Imperial College London

ProfessorArashMostofi

Faculty of EngineeringDepartment of Materials

Professor of Theory and Simulation of Materials
 
 
 
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Contact

 

+44 (0)20 7594 8154a.mostofi Website

 
 
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Location

 

Bessemer B332Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

82 results found

Andersen CW, Armiento R, Blokhin E, Conduit GJ, Dwaraknath S, Evans ML, Fekete Á, Gopakumar A, Gražulis S, Merkys A, Mohamed F, Oses C, Pizzi G, Rignanese G-M, Scheidgen M, Talirz L, Toher C, Winston D, Aversa R, Choudhary K, Colinet P, Curtarolo S, Di Stefano D, Draxl C, Er S, Esters M, Fornari M, Giantomassi M, Govoni M, Hautier G, Hegde V, Horton MK, Huck P, Huhs G, Hummelshøj J, Kariryaa A, Kozinsky B, Kumbhar S, Liu M, Marzari N, Morris AJ, Mostofi AA, Persson KA, Petretto G, Purcell T, Ricci F, Rose F, Scheffler M, Speckhard D, Uhrin M, Vaitkus A, Villars P, Waroquiers D, Wolverton C, Wu M, Yang Xet al., 2021, OPTIMADE, an API for exchanging materials data, Scientific Data, Vol: 8, ISSN: 2052-4463

The Open Databases Integration for Materials Design (OPTIMADE) consortium has designed a universal applicationprogramming interface (API) to make materials databases accessible and interoperable. We outline the first stablerelease of the specification, v1.0, which is already supported by many leading databases and several softwarepackages. We illustrate the advantages of the OPTIMADE API through worked examples on each of the publicmaterials databases that support the full API specification.

Journal article

Prentice J, Mostofi AA, 2021, Accurate and efficient computation of optical absorption spectra of molecular crystals: the case of the polymorphs of ROY, Journal of Chemical Theory and Computation, Vol: 17, Pages: 5214-5224, ISSN: 1549-9618

When calculating the optical absorption spectra of molecular crystals from first principles, the influence of the crystalline environment on the excitations is of significant importance. For such systems, however, methods to describe the excitations accurately can be computationally prohibitive due to the relatively large system sizes involved. In this work, we demonstrate a method that allows optical absorption spectra to be computed both efficiently and at high accuracy. Our approach is based on the spectral warping method successfully applied to molecules in solvent. It involves calculating the absorption spectrum of a supercell of the full molecular crystal using semi-local time-dependent density functional theory (TDDFT), before warping the spectrum using a transformation derived from smaller-scale semi-local and hybrid TDDFT calculations on isolated dimers. We demonstrate the power of this method on three polymorphs of the well-known color polymorphic compound ROY and find that it outperforms both small-scale hybrid TDDFT dimer calculations and large-scale semi-local TDDFT supercell calculations, when compared to the experiment.

Journal article

Vitale V, Atalar K, Mostofi AA, Lischner Jet al., 2021, Flat band properties of twisted transition metal dichalcogenide homo- andheterobilayers of MoS2, MoSe2, WS2 and WSe2, 2D Materials, Vol: 8, ISSN: 2053-1583

Twisted bilayers of two-dimensional materials, such as twisted bilayergraphene, often feature flat electronic bands that enable the observation ofelectron correlation effects. In this work, we study the electronic structureof twisted transition metal dichalcogenide (TMD) homo- and heterobilayers thatare obtained by combining MoS$_2$, WS$_2$, MoSe$_2$ and WSe$_2$ monolayers, andshow how flat band properties depend on the chemical composition of the bilayeras well as its twist angle. We determine the relaxed atomic structure of thetwisted bilayers using classical force fields and calculate the electronic bandstructure using a tight-binding model parametrized from first-principlesdensity-functional theory. We find that the highest valence bands in thesesystems can derive either from $\Gamma$-point or $K$/$K'$-point states of theconstituent monolayers. For homobilayers, the two highest valence bands arecomposed of monolayer $\Gamma$-point states, exhibit a graphene-like dispersionand become flat as the twist angle is reduced. The situation is morecomplicated for heterobilayers where the ordering of $\Gamma$-derived and$K$/$K'$-derived states depends both on the material composition and also thetwist angle. In all systems, qualitatively different band structures areobtained when atomic relaxations are neglected.

Journal article

Goodwin Z, Klebl L, Vitale V, Liang X, Gogtay V, Gorp XV, Kennes DM, Mostofi AA, Lischner Jet al., 2021, Flat bands, electron interactions and magnetic order in magic-angle mono-trilayer graphene, Physical Review Materials, ISSN: 2475-9953

Starting with twisted bilayer graphene, graphene-based moir\'e materials haverecently been established as a new platform for studying strong electroncorrelations. In this paper, we study twisted graphene monolayers on trilayergraphene and demonstrate that this system can host flat bands when the twistangle is close to the magic-angle of 1.16$^\circ$. When monolayer graphene istwisted on ABA trilayer graphene (denoted AtABA), the flat bands are notisolated, but are intersected by a dispersive Dirac cone. In contrast, graphenetwisted on ABC trilayer graphene (denoted AtABC) exhibits a gap between flatand remote bands. Since ABC trilayer graphene and twisted bilayer graphene areknown to host broken symmetry phases, we further investigate magic-angle AtABC.We study the effect of electron-electron interactions in AtABC using bothHartree theory, and an atomic Hubbard theory to calculate the magnetic phasediagram as a function of doping, twist angle and perpendicular electric field.Our analysis reveals a rich variety of magnetic orderings, includingferromagnetism and ferrimagnetism, and demonstrates that a perpendicularelectric field makes AtABC more susceptible to magnetic ordering.

Journal article

Klebl L, Goodwin Z, Mostofi AA, Kennes DM, Lischner Jet al., 2021, Importance of long-ranged electron-electron interactions for the magnetic phase diagram of twisted bilayer graphene, Physical Review B, Vol: 103, Pages: 1-7, ISSN: 2469-9950

Electron-electron interactions are intrinsically long ranged, but many models of strongly interacting electrons only take short-ranged interactions into account. Here, we present results of atomistic calculations including both long-ranged and short-ranged electron-electron interactions for the magnetic phase diagram of twisted bilayer graphene and demonstrate that qualitatively different results are obtained when long-ranged interactions are neglected. In particular, we use Hartree theory augmented with Hubbard interactions and calculate the interacting spin susceptibility at a range of doping levels and twist angles near the first magic angle to identify the dominant magnetic instabilities. At the magic angle, mostly antiferromagnetic order is found, while ferromagnetism dominates at other twist angles. Moreover, long-ranged interactions significantly increase the twist angle window in which strong correlation phenomena can be expected. These findings are in good agreement with available experimental data.

Journal article

Liang X, Goodwin ZAH, Vitale V, Corsetti F, Mostofi AA, Lischner Jet al., 2020, Effect of bilayer stacking on the atomic and electronic structure of twisted double bilayer graphene, Physical Review B, Vol: 102, Pages: 155146 – 1-155146 – 12, ISSN: 2469-9950

Twisted double bilayer graphene has recently emerged as an interesting moiré material that exhibits strong correlation phenomena that are tunable by an applied electric field. Here we study the atomic and electronic properties of three different graphene double bilayers: double bilayers composed of two AB stacked bilayers (AB/AB), double bilayers composed of two AA stacked bilayers (AA/AA), as well as heterosystems composed of one AB and one AA bilayer (AB/AA). The atomic structure is determined using classical force fields. We find that the inner layers of the double bilayer exhibit significant in-plane and out-of-plane relaxations, similar to twisted bilayer graphene. The relaxations of the outer layers depend on the stacking: atoms in AB bilayers follow the relaxations of the inner layers, while atoms in AA bilayers attempt to avoid higher-energy AA stacking. For the relaxed structures, we calculate the electronic band structures using the tight-binding method. All double bilayers exhibit flat bands at small twist angles, but the shape of the bands depends sensitively on the stacking of the outer layers. To gain further insight, we study the evolution of the band structure as the outer layers are rigidly moved away from the inner layers, while preserving their atomic relaxations. This reveals that the hybridization with the outer layers results in an additional flattening of the inner-layer flat band manifold. Our results establish AA/AA and AB/AA twisted double bilayers as interesting moiré materials with different flat band physics compared to the widely studied AB/AB system.

Journal article

Goodwin Z, Vitale V, Liang X, Mostofi AA, Lischner Jet al., 2020, Hartree theory calculations of quasiparticle properties in twisted bilayer graphene, Physical Review B: Condensed Matter and Materials Physics, Vol: 2, ISSN: 1098-0121

A detailed understanding of interacting electrons in twisted bilayer graphene(tBLG) near the magic angle is required to gain insights into the physicalorigin of the observed broken symmetry phases including correlated insulatorstates and superconductivity. Here, we present extensive atomistic Hartreetheory calculations of the electronic properties of tBLG in the (semi-)metallicphase as function of doping and twist angle. Specifically, we calculatequasiparticle properties, such as the band structure, density of states (DOS)and local density of states (LDOS), which are directly accessible inphotoemission and tunnelling spectroscopy experiments. We find thatquasiparticle properties change significantly upon doping - an effect which isnot captured by tight-binding theory. In particular, we observe that thepartially occupied bands flatten significantly which enhances the density ofstates at the Fermi level and explains the experimentally observed Fermi levelpinning. We predict a clear signature of this band flattening in the LDOS inthe AB/BA regions of tBLG which can be tested in scanning tunnelingexperiments. We also study the dependence of quasiparticle properties on thedielectric environment of tBLG and discover that these properties aresurprisingly robust as a consequence of the strong internal screening. Finally,we present a simple analytical expression for the Hartree potential whichenables the determination of quasiparticle properties without the need forself-consistent calculations.

Journal article

Skylaris C-K, Haynes PD, Mostofi AA, Payne MCet al., 2020, Recent progress in linear-scaling density functional calculations with plane waves and pseudopotentials: the ONETEP code (vol 20, 064209, 2008), JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 32, ISSN: 0953-8984

Journal article

Skylaris C-K, Haynes PD, Mostofi AA, Payne MCet al., 2020, Using ONETEP for accurate and efficient O(N) density functional calculations (vol 17, 175757, 2005), JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 32, ISSN: 0953-8984

Journal article

Haynes PD, Skylaris C-K, Mostofi AA, Payne MCet al., 2020, Density kernel optimization in the ONETEP code (vol 20, 294207, 2008), JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 32, ISSN: 0953-8984

Journal article

Oliveira MJT, Papior N, Pouillon Y, Blum V, Artacho E, Caliste D, Corsetti F, Gironcoli SD, Elena AM, Garcia A, Garcia-Suarez VM, Genovese L, Huhn WP, Huhs G, Kokott S, Kucukbenli E, Larsen AH, Lazzaro A, Lebedeva IV, Li Y, Lopez-Duran D, Lopez-Tarifa P, Luders M, Marques MAL, Minar J, Mohr S, Mostofi AA, O'Cais A, Payne MC, Ruh T, Smith DGA, Soler JM, Strubbe DA, Tancogne-Dejean N, Tildesley D, Torrent M, Yu VW-Zet al., 2020, The CECAM Electronic Structure Library and the modular software development paradigm, Journal of Chemical Physics, Vol: 153, Pages: 024117-1-024117-23, ISSN: 0021-9606

First-principles electronic structure calculations are very widely usedthanks to the many successful software packages available. Their traditionalcoding paradigm is monolithic, i.e., regardless of how modular its internalstructure may be, the code is built independently from others, from thecompiler up, with the exception of linear-algebra and message-passinglibraries. This model has been quite successful for decades. The rapid progressin methodology, however, has resulted in an ever increasing complexity of thoseprograms, which implies a growing amount of replication in coding and in therecurrent re-engineering needed to adapt to evolving hardware architecture. TheElectronic Structure Library (\esl) was initiated by CECAM (European Centre forAtomic and Molecular Calculations) to catalyze a paradigm shift away from themonolithic model and promote modularization, with the ambition to extractcommon tasks from electronic structure programs and redesign them as free,open-source libraries. They include ``heavy-duty'' ones with a high degree ofparallelisation, and potential for adaptation to novel hardware within them,thereby separating the sophisticated computer science aspects of performanceoptimization and re-engineering from the computational science done byscientists when implementing new ideas. It is a community effort, undertaken bydevelopers of various successful codes, now facing the challenges arising inthe new model. This modular paradigm will improve overall coding efficiency andenable specialists (computer scientists or computational scientists) to usetheir skills more effectively. It will lead to a more sustainable and dynamicevolution of software as well as lower barriers to entry for new developers.

Journal article

Pomiro F, Ablitt C, Bristowe NC, Mostofi AA, Won C, Cheong S-W, Senn MSet al., 2020, From first- to second-order phase transitions in hybrid improper ferroelectrics through entropy stabilization, Physical Review B, Vol: 102, Pages: 014101 – 1-014101 – 8, ISSN: 2469-9950

Hybrid improper ferroelectrics (HIFs) have been studied intensively over the past few years to gain an understanding of their temperature-induced phase transitions and ferroelectric switching pathways. Here we report a switching from a first- to a second-order phase transition pathway for HIFs Ca3−xSrxTi2O7, which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that are coupled to specific symmetries and octahedral rotation and tilt distortions—such as superconductivity, negative thermal expansion, fast ion conductivity, ferroelectricity, among others—is a crucial step in creating novel functional materials by design.

Journal article

Gołębiowski JR, Kermode JR, Haynes PD, Mostofi AAet al., 2020, Correction: Atomistic QM/MM simulations of the strength of covalent interfaces in carbon nanotube-polymer composites., Physical Chemistry Chemical Physics, Vol: 22, Pages: 14375-14375, ISSN: 1463-9076

Correction for 'Atomistic QM/MM simulations of the strength of covalent interfaces in carbon nanotube-polymer composites' by Jacek R. Gołębiowski et al., Phys. Chem. Chem. Phys., 2020, 22, 12007-12014, DOI: 10.1039/d0cp01841d.

Journal article

Vitale V, Pizzi G, Marrazzo A, Yates J, Marzari N, Mostofi Aet al., 2020, Automated high-throughput Wannierisation, npj Computational Materials, Vol: 6, ISSN: 2057-3960

Maximally-localised Wannier functions (MLWFs) are routinely used to compute from first-principles advanced materials properties that require very dense Brillouin zone integration and to build accurate tight-binding models for scale-bridging simulations. At the same time, high-throughput (HT) computational materials design is an emergent field that promises to accelerate reliable and cost-effective design and optimisation of new materials with target properties. The use of MLWFs in HT workflows has been hampered by the fact that generating MLWFs automatically and robustly without any user intervention and for arbitrary materials is, in general, very challenging. We address this problem directly by proposing a procedure for automatically generating MLWFs for HT frameworks. Our approach is based on the selected columns of the density matrix method and we present the details of its implementation in an AiiDA workflow. We apply our approach to a dataset of 200 bulk crystalline materials that span a wide structural and chemical space. We assess the quality of our MLWFs in terms of the accuracy of the band-structure interpolation that they provide as compared to the band-structure obtained via full first-principles calculations. Finally, we provide a downloadable virtual machine that can be used to reproduce the results of this paper, including all first-principles and atomistic simulations as well as the computational workflows.

Journal article

Prentice J, Aarons J, Womack JC, Allen AEA, Andrinopoulos L, Anton L, Bell RA, Bhandari A, Bramley GA, Charlton R, Clements RJ, Cole DJ, Constantinescu G, Corsetti F, Dubois SM-M, Duff KKB, Escartín JM, Greco A, Hill Q, Lee LP, Linscott E, ORegan DD, Phipps MJS, Ratcliff L, Serrano ÁR, Tait EW, Teobaldi G, Vitale V, Yeung N, Zuehlsdorff T, Dziedzic J, Haynes PD, Hine N, Mostofi AA, Payne MC, Skylaris C-Ket al., 2020, The ONETEP linear-scaling density functional theory program, The Journal of Chemical Physics, Vol: 152, Pages: 174111-1-174111-36, ISSN: 0021-9606

We present an overview of the onetep program for linear-scaling density functional theory (DFT) calculations with large basis set (plane-wave) accuracy on parallel computers. The DFT energy is computed from the density matrix, which is constructed from spatially localized orbitals we call Non-orthogonal Generalized Wannier Functions (NGWFs), expressed in terms of periodic sinc (psinc) functions. During the calculation, both the density matrix and the NGWFs are optimized with localization constraints. By taking advantage of localization, onetep is able to perform calculations including thousands of atoms with computational effort, which scales linearly with the number or atoms. The code has a large and diverse range of capabilities, explored in this paper, including different boundary conditions, various exchange–correlation functionals (with and without exact exchange), finite electronic temperature methods for metallic systems, methods for strongly correlated systems, molecular dynamics, vibrational calculations, time-dependent DFT, electronic transport, core loss spectroscopy, implicit solvation, quantum mechanical (QM)/molecular mechanical and QM-in-QM embedding, density of states calculations, distributed multipole analysis, and methods for partitioning charges and interactions between fragments. Calculations with onetep provide unique insights into large and complex systems that require an accurate atomic-level description, ranging from biomolecular to chemical, to materials, and to physical problems, as we show with a small selection of illustrative examples. onetep has always aimed to be at the cutting edge of method and software developments, and it serves as a platform for developing new methods of electronic structure simulation. We therefore conclude by describing some of the challenges and directions for its future developments and applications.

Journal article

Golebiowski J, Kermode J, Haynes P, Mostofi AAet al., 2020, Atomistic QM/MM simulations of the strength of covalent interfaces in carbon nanotube–polymer composites, Physical Chemistry Chemical Physics, Vol: 22, Pages: 12007-12014, ISSN: 1463-9076

We investigate the failure of carbon-nanotube/polymer composites by using a recently-developed hybrid quantum-mechanical/molecular-mechanical (QM/MM) approach to simulate nanotube pull-out from a cross-linked polyethene matrix. Our study focuses on the strength and failure modes of covalently-bonded nanotube–polymer interfaces based on amine, carbene and carboxyl functional groups and a [2+1] cycloaddition. We find that the choice of the functional group linking the polymer matrix to the nanotube determines the effective strength of the interface, which can be increased by up to 50% (up to the limit dictated by the strength of the polymer backbone itself) by choosing groups with higher interfacial binding energy. We rank the functional groups presented in this work based on the strength of the resulting interface and suggest broad guidelines for the rational design of nanotube functionalisation for nanotube–polymer composites.

Journal article

Goodwin Z, Vitale V, Corsetti F, Efetov DK, Mostofi AA, Lischner Jet al., 2020, Critical role of device geometry for the phase diagram of twisted bilayer graphene, Physical Review B: Condensed Matter and Materials Physics, Vol: 101, Pages: 1-8, ISSN: 1098-0121

The effective interaction between electrons in two-dimensional materials can be modified by their environment, enabling control of electronic correlations and phases. Here, we study the dependence of electronic correlations in twisted bilayer graphene (tBLG) on the separation to the metallic gate(s) in two device configurations. Using an atomistic tight-binding model, we determine the Hubbard parameters of the flat bands as a function of gate separation, taking into account the screening from the metallic gate(s), the dielectric spacer layers, and the tBLG itself. We determine the critical gate separation at which the Hubbard parameters become smaller than the critical value required for a transition from a correlated insulator state to a (semi)metallic phase. We show how this critical gate separation depends on twist angle, doping, and the device configuration. These calculations may help rationalize the reported differences between recent measurements of tBLG's phase diagram and suggest that correlated insulator states can be screened out in devices with thin dielectric layers.

Journal article

Aghajanian M, Schuler B, Cochrane KA, Lee J-H, Kastl C, Neaton JB, Weber-Bargioni A, Mostofi AA, Lischner Jet al., 2020, Resonant and bound states of charged defects in two-dimensional semiconductors, Physical Review B: Condensed Matter and Materials Physics, Vol: 101, Pages: 1-6, ISSN: 1098-0121

A detailed understanding of charged defects in two-dimensional semiconductors is needed for the development of ultrathin electronic devices. Here, we study negatively charged acceptor impurities in monolayer WS2 using a combination of scanning tunneling spectroscopy and large-scale atomistic electronic structure calculations. We observe several localized defect states of hydrogenic wave function character in the vicinity of the valence band edge. Some of these defect states are bound, while others are resonant. The resonant states result from the multivalley valence band structure of WS2, whereby localized states originating from the secondary valence band maximum at Γ hybridize with continuum states from the primary valence band maximum at K/K′. Resonant states have important consequences for electron transport as they can trap mobile carriers for several tens of picoseconds.

Journal article

Pizzi G, Vitale V, Arita R, Bluegel S, Freimuth F, Géranton G, Gibertini M, Gresch D, Johnson C, Koretsune T, Ibanez J, Lee H, Lihm J-M, Marchand D, Marrazzo A, Mokrousov Y, Mustafa JI, Nohara Y, Nomura Y, Paulatto L, Ponce S, Ponweiser T, Qiao J, Thöle F, Tsirkin SS, Wierzbowska M, Marzari N, Vanderbilt D, Souza I, Mostofi AA, Yates JRet al., 2020, Wannier90 as a community code: new features and applications, Journal of Physics: Condensed Matter, Vol: 32, Pages: 1-25, ISSN: 0953-8984

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.

Journal article

Ablitt C, McCay H, Craddock S, Cooper L, Reynolds E, Mostofi AA, Bristowe N, Murray CA, Senn Met al., 2019, Tolerance factor control of uniaxial negative thermal expansion in a layered perovskite, Chemistry of Materials, Vol: 32, Pages: 605-610, ISSN: 0897-4756

By tuning the tolerance factor, $t$, of the Ruddlesden--Popper oxide Ca$_2$MnO$_4$ through isovalent substitutions we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems can be systematically changed through large negative to positive values. High-resolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as $t$ decreases across the stability window of the NTE phase. Transitions to phases with positive thermal expansion (PTE) are found to occur at both the high-$t$ and low-$t$ limits of stability. First-principles calculations demonstrate that reducing $t$ enhances the contribution to thermal expansion from the lowest frequency phonons, which have the character of octahedral tilts and have negative mode Gr\"uneisen parameter components along the NTE axis. By tuning $t$ to the lower edge of the NTE phase stability window, we are hence able to maximise the amplitudes of these vibrations and thereby maximise NTE with a CLTE of -8.1~ppm/K at 125~K. We also illustrate, at the other end of the phase diagram, that an enhancement in compliance of these materials associated with the rotational instability provides another mechanism by which NTE could be yet further enhanced in this and related systems.

Journal article

Goodwin Z, Corsetti F, Mostofi A, Lischner Jet al., 2019, Attractive electron-electron interactions from internal screening in magic angle twisted bilayer graphene, Physical Review B: Condensed Matter and Materials Physics, Vol: 100, ISSN: 1098-0121

Twisted bilayer graphene (tBLG) has recently emerged as a new platform for studying electroncorrelations, the strength of which can be controlled via the twist angle. Here, we study the effectof internal screening on electron-electron interactions in undoped tBLG. Using the random phaseapproximation, we find that the dielectric response of tBLG drastically increases near the magicangle and is highly twist-angle dependent. As a consequence of the abrupt change of the Fermivelocity as a function of wave vector, the screened interaction in real space exhibits attractiveregions for certain twist angles near the magic angle. Attractive interactions can induce chargedensity waves and superconductivity and therefore our findings could be relevant to understand themicroscopic origins of the recently observed strong correlation phenomena in undoped tBLG. Theresulting screened Hubbard parameters are strongly reduced and exhibit a non-linear dependence onthe twist angle. We also carry out calculations with the constrained random phase approximationand parametrize a twist-angle dependent Keldysh model for the resulting effective interaction.

Journal article

Prentice J, Charlton R, Mostofi AA, Haynes Pet al., 2019, Combining embedded mean-field theory with linear-scaling density-functional theory, Journal of Chemical Theory and Computation, Vol: 16, Pages: 354-365, ISSN: 1549-9618

We demonstrate the capability of embedded mean field theory (EMFT) within the linear-scaling density-functional theory code ONETEP, which enables DFT-in-DFT quantum embedding calculations on systems containing thousands of atoms at a fraction of the cost of a full calculation. We perform simulations on a wide range of systems from molecules to complex nanostructures to demonstrate the performance of our implementation with respect to accuracy and efficiency. This work paves the way for the application of this class of quantum embedding method to large-scale systems that are beyond the reach of existing implementations.

Journal article

Goodwin ZAH, Corsetti F, Mostofi AA, Lischner Jet al., 2019, Twist-angle sensitivity of electron correlations in moiré graphene bilayers, Physical Review B, Vol: 100, ISSN: 2469-9950

Motivated by the recent observation of correlated insulator states and unconventional superconductivity in twisted bilayer graphene, we study the dependence of electron correlations on the twist angle and reveal the existence of strong correlations over a narrow range of twist angles near the magic angle. Specifically, we determine the on-site and extended Hubbard parameters of the low-energy Wannier states using an atomistic quantum-mechanical approach. The ratio of the on-site Hubbard parameter and the width of the flat bands, which is an indicator of the strength of electron correlations, depends sensitively on the screening by the semiconducting substrate and the metallic gates. Including the effect of long-ranged Coulomb interactions significantly reduces electron correlations and explains the experimentally observed sensitivity of strong-correlation phenomena on twist angles.

Journal article

Mostofi A, Ablitt C, Bristowe N, Senn M, Saito T, Shimakawa Y, Chen W-Tet al., 2019, Negative thermal expansion in high pressure layered perovskite Ca2GeO4, Chemical Communications, Vol: 55, Pages: 2984-2987, ISSN: 1359-7345

We report the high pressure synthesis of a layered perovskite Ca2GeO4 which is found to have the Ruddlesden–Popper structure with I41/acd symmetry. Consonant with our recent predictions [Ablitt et al., npj Comput. Mater., 2017, 3, 44], the phase displays pronounced uniaxial negative thermal expansion over a large temperature range. Negative thermal expansion that persists over a large temperature range is very unusual in the perovskite structure, and its occurrence in this instance can be understood to arise due to both soft lattice vibrations associated with a phase competition and the unusually compliant nature of this structure, which effectively couples thermal expansion in the layer plane to lattice contractions perpendicular to the layering direction via a “corkscrew” mechanism.

Journal article

Golebiowski J, Kermode J, Mostofi A, Haynes Pet al., 2018, Multiscale simulations of critical interfacial failure in carbon nanotube-polymer composites, Journal of Chemical Physics, Vol: 149, ISSN: 0021-9606

Computational investigation of interfacial failure in composite materials is challenging because it is inherently multi-scale: the bond-breaking processes that occur at the covalently bonded interface and initiate failure involve quantum mechanical phenomena, yet the mechanisms by which external stresses are transferred through the matrix occur on length and time scales far in excess of anything that can be simulated quantum mechanically. In this work, we demonstrate and validate an adaptive quantum mechanics (QM)/molecular mechanics simulation method that can be used to address these issues and apply it to study critical failure at a covalently bonded carbon nanotube (CNT)-polymer interface. In this hybrid approach, the majority of the system is simulated with a classical forcefield, while areas of particular interest are identified on-the-fly and atomic forces in those regions are updated based on QM calculations. We demonstrate that the hybrid method results are in excellent agreement with fully QM benchmark simulations and offers qualitative insights missing from classical simulations. We use the hybrid approach to show how the chemical structure at the CNT-polymer interface determines its strength, and we propose candidate chemistries to guide further experimental work in this area.

Journal article

Mostofi AA, Ablitt C, Senn M, Bristowe Net al., 2018, Control of uniaxial negative thermal expansion in layered Perovskites by tuning layer thickness, Frontiers in Chemistry, Vol: 6, ISSN: 2296-2646

Uniaxial negative thermal expansion (NTE) is known to occur in low n members of the An+1BnO3n+1 Ruddlesden–Popper (RP) layered perovskite series with a frozen rotation of BO6 octahedra about the layering axis. Previous work has shown that this NTE arises due to the combined effects of a close proximity to a transition to a competing phase, so called “symmetry trapping”, and highly anisotropic elastic compliance specific to the symmetry of the NTE phase. We extend this analysis to the broader RP family (n = 1, 2, 3, 4, …, ∞), demonstrating that by changing the fraction of layer interface in the structure (i.e., the value of 1/n) one may control the anisotropic compliance that is necessary for the pronounced uniaxial NTE observed in these systems. More detailed analysis of how the components of the compliance matrix develop with 1/n allows us to identify different regimes, linking enhancements in compliance between these regimes to the crystallographic degrees of freedom in the structure. We further discuss how the perovskite layer thickness affects the frequencies of soft zone boundary modes with large negative Grüneisen parameters, associated with the aforementioned phase transition, that constitute the thermodynamic driving force for NTE. This new insight complements our previous work—showing that chemical control may be used to switch from positive to negative thermal expansion in these systems—since it makes the layer thickness, n, an additional design parameter that may be used to engineer layered perovskites with tuneable thermal expansion. In these respects, we predict that, with appropriate chemical substitution, the n = 1 phase will be the system in which the most pronounced NTE could be achieved.

Journal article

Wong D, Wang Y, Jin W, Tsai H-Z, Bostwick A, Rotenberg E, Kawakami R, Zettl A, Mostofi A, Lischner J, Crommie Met al., 2018, Microscopy of hydrogen and hydrogen-vacancy defect structures on graphene devices, Physical review B: Condensed matter and materials physics, Vol: 98, ISSN: 1098-0121

We have used scanning tunneling microscopy (STM) to investigate two types of hydrogen defect structures on monolayer graphene supported by hexagonal boron nitride (h−BN) in a gated field-effect transistor configuration. The first H-defect type is created by bombarding graphene with 1-keV ionized hydrogen and is identified as two hydrogen atoms bonded to a graphene vacancy via comparison of experimental data to first-principles calculations. The second type of H defect is identified as dimerized hydrogen and is created by depositing atomic hydrogen having only thermal energy onto a graphene surface. Scanning tunneling spectroscopy (STS) measurements reveal that hydrogen dimers formed in this way open a new elastic channel in the tunneling conductance between an STM tip and graphene.

Journal article

Molinari N, Sutton A, Mostofi AA, 2018, Mechanisms of reinforcement in polymer nanocomposites, Physical Chemistry Chemical Physics, Vol: 20, Pages: 23085-23094, ISSN: 1463-9076

Coarse-grained molecular dynamics simulations are used to elucidate molecular mechanisms responsible for different mechanical behaviours of elastomers containing spherical particles with different volume fractions. We observe that different filler volume fractions result in qualitatively different responses of the polymer nanocomposite to tensile strain. At relatively low filler volume fraction a yield drop appears in the stress–strain curve. As the filler volume fraction increases there is a reduction in the rate of plastic hardening, becoming plastic softening at sufficiently high filler volume fraction. We demonstrate that these behaviours are a result of the network formed by the polymer chains and filler particles. We identify three distinct molecular structural motifs between polymer and filler particles whose relative prevalence varies with the filler volume fraction and as the system is dynamically strained. We show how this evolution in molecular structure is directly linked to the observed mechanical response.

Journal article

Lischner JC, Mostofi AA, Aghajanian M, 2018, Tuning electronic properties of transition-metal dichalcogenides via defect charge, Scientific Reports, Vol: 8, ISSN: 2045-2322

Defect engineering is a promising route for controlling the electronic properties of monolayer transition-metal dichalcogenide (TMD) materials. Here, we demonstrate that the electronic structure of MoS2 depends sensitively on the defect charge, both its sign and magnitude. In particular, we study shallow bound states induced by charged defects using large-scale tight-binding simulations with screened defect potentials and observe qualitative changes in the orbital character of the lowest lying impurity states as function of the impurity charge. To gain further insights, we analyze the competition of impurity states originating from different valleys of the TMD band structure using effective mass theory and find that impurity state binding energies are controlled by the effective mass of the corresponding valley, but with significant deviations from hydrogenic behaviour due to unconventional screening of the defect potential.

Journal article

Ablitt C, Craddock S, Senn MS, Mostofi AA, Bristowe NCet al., 2017, The origin of uniaxial negative thermal expansion in layered perovskites, npj Computational Materials, Vol: 3, ISSN: 2057-3960

Why is it that ABO3 perovskites generally do not exhibit negative thermal expansion (NTE) over a wide temperature range, whereas layered perovskites of the same chemical family often do? It is generally accepted that there are two key ingredients that determine the extent of NTE: the presence of soft phonon modes that drive contraction (have negative Grüneisen parameters); and anisotropic elastic compliance that predisposes the material to the deformations required for NTE along a specific axis. This difference in thermal expansion properties is surprising since both ABO3 and layered perovskites often possess these ingredients in equal measure in their high-symmetry phases. Using first principles calculations and symmetry analysis, we show that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and condensed rotations of oxygen octahedra. This feature, unique to layered perovskites of certain symmetry, is what allows uniaxial NTE to persist over a large temperature range. This fundamental insight means that symmetry and the elastic tensor can be used as descriptors in high-throughput screening and to direct materials design.

Journal article

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