518,244 research outputs found
Isotope dependence of band-gap energy
The results of the quantitative investigations of the renormalization of the
absorption edge of different compounds by the isotope effect are described.Comment: 7 pages, 3 figure
Fractional charge perspective on the band-gap in density-functional theory
The calculation of the band-gap by density-functional theory (DFT) methods is
examined by considering the behavior of the energy as a function of number of
electrons. It is found that the incorrect band-gap prediction with most
approximate functionals originates mainly from errors in describing systems
with fractional charges. Formulas for the energy derivatives with respect to
number of electrons are derived which clarify the role of optimized effective
potentials in prediction of the band-gap. Calculations with a recent functional
that has much improved behavior for fractional charges give a good prediction
of the energy gap and also for finite systems.
Our results indicate it is possible, within DFT, to have a functional whose
eigenvalues or derivatives accurately predict the band-gap
Correlations in a band insulator
We study a model of a covalent band insulator with on-site Coulomb repulsion
at half-filling using dynamical mean-field theory. Upon increasing the
interaction strength the system undergoes a discontinuous transition from a
correlated band insulator to a Mott insulator with hysteretic behavior at low
temperatures. Increasing the temperature in the band insulator close to the
insulator-insulator transition we find a crossover to a Mott insulator at
elevated temperatures. Remarkably, correlations decrease the energy gap in the
correlated band insulator. The gap renormalization can be traced to the
low-frequency behavior of the self-energy, analogously to the quasiparticle
renormalization in a Fermi liquid. While the uncorrelated band insulator is
characterized by a single gap for both charge and spin excitations, the spin
gap is smaller than the charge gap in the correlated system.Comment: 7 pages, 7 figure
Vacancy induced energy band gap changes of semiconducting zigzag single walled carbon nanotubes
In this work, we have examined how the multi-vacancy defects induced in the
horizontal direction change the energetics and the electronic structure of
semiconducting Single-Walled Carbon Nanotubes (SWCNTs). The electronic
structure of SWCNTs is computed for each deformed configuration by means of
real space, Order(N) Tight Binding Molecular Dynamic (O(N) TBMD) simulations.
Energy band gap is obtained in real space through the behavior of electronic
density of states (eDOS) near the Fermi level. Vacancies can effectively change
the energetics and hence the electronic structure of SWCNTs. In this study, we
choose three different kinds of semiconducting zigzag SWCNTs and determine the
band gap modifications. We have selected (12,0), (13,0) and (14,0) zigzag
SWCNTs according to n (mod 3) = 0, n (mod 3) = 1 and n (mod 3) = 2
classification. (12,0) SWCNT is metallic in its pristine state. The application
of vacancies opens the electronic band gap and it goes up to 0.13 eV for a di-
vacancy defected tube. On the other hand (13,0) and (14,0) SWCNTs are
semiconductors with energy band gap values of 0.44 eV and 0.55 eV in their
pristine state, respectively. Their energy band gap values decrease to 0.07 eV
and 0.09 eV when mono-vacancy defects are induced in their horizontal
directions. Then the di-vacancy defects open the band gap again. So in both
cases, the semiconducting-metallic - semiconducting transitions occur. It is
also shown that the band gap modification exhibits irreversible
characteristics, which means that band gap values of the nanotubes do not reach
their pristine values with increasing number of vacancies
Semiconductor resonator solitons above band gap
We show experimentally the existence of bright and dark spatial solitons in
semiconductor resonators for excitation above the band gap energy. These
solitons can be switched on, both spontaneously and with address pulses,
without the thermal delay found for solitons below the band gap which is
unfavorable for applications. The differences between soliton properties above
and below gap energy are discussed.Comment: 4 pages, 7 figure
Energy Band Gap Engineering of Graphene Nanoribbons
We investigate electronic transport in lithographically patterned graphene
ribbon structures where the lateral confinement of charge carriers creates an
energy gap near the charge neutrality point. Individual graphene layers are
contacted with metal electrodes and patterned into ribbons of varying widths
and different crystallographic orientations. The temperature dependent
conductance measurements show larger energy gaps opening for narrower ribbons.
The sizes of these energy gaps are investigated by measuring the conductance in
the non-linear response regime at low temperatures. We find that the energy gap
scales inversely with the ribbon width, thus demonstrating the ability to
engineer the band gap of graphene nanostructures by lithographic processes.Comment: 7 pages including 4 figure
The effect of substituted benzene dicarboxylic acid linkers on the optical band gap energy and magnetic coupling in manganese trimer metal organic frameworks
We have systematically studied a series of eight metal-organic frameworks (MOFs) in which the secondary building unit is a manganese trimer cluster, and the linkers are differently substituted benzene dicarboxylic acids (BDC). The optical band gap energy of the compounds vary from 2.62 eV to 3.57 eV, and theoretical studies find that different functional groups result in new states in the conduction band, which lie in the gap and lower the optical band gap energy. The optical absorption between the filled Mn 3d states and the ligands is weak due to minimal overlap of the states, and the measured optical band gap energy is due to transitions on the BDC linker. The Mn atoms in the MOFs have local moments of 5 mu B, and selected MOFs are found to be antiferromagnetic, with weak coupling between the cluster units, and paramagnetic above 10 K
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