23 research outputs found
Vibrational and dielectric properties of the bulk transition metal dichalcogenides
Interest in the bulk transition metal dichalcogenides for their electronic,
photovoltaic, and optical properties has grown and led to their use in many
technological applications. We present a systematic investigation of their
interlinked vibrational and dielectric properties, using density functional
theory and density functional perturbation theory, studying the effects of the
spin-orbit interaction and of the long-range e- e correlation as part
of our investigation. This study confirms that the spin-orbit interaction plays
a small role in these physical properties, while the direct contribution of
dispersion corrections is of crucial importance in the description of the
interatomic force constants. Here, our analysis of the structural and
vibrational properties, including the Raman spectra, compare well to
experimental measurement. Three materials with different point groups are
showcased and data trends on the full set of fifteen existing hexagonal,
trigonal, and triclinic materials are demonstrated. This overall picture will
enable the modeling of devices composed of these materials for novel
applications.Comment: 11 pages, 6 figure
Vibrational and dielectric properties of monolayer transition metal dichalcogenides
First-principles studies of two-dimensional transition metal dichalcogenides
have contributed considerably to the understanding of their dielectric,
optical, elastic, and vibrational properties. The majority of works to date
focus on a single material or physical property. Here we use a single
first-principles methodology on the whole family of systems, to investigate in
depth the relationships between different physical properties, the underlying
symmetry and the composition of these materials, and observe trends. We compare
to bulk counterparts to show strong interlayer effects in triclinic compounds.
Previously unobserved relationships between these monolayer compounds become
apparent. These trends can then be exploited by the materials science,
nanoscience, and chemistry communities to better design devices and
heterostructures for specific functionalities.Comment: 4 figures, 11 page
Large phosphorene in-plane contraction induced by interlayer interactions in graphene-phosphorene heterostructures
Intralayer deformation in van der Waals (vdW) heterostructures is generally
assumed to be negligible due to the weak nature of the interactions between the
layers, especially when the interfaces are found incoherent. In the present
work, graphene-phosphorene vdW-heterostructures are investigated with the
Density Functional Theory (DFT). The challenge of treating nearly
incommensurate (very large) supercell in DFT is bypassed by considering
different energetic quantities in the grand canonical ensemble, alternative to
the formation energy, in order to take into account the mismatch elastic
contribution of the different layers. In the investigated heterostructures, it
is found that phosphorene contracts by ~4% in the armchair direction when
compared to its free-standing form. This large contraction leads to important
changes in term of electronic properties, with the direct electronic optical
transition of phosphorene becoming indirect in specific vdW-heterostructures.
More generally, such a contraction indicates strong substrate effects in
supported or encapsulated phosphorene -neglected hitherto- and paves the way to
substrate-controlled stress- tronic in such 2D crystal. In addition, the
stability of these vdW-heterostructures are investigated as a function of the
rotation angle between the layers and as a function of the stacking
composition. The alignment of the specific crystalline directions of graphene
and phosphorene is found energetically favored. In parallel, several several
models based on DFT-estimated quantities are presented; they allow notably a
better understanding of the global mutual accommodation of 2D materials in
their corresponding interfaces, that is predicted to be non-negligible even in
the case of incommensurate interfaces.Comment: 33 pages, 6 figure
First-principle study of paraelectric and ferroelectric CsHPO including dispersion forces: stability and related vibrational, dielectric and elastic properties
Using density functional theory (DFT) and density functional perturbation
theory (DFPT), we investigate the stability and response functions of
CsHPO, a ferroelectric material at low temperature. This material
cannot be described properly by the usual (semi-)local approximations within
DFT. The long-range e-e correlation needs to be properly taken into
account, using, for instance, Grimme's DFT-D methods, as investigated in this
work. We find that DFT-D3(BJ) performs the best for the members of the
dihydrogenated alkali phosphate family (KHPO, RbHPO,
CsHPO), leading to experimental lattice parameters reproduced with an
average deviation of 0.5 %. With these DFT-D methods, the structural,
dielectric, vibrational and mechanical properties of CsHPO are globally
in excellent agreement with the available experiments ( 2% MAPE for
Raman-active phonons). Our study suggests the possible existence of a new
low-temperature phase for CsHPO, not yet reported experimentally.
Finally, we report the implementation of DFT-D contributions to elastic
constants within DFPT.Comment: This paper was published in Physical Review B the 25 January 2017 (21
pages, 4 figures
First-principles prediction of lattice coherency in van der Waals heterostructures
The emergence of superconductivity in slightly-misaligned graphene bilayer
[1] and moir\'e excitons in MoSe-WSe van der Waals (vdW)
heterostructures [2] is intimately related to the formation of a 2D
superlattice in those systems. At variance, perfect primitive lattice matching
of the constituent layers has also been reported in some vdW-heterostructures
[3-5], highlighting the richness of interfaces in the 2D world. In this work,
the determination of the nature of such interface, from first principles, is
demonstrated. To do so, an extension of the Frenkel-Kontorova (FK) model [6] is
presented, linked to first-principles calculations, and used to predict lattice
coherency for a set of 56 vdW-heterostructures. Computational predictions agree
with experiments, when available. New superlattices as well as
perfectly-matching interfaces are predicted.Comment: 16 pages, 3 figure
Lattice dynamics localization in low-angle twisted bilayer graphene
A low twist angle between the two stacked crystal networks in bilayer
graphene enables self-organized lattice reconstruction with the formation of a
periodic domain. This superlattice modulates the vibrational and electronic
structures, imposing new rules for electron-phonon coupling and the eventual
observation of strong correlation and superconductivity. Direct optical images
of the crystal superlattice in reconstructed twisted bilayer graphene are
reported here, generated by the inelastic scattering of light in a nano-Raman
spectroscope. The observation of the crystallographic structure with visible
light is made possible due to lattice dynamics localization, the images
resembling spectral variations caused by the presence of strain solitons and
topological points. The results are rationalized by a nearly-free-phonon model
and electronic calculations that highlight the relevance of solitons and
topological points, particularly pronounced for structures with small twist
angles. We anticipate our discovery to play a role in understanding Jahn-Teller
effects and electronic Cooper pairing, among many other important
phonon-related effects, and it may be useful for characterizing devices in the
most prominent platform for the field of twistronics.Comment: 9 pages, 8 figure
ABINIT: Overview and focus on selected capabilities
Paper published as part of the special topic on Electronic Structure SoftwareABINIT is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and
Bethe–Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the “temperaturedependent effective potential” approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and
other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic
materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which ABINIT relies, and a list of the associated capabilities. It then focuses on selected
capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment
of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts,
and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap,
and spin-magnetic-field perturbation. The ABINIT DFPT implementation is very general, including systems with van der Waals interaction or
with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput
calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library LIBPAW.
ABINIT has strong links with many other software projects that are briefly mentioned.This work (A.H.R.) was supported by the DMREF-NSF Grant No. 1434897, National Science Foundation OAC-1740111, and U.S. Department of Energy DE-SC0016176 and DE-SC0019491 projects.
N.A.P. and M.J.V. gratefully acknowledge funding from the Belgian Fonds National de la Recherche Scientifique (FNRS) under Grant No. PDR T.1077.15-1/7. M.J.V. also acknowledges a sabbatical “OUT” grant at ICN2 Barcelona as well as ULiège and the Communauté Française de Belgique (Grant No. ARC AIMED G.A. 15/19-09).
X.G. and M.J.V. acknowledge funding from the FNRS under Grant No. T.0103.19-ALPS.
X.G. and G.-M. R. acknowledge support from the Communauté française de Belgique through the SURFASCOPE Project (No. ARC 19/24-057).
X.G. acknowledges the hospitality of the CEA DAM-DIF during the year 2017.
G.H. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 (Materials Project Program No. KC23MP).
The Belgian authors acknowledge computational resources from supercomputing facilities of the University of Liège, the Consortium des Equipements de Calcul Intensif (Grant No. FRS-FNRS G.A. 2.5020.11), and Zenobe/CENAERO funded by the Walloon Region under Grant No. G.A. 1117545.
M.C. and O.G. acknowledge support from the Fonds de Recherche du Québec Nature et Technologie (FRQ-NT), Canada, and the Natural Sciences and Engineering Research Council of Canada (NSERC) under Grant No. RGPIN-2016-06666.
The implementation of the libpaw library (M.T., T.R., and D.C.) was supported by the ANR NEWCASTLE project (Grant No. ANR-2010-COSI-005-01) of the French National Research Agency.
M.R. and M.S. acknowledge funding from Ministerio de Economia, Industria y Competitividad (MINECO-Spain) (Grants Nos. MAT2016-77100-C2-2-P and SEV-2015-0496) and Generalitat de Catalunya (Grant No. 2017 SGR1506). This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation program (Grant Agreement No. 724529).
P.G. acknowledges support from FNRS Belgium through PDR (Grant No. HiT4FiT), ULiège and the Communauté française de Belgique through the ARC project AIMED, the EU and FNRS through M.ERA.NET project SIOX, and the European Funds for Regional Developments (FEDER) and the Walloon Region in the framework of the operational program “Wallonie-2020.EU” through the project Multifunctional thin films/LoCoTED.
The Flatiron Institute is a division of the Simons Foundation.
A large part of the data presented in this paper is available directly from the Abinit Web page www.abinit.org. Any other data not appearing in this web page can be provided by the corresponding author upon reasonable request.Peer reviewe