Forget graphene: More exotic moiré physics with TMDs

On Friday 7 October 2022 I had the pleasure of giving a presentation at the LPS Orsay, just south of Paris, France. Since the group there has a lot of interest in strong correlations and topology, it was a good time to present an overview of my TMD-related work from last year.

Abstract: The recent revolution in moiré materials started with the discovery of correlated insulators and superconductivity in twisted bilayer graphene. I will show that much stronger electron correlations appear in moiré bilayers of transition-metal dichalcogenides (TMDs).
After introducing the origin of flat bands in TMD moirés (including ARPES results), I will discuss theoretical predictions for a range of exotic phenomena: the amorphous Wigner-Mott electron slush; the 3/4 chiral spin liquid; and metal-insulator criticality.

Download here the PDF file of the presentation slides.

Fake Insulators: What are they, and how to spot them?

The traditional view of a metal-insulator transition is wrong, instead, at a MIT the critical curve diverges and there is a metallic regime with negative dR/dT: “fake insulators”.

While the difference between insulators and metals is strictly speaking only defined at zero temperature, it has become commonplace to identify systems with a negative temperature-derivative of the resistivity (dR/dT < 0) as insulators. This is, however, misleading. In particular, sufficiently close to a metal-insulator transition a system can have dR/dT < 0 yet reach a finite zero-temperature resistivity, meaning it is actually a metal. Such ‘fake insulators’ can obscure the interpretation of Mott- and band metal-insulator transitions.

In a recent presentation at the workshop “New Spin on Molecular Quantum Materials” held by SPICE in Mainz (Germany), I discussed this phenomenon of ‘fake insulators’ in depth, and comparing it to recent experimental results in graphene and TMD bilayers. The presentation can be watched online:

You can also download the slides of this presentation (in pptx, 51 MB).

Spin-valley symmetry breaking and Chern insulators in twisted graphene structures

Watch my 15-minute presentation on spin-valley symmetry breaking and Chern insulators, given on 13 August on the online Moiré-Twistronics workshop.

At the online Moiré-Twistronics workshop last week I gave a short presentation on our work on Chern insulators in twisted bilayer graphene and twisted mono-bilayer graphene. It was a great online conference, so check their website for recordings of most other talks there.

Download the slides for this presentation (pdf, 24 MB).

A Practical Introduction to Density Functional Theory

At the University of Geneva a group of enthusiastic graduate students have instigated a seminar series devoted to ‘relevant techniques in many-body physics’: the “ToolBoX“. I had the honor of providing the first set of lectures on density functional theory, for which I prepared some notes with exercises.. Download it here below!

For the lazy students, you can download all the pseudopotentials and input files here.

Quantum Anomalous Hall effect in Twisted Bilayer Graphene

At our weekly “Flat Club” in Geneva I presented on Friday 11 October the latest experiments from Andrea Young’s group, who observed the quantum anomalous Hall effect in twisted bilayer graphene. Download my slides here (PDF).

Title: Quantum Anomalous Hall effect in Twisted Bilayer Graphene

Abstract: A recent experiment brings together many topics discussed in earlier Flat Club meetings: namely the observation of a Quantum Anomalous Hall effect in magic angle twisted bilayer graphene aligned with hBN. In order to understand these results, we will discuss first the concept of a Chern insulator, its response in a magnetic field, and the role of ferromagnetism. Next, we will discuss how the alignment of hBN with twisted graphene opens up the possibility of creating a ferromagnetic Chern insulator. We will end with the experiments by the Young group who observed a QAH with a quantized rho_xy within 0.1% of h/e^2.

To read:
Serlin et al., Intrinsic quantized anomalous Hall effect in a moiré heterostructure,

Unconventional Many-Body Localization in Long-Range Quantum Spin Glasses

At the Conference on “Complex Quantum Systems out of Equilibrium” in Murcia, Spain I presented my latest work with Dmitry Abanin on the relation between MBL and spin glasses. This work has been published on the arXiv now as well. The full presentation slides can be downloaded here (PDF).

Title: Unconventional Many-Body Localization in Long-Range Quantum Spin Glasses

Abstract: Spin glasses are a well-studied class of classical systems where random interactions lead to spin freezing at low temperatures. On the other hand, many-body localization is a quantum phe- nomenon where randomness and interactions lead to localization characterized by, amongst others, area law entanglement entropy and local integrals of motion. We show that a third, intermediate, state can emerge in a long-range one-dimensional spin glass under the applica- tion of a transverse field. At small applied fields and low temperatures the spin glass order remains, as characterized by the Edwards-Anderson order parameter. However, interacting low-energy spin resonances at large distances create unconventional long-range entanglement in eigenstates. The quench dynamics therefore display a wide variety in possible results: while some spins remain frozen, others ‘thaw’. The ”quantum spin glass” is therefore neither ergodic, nor many-body localized.

The nu=-2 state in Twisted Bilayer Graphene: A Bad Insulator?

At the “Mottness, Poor Conductors, and Strange Metals” Workshop at the Tsung-Dao Lee Institute, Shanghai, China I presented my work and some new speculations on twisted bilayer graphene. The full presentation can be downloaded here (PDF).

Title: The nu = -2 state in Twisted Bilayer Graphene: a bad Mott insulator?

Abstract: Twisted bilayer graphene near the ‘magic angle’ has shown a wealth of interesting states: superconductivity, ferromagnetism, correlated insulator states and a linear resistivity ‘strange metal’. I will focus on the state at carrier density nu = -2 relative to charge neutrality. At this filling the resistivity is minimal at around 4 K, above which there is reported linear resistivity and below which it is insulating. Using unbiased real-space Hartree-Fock calculations, we show that the nu = -2 state undergoes a charge transfer between “ring” and “center” orbitals leading to an even further flattening of the bands. Including a Hubbard interaction will then lead to a Mott insulator. However, unlike ‘strong’ Mott insulators like the cuprate parent compounds, this Mott state can easily be destroyed by temperature or magnetic field. I will discuss possible mechanisms for this ‘bad insulator’ behavior, including its relation to the multi-channel Kondo effect