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.

Quenching the Kitaev honeycomb model

This work was presented first in a seminar at the University of Toronto on 18 October 2017. The slides of this presentation can be downloaded here (pptx, 13 MB).

Title: Quenching the Kitaev honeycomb model

Author: Louk Rademaker


Abstract: I studied the non-equilibrium response of an initial Néel state under time evolution with the Kitaev honeycomb model. This time evolution can be computed using a random sampling over all relevant flux configurations. With isotropic interactions the system quickly equilibrates into a steady state valence bond solid. Anisotropy induces an exponentially long prethermal regime whose dynamics are governed by an effective toric code. Signatures of topology are absent, however, due to the high energy density nature of the initial state.


Quantum Thermalization and the Expansion of Atomic Clouds

I also presented this work in the form of a poster at the SPICE workshop ’Non-equilibrium Quantum Matter’, Mainz, Germany, from 30 May to 2 June 2017. The poster can be downloaded here (pdf, 1.3 MB).

Title: Quantum Thermalization and the Expansion of Atomic Clouds

Authors: Louk Rademaker, Jan Zaanen


Abstract: The ultimate consequence of quantum many-body physics is that even the air we breathe is governed by strictly unitary time evolution. The reason that we perceive it nonetheless as a completely classical high temperature gas is due to the incapacity of our measurement machines to keep track of the dense many-body entanglement of the gas molecules. The question thus arises whether there are instances where the quantum time evolution of a macroscopic system is qualitatively different from the equivalent classical system? Here we study this question through the expansion of noninteracting atomic clouds. While in many cases the full quantum dynamics is indeed indistinguishable from classical ballistic motion, we do find a notable exception. The subtle quantum correlations in a Bose gas approaching the condensation temperature appear to affect the expansion of the cloud, as if the system has turned into a diffusive collision-full classical system.



Phonon linewidth due to electron-phonon interactions with strong forward scattering in FeSe thin films on oxide substrates

Title: Phonon linewidth due to electron-phonon interactions with strong forward scattering in FeSe thin films on oxide substrates

Authors: Yan Wang, Louk Rademaker, Elbio Dagotto, Steven Johnston

Abstract: The discovery of an enhanced superconducting transition temperature Tc in monolayers of FeSe grown on several oxide substrates has opened a new route to high-Tc superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electron-phonon (e-ph) interaction across the interface that peaked at small momentum transfers. In this paper, we examine the implications of such a coupling on the phononic properties of the system. We show that a strong forward scattering leads to a sizable broadening of phonon lineshape, which may result in charge instabilities at long-wavelengths. However, we further find that the inclusion of Coulombic screening significantly reduces the phonon broadening. Our results show that one might not expect anomalously broad phonon linewidths in the FeSe interface systems, despite the fact that the e-ph interaction has a strong peak in the forward scattering (small q) direction.


Publication: Bilayer Excitons in Two-Dimensional Nanostructures for Greatly Enhanced Thermoelectric Efficiency

Title: Bilayer Excitons in Two-Dimensional Nanostructures for Greatly Enhanced Thermoelectric Efficiency

thermoAbstract: Currently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer-exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer-exciton thermocouple. Based on current experimental parameters, a bilayer-exciton heterostructure of p- and n-doped Bi2Te3 can enhance the figure of merit an order of magnitude compared to bulk Bi2Te3. Another material suggestion is to make a bilayer out of electron-doped SrTiO3 and hole-doped Ca3Co4O9.

Reference: Kai Wu, Louk Rademaker, and Jan Zaanen, Phys. Rev. Applied 2, 054013 (2014).

Publication: Exciton condensation in strongly correlated electron bilayers

Title: Exciton condensation in strongly correlated electron bilayers


Abstract: We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems, an exciton is the bound state of a double occupied and empty site. In the strong coupling limit, the exciton acts as a hard-core boson. Its physics is captured by the exciton t-J model, containing an effective XXZ model describing the exciton dynamics only. Using numerical simulations and analytical mean-field theory, we constructed the ground-state phase diagram. Three homogeneous phases can be distinguished: the antiferromag- net, the exciton checkerboard crystal, and the exciton superfluid. For most model parameters, however, we predict macroscopic phase separation between these phases. The exciton superfluid exists only for large exciton hopping energy. Additionally, we studied the collective modes and susceptibilities of the three phases. In the superfluid phase, we find the striking feature that the bandwidth of the spin-triplet excitations, potentially detectable by resonant inelastic x-ray scattering (RIXS), is proportional to the superfluid density. The superfluid phase mode is visible in the charge susceptibility, measurable by RIXS or electron energy loss spectroscopy (EELS).

Reference: Louk Rademaker, Jeroen van den Brink, Jan Zaanen, and Hans Hilgenkamp, Phys. Rev. B 88, 235127 (2013).

Publication: Determinant quantum Monte Carlo study of exciton condensation in the bilayer Hubbard model

Title: Determinant quantum Monte Carlo study of exciton condensation in the bilayer Hubbard model


Abstract: We studied the possibility of exciton condensation in a strongly correlated bilayer extended Hubbard model using determinant quantum Monte Carlo. To model both the on-site repulsion U and the interlayer interaction V we introduced an update scheme extending the standard Sherman-Morrison update. We observe that the sign problem increases dramatically with the inclusion of the interlayer interaction V, which prohibits at this stage an unequivocal conclusion regarding the presence of exciton condensation. However, enhancement of the interlayer tunneling results suggest that the strongest exciton condensation tendency lies around 10–20% p/n doping. Magnetic properties and conductivity turn out to be relatively independent of the interlayer interaction.

Reference: Louk Rademaker, Steve Johnston, Jan Zaanen, and Jeroen van den Brink, Phys. Rev. B 88, 235115 (2013)

Publication: Influence of long-range interactions on charge ordering phenomena on a square lattice

TitleInfluence of long-range interactions on charge ordering phenomena on a square lattice

Abstract: Usually complex charge ordering phenomena arise due to competing interactions. We have studied how such ordered patterns emerge from the frustration of a long-ranged interaction on a lattice. Using the lattice gas model on a square lattice with fixed particle density, we have identified several interesting phases, such as a generalization of Wigner crystals at low particle densities and stripe phases at densities between ρ=1/3 and 1/2. These stripes act as domain walls in the checkerboard phase present at half-filling. The phases are characterized at zero temperatures using numerical simulations, and mean field theory is used to construct a finite temperature phase diagram.
ReferenceLouk Rademaker, Yohanes Pramudya, Jan Zaanen, and Vladimir Dobrosavljević, Phys. Rev. E 88, 032121 (2013)

Publication: Enhancement of spin propagation due to interlayer exciton condensation

Our latest paper, just published on-line in Phys. Rev. B Rapid Communications!

Title: Enhancement of spin propagation due to interlayer exciton condensation


Abstract: We show that an interlayer exciton condensate doped into a strongly correlated Mott insulator exhibits a remarkable enhancement of the bandwidth of the magnetic excitations (triplons). This triplon is visible in the dynamical magnetic susceptibility and can be measured using resonant inelastic x-ray scattering. The bandwidth of the triplon scales with the exciton superfluid density, but only in the limit of strong correlations. As such the triplon bandwidth acts as a probe of exciton-spin interactions in the condensate.

Reference: Louk Rademaker, Jeroen van den Brink, Hans Hilgenkamp and Jan Zaanen, Phys. Rev. B 88, 121101(R) (2013).