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Title: Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene

Abstract

A remarkable recent experiment has observed Mott insulator and proximate superconductor phases in twisted bilayer graphene when electrons partly fill a nearly flat miniband that arises a “magic” twist angle. However, the nature of the Mott insulator, the origin of superconductivity, and an effective low-energy model remain to be determined. We propose a Mott insulator with intervalley coherence that spontaneously breaks U(1) valley symmetry and describe a mechanism that selects this order over the competing magnetically ordered states favored by the Hund’s coupling. We also identify symmetry-related features of the nearly flat band that are key to understanding the strong correlation physics and constrain any tight-binding description. First, although the charge density is concentrated on the triangular-lattice sites of the moiré pattern, the Wannier states of the tight-binding model must be centered on different sites which form a honeycomb lattice. Next, spatially localizing electrons derived from the nearly flat band necessarily breaks valley and other symmetries within any mean-field treatment, which is suggestive of a valley-ordered Mott state, and also dictates that additional symmetry breaking is present to remove symmetry-enforced band contacts. Tight-binding models describing the nearly flat miniband are derived, which highlight the importance of further neighbor hopping andmore » interactions. We discuss consequences of this picture for superconducting states obtained on doping the valley-ordered Mott insulator. We show how important features of the experimental phenomenology may be explained and suggest a number of further experiments for the future. We also describe a model for correlated states in trilayer graphene heterostructures and contrast it with the bilayer case.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1474784
Alternate Identifier(s):
OSTI ID: 1499093
Grant/Contract Number:  
SC0008739
Resource Type:
Published Article
Journal Name:
Physical Review. X
Additional Journal Information:
Journal Name: Physical Review. X Journal Volume: 8 Journal Issue: 3; Journal ID: ISSN 2160-3308
Publisher:
American Physical Society
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY

Citation Formats

Po, Hoi Chun, Zou, Liujun, Vishwanath, Ashvin, and Senthil, T. Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene. United States: N. p., 2018. Web. doi:10.1103/PhysRevX.8.031089.
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Po, Hoi Chun, Zou, Liujun, Vishwanath, Ashvin, & Senthil, T. Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene. United States. https://doi.org/10.1103/PhysRevX.8.031089
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Po, Hoi Chun, Zou, Liujun, Vishwanath, Ashvin, and Senthil, T. Fri . "Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene". United States. https://doi.org/10.1103/PhysRevX.8.031089.
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@article{osti_1474784,
title = {Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene},
author = {Po, Hoi Chun and Zou, Liujun and Vishwanath, Ashvin and Senthil, T.},
abstractNote = {A remarkable recent experiment has observed Mott insulator and proximate superconductor phases in twisted bilayer graphene when electrons partly fill a nearly flat miniband that arises a “magic” twist angle. However, the nature of the Mott insulator, the origin of superconductivity, and an effective low-energy model remain to be determined. We propose a Mott insulator with intervalley coherence that spontaneously breaks U(1) valley symmetry and describe a mechanism that selects this order over the competing magnetically ordered states favored by the Hund’s coupling. We also identify symmetry-related features of the nearly flat band that are key to understanding the strong correlation physics and constrain any tight-binding description. First, although the charge density is concentrated on the triangular-lattice sites of the moiré pattern, the Wannier states of the tight-binding model must be centered on different sites which form a honeycomb lattice. Next, spatially localizing electrons derived from the nearly flat band necessarily breaks valley and other symmetries within any mean-field treatment, which is suggestive of a valley-ordered Mott state, and also dictates that additional symmetry breaking is present to remove symmetry-enforced band contacts. Tight-binding models describing the nearly flat miniband are derived, which highlight the importance of further neighbor hopping and interactions. We discuss consequences of this picture for superconducting states obtained on doping the valley-ordered Mott insulator. We show how important features of the experimental phenomenology may be explained and suggest a number of further experiments for the future. We also describe a model for correlated states in trilayer graphene heterostructures and contrast it with the bilayer case.},
doi = {10.1103/PhysRevX.8.031089},
journal = {Physical Review. X},
number = 3,
volume = 8,
place = {United States},
year = {Fri Sep 28 00:00:00 EDT 2018},
month = {Fri Sep 28 00:00:00 EDT 2018}
}
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https://doi.org/10.1103/PhysRevX.8.031089
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