106 research outputs found
Towards improved exact exchange functionals relying on GW quasiparticle methods for parametrization
We use fully self-consistent GW calculations on diamond and silicon carbide
to reparametrize the Heyd-Scuseria-Ernzerhof exact exchange density functional
for use in band structure calculations of semiconductors and insulators. We
show that the thus modified functional is able to calculate the band structure
of bulk Si, Ge, GaAs, and CdTe with good quantitative accuracy at a
significantly reduced computational cost as compared to GW methods. We discuss
the limitations of this functional in low-dimensions by calculating the band
structures of single-layer hexagonal BN and MoS, and by demonstrating
that the diameter scaling of curvature induced band gaps in single-walled
carbon nanotubes is still physically incorrect using our functional; we
consider possible remedies to this problem.Comment: Submitted to Physical Review
Silicane and germanane: tight-binding and first-principles studies
We present a first-principles and tight-binding model study of silicane and
germanane, the hydrogenated derivatives of two-dimensional silicene and
germanene. We find that the materials are stable in freestanding form, analyse
the orbital composition, and derive a tight-binding model using
first-principles calculations to fit the parameters.Comment: Published in "2D Materials
Stacking domains and dislocation networks in marginally twisted bilayers of transition metal dichalcogenides
We apply a multiscale modeling approach to study lattice reconstruction in
marginally twisted bilayers of transition metal dichalcogenides (TMD). For
this, we develop DFT-parametrized interpolation formulae for interlayer
adhesion energies of MoSe, WSe, MoS, and WS, combine those with
elasticity theory, and analyze the bilayer lattice relaxation into mesoscale
domain structures. Paying particular attention to the inversion asymmetry of
TMD monolayers, we show that 3R and 2H stacking domains, separated by a network
of dislocations develop for twist angles and for,
respectively, bilayers with parallel (P) and antiparallel (AP) orientation of
the monolayer unit cells and suggest how the domain structures would manifest
itself in local probe scanning of marginally twisted P- and AP-bilayers
Diffusion quantum Monte Carlo and GW study of the electronic properties of monolayer and bulk hexagonal boron nitride
© 2020 American Physical Society. We report diffusion quantum Monte Carlo (DMC) and many-body GW calculations of the electronic band gaps of monolayer and bulk hexagonal boron nitride (hBN). We find the monolayer band gap to be indirect. GW predicts much smaller quasiparticle gaps at both the single-shot G0W0 and the partially self-consistent GW0 levels. In contrast, solving the Bethe-Salpeter equation on top of the GW0 calculation yields an exciton binding energy for the direct exciton at the K point in close agreement with the DMC value. Vibrational renormalization of the electronic band gap is found to be significant in both the monolayer and the bulk. Taking vibrational effects into account, DMC overestimates the band gap of bulk hBN, while GW theory underestimates it
Multifaceted moir\'e superlattice physics in twisted WSe bilayers
Lattice reconstruction in twisted transition-metal dichalcogenide (TMD)
bilayers gives rise to piezo- and ferroelectric moir\'e potentials for
electrons and holes, as well as a modulation of the hybridisation across the
bilayer. Here, we develop hybrid tight-binding
models to describe electrons and holes in the relevant valleys of twisted TMD
homobilayers with parallel (P) and anti-parallel (AP) orientations of the
monolayer unit cells. We apply these models to describe moir\'e superlattice
effects in twisted WSe bilayers, in conjunction with microscopic \emph{ab
initio} calculations, and considering the influence of encapsulation, pressure
and an electric displacement field. Our analysis takes into account mesoscale
lattice relaxation, interlayer hybridisation, piezopotentials, and a weak
ferroelectric charge transfer between the layers, and describes a multitude of
possibilities offered by this system, depending on the choices of P or AP
orientation, twist angle magnitude, and electron/hole valley.Comment: 44 pages, 27 figures, 6 appendices. For v2: Modelling and analysis
for Q-point bands and minibands adde
Fine-tuning the functional properties of carbon nanotubes via the interconversion of encapsulated molecules
Tweaking the properties of carbon nanotubes is a prerequisite for their
practical applications. Here we demonstrate fine-tuning the electronic
properties of single-wall carbon nanotubes via filling with ferrocene
molecules. The evolution of the bonding and charge transfer within the tube is
demonstrated via chemical reaction of the ferrocene filler ending up as
secondary inner tube. The charge transfer nature is interpreted well within
density functional theory. This work gives the first direct observation of a
fine-tuned continuous amphoteric doping of single-wall carbon nanotubes
Doped carbon nanotubes as a model system of biased graphene
Albeit difficult to access experimentally, the density of states (DOS) is a
key parameter in solid state systems which governs several important phenomena
including transport, magnetism, thermal, and thermoelectric properties. We
study DOS in an ensemble of potassium intercalated single-wall carbon nanotubes
(SWCNT) and show using electron spin resonance spectroscopy that a sizeable
number of electron states are present, which gives rise to a Fermi-liquid
behavior in this material. A comparison between theoretical and the
experimental DOS indicates that it does not display significant correlation
effects, even though the pristine nanotube material shows a Luttinger-liquid
behavior. We argue that the carbon nanotube ensemble essentially maps out the
whole Brillouin zone of graphene thus it acts as a model system of biased
graphene
Electron spin resonance signal of Luttinger liquids and single-wall carbon nanotubes
A comprehensive theory of electron spin resonance (ESR) for a Luttinger
liquid (LL) state of correlated metals is presented. The ESR measurables such
as the signal intensity and the line-width are calculated in the framework of
Luttinger liquid theory with broken spin rotational symmetry as a function of
magnetic field and temperature. We obtain a significant temperature dependent
homogeneous line-broadening which is related to the spin symmetry breaking and
the electron-electron interaction. The result crosses over smoothly to the ESR
of itinerant electrons in the non-interacting limit. These findings explain the
absence of the long-sought ESR signal of itinerant electrons in single-wall
carbon nanotubes when considering realistic experimental conditions.Comment: 5 pages, 1 figur
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