Thibault Sohier

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.The largest available database of potentially exfoliable 2D materials has been obtained via high-throughput calculations using van der Waals density functional theory.

Electron–Phonon Interactions and the Intrinsic Electrical Resistivity of Graphene

We present a first-principles study of the temperature- and density-dependent intrinsic electrical resistivity of graphene. We use density-functional theory and density-functional perturbation theory together with very accurate Wannier interpolations to compute all electronic and vibrational properties and electron-phonon coupling matrix elements; the phonon-limited resistivity is then calculated within a Boltzmann-transport approach. An effective tight-binding model, validated against first-principles results, is also used to study the role of electron-electron interactions at the level of many-body perturbation theory. The results found are in excellent agreement with recent experimental data on graphene samples at high carrier densities and elucidate the role of the different phonon modes in limiting electron mobility. Moreover, we find that the resistivity arising from scattering with transverse acoustic phonons is 2.5 times higher than that from longitudinal acoustic phonons. Last, high-energy, optical, and zone-boundary phonons contribute as much as acoustic phonons to the intrinsic electrical resistivity even at room temperature and become dominant at higher temperatures.

Density functional perturbation theory for gated two-dimensional heterostructures: Theoretical developments and application to flexural phonons in graphene

In the search for exciting new physics and the design of next-generation devices, gated two-dimensional heterostructures are becoming omnipresent. As the fabrication and characterization techniques in this field improve, first-principles methods must follow suit. The authors present here an implementation of density functional perturbation theory, tailored to simulate the electronic and vibrational properties of two-dimensional heterostructures in the field-effect configuration. They apply the method to gated graphene and show that while the field effect activates the coupling between electrons and flexural phonons, this coupling is strongly screened by the electrons of doped graphene.

Breakdown of Optical Phonons’ Splitting in Two-Dimensional Materials

We investigate the long-wavelength dispersion of longitudinal and transverse optical phonon modes in polar two-dimensional materials, multilayers, and their heterostructures. Using analytical models and density-functional perturbation theory in a two-dimensional framework, we show that at variance with the three-dimensional case these modes are degenerate at the zone center but the macroscopic electric field associated with the longitudinal-optical modes gives rise to a finite slope at the zone center in their corresponding phonon dispersions. This slope increases linearly with the number of layers and it is determined solely by the Born effective charges of the material and the dielectric properties of the surrounding media. Screening from the environment can greatly reduce the slope splitting between the longitudinal and transverse optical modes and can be seen in the experimentally relevant case of boron nitride-graphene heterostructures. As the phonon momentum increases, the intrinsic screening properties of the two-dimensional material dictate the transition to a momentum-independent splitting similar to that of three-dimensional materials. These considerations are essential to understand electrical transport and optical coupling in two-dimensional systems.

Density-functional calculation of static screening in two-dimensional materials: The long-wavelength dielectric function of graphene

We calculate the long-wavelength static screening properties of both neutral and doped graphene in the framework of density-functional theory. We use a plane-wave approach with periodic images in the third dimension and truncate the Coulomb interactions to eliminate spurious interlayer screening. We carefully address the issue of extracting two dimensional dielectric properties from simulated three-dimensional potentials. We compare this method with analytical expressions derived for two dimensional massless Dirac fermions in the random phase approximation. We evaluate the contributions of the deviation from conical bands, exchange-correlation and local-fields. For momenta smaller than twice the Fermi wavevector, the static screening of graphene within the density-functional perturbative approach agrees with the results for conical bands within random phase approximation and neglecting local fields. For larger momenta, we find that the analytical model underestimates the static dielectric function by

Phonon-limited resistivity of graphene by first-principles calculations: Electron-phonon interactions, strain-induced gauge field, and Boltzmann equation

\approx 10%

Two-dimensional Frohlich interaction in transition-metal dichalcogenide monolayers: Theoretical modeling and first-principles calculations

, mainly due to the conical band approximation.