arXiv subjects

Thibault Sohier

Publications and source records attributed to Thibault Sohier.

24 records · Page 2Linked to original sources

arXiv2017

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

The ability to perform first-principles calculations of electronic and vibrational properties of two-dimensional heterostructures in a field-effect setup is crucial for the understanding and design of next-generation devices. We present here an implementation of density functional perturbation theories tailored for the case of two-dimensional heterostructures in field-effect configuration. Key ingredients are the inclusion of a truncated Coulomb interaction in the direction perpendicular to the slab and the possibility of simulating charging of the slab via field-effects. With this implementation we can access total energies, force and stress tensors, the vibrational properties and the electron-phonon interaction. We demonstrate the relevance of the method by studying flexural acoustic phonons and their coupling to electrons in graphene doped by field-effect. In particular, we show that while the electron-phonon coupling to those phonons can be significant in neutral graphene, it is strongly screened and negligible in doped graphene, in disagreement with other recent first-principles reports. Consequently, the gate-induced coupling with flexural acoustic modes would not be detectable in transport measurements on doped graphene.

cond-mat.mtrl-sci↗

arXiv2017

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.

cond-mat.mtrl-sci↗

arXiv2016

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

We perform ab initio calculations of the coupling between electrons and small-momentum polar-optical phonons in monolayer transition metal dichalcogenides of the 2H type: MoS2, MoSe2, MoTe2, WS2, and WSe2. The so-called Fröhlich interaction is fundamentally affected by the dimensionality of the system. In a plane-wave framework with periodic boundary conditions, this coupling is affected by the spurious interaction between the 2D material and its periodic images. To overcome this, we perform density functional perturbation theory calculations with a truncated Coulomb interaction in the out-of-plane direction. We show that the 2D Fröhlich interaction is much stronger than assumed in previous ab initio studies. We provide analytical models depending on the effective charges and dielectric properties of the materials to interpret our ab initio calculations. Screening is shown to play a fundamental role in the phonon-momentum dependency of the polar-optical coupling, with a crossover between two regimes depending on the dielectric properties of the material relative to its environment. The Fröhlich interaction is screened by the dielectric environment in the limit of small phonon momenta and sharply decreases due to stronger screening by the monolayer at finite momenta. The small-momentum regime of the ab initio Fröhlich interaction is reproduced by a simple analytical model, for which we provide the necessary parameters. At larger momenta, however, direct ab initio calculations of electron-phonon interactions are necessary to capture band-specific effects. We compute and compare the carrier relaxation times associated to the scattering by both LO and A1 phonon modes. While both modes are capable of relaxing carriers on timescales under the picosecond at room temperature, their absolute and relative importance vary strongly depending on the material, the band, and the substrate.

cond-mat.mtrl-sci↗

arXiv2015

Density-functional calculation of static screening in 2D 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 $\approx 10%$, mainly due to the conical band approximation.

cond-mat.mtrl-sci↗

arXiv2014

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

Electron-phonon coupling in graphene is extensively modeled and simulated from first principles. We find that using an accurate model for the polarizations of the acoustic phonon modes is crucial to obtain correct numerical results. The interactions between electrons and acoustic phonon modes, the gauge field and deformation potential, are calculated at the DFT level in the framework of linear response. The zero-momentum limit of acoustic phonons is interpreted as a strain pattern, allowing the calculation of the acoustic gauge field parameter in the GW approximation. The role of electronic screening on the electron-phonon matrix elements is investigated. We then solve the Boltzmann equation semi-analytically in graphene, including both acoustic and optical phonon scattering. We show that, in the Bloch-Grüneisen and equipartition regimes, the electronic transport is mainly ruled by the unscreened acoustic gauge field, while the contribution due to the deformation potential is negligible and strongly screened. By comparing with experimental data, we show that the contribution of acoustic phonons to resistivity is doping- and substrate-independent. The DFT+GW approach underestimates this contribution to resistivity by about 30 %. Above 270K, the calculated resistivity underestimates the experimental one more severely, the underestimation being larger at lower doping. We show that, beside remote phonon scattering, a possible explanation for this disagreement is the electron-electron interaction that strongly renormalizes the coupling to intrinsic optical-phonon modes. Finally, after discussing the validity of the Matthiessen rule in graphene, we derive simplified analytical solutions of the Boltzmann equation to extract the coupling to acoustic phonons, related to the strain-induced gauge field, directly from experimental data.

cond-mat.mtrl-sci↗

arXiv2014

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.

cond-mat.mtrl-sci↗