Strongly correlated phenomena in twisted bilayers of graphene and transition metal dichacogenides

Project Information

TWISTM

Grant agreement ID: 844271

DOI

10.3030/844271

Project closed

EC signature date 24 April 2019

Start date 1 February 2020

End date 5 February 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 246 669,12

EU contribution

€ 246 669,12

Coordinated by

FREIE UNIVERSITAET BERLIN
Germany

Periodic Reporting for period 2 - TWISTM (Strongly correlated phenomena in twisted bilayers of graphene and transition metal dichacogenides)

Reporting period: 2022-02-01 to 2023-01-31

Summary of the context and overall objectives of the project

The electronic properties of materials determine their potential use in commercial devices. Such electronic properties can be inferred by studying the electronic band structure - the allowed energy levels where electrons are. One way to create new band structures that host new states of matter is to change the crystalline environment. A crystal is a material where the atoms are arranged in an ordered manner. Depending on the details of the atoms environment, the band structure is going to be different.


Two-dimensional materials, such as graphene and transition metal dichalcogenides, are crystalline structures that consist of a stack of one-atom-thick layers. They constitute a great platform to control the crystalline environment. Their layered structure enables their exfoliation down to the monolayer limit and their subsequent stacking to form vertical structures with novel properties. Both the choice of the materials that constitute each layer and how their are placed on top of each other allows for the design of the electronic properties of the full structure.

Recent progress in the field has demonstrated that the combination of the constituents in the heterostructure is as important as the misalignment between the atomic layers for their performance. When to atomic layers are misaligned, a moiré pattern emerges. Such twist angle between the atomic layers strongly modifies the electronic band structure and new and exotic phases emerge that where absent in the individual elements. Correlated insulating states as well as superconductivity arises in twisted graphene structures at specific twist angles called 'magic angle'. Despite all this progress, however, research in this field is in its nascent stage and many exciting phenomena remain to be explored. In particular, the presence of other correlation-driven phases in twisted graphitic systems is unclear and the field needs a comprehensive understanding of the electronic phase diagram.

The overall objective of TWISTM is to engineer samples with a twist angle between the constituents and study their electronic properties by means of low-temperature Scanning Tunneling Microscopy and Spectroscopy. Multiple two-dimensional materials will be explored, from semimetalic graphene to semiconducting transition metal dichalcogenides. The final goal is to understand the states of matter that emerge in moiré materials.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The results obtained have been disseminated in several scientific articles. The most important publication, which provides a holistic understanding of the phase diagram in moiré materials is:

Carmen Rubio-Verdú et al., 'Moiré nematic phase in twisted double bilayer graphene', Nature Physics 18 196 (2022)

Experimental results concerning the collective phases that emerge in twisted heterostructures are:

Daniel J. Rizzo, Sara Shabani, Bjarke S. Jessen, Jin Zhang, Alexander S. McLeod, Carmen Rubio-Verdú, Francesco L. Ruta, Matthew Cothrine, Jiaqiang Yan, David G. Mandrus, Stephen E. Nagler, Angel Rubio*, James C. Hone, Cory R. Dean, Abhay N. Pasupathy, and D. N. Basov, Nano Letters 22, 1946 (2022)

Rhine Samajdar, Mathias S Scheurer, Simon Turkel, Carmen Rubio-Verdú, Abhay N Pasupathy, Jörn WF Venderbos, Rafael M Fernandes, 2D Materials 8, 034005 (2021)

Pasqual Rivera, Minhao He, Bumho Kim, Song Liu, Carmen Rubio-Verdú, Hyowon Moon, Lukas Mennel, Daniel A Rhodes, Hongyi Yu, Takashi Taniguchi, Kenji Watanabe, Jiaqiang Yan, David G Mandrus, Hanan Dery, Abhay Pasupathy, Dirk Englund, James Hone, Wang Yao, Xiaodong Xu, Nature Communications 12 (2021)

Alexander Kerelsky*, Carmen Rubio-Verdú*, Lede Xian, Dante M Kennes, Dorri Halbertal, Nathan Finney, Larry Song, Simon Turkel, Lei Wang, Kenji Watanabe, Takashi Taniguchi, James Hone, Cory Dean, Dmitri N Basov, Angel Rubio, Abhay N Pasupathy, PNAS 118 (2021)

Dorri Halbertal, Nathan R Finney, Sai S Sunku, Alexander Kerelsky, Carmen Rubio-Verdú, Sara Shabani, Lede Xian, Stephen Carr, Shaowen Chen, Charles Zhang, Lei Wang, Derick Gonzalez-Acevedo, Alexander S McLeod, Daniel Rhodes, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Cory R Dean, James C Hone, Abhay N Pasupathy, Dante M Kennes, Angel Rubio, DN Basov, Nature Communications 12 (2021)

The fellow was invited to write several perspective articles:

Carmen Rubio-Verdú, 'Electron crystals come under the microscope', Nature 597, 640 (2021)
Carmen Rubio-Verdú and Abhay N. Pasupathy, 'A tell-tale wiggle', Nature Physics 17, 1082 (2021)
Carmen Rubio-Verdú and M. Reyes Calvo, 'New Moiré Landscapes for Atomic Spins', Physics 14, 165 (2021)

Besides the publications above, the fellow has given several invited talks at international conferences and seminars at academic institutions including Stony Brook University, New York University and ICFO.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

During the first part of the action, the fellow has demonstrated that exotic electronic phases emerge in moiré materials. Graphene moiré superlattices display electronic flat bands, where electronic interactions become strong and correlated phases such as superconductivity emerge. Understanding the mechanism underlying such correlated phases is the main open question in the field. Working in the group of Prof. Pasupathy at Columbia University, the fellow has reported the first experimental observation of an electronic nematic phase in a moiré material.

In addition, part of the efforts in this action have been directed to the design of heterostructures with different functionalities.

Nanometer-scale laterial p-n junctions are essential for the next generation of two-dimensional devices. Using a charge-transfer heterostructure graphene/RuCl3, the fellow and coworkers realize a nanoscale lateral p-n junction providing a conceptual foundation for their use in future device design for applications in electronics.

Two-dimensional transition metal dichalcogenides are a promising platform for optical applications. In these semiconducting materials, electrons and holes bound in pairs called excitons. Despite the rapid progress in the field, these materials are not perfect and the presence of atomic defects plays a role in the underlying physics. In a collaboration across institutions, the fellow reported the observation of donor bound dark excitons in ultraclean monolayer WSe2 samples.

Twisted van der Waals materials are an excellent platform to achieve correlated electronic phases. Recent experiments showed the interplay of two phenomena: the presence of flat bands and the existence of a moiré pattern. A natural question that arises is whether the moiré pattern is a necessary condition for the observation of correlated phases or whether it is simply sufficient to reduce the flat-band bandwidth. Multilayer rhobohedral graphene offers a different perspective toward achieving a flat-bands without the use of a moiré potential. Unfortunately, isolating rhombohedral graphene is extremely difficult as it is less energetically favorable than the multilayer counterpart, Bernal graphene. The fellow and collaborators realized micrometer-sized uniform four-layer rhombohedral graphene by introducing a tiny twist angle between two graphene bilayers, thus demonstrating that a material that host extremely sharp flat bands and that realize electronic correlations in the absence of a moiré potential.

Density of states maps (a)out and (b)in the nematic phase.