MIT News November 18, 2024 Recent experiments on the moiré structure formed by pentalayer rhombohedral graphene aligned with a hexagonal boron nitride substrate report the discovery of a zero-field fractional quantum Hall effect. These “(fractional) quantum anomalous Hall” [(F)QAH] phases occur for one sign of a perpendicular displacement field, and correspond, experimentally, to full or partial filling of a valley polarized Chern-1 band. Such a band is absent in the noninteracting band structure. Researchers at MIT showed that electron-electron interactions play a crucial role, and presented microscopic theoretical calculations demonstrating the emergence of a nearly flat, isolated, Chern-1 band and […]
Category Archives: Graphene
Physicists reveal how layers and twists impact graphene’s optical conductivity
Phys.org November , 2024 An international team of researchers (USA – Florida State University, China) explored the advantages of moiré superlattices in twisted bilayer graphene (TBG) aligned with hexagonal boron nitride (hBN) for passively enhancing optical conductivity in the low-energy regime. To probe the local optical response of TBG/hBN double-moiré lattices, they used infrared nano-imaging in conjunction with nanocurrent imaging to examine local optical conductivity over a wide range of TBG twist angles. They showed that interband transitions associated with the multiple moiré flat and dispersive bands produced tunable transparent IR responses even at finite carrier densities, which is in […]
Graphene-based memristors move a step closer to benefiting next-generation computing
Phys.org October 22, 2024 Memristors based on graphene is a promising alternative to contemporary field-effect transistor (FET) technology achieves higher integration density and lower power consumption. The use of graphene as electrodes in memristors could also increase robustness against degradation mechanisms. To realize this researchers in the UK have developed a process for direct growth of high-quality monolayer graphene on sapphire wafers in a mass-producible, contamination-free, and transfer-free manner using a commercially available metal–organic chemical vapor deposition (MOCVD) system. Using their process they developed graphene-electrode based memristors incorporating graphene electrodes at wafer scale. The memristor demonstrated promising characteristics in terms […]
High-selectivity graphene membranes enhance CO₂ capture efficiency
Phys.org July 6, 2024 Although membranes based on a porous two-dimensional selective layer offer the potential to achieve exceptional performance to improve energy efficiency and reduce the cost for carbon capture, competitive sorption of CO2 with the potential to yield high permeance and selectivity has remained elusive. Researchers in Switzerland showed that a simple exposure of ammonia to oxidized single-layer graphene at room temperature incorporates pyridinic nitrogen at the pore edges. This led to a highly competitive but quantitatively reversible binding of CO2 with the pore. A combination of CO2/N2 separation factor and CO2 permeance from a stream containing 20 vol% […]
Stacking three layers of graphene with a twist speeds up electrochemical reactions
Phys.org June 21, 2024 A team of researchers in the US (University of Michigan, SLAC National Accelerator Laboratory, Carnegie Mellon University, MIT) developed a twist-dependent electrochemical activity map, combining a low-energy continuum electronic structure model with modified Marcus–Hush–Chidsey kinetics in Tri layer graphene. They identified a counterintuitive rate enhancement region spanning the magic angle curve and incommensurate twists in the system geometry. They found a broad activity peak with a ruthenium hexamine redox coupled in regions corresponding to both magic angles and incommensurate angles, a result qualitatively distinct from the twisted bilayer case. According to the researchers flat bands and […]
Ultrafast lasers map electrons ‘going ballistic’ in graphene with implications for next-gen electronic devices
Phys.org December 15, 2023 Researchers at the University of Kansas formed heterostructure by sandwiching a MoS2 and MoSe2 heterobilayer between two graphene monolayers. Transient absorption measurements revealed that the electrons and holes separated by the type-II interface between MoS2 and MoSe2 could transfer to the two graphene layers, respectively. With high spatial and temporal resolution, they found that while the holes in one graphene layer undergoes a classical diffusion process with a large diffusion coefficient of 65 cm2 s–1 and a charge mobility of 5000 cm2 V–1 s–1, the electrons in the other graphene layer exhibits a quasi-ballistic transport feature. […]
Physicists turn pencil lead into metaphorical ‘gold’
Phys.org October 30, 2023 Rhombohedral-stacked multilayer graphene hosts a pair of flat bands touching at zero energy, which should give rise to correlated electron phenomena that can be tuned further by an electric field. An international team of researchers (USA – MIT, Harvard University, Japan) measured electron transport through hexagonal boron nitride-encapsulated pentalayer graphene down to 100 mK and observed a correlated insulating state with resistance at the megaohm level or greater at charge density n = 0 and displacement field D = 0. By increasing D, they observed a Chern insulator state with C = −5 and two other states with C = −3 at a magnetic field […]
The right twist and strain for graphene to form 1D moirés
Phys.org October 30, 2023 The formation of one-dimensional moiré patterns is a consequence of an interplay between twist and strain which gives rise to a collapse of the reciprocal space unit cell. An international team of researchers (Spain, Poland) found a simple relation between the two quantities and the material specific Poisson ratio. The induced one-dimensional behavior was characterized by two, usually incommensurate, periodicities. According to the researchers their results offer explanations for the complex patterns of one-dimensional channels observed in low angle twisted bilayer graphene systems and twisted bilayer dichalcogenides. Their findings can be applied to any hexagonal twisted […]
From a five-layer graphene sandwich, a rare electronic state emerges
Science Daily October 18, 2023 Multiferroics have multifunctional electrical and magnetic device applications. Two-dimensional materials with honeycomb lattices offer opportunities to engineer unconventional multiferroicity. Orbital multiferroics could offer strong valley–magnetic couplings and large responses to external fields—enabling device applications such as multiple-state memory elements and electric control of the valley and magnetic states. An international team of researchers (USA – MIT, Harvard University, Japan) has shown orbital multiferroicity in Penta layer rhombohedral graphene. They observed anomalous Hall signals Rxy with an exceptionally large Hall angle and orbital magnetic hysteresis at hole doping. There were four such states with different valley […]
Experiments reveal that water can ‘talk’ to electrons in graphene
Nanowerk June 23, 2023 Liquids induce electronic polarization and drive electric currents as they flow; electronic excitations, in turn, participate in hydrodynamic friction. Yet, the underlying solid–liquid interactions have been lacking a direct experimental probe. An international team of researchers (Germany, UK, Spain) heated graphene electron quasi-instantaneously by a visible excitation pulse and monitored the time evolution of the electronic temperature with a terahertz pulse. They observed that water accelerated the cooling of the graphene electrons, whereas other polar liquids leave the cooling dynamics largely unaffected. A quantum theory of solid–liquid heat transfer accounts for the water-specific cooling enhancement through […]