; five Fields Medalists; 122 National Academy of Sciences members; 53 living billionaires; eleven Olympic medalists; 33 Academy Award winners; and 125 Pulitzer Prize recipients.
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