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Physicists find misaligned carbon sheets yield unparalleled properties by Staff Writers Dallas TX (SPX) Aug 03, 2020
A material composed of two one-atom-thick layers of carbon has grabbed the attention of physicists worldwide for its intriguing - and potentially exploitable - conductive properties. Dr. Fan Zhang, assistant professor of physics in the School of Natural Sciences and Mathematics at The University of Texas at Dallas, and physics doctoral student Qiyue Wang published an article in June with Dr. Fengnian Xia's group at Yale University in Nature Photonics that describes how the ability of twisted bilayer graphene to conduct electrical current changes in response to mid-infrared light.
From One to Two Layers If two sheets of graphene are stacked on top of one another, and one layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graphene, yields electronic properties that differ significantly from those exhibited by a single layer alone or by two aligned layers. "Graphene has been of interest for about 15 years," Zhang said. "A single layer is interesting to study, but if we have two layers, their interaction should render much richer and more interesting physics. This is why we want to study bilayer graphene systems."
A New Field Emerges "The angle at which the two layers of graphene are misaligned - the twist angle - is critically important to the material's electronic properties," Wang said. "The smaller the twist angle, the larger the moire periodicity." The unusual effects of specific twist angles on electron behavior were first proposed in a 2011 article by Dr. Allan MacDonald, professor of physics at UT Austin, and Dr. Rafi Bistritzer. Zhang witnessed the birth of this field as a doctoral student in MacDonald's group. "At that time, others really paid no attention to the theory, but now it has become arguably the hottest topic in physics," Zhang said. In that 2011 research MacDonald and Bistritzer predicted that electrons' kinetic energy can vanish in a graphene bilayer misaligned by the so-called "magic angle" of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory, finding that offsetting two graphene layers by 1.1 degrees produced a two-dimensional superconductor, a material that conducts electrical current with no resistance and no energy loss. In a 2019 article in Science Advances, Zhang and Wang, together with Dr. Jeanie Lau's group at The Ohio State University, showed that when offset by 0.93 degrees, twisted bilayer graphene exhibits both superconducting and insulating states, thereby widening the magic angle significantly. "In our previous work, we saw superconductivity as well as insulation. That's what's making the study of twisted bilayer graphene such a hot field - superconductivity. The fact that you can manipulate pure carbon to superconduct is amazing and unprecedented," Wang said.
New UT Dallas Findings Zhang is a theoretical physicist, so he and Wang set out to determine how mid-infrared light might affect the conductance of electrons in twisted bilayer graphene. Their work involved calculating the light absorption based on the moire pattern's band structure, a concept that determines how electrons move in a material quantum mechanically. "There are standard ways to calculate the band structure and light absorption in a regular crystal, but this is an artificial crystal, so we had to come up with a new method," Wang said. Using resources of the Texas Advanced Computing Center, a supercomputer facility on the UT Austin campus, Wang calculated the band structure and showed how the material absorbs light. The Yale group fabricated devices and ran experiments showing that the mid-infrared photoresponse - the increase in conductance due to the light shining - was unusually strong and largest at the twist angle of 1.8 degrees. The strong photoresponse vanished for a twist angle less than 0.5 degrees. "Our theoretical results not only matched well with the experimental findings, but also pointed to a mechanism that is fundamentally connected to the period of moire pattern, which itself is connected to the twist angle between the two graphene layers," Zhang said.
Next Step "This new breakthrough will potentially enable a new class of infrared detectors based on graphene with high sensitivity," said Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics at the U.S. Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Development Command's Army Research Laboratory. "These new detectors will potentially impact applications such as night vision, which is of critical importance for the U.S. Army."
White dwarfs reveal new insights into the origin of carbon in the universe Santa Cruz CA (SPX) Jul 07, 2020 A new analysis of white dwarf stars supports their role as a key source of carbon, an element crucial to all life, in the Milky Way and other galaxies. Approximately 90 percent of all stars end their lives as white dwarfs, very dense stellar remnants that gradually cool and dim over billions of years. With their final few breaths before they collapse, however, these stars leave an important legacy, spreading their ashes into the surrounding space through stellar winds enriched with chemical elemen ... read more
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