87 results found
Fischer A, Goodwin ZAH, Mostofi AA, et al., 2022, Unconventional superconductivity in magic-angle twisted trilayer graphene, npj Quantum Materials, Vol: 7
<jats:title>Abstract</jats:title><jats:p>Magic-angle twisted trilayer graphene (MATTG) recently emerged as a highly tunable platform for studying correlated phases of matter, such as correlated insulators and superconductivity. Superconductivity occurs in a range of doping levels that is bounded by van Hove singularities, which stimulates the debate of the origin and nature of superconductivity in this material. In this work, we discuss the role of spin-fluctuations arising from atomic-scale correlations in MATTG for the superconducting state. We show that in a phase diagram as a function of doping (<jats:italic>ν</jats:italic>) and temperature, nematic superconducting regions are surrounded by ferromagnetic states and that a superconducting dome with <jats:italic>T</jats:italic><jats:sub>c</jats:sub> ≈ 2 K appears between the integer fillings <jats:italic>ν</jats:italic> = −2 and <jats:italic>ν</jats:italic> = −3. Applying a perpendicular electric field enhances superconductivity on the electron-doped side which we relate to changes in the spin-fluctuation spectrum. We show that the nematic unconventional superconductivity leads to pronounced signatures in the local density of states detectable by scanning tunneling spectroscopy measurements.</jats:p>
Pizzochero M, Tepliakov N, Mostofi AA, et al., 2021, Electrically Induced Dirac Fermions in Graphene Nanoribbons, NANO LETTERS, Vol: 21, Pages: 9332-9338, ISSN: 1530-6984
Wentink M, Gaberle J, Aghajanian M, et al., 2021, Substitutional tin acceptor states in black phosphorus, The Journal of Physical Chemistry C, Vol: 125, Pages: 22883-22889, ISSN: 1932-7447
Nominally pure black phosphorus (BP) is commonly found to be a p-type semiconductor, suggesting the ubiquitious presence of impurity species or intrinsic, charged defects. Moreover, scanning tunneling microscopy (STM) images of black phosphorus reveal the presence of long-range double-lobed defect features superimposed onto the surface atomic lattice. We show that both the p-type doping of BP and the defect features observed in STM images can be attributed to substitutional tin impurities. We show that black phosphorus samples produced through two common synthesis pathways contain tin impurities, and we demonstrate that the ground state of substitutional tin impurities is negatively charged for a wide range of Fermi level positions within the BP band gap. The localized negative charge of the tin impurities induces hydrogenic states in the band gap, and it is the 2p level that sits at the valence band edge that gives rise to the double-lobed features observed in STM images.
Goodwin Z, Klebl L, Vitale V, et al., 2021, Flat bands, electron interactions and magnetic order in magic-angle mono-trilayer graphene, Physical Review Materials, Vol: 5, ISSN: 2475-9953
Starting with twisted bilayer graphene, graphene-based moiré materials have recently been established as a new platform for studying strong electron correlations. In this paper, we study twisted graphene monolayers on trilayer graphene and demonstrate that this system can host flat bands when the twist angle is close to the magic angle of 1.16∘. When monolayer graphene is twisted on ABA trilayer graphene, the flat bands are not isolated, but are intersected by a Dirac cone with a large Fermi velocity. In contrast, graphene twisted on ABC trilayer graphene (denoted AtABC) exhibits a gap between flat and remote bands. Since ABC trilayer graphene and twisted bilayer graphene are known to host broken-symmetry phases, we further investigate the ostensibly similar magic-angle AtABC system. We study the effect of electron-electron interactions in AtABC using both Hartree theory and an atomic Hubbard theory to calculate the magnetic phase diagram as a function of doping, twist angle, and perpendicular electric field. Our analysis reveals a rich variety of magnetic orderings, including ferromagnetism and ferrimagnetism, and demonstrates that a perpendicular electric field makes AtABC more susceptible to magnetic ordering.
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.
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.
Vitale V, Atalar K, Mostofi AA, et 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.
Klebl L, Goodwin Z, Mostofi AA, et 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.
Liang X, Goodwin ZAH, Vitale V, et 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.
Goodwin Z, Vitale V, Liang X, et 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.
Skylaris C-K, Haynes PD, Mostofi AA, et 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
Skylaris C-K, Haynes PD, Mostofi AA, et 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
Haynes PD, Skylaris C-K, Mostofi AA, et al., 2020, Density kernel optimization in the ONETEP code (vol 20, 294207, 2008), JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 32, ISSN: 0953-8984
Oliveira MJT, Papior N, Pouillon Y, et 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.
Pomiro F, Ablitt C, Bristowe NC, et 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.
Gołębiowski JR, Kermode JR, Haynes PD, et 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.
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.
Prentice J, Aarons J, Womack JC, et 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.
Golebiowski J, Kermode J, Haynes P, et 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.
Goodwin Z, Vitale V, Corsetti F, et 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.
Aghajanian M, Schuler B, Cochrane KA, et 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.
Pizzi G, Vitale V, Arita R, et 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.
Ablitt C, McCay H, Craddock S, et 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.
Goodwin Z, Corsetti F, Mostofi A, et 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.
Prentice J, Charlton R, Mostofi AA, et 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.
Goodwin ZAH, Corsetti F, Mostofi AA, et 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.
Mostofi A, Ablitt C, Bristowe N, et 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.
Golebiowski J, Kermode J, Mostofi A, et 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.
Mostofi AA, Ablitt C, Senn M, et 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.
Wong D, Wang Y, Jin W, et 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.
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