SNSF Professorship!

I received an SNSF Starting Grant for the project « Quantum Matter with a Twist – The Interplay of Correlations and Topology in Moiré Materials ». This means I will be an SNSF Professor at the Department of Quantum Matter Physics starting in 2023. Needless to say, I’m very excited about this opportunity!

Moreover, this means I have the opportunity to hire two PhD students and one postdoc. You can apply and find more information on AcademicJobsOnline:
PhD student positions
Postdoc position

The press release from my new department can be found here.

Kagome Chiral Spin Liquid in Transition Metal Dichalcogenide Moiré Bilayers

At n=3/4 filling of the moiré flat band, transition metal dichalcogenide moiré bilayers will develop kagome charge order. We derived an effective spin model for the resulting localized spins and study its phase diagram using density matrix renormalization group simulations. Using realistic model parameters relevant for WSe2/WS2, we showed that this material can realize the exotic chiral spin liquid phase and the highly debated kagome spin liquid. Our work thus shows that the frustration and strong interactions present in TMD heterobilayers provide an exciting platform to study spin liquid physics.
Johannes MotrukDario RossiDmitry A. AbaninLouk Rademaker

Observation of flat Γ moiré bands in twisted bilayer WSe2

I provided the theory for a wonderful ARPES experiment that revealed the presence of flat bands in twisted bilayer WSe2. To our surprise, we only saw the flat bands around Γ and not at K — where they were to be expected based on all other (transport, exciton, STM) experiments. So some new mysteries to solve! Now available on the arXiv:

Title: Observation of flat Γ moiré bands in twisted bilayer WSe2
Reference: arXiv:2211.01192
Gianmarco Gatti, Julia Issing, Louk Rademaker, Florian Margot, Tobias A. de Jong, Sense Jan van der Molen, Jérémie Teyssier, Timur K. Kim, Matthew D. Watson, Cephise Cacho, Pavel Dudin, José Avila, Kumara Cordero Edwards, Patrycja Paruch, Nicolas Ubrig, Ignacio Gutiérrez-Lezama, Alberto Morpurgo, Anna Tamai, Felix Baumberger
The recent observation of correlated phases in transition metal dichalcogenide moiré systems at integer and fractional filling promises new insight into metal-insulator transitions and the unusual states of matter that can emerge near such transitions. Here, we combine real- and momentum-space mapping techniques to study moiré superlattice effects in 57.4∘ twisted WSe2 (tWSe2). Our data reveal a split-off flat band that derives from the monolayer Γ states. Using advanced data analysis, we directly quantify the moiré potential from our data. We further demonstrate that the global valence band maximum in tWSe2 is close in energy to this flat band but derives from the monolayer K-states which show weaker superlattice effects. These results constrain theoretical models and open the perspective that Γ-valley flat bands might be involved in the correlated physics of twisted WSe2.

Doping a Wigner-Mott insulator: Electron slush in transition-metal dichalcogenide moiré heterobilayers

Authors: Yuting Tan, Pak Ki Henry Tsang, Vladimir Dobrosavljević, Louk Rademaker
Abstract: The moiré pattern induced by lattice mismatch in transition-metal dichalcogenide heterobilayers causes the formation of flat bands, where interactions dominate the kinetic energy. At fractional fillings of the flat valence band, the long-range electron interactions then induce Wigner-Mott crystals. In this Letter we investigate the nontrivial electronic phases appearing away from commensurate fillings. Here, competing phases arise that are either characterized as doped Wigner-Mott charge transfer insulators or alternatively, a novel state with frozen charge order yet is conducting: the ‘electron slush’. We propose that an extremely spatially inhomogeneous local density of states can serve as a key signature of the electron slush.
Reference: arXiv:2210.07926 (2022)

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).

How to recognize the universal aspects of Mott criticality?

Similar quantum critical scaling behavior close to a interaction-driven Metal-Insulator Transition (MIT) has been observed in dilute 2DEGs, organic materials and recently TMD moiré bilayers.

In our latest paper we compare several examples of two-dimensional electronic systems displaying interaction-driven metal-insulator transitions of the Mott type, including dilute two-dimension electron gases (2DEG) in semiconductors, Mott organic materials, as well as the recently discovered transition-metal dichalcogenide (TMD) moiré bilayers.

The amazing thing is: remarkably similar behavior is found in all these systems, which is starting to paint a robust picture of Mott criticality. Most notable, on the metallic side a resistivity maximum is observed whose temperature scale vanishes at the transition.

In this paper we compare the available experimental data on these systems to three existing theoretical scenarios: spinon theory, Dynamical Mean Field Theory (DMFT) and percolation theory. We show that the DMFT and percolation pictures for Mott criticality can be distinguished by studying the origins of the resistivity maxima using an analysis of the dielectric response.

Published in a special edition of Crystals.
Yuting Tan, Vladimir Dobrosavljevic, Louk Rademaker, How to recognize the universal aspects of Mott criticality?, Crystals 12, 932 (2022)