Publication in NRC

The Dutch newspaper NRC Handelsblad covered our recent work on a direct detection of flat bands in twisted bilayer graphene!

Slow Nonthermalizing Dynamics in a Quantum Spin Glass

Most spins remain localized and only a small fraction of ‘resonant’ spins become ergodic, in a one-dimensional long-range quantum spin glass.

Just published in Physical Review Letters, our latest work on a quantum spin glass with unusual dynamics. Spin glasses and many-body localization (MBL) are prime examples of ergodicity breaking, yet their physical origin is quite different: the former phase arises due to rugged classical energy landscape, while the latter is a quantum-interference effect. Here, we study quantum dynamics of an isolated 1D spin glass under application of a transverse field. At high energy densities, the system is ergodic, relaxing via a resonance avalanche mechanism, that is also responsible for the destruction of MBL in nonglassy systems with power-law interactions. At low energy densities, the interaction-induced fields obtain a power-law soft gap, making the resonance avalanche mechanism inefficient. This leads to the persistence of the spin-glass order, as demonstrated by resonance analysis and by numerical studies. A small fraction of resonant spins forms a thermalizing system with long-range entanglement, making this regime distinct from the conventional MBL. The model considered can be realized in systems of trapped ions, opening the door to investigating slow quantum dynamics induced by glassiness.

Rademaker and Abanin, Phys. Rev. Lett. 125, 260405 (2020)

Study the Collapse of the Wave Function in Traveling‐Wave Parametric Amplifiers

Our experimental proposal for testing the collapse of the wavefunction during amplification.

The readout of a microwave qubit state occurs using an amplification chain that enlarges the quantum state to a signal detectable with a classical measurement apparatus. However, at what point in this process is the quantum state really “measured”? To investigate whether the “measurement” takes place in the amplification chain, in which a parametric amplifier is often chosen as the first amplifier, we proposed to construct a microwave interferometer that has such an amplifier added to each of its arms. Feeding the interferometer with single photons, the interference visibility depends on the gain of the amplifiers and whether a measurement collapse has taken place during the amplification process. 

Van der Reep et al., Phys. Status Solidi B 2020, 2000567

Observation of flat bands in twisted bilayer graphene

My first paper with experimental groups is now published in Nature Physics. Our work is a detailed characterization of twisted bilayer graphene, with as highlight the direct observation of flat bands using nano-ARPES. I calculated the expected ARPES spectra, which can be seen in Fig 3 and 4 of the paper.

The full paper is here: Observation of flat bands in twisted bilayer graphene

Also, you can read more about it on the website of the DQMP in Geneva.

Hofstadter subband ferromagnetism in twisted bilayer graphene

The beautiful measurements of Yu Saito and Andrea Young display clear Chern insulator states. The sequence of C=-1, -2, -3, -4 states from the left can be understood as arising from spontaneous valley and spin polarization.

Together with the Yu Saito and Andrea Young (UC Santa Barbara) we wrote a paper about symmetry broken states in the presence of large magnetic fields in twisted bilayer graphene.

Read here more about it, on the arXiv.

Topological Flat Bands and Correlated States in Twisted Monolayer-Bilayer Graphene

We have been working on twisted monolayer bilayer graphene (tMBG) for a while when suddenly three groups put their experimental results last week on the arXiv (UCSB, Columbia, Manchester). So we had to rush writing up everything we had, and now you can read our postdictions about the quantum anomalous Hall effect in tMBG!

Title: Topological Flat Bands and Correlated States in Twisted Monolayer-Bilayer Graphene
Authors: Louk Rademaker, Ivan Protopopov, Dmitry Abanin
Abstract: Monolayer graphene placed with a twist on top of AB-stacked bilayer graphene hosts topological flat bands in a wide range of twist angles. The dispersion of these bands and gaps between them can be efficiently controlled by a perpendicular electric field, which induces topological transitions accompanied by changes of the Chern numbers. In the regime where the applied electric field induces gaps between the flat bands, we find a relatively uniform distribution of the Berry curvature. Consequently, interaction-induced valley- and/or spin-polarized states at integer filling factors are energetically favorable. In particular, we predict a quantum anomalous Hall state at filling factor ν=1 for a range of twist angles 1<θ<1.4. Furthermore, to characterize the response of the system to magnetic field, we computed the Hofstadter butterfly and the Wannier plot, which can be used to probe the dispersion and topology of the flat bands in this material.
Reference: arXiv:2004.14964

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.

Direct evidence for flat bands in twisted bilayer graphene from nano-ARPES

Constant energy cuts of the ARPES intensity near the K-point shows four circles centered around the Gamma points of the mini-Brillouin zones. Top row shows the experimental data, bottom row are my theoretical predictions.

Today our paper appeared on the arXiv describing the evidence for flat bands in twisted bilayer graphene. A really great effort, with experiments in Leiden and Barcelona and with our local Genevan Simone Lisi leading the ARPES effort. I’m very happy to have provided some theoretical work on this.

Read more on the arXiv…

Twisted bilayer graphene, Kitaev quenches and spontaneous symmetry breaking

The month november was quite successful publication-wise, as four of my publications have been accepted.

Quenching the Kitaev honeycomb model
Starting from an antiferromagnet, let it evolve with Kitaev’s spin liquid honeycomb model. I developed the technique to study what happens next, a combination of Majorana Loschmidt echo’s and gauge field Monte Carlo. A prethermal regime appears, characterized by magnetization oscillations that can be described by the toric code. Accepted by SciPost.

An Introduction to Spontaneous Symmetry Breaking
Together with Aron Beekman and Jasper van Wezel, I wrote a lecture notes on the basics of spontaneous symmetry breaking. Aimed at graduate students, it is a modern overview containing both the classics (Mermin-Wagner theorem, Nambu-Goldstone modes) as well as modern notions (tower of states, type A/B symmetry breaking, topology). Accepted by SciPost Lecture Notes.

Exact ground state of the Lieb-Mattis Hamiltonian as a superposition of Néel states
The Lieb-Mattis Hamiltonian was originally used to show that the ground state of the Heisenberg antiferromagnet is a symmetric singlet state. How then, one might wonder, does the symmetry broken antiferromagnetic state arise? Well, read my published work in Physical Review Research. 

Charge smoothening and band flattening due to Hartree corrections in twisted bilayer graphene
Together with Paula Mellado I calculated the Hartree corrections in twisted bilayer graphene when doping away from charge neutrality. Surprisingly, the charge transfer we observe between AB/BA and AA regions of the Moiré unit cell makes the flat bands even flatter. Published in Physical Review B.

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, https://arxiv.org/abs/1907.00261