132 research outputs found
Ab initio GW many-body effects in graphene
We present an {\it ab initio} many-body GW calculation of the self-energy,
the quasiparticle band plot and the spectral functions in free-standing undoped
graphene. With respect to other approaches, we numerically take into account
the full ionic and electronic structure of real graphene and we introduce
electron-electron interaction and correlation effects from first principles.
Both non-hermitian and also dynamical components of the self-energy are fully
taken into account. With respect to DFT-LDA, the Fermi velocity is
substantially renormalized and raised by a 17%, in better agreement with
magnetotransport experiments. Furthermore, close to the Dirac point the linear
dispersion is modified by the presence of a kink, as observed in ARPES
experiments. Our calculations show that the kink is due to low-energy single-particle excitations and to the plasmon. Finally, the GW
self-energy does not open the band gap.Comment: 5 pages, 4 figures, 1 tabl
Optical spectra of solids obtained by time-dependent density-functional theory with the jellium-with-gap model exchange-correlation kernel
Within the framework of ab initio time-dependent density-functional theory
(TD-DFT), we propose a static approximation to the exchange-correlation kernel
based on the jellium-with-gap model. This kernel accounts for electron-hole
interactions and it is able to address both strongly bound excitons and weak
excitonic effects. TD-DFT absorption spectra of several bulk materials (both
semiconductor and insulators) are reproduced in very good agreement with the
experiments and with a low computational cost.Comment: 5 pages, 3 figures, 1 tabl
Energy bands of atomic monolayers of various materials: Possibility of energy gap engineering
The mobility of graphene is very high because the quantum Hall effects can be
observed even at room temperature. Graphene has the potential of the material
for novel devices because of this high mobility. But the energy gap of graphene
is zero, so graphene can not be applied to semiconductor devices such as
transistors, LEDs, etc. In order to control the energy gaps, we propose atomic
monolayers which consist of various materials besides carbon atoms. To examine
the energy dispersions of atomic monolayers of various materials, we calculated
the electronic states of these atomic monolayers using density functional
theory with structural optimizations. The quantum chemical calculation software
"Gaussian 03" was used under periodic boundary conditions. The calculation
method is LSDA/6-311G(d,p), B3LYP/6-31G(d), or B3LYP/6-311G(d,p). The
calculated materials are C (graphene), Si (silicene), Ge, SiC, GeC, GeSi, BN,
BP, BAs, AlP, AlAs, GaP, and GaAs. These atomic monolayers can exist in the
flat honeycomb shapes. The energy gaps of these atomic monolayers take various
values. Ge is a semimetal; AlP, AlAs, GaP, and GaAs are indirect
semiconductors; and others are direct semiconductors. We also calculated the
change of energy dispersions accompanied by the substitution of the atoms. Our
results suggest that the substitution of impurity atoms for monolayer materials
can control the energy gaps of the atomic monolayers. We conclude that atomic
monolayers of various materials have the potential for novel devices.Comment: This paper was first presented at the 14th International Conference
on Modulated Semiconductor Structures (MSS14) held in Kobe, Japan, on 23 July
200
Generation of multiple plasmons in strontium niobates mediated by local field effects
Recently, an anomalous generation of multiple plasmons with large spectral
weight transfer in the visible to ultraviolet range (energies below the band
gap) has been experimentally observed in the insulating-like phase of
oxygen-rich strontium niobium oxides (SrNbO). Here, we investigate
the ground state and dielectric properties of SrNbO as a function
of by means of extensive first principle calculations. We find that in
the random phase approximation by taking into account the local field effects
(LFEs), our calculations are able to reproduce both the unconventional multiple
generations of plasmons and spectral weight transfers, consistent with
experimental data. Interestingly, these unconventional plasmons can be tuned by
oxygen stoichiometry as well as microscopic superstructure. This unusual
predominance of LFEs in this class of materials is ascribed to the strong
electronic inhomogeneity and high polarizability and paves a new path to induce
multiple plasmons in the untapped visible to ultraviolet ranges of
insulating-like oxides
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