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Welcome to my webpage.

I am a theoretical physicist interested in the physical properties of new class of materials, two-dimensional crystals, as well as their stacks, often referred to as van der Waals heterostructures. Most of my work concerns graphene, the best known two-dimensional crystal first obtained by mechanical exfoliation of graphite and made of a single layer of carbon atoms arranged in regular hexagons, and its heterostructures (for example, graphene on hexagonal boron nitride or twisted bilayer graphene). I am also interested in layered transition metal dichalcogenides, especially those with Jahn-Teller/Peierls instabilities. As a side interest, I explore the benefits of applying the physicist's mindset to economics (vide econophysics).

Information for potential PhD students

If you are interested in my research and would like to join me as a PhD student, please get in touch with me to discuss potential projects and available funding. Bear in mind that the possibility of a scholarship is much higher if you are a British/EU citizen. Also, early contact is crucial as some funding sources require applications submitted as early as December (for start in October the following year).

News

26/05/2023 - Learning about electronic states in graphene stacks with a twist

Our work in collaboration with researchers from the University of Warwick and the University of Manchester (including a former student of mine, Aitor, who moved on to a postdoc in Manchester after his PhD in Bath) has been published in Nano Letters! We use angle-resolved photoemission spectroscopy to study the twist-dependent band structure of graphene stacks of various thicknesses, from two to four atomic layers. We show that our theoretical model describes all samples, irrespectively of the twist angle, thickness or the applied external out-of-plane electric field, very well. For the four-layer crystal (so called twisted double layer graphene) with a twist of 1.5°, we observe a flat band near the Fermi level with a bandwidth of about 30 meV.

Full version of the article can be accessed here (the article is open access; preprint version available here).

28/04/2023 - From model building to forecasting

Our work lead by Matthew has been finally published in the International Journal of Forecasting! He has used the model he developed earlier to forecast two important economic indicators of risk, value at risk and expected shortfall. He has shown that his novel Hawkes-type model performs better in predicting the most extreme gains and losses than the established industry models.

Full version of the article can be accessed here (the article is open access; preprint version available here).

19/01/2023 - Congratulations to Will!

Today, Will has successfully defended his thesis studying the charge density waves in 2H-TaSe₂. Will has done an excellent job, with his paper under review at Physical Review Research and one more in the pipeline if he finds the time while working as a patent lawyer in London.

01/10/2022 - Welcome to Luke Soneji

Welcome to Luke Soneji who is starting his PhD in my group. He is supported by EPSRC and will be studying the survival and impact of moiré effects in bulk layered crystals.

24/01/2022 - Topology of near-30-degree twisted bilayer graphene

Our work in collaboration with experimentalists from the University of Manchester and the Elettra Synchrotrone has been published in ACS Nano!

In stacks of two-dimensional crystals, mismatch of their lattice constants and misalignment of crystallographic axes lead to formation of moiré patterns. We show that moiré superlattice effects persist in twisted bilayer graphene (tBLG) with large twists and short moiré periods. Using angle-resolved photoemission, we observe dramatic changes in valence band topology across large regions of the Brillouin zone, including the vicinity of the saddle point at M and across 3 eV from the Dirac points. In this energy range, we resolve several moiré minibands and detect signatures of secondary Dirac points in the reconstructed dispersions. For twists θ > 21.8°, the low-energy minigaps are not due to cone anticrossing as is the case at smaller twist angles but rather due to moiré scattering of electrons in one graphene layer on the potential of the other which generates intervalley coupling. Our work demonstrates the robustness of the mechanisms which enable engineering of electronic dispersions of stacks of two-dimensional crystals by tuning the interface twist angles. It also shows that large-angle tBLG hosts electronic minigaps and van Hove singularities of different origin which, given recent progress in extreme doping of graphene, could be explored experimentally.

Full version of the article can be accessed here (the article is open access; preprint version available here).

10/08/2021 - Earthquakes and financial markets

Our work connecting physics and financial markets has been published in Physical Review E - congratulations to Matthew who bravely ventured into the unknown lands of econophysics! In this paper, he has applied a model developed to describe the fore- and aftershocks in earthquake sequences to the extreme price changes of the S&P 500 stock market index. In both of these cases, the extreme events tend to follow each other closely, arriving in clusters. However, where the earthquakes are characterised only by their magnitude, financial returns can be both positive (gains) or negative (losses) - up to now, the question of how these two types of extreme event interact has largely been neglected. We have constructed a model describing separately the positive and negative extremes as well as the interaction between them and found that extreme losses excite the market more than twice as much as do extreme gains, and they do so with more than five times the immediacy. This result reflects an underlying negativity bias among market traders, while also providing a new tool to probe and predict extreme fluctuations within both financial markets as well as other drift-diffusion-like processes.

15/07/2021 - Choosing a direction in Flatland

Our work (in collaboration with the experimental group of Dan Wolverson here in Bath as well as photoemission experts at Soleil and ICMM Madrid) showing coupling between crystal thickness and in-plane anisotropy in mono- and few-layer ReSe₂ has been published in Physical Review B. Congratulations to Surani who contributed her theoretical calculations to this work!

11/05/2021 - Congratulations to Surani!

Today, Surani has successfuly defended her thesis discussing the electronic structure of rhenium dichalcogenides. During her PhD, Surani published two papers (one in ACS Nano and one in Journal of Electronic Materials). One more paper is currently under review in Physical Review B and one is being prepared for submission (yes, this certainly is a trend).

06/11/2020 - Raman spectroscopy for in situ optical control of angles in twistronic graphene

Our work in collaboration with researchers from the University of Manchester has been published in Physical Review Letters! Congratulations to Aitor and Joshua, who started this project while still PhD students in my group, for such a high-impact publication!

After the discovery of superconductivity in magic-angle twisted bilayer graphene, other stacks of graphene layers with twisted interfaces have also been found to possess exotic electronic properties. We propose a non-destructive and quick method to characterize the twist angle between two graphene layers in twistronic structures with high sensitivity.

Graphenes are layers of carbon atoms extracted from graphite and they commonly display semimetallic properties. When placed on top of each other with a small twist, they qualitatively change their electronic properties due to the formation of a periodic moiré pattern which strongly depends on the misalignment angle. We show that Raman spectra of electronic excitations in twistronic graphenes display features which reflect peculiar band structure properties of these stacks such as flat band dispersion intervals. Our proposal paves the way for in situ control of the twist angle in twistronic graphene devices over a broad range of angles.

Full version of the article can be accessed here (article is open access; preprint version available here).

17/07/2020 - Determining interatomic coupling at van der Waals interfaces

Our work in collaboration with researchers from the Oxford University, Peking University and Elettra Synchrotron in Italy has been published in Nature Communications! Congratulations to Joshua, who started this project while still a PhD student in my group, for such a high-impact publication!

Following the isolation of graphene, many other atomically thin two-dimensional crystals have been produced and can even be stacked on top of each other in a desired order to form so called van der Waals heterostructures. In contrast to conventional heterostructures, in which chemical bonding at interfaces between two materials modifies their properties and requires lattice matching thus restricting what materials can be placed next to each other, stacks of two-dimensional crystals are held together by weak forces without directional bonding. As a result, any two can be stacked together, providing extraordinary flexibility in the design of vertical structures. Moreover, subtle changes in the stacking, especially the angle between the crystallographic axes (special directions in the arrangement of atoms) of two adjacent layers, can have big impact on the properties of the whole heterostructure, with examples including appearance of superconductivity in twisted bilayer graphene with twist angle around 1.1 degrees (known as magic-angle twisted bilayer graphene). In our work, we show that the coupling between atoms in two two-dimensional crystals, knowledge of which is necessary to describe the properties of the stack, can be determined by studying a trilayer structure with two similar interfaces but one with crystallographic axes aligned and one twisted. This is because each of the interfaces provides complementary information and together they enable self-consistent determination of the coupling. We use angle-resolved photoemission spectroscopy and obtain interatomic coupling for carbon atoms by studying twisted trilayer graphene. We also show that our result can be used to predict photoemission spectra of structures with different twist angles and number of layers. Our approach demonstrates how to extract fundamental information about interlayer coupling in a stack of two-dimensional crystals and can be applied to many other van der Waals interfaces.

Full version of the article can be accessed here (article is open access; preprint version available here). It was also selected as a highlight of research performed using ELETTRA Synchrotron resources and made University of Bath news.

28/05/2020 - Visualizing Orbital Content of Electronic Bands in ReSe₂

Our work in collaboration with researchers from South Korea including the University of Seoul, Yonsei University, Sogang University and Sejong University as well as the Lawrence Berkeley National Laboratory in the US has been published in ACS Nano! Congratulations to Surani for such a high-impact publication!

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. We use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe₂, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe₂, in contrast to the 2H materials, the out-of-plane transition metal d_z² and chalcogen p_z orbitals do not contribute significantly to the top of the valence band which explains the reported weak changes in the electronic structure of this compound as a function of layer number. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place amongst two-dimensional crystals.

Full version of the article can be accessed here (subscription required; preprint version available here).

06/12/2019 - Congratulations to Aitor!

Today, Aitor has successfuly defended his thesis discussing Raman spectroscopy of electronic excitations in graphene materials. During his PhD, Aitor published two papers (one in Physical Review B and one in Nano Letters) and is writing up a third one (this does seem to be a trend). He will be now moving on to a postdoctoral position at the National Graphene Institute at the University of Manchester.

01/10/2019 - Welcome to Matthew Tomlinson

Welcome to Matthew Tomlinson who is starting his PhD in my group. Matthew, supported by CheckRisk LLP and EPSRC through a CASE scholarship, will be studying phase transitions in financial markets using the approaches from the physics of complex systems.

30/07/2019 - Electronic Raman scattering features of rhombohedral graphite

Our work in collaboration with researchers from the National Graphene Institute at the University of Manchester in the UK has been published in Nano Letters! Congratulations to Aitor for such a high-impact publication!

Rhombohedral graphite features peculiar electronic properties, including persistence of low-energy surface bands of a topological nature. Here, we study the contribution of electron–hole excitations toward inelastic light scattering in thin films of rhombohedral graphite. We show that, in contrast to the featureless electron–hole contribution toward Raman spectrum of graphitic films with Bernal stacking, the inelastic light scattering accompanied by electron–hole excitations in crystals with rhombohedral stacking produces distinct features in the Raman signal which can be used both to identify the stacking and to determine the number of layers in the film.

Full version of the article can be accessed here (the article is open access; preprint version available here).

01/07/2019 - Senior Lectureship

From the beginning of July, I have been promoted to the post of Senior Lecturer (Associate Professor)!

07/06/2019 - Congratulations to Joshua!

Today, Joshua Thompson has successfully defended his PhD thesis on the electronic properties and electron transport in van der Waals heterostructures containing graphene. During his PhD, Joshua published two papers (one in Physical Review B and one in Physical Review Applied, both with valuable input from Damien) and is writing up a third one. He will be moving to a postdoctoral position at the Chalmers University of Technology in Sweden.

01/05/2019 - Welcome to Will Luckin

Welcome to Will Luckin who is starting his PhD in my group. Will, supported by the Bristol/Bath Centre for Doctoral Training in Condensed Matter Physics, will investigate twisted homo- and heterostructures of transition metal dichalcogenides.

01/12/2018 - Excellence Award

I have been awarded the University of Bath Excellence Award for 2017/18!

30/08/2018 - Congratulations to Damien!

After four years of hard work, Damien completed his PhD on the electronic properties of bilayer graphene-based van der Waals heterostructures! During his PhD, Damien published two papers (one in Physical Review B and one in Physical Review Applied with Joshua) and is writing up a third one. He will be moving on to a postdoctoral position at the Centre for Fine Print Research at the University of the West of England.

05/01/2018 - Interfacial polarons in graphene/hBN

Our work in collaboration with researchers from SOLEIL Synchrotron in France, Institute of Physics of the Chinese Academy of Sciences in Beijing, Stanford University and Harvard University has been published in Nano Letters!

van der Waals heterostructures, vertical stacks of layered materials, offer new opportunities for novel quantum phenomena which are absent in their constituent components. Here we report the emergence of polaron quasiparticles at the interface of graphene/hexagonal boron nitride (h-BN) heterostructures. Using nanospot angle-resolved photoemission spectroscopy, we observe zone-corner replicas of h-BN valence band maxima, with energy spacing coincident with the highest phonon energy of the heterostructure, an indication of Fröhlich polaron formation due to forward-scattering electron–phonon coupling. Parabolic fitting of the h-BN bands yields an effective mass enhancement of ∼2.3, suggesting an intermediate coupling strength. Our theoretical simulations based on Migdal–Eliashberg theory corroborate the experimental results, allowing the extraction of microscopic physical parameters. Moreover, renormalization of graphene π-band is observed due to the hybridization with the h-BN band. Our work generalizes the polaron study from transition metal oxides to van der Waals heterostructures with higher material flexibility, highlighting interlayer coupling as an extra degree of freedom to explore emergent phenomena.

Full version of the article can be accessed here (subscription required; preprint version available here).

30/06/2017 - I won the Maxwell Medal!

In recognition of my work on the electronic properties of graphene, I have been awarded the James Clerk Maxwell Medal and Prize by the Institute of Physics. It feels very rewarding but is also a big motivation for me to continue my research. It also is a little intimidating, as the list of past recipients of this medal contains some very recognizable names in theoretical physics, including some Nobel Prize winners.

You can read more about the award here.

01/05/2017 - Welcome to Surani Gunasekera

Surani, funded partly by the University of Bath, started her PhD in my group after finishing her first year of lectures and projects within the Bristol/Bath CDT in Condensed Matter Physics. Her research will focus on the electronic properties of rhenium dichalcogenides.

01/05/2016 - Welcome to Aitor Garcia Ruiz-Fuentes

Aitor started his PhD in my group after finishing his first year of lectures and projects within the Bristol/Bath CDT in Condensed Matter Physics. He will study the electronic properties of graphene with proximity-induced superconductivity.

01/04/2016 - Centre for Nanoscience and Nanotechnology

A new research centre focused on nanoscience and nanotechnology has been established at the University of Bath. I have joined it as one of the founding Principal Investigators.

01/10/2015 - Welcome to Joshua Thompson

With the beginning of the 2015/16 academic year, Joshua Thompson starts his EPSRC-funded PhD with me. Joshua will investigate the electronic properties of the graphene/hexagonal boron nitride heterostructures.

20/12/2014 - New chapter in Bath

It's official! After finishing my two years as the University of Bath Prize Fellow in February 2015, I will continue my work at the University of Bath as a Lecturer (Assistant Professor).

01/12/2014 - Excellence Award

I have been awarded the University of Bath Excellence Award for 2013/14!

01/10/2014 - Welcome to Damien Leech

With the beginning of a new academic year, Damien Leech joins me as my first PhD student. His research into the electronic properties of two-dimensional crystals is funded by EPSRC.

12/09/2014 - Fermi surface of bilayer graphene breaks into pieces

Our work in collaboration with researchers from ETH Zurich, NIMS Tsukuba and Lancaster University has been published as a cover article in Physical Review Letters!

Changing the topology of an object can significantly improve its functionality. A well-known example is changing a mug without a handle (topology of a sphere) into one with an arched handle (topology of a doughnut). In electronic materials, it is the more abstract topology of constant-energy surfaces for electrons that determines their potential for functional uses. The shape of the Fermi surface (an abstract boundary in the momentum space separating filled electron states from the empty ones) determines for example electron transport (current) or propagation of sound in a metal. Unfortunately, for bulk materials, the Fermi energy (energy of the most energetic electron in the material) cannot be easily altered to reach points in the band structure where the connectivity of the Fermi surface changes causing singular behaviour of the dependent properties.

Bilayer graphene turns out to be a very special case because its electronic spectrum can be tuned by applying an external electric field. By studying bilayer graphene in strong electric fields, we identified signatures of its Fermi surface changing from a singly-connected one into one composed of three separate pieces. Interestingly, our team observed that in external magnetic field the electron topological transition leads to other, interaction-driven transitions between electron states as the change in topology affects the repulsion between electrons.

Full version of the article can be accessed here (subscription required; preprint version available here).