N. V. Tepliakov, R. Ma, J. Lischner, E. Kaxiras, A. A. Mostofi and M. Pizzochero, Dirac half-semimetallicity and antiferromagnetism in graphene nanoribbon/hexagonal boron nitride heterojunctions, Nano Letters 23, 6698 (2023).

Spintronics is an emerging branch of electronics that aims to harness the spin of electrons, a purely quantum mechanical feature, for storing and processing information. The development of spintronic devices relies on discovering materials in which electrons with opposite spin orientations behave differently. In our recent work, carried out in collaboration with researchers at Harvard University, we examined graphene nanoribbons, one-dimensional strips of carbon atoms, embedded in hexagonal boron nitride, a two-dimensional insulator. Using advanced computer simulations, we revealed that this heterojunction is half-metallic, that is, electrons exhibit conducting behaviour for electrons of one spin orientation and insulating behaviour for those with the opposite spin orientation. Further, we found that charge doping the heterojunction can drive a transition between two different magnetic states of the nanoribbon (an antiferromagnetic and a ferrimagnetic one). Our results show that heterojunctions comprising graphene nanoribbons embedded in hexagonal boron nitride are a promising platform for future spintronic applications thanks to their combination of half-metallic behaviour and electrically tunable magnetism.

This work was part of the PhD work of Nikita Tepliakov and was in collaboration with colleagues Michele Pizzochero and Efthimios Kaxiras at Harvard University.

I. Maity, A. A. Mostofi and J. Lischner, Electrons surf phason waves in moiré bilayers,
Nano Letters 23, 4870 (2023).

Phasons are a particular type of collective motion of atoms that occur in aperiodic structures such as quasicrystals. The interaction between phasons and other collective excitations within a material leads to fascinating thermal and electronic transport phenomena.

Recently, twisted moiré materials, in which two or more two-dimensional (2D) materials are stacked and rotated relative to one another, have also been predicted to host phasons. Moiré systems have emerged as a new platform to discover, understand and manipulate novel quantum phases of matter, including superconducting and correlated insulating states. As a result, moiré materials provide a promising material platform to study the interaction of phasons and electrons.

In this work, we discover a fascinating consequence of this interplay. Focussing on a MoSe₂/WSe₂ heterobilayer, we have shown that thermally excited phason modes give rise to an almost rigid motion of the moiré pattern. Low-energy states associated with electrons and holes are localized in specific regions of the moiré material and follow the thermal motion of these regions, and therefore appear to “surf” the phason waves.

Our findings provide new insights into the interplay of electrons and phasons and have implications for the design of efficient charge and exciton transport devices based on moiré materials.

This work was done in collaboration with Dr Indrajit Maity and Prof Johannes Lischner at Imperial College London.

N. V. Tepliakov, J. Lischner, E. Kaxiras, A. A. Mostofi and M. Pizzochero, Unveiling and Manipulating Hidden Symmetries in Graphene Nanoribbons, Phys. Rev. Lett. 130, 026401 (2023).

Symmetry is an ubiquitous concept in physics that helps us rationalise a wide spectrum of physical properties and phenomena. Certain materials exhibit a higher degree of symmetry than is evident from their spatial or atomic structure. In such cases, it is said that these materials feature a hidden symmetry, which reveals itself in a higher-dimensional space. In a paper published in Physical Review Letters, working in collaboration with researchers at Harvard University, we discovered the existence of a previously overlooked hidden symmetry in graphene nanoribbons, a class of carbon-based materials that hold promise for future nanoelectronics beyond silicon. We showed that the breaking of this hidden symmetry is responsible for the semiconducting properties of graphene nanoribbons, thus providing a novel microscopic explanation of their electronic structure. Furthermore, we demonstrated that control of this hidden symmetry by straining the nanoribbon results in the emergence of topological and semimetallic phases.

This work was part of the PhD work of Nikita Tepliakov and was in collaboration with colleagues Michele Pizzochero and Efthimios Kaxiras at Harvard University.

I. Maity, A. A. Mostofi and J. Lischner, Chiral valley phonons and flat phonon bands in moiré materials, Phys. Rev. B Letters 105, L041408 (2022).

Chirality–the characteristic of an object that can be distinguished from its mirror image–plays a fundamental role in understanding many phenomena in physics, chemistry, and biology. Recently, the study of chirality in phonons has attracted significant interest. The interaction of chiral phonons with other quasiparticles present in a crystal is relevant for understanding and controlling many electronic and optical phenomena.

A particularly promising platform for observing and manipulating chiral phonons are twisted bilayers of two-dimensional (2D) materials. Since the discovery of flat electronic bands in twisted bilayer graphene, such moiré materials have served as a rich playground for investigating the properties of correlated electrons, excitons and phonons. In this work, we demonstrate that phonons in moiré materials can also be chiral. Focusing on twisted WSe₂ at twist angles close to 60 degress, we find two sets of emergent chiral moiré valley phonons that originate from symmetry breaking at the moiré scale. We also discover flat chiral phonon bands in the energy gap between the acoustic and optical phonon modes, which, similar to the flat electronic bands in this system, occur over a range of twist angles and without the requirement of a magic angle.

Our findings, which are expected to be generic for moiré systems composed of two-dimensional materials that break inversion symmetry, are relevant for understanding electron-phonon and exciton-phonon scattering, and also for the design of phononic analogues of flat band electrons.

M. Pizzochero, N. V. Tepliakov, A. A. Mostofi and E. Kaxiras, Electrically Induced Dirac Fermions in Graphene Nanoribbons, Nano Letters 21, 9332 (2021).

Graphene nanoribbons (GNRs) are few-nanometer wide strips of graphene. Unlike graphene, graphene nanoribbons have a band-gap, while still maintaining high electron and hole mobility. This makes them promising candidates for beyond-silicon nanoelectronic devices.

In this work we used first-principles calculations and simpler tight-binding models to explore whether and to what degree the properties of graphene nanoribbons can be controlled by an electric field. This is of crucial importance for developing electronic devices based on graphene nanoribbons. We showed that applying an electric field across the width of the nanoribbon modulates the size of the band-gap, and can be used to turn a normally semiconducting nanoribbon into a semimetal. The electrically-induced semimetallic phase features zero-energy massless Dirac fermions, analogous to graphene. The electric field can be applied externally, i.e., by contacting the edges of the nanoribbon to source and drain electrodes, or internally, by incorporating doping elements of opposite polarity in the opposite edges of the nanoribbon. These findings generalize to other group-IV nanoribbons, including recently fabricated silicene nanoribbons, which hold great promise due to their potential for integration into current silicon electronics.

This work was part of the PhD work of Nikita Tepliakov and was in collaboration with colleagues Michele Pizzochero and Efthimios Kaxiras at Harvard University.

J. C. A. Prentice and A. A. Mostofi, Accurate and efficient computation of optical absorption spectra of molecular crystals: the case of the polymorphs of ROY, Journal of Chemical Theory and Computation 17, 5214 (2021)

Molecular crystals are a class of solids that have applications in organic LEDs, lasers, and solar cells, and optical absorption is among their most important properties. The optical absorption behaviour can be strongly affected by changes in the conformation or packing of the molecule, leading to phenomena such as colour polymorphism, where the same molecule can form different crystal structures that have different colours. 

In this work, we present a ‘crystal spectral warping’ method that allows the accurate and efficient prediction of optical absorption spectra of molecular crystals, including both long-range interactions, and the accuracy of hybrid TDDFT calculations. We have applied this approach to the well-known colour polymorphic system ROY, and show that our results give a better description of experimental observations than previous attempts.

This research was done by Joseph Prentice, who was a post-doctoral research associate funded by EPSRC (EP/P02209X/1).

Z. A. H. Goodwin, V. Vitale, X. Liang, A. A. Mostofi and J. Lischner, Hartree theory calculations of quasiparticle properties in twisted bilayer graphene, Electronic Structure 2, 034001 (2020)

Twisted bilayer graphene (tBLG) has generated a tremendous amount of interest in the condensed matter physics community (read our earlier research spotlights on the twist angle dependence of electron correlations in tBLG and the critical role of device geometry on the phase diagram of tBLG). Recent tunnelling experiments have found signatures corresponding to correlated insulator states as well as a number of other observations that cannot be understood within the framework of non-interacting electrons.

In a paper appearing in Physical Review B, we study the behaviour of interacting electrons in twisted bilayer graphene near the magic angle using atomistic Hartree theory. In particular, we calculate band structures and (local) densities of states in the metallic state as function of doping for a variety of twist angles. Our results explain several features of the experimentally observed tunnelling spectra. We also provide a simple parametrisation of the Hartree potential which enables Hartree band structures to be calculated without the need for self-consistency.

This work was part of Zachary Goodwin’s PhD in the Centre for Doctoral Training in Theory and Simulation of Materials. Zachary is supervised by Johannes Lischner and Arash Mostofi.

Prentice et al., The ONETEP linear-scaling density functional theory program, J. Chem. Phys. 152, 174111 (2020)

Oliveira et al., The CECAM electronic structure library and the modular software development paradigm, J. Chem. Phys. 153, 024117 (2020)

In the summer of 2020 the Journal of Chemical Physics published a special issue on the topic of electronic structure software. This issue highlights the excellent ecosystem of electronic structure codes that are developed in the community and includes papers on some very well-established codes as well as more recent ones. It is these advances in electronic structure software, hand-in-hand with developments in computer architecture and compute capacity, that drive our progress towards a predictive, first-principles-based understanding of materials and molecular systems.

We have contributed to two papers in this special topic issue. The first paper is on the ONETEP software package, which is a linear-scaling density-functional theory code that is able to bring to bear the predictive power of first-principles simulations on systems that are typically well beyond the capabilities of conventional approaches, such as the plane-wave pseudopotential method. The second paper is about the CECAM Electronic Structure Library (ESL). The ESL was founded to catalyse a transition away from the traditional “monolithic” model of electronic structure software, towards a more modular structure that incorporates interoperable and reusable modules and libraries that are open-source and available to the general community. The aim is that this will lead to better optimisation, more agile code development, and lower barriers to entry for new developers. One of the other codes developed in our group, Wannier90, and the subject of an earlier research spotlight, is a paradigmatic example of the ESL philosophy. You can also read the special AIP Scilight on the ESL effort.

V. Vitale, G. Pizzi, A. Marrazzo, J. R. Yates, N. Marzari and A. A. Mostofi, Automated high-throughput wannierisation, npj Computational Materials 6, 66 (2020).

Maximally-localised Wannier functions (MLWFs) are routinely used to compute a wide range of advanced materials properties, including highly accurate band structures, gyrotropic effects, spin Hall effects, thermoelectric properties and electron-phonon interactions. They are also an important scale-bridging tool for transferring the accuracy and transferability of first-principles methods at atomic scales to tight-binding models that can be used for mesoscale simulations (read our review article in Reviews of Modern Physics for more details).

The impact of MLWFs on electronic structure research is exemplified by the widespread usage of the Wannier90 code for generating and using MLWFs, which is interfaced to almost every major electronic structure code and is used by thousands of researchers worldwide.

In a paper appearing in npj Computational Materials, we present a general automated high-throughput framework for computing MLWFs and using them to compute material properties. Our approach is based on the recently proposed selected columns of the density matrix algorithm. The process does not require any a priori chemical intuition about the system being studied, and is therefore ideally suited to high-throughput calculations. Our workflows have been implemented within the AiiDA framework, and the full dataset of ~200 materials as well as the full provenance of how to generate the data shown in this work are provided via a publicly available Virtual Machine that can be downloaded from the Materials Cloud. To the best of our knowledge, this is the first time that such a level of reproducibility has been made available in the field of first-principles electronic structure simulations.

This work was part of Dr Valerio Vitale’s postdoctoral work as part of the European Union’s Centre of Excellence E-CAM, supervised by Jonathan Yates and Arash Mostofi.

J. R. Golebiowski, J. R. Kermode, P. D. Haynes and A. A. Mostofi, Atomistic QM/MM simulations of the strength of covalent interfaces in carbon nanotube-polymer composites, Phys. Chem. Chem. Phys. 22, 12007 (2020).

There is a “tyranny of scales” associated with the theoretical prediction of complex processes in materials and molecular systems. Often there is a small “active region” that governs the physics and chemistry of the phenomenon being investigated, but this region interacts and is influenced by the rest of the system, the “environment”, which is generally much larger and structurally more complex. Carbon nanotube-polymer composites are one such material. They are strong and lightweight with applications in demanding areas of industry such as ballistic protection and aerospace. Properties of the interface between carbon nanotubes (CNT) and polymers are a crucial factor affecting critical mechanical failure in these materials and chemical functionalisation of the CNTs can be used to promote strong interfaces based on CNT-polymer covalent crosslinks.

Developing a fundamental understanding of interfacial failure in CNT-polymer composites is challenging because the bond-breaking processes at the polymer-CNT attachment point that initiate failure are quantum-mechanical in nature, yet the mechanisms by which stresses are transferred through the disordered polymer occur on length-scales far in excess of anything that can be simulated quantum-mechanically.

In a paper appearing in Phys. Chem. Chem. Phys., we address the multi-scale nature of critical failure using a concurrent hybrid method that couples a quantum mechanical description of the bond-breaking process at the interface to a classical description of the remainder of the system. Using this “QM/MM” approach, which we first proposed and validated in an earlier work, we investigated the effects of different interfacial chemistries on mechanical strength and mechanisms mechanical failure in model CNT-polyethylene composites by simulating a CNT pull-out experiment. The predictions from our simulations can help to guide further experimental work with the aim of accelerating the design of high-performance CNT-polymer composites.

This work was part of Jacek Golebiowski’s PhD in the Centre for Doctoral Training in Theory and Simulation of Materials. Jacek was supervised by Peter Haynes, James Kermode and Arash Mostofi.