51 research outputs found
Two-dimensional Fr\"ohlich 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 Fr\"ohlich 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 Fr\"ohlich
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
Fr\"ohlich 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
Fr\"ohlich 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.Comment: 14 pages, 8 figure
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 , mainly due to the conical band approximation
Density-functional perturbation theory for one-dimensional systems: implementation and relevance for phonons and electron-phonon interactions
The electronic and vibrational properties and electron-phonon couplings of
one-dimensional materials will be key to many prospective applications in
nanotechnology. Dimensionality strongly affects these properties and has to be
correctly accounted for in first-principles calculations. Here we develop and
implement a formulation of density-functional and density-functional
perturbation theory that is tailored for one-dimensional systems. A key
ingredient is the inclusion of a Coulomb cutoff, a reciprocal-space technique
designed to correct for the spurious interactions between periodic images in
periodic-boundary conditions. This restores the proper one-dimensional
open-boundary conditions, letting the true response of the isolated
one-dimensional system emerge. In addition to total energies, forces and stress
tensors, phonons and electron-phonon interactions are also properly accounted
for. We demonstrate the relevance of the present method on a portfolio of
realistic systems: BN atomic chains, BN armchair nanotubes, and GaAs nanowires.
Notably, we highlight the critical role of the Coulomb cutoff by studying
previously inaccessible polar-optical phonons and Frohlich electron-phonon
couplings. We also develop and apply analytical models to support the physical
insights derived from the calculations and we discuss their consequences on
electronic lifetimes. The present work unlocks the possibility to accurately
simulate the linear response properties of one-dimensional systems, sheds light
on the transition between dimensionalities and paves the way for further
studies in several fields, including charge transport, optical coupling and
polaritronics.Comment: 15 pages, 7 figure
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.Comment: 7 pages 5 figure
Theory of infrared double-resonance Raman spectrum in graphene: the role of the zone-boundary electron-phonon enhancement
We theoretically investigate the double-resonance Raman spectrum of monolayer
graphene down to infrared laser excitation energies. By using first-principles
density functional theory calculations, we improve upon previous theoretical
predictions based on conical models or tight-binding approximations, and
rigorously justify the evaluation of the electron-phonon enhancement found in
Ref. [Venanzi, T., Graziotto, L. et al., Phys. Rev. Lett. 130, 256901 (2023)].
We proceed to discuss the effects of such enhancement on the room temperature
graphene resistivity, hinting towards a possible reconciliation of theoretical
and experimental discrepancies.Comment: 19 pages, 18 figure
Gate control of spin-layer-locking FETs and application to monolayer LuIO
A recent 2D spinFET concept proposes to switch electrostatically between two
separate sublayers with strong and opposite intrinsic Rashba effects. This
concept exploits the spin-layer locking mechanism present in centrosymmetric
materials with local dipole fields, where a weak electric field can easily
manipulate just one of the spin channels. Here, we propose a novel monolayer
material within this family, lutetium oxide iodide (LuIO). It displays one of
the largest Rashba effects among 2D materials (up to
{\AA}), leading to a rotation of the spins over just 1 nm. The
monolayer had been predicted to be exfoliable from its experimentally-known 3D
bulk counterpart, with a binding energy even lower than graphene. We
characterize and model with first-principles simulations the interplay of the
two gate-controlled parameters for such devices: doping and spin channel
selection. We show that the ability to split the spin channels in energy
diminishes with doping, leading to specific gate-operation guidelines that can
apply to all devices based on spin-layer locking.Comment: 11 pages, 9 figure
Remote free-carrier screening to boost the mobility of Fröhlich-limited two-dimensional semiconductors
Van der Waals heterostructures provide a versatile tool to not only protect
or control, but also enhance the properties of a 2D material. We use ab initio
calculations and semi-analytical models to find strategies which boost the
mobility of a current-carrying 2D semiconductor within an heterostructure.
Free-carrier screening from a metallic "screener" layer remotely suppresses
electron-phonon interactions in the current-carrying layer. This concept is
most effective in 2D semiconductors whose scattering is dominated by screenable
electron-phonon interactions, and in particular the Fr\"ohlich coupling to
polar-optical phonons. Such materials are common and characterised by overall
low mobilities in the small doping limit, and much higher ones when the 2D
material is doped enough for electron-phonon interactions to be screened by its
own free carriers. We use GaSe as a prototype and place it in a heterostructure
with doped graphene as the "screener" layer and BN as a separator. We develop
an approach to determine the electrostatic response of any heterostructure by
combining the responses of the individual layers computed within
density-functional perturbation theory. Remote screening from graphene can
suppress the long-wavelength Fr\"ohlich interaction, leading to a consistently
high mobility around to cm/Vs for carrier densities in GaSe
from to cm. Notably, the low-doping mobility is
enhanced by a factor 2.5. This remote free-carrier screening is more efficient
than more conventional manipulation of the dielectric environment, and it is
most effective when the separator (BN) is thin.Comment: 20 pages, 14 figure
Gate control of spin-layer-locking FETs and application to monolayer LuIO
peer reviewedA recent 2D spinFET concept proposes to switch electrostatically between two separate sublayers with strong and opposite intrinsic Rashba effects, exploiting the spin-layer locking mechanism in centrosymmetric materials with local dipole fields.
Here, we propose a novel monolayer material within this family, lutetium oxide iodide (LuIO). It displays one of the largest Rashba effects among 2D materials (up to k_R = 0.08 \si{\angstrom}^{-1}), leading to a rotation of the spins over just 1 nm. The monolayer was predicted to be exfoliable from its experimentally-known 3D bulk counterpart, with a binding energy lower than graphene. We characterize and simulate the interplay of the two gate-controlled parameters for such devices: doping and spin channel selection. We show that the ability to split the spin channels in energy diminishes with doping, leading to specific gate-operation guidelines that can apply to all devices based on spin-layer locking
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