twisted bilayer graphene

twisted bilayer graphene
Name	Twisted bilayer graphene
Crystal system	Hexagonal (moire superlattice)
Discovered	2018
Discoverers	Pablo Jarillo-Herrero; Emanuel Tutuc; Philip Kim; Yuan Cao

twisted bilayer graphene

Twisted bilayer graphene is a van der Waals heterostructure formed by stacking two Graphene sheets with a relative rotational misorientation, producing a long-wavelength moiré pattern that dramatically alters electronic properties. Since the 2018 discovery by groups led by Pablo Jarillo-Herrero and Emanuel Tutuc with contributions from Philip Kim and Yuan Cao, it has become a focal point linking experiments at institutions such as Massachusetts Institute of Technology, Harvard University, Columbia University, and Swiss Federal Institute of Technology in Zurich to theoretical work from laboratories at Princeton University, University of Cambridge, and École Normale Supérieure.

Introduction

Twisted bilayer graphene arises when two Graphene layers are rotated by a small angle relative to one another, creating emergent physics near so-called "magic angles" observed in experiments by teams at MIT and Harvard and analyzed by theorists from Perimeter Institute and Max Planck Institute. The system links fields including condensed matter physics practiced at Bell Labs and IBM Research to materials science at Argonne National Laboratory and Lawrence Berkeley National Laboratory, and it has stimulated cross-disciplinary collaborations with researchers affiliated with Columbia University and University of California, Berkeley.

Crystallography and Moiré Geometry

The crystallography of twisted bilayer graphene is governed by the relative rotation between two Graphene honeycomb lattices, producing a moiré superlattice that can be indexed by commensurate lattice vectors used in studies at CERN-affiliated collaborations and crystallography groups at University of Oxford. Geometry analysis draws on methods developed at Stanford University and University of Chicago and connects to structural studies at Brookhaven National Laboratory. Key concepts such as moiré wavelength, registry, and domain formation are central to experiments from National Institute for Materials Science and simulations from Los Alamos National Laboratory. Atomic relaxation, strain, and reconstruction phenomena have been characterized in work by groups at University of Tokyo and University of California, Santa Barbara, linking to electron microscopy performed at Argonne National Laboratory and Max Planck Institute for Solid State Research.

Electronic Structure and Magic Angle Phenomena

Electronic structure of twisted bilayer graphene features flat electronic bands at specific twist angles (notably near ~1.1°), a discovery rediscovered experimentally by groups at MIT and analyzed theoretically by researchers at Columbia University and Princeton University. Band flattening leads to enhanced density of states and strong correlations reminiscent of findings in High-temperature superconductivity research from Bell Labs and comparisons to models used at Los Alamos National Laboratory. Calculations employ continuum models introduced by teams at University of Texas at Austin and École Polytechnique, and advanced spectroscopic measurements have been performed by groups at Argonne National Laboratory and Lawrence Berkeley National Laboratory.

Correlated Phases and Superconductivity

Correlated insulating states and unconventional superconductivity were reported by experimental groups led by Yuan Cao and Pablo Jarillo-Herrero at MIT and Harvard University, sparking comparisons to phenomena studied at Stanford University and Princeton University in the context of strong-correlation physics. The phase diagram includes Mott-like insulators, superconducting domes, nematicity, and ferromagnetism probed by teams at Columbia University, Cornell University, and University of Illinois Urbana-Champaign. Studies link to many-body approaches developed at Perimeter Institute, Max Planck Institute, and Institute for Advanced Study, and to experimental techniques from National Institute of Standards and Technology and Oak Ridge National Laboratory.

Experimental Fabrication and Characterization Techniques

Fabrication relies on deterministic stacking and tear-and-stack methods pioneered in labs at University of Manchester and Columbia University, with rotational control achieved using setups from MIT and Harvard. Characterization employs scanning tunneling microscopy used at University of California, Irvine and University of Basel, angle-resolved photoemission spectroscopy performed at Lawrence Berkeley National Laboratory and Stanford Synchrotron Radiation Lightsource, and transport measurements carried out in cryogenic facilities at CERN-collaborating institutions and National High Magnetic Field Laboratory. Advanced transmission electron microscopy at Brookhaven National Laboratory and Raman mapping at University of Tokyo elucidate lattice relaxation and strain patterns.

Theoretical Models and Computational Methods

Theoretical descriptions include continuum models developed by researchers at Massachusetts Institute of Technology and Columbia University, tight-binding Hamiltonians used by groups at University of Cambridge and Princeton University, and many-body techniques employed at Perimeter Institute and Max Planck Institute. Numerical approaches such as exact diagonalization, density matrix renormalization group applied by teams at Rutgers University and ETH Zurich, and mean-field theories from École Normale Supérieure are central, alongside first-principles density functional theory calculations carried out at Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory. Machine learning approaches from Google and DeepMind have also been applied to moiré materials.

Applications and Future Directions

Potential applications span quantum simulation pursued at IBM Research and Microsoft Research, quantum information platforms explored at National Institute of Standards and Technology, and tunable optoelectronics studied in collaborations between Stanford University and Harvard University. Future directions involve engineering heterostructures combining twisted layers with Transition metal dichalcogenides investigated at Rutgers University and scalable fabrication efforts at Tsinghua University and Korean Advanced Institute of Science and Technology. The field continues to connect experimental programs at Lawrence Berkeley National Laboratory and Argonne National Laboratory with theoretical initiatives at Perimeter Institute and Institute for Advanced Study to explore topology, magnetism, and designer correlated materials.

Category:Graphene Category:Condensed matter physics