202 research outputs found
Diameter and Chirality Dependence of Exciton Properties in Carbon Nanotubes
We calculate the diameter and chirality dependences of the binding energies,
sizes, and bright-dark splittings of excitons in semiconducting single-wall
carbon nanotubes (SWNTs). Using results and insights from {\it ab initio}
calculations, we employ a symmetry-based, variational method based on the
effective-mass and envelope-function approximations using tight-binding
wavefunctions. Binding energies and spatial extents show a leading dependence
with diameter as and , respectively, with chirality corrections
providing a spread of roughly 20% with a strong family behavior. Bright-dark
exciton splittings show a leading dependence. We provide analytical
expressions for the binding energies, sizes, and splittings that should be
useful to guide future experiments
Energy-Materials Nexus for a Low Carbon Energy System
OUTLINE OF THE PRESENTATION:
1. Introduction
2. The Energy-Materials Nexus
3. Research Approach
4. Challenges of the Energy-Materials Nexus
5. The Energy-Materials Nexus & the UN SDGs
6. Further Research, Development & Action
7. Conclusio
Scaling of excitons in carbon nanotubes
Light emission from carbon nanotubes is expected to be dominated by excitonic
recombination. Here we calculate the properties of excitons in nanotubes
embedded in a dielectric, for a wide range of tube radii and dielectric
environments. We find that simple scaling relationships give a good description
of the binding energy, exciton size, and oscillator strength.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Let
Fano resonances in a three-terminal nanodevice
The electron transport through a quantum sphere with three one-dimensional
wires attached to it is investigated. An explicit form for the transmission
coefficient as a function of the electron energy is found from the first
principles. The asymmetric Fano resonances are detected in transmission of the
system. The collapse of the resonances is shown to appear under certain
conditions. A two-terminal nanodevice with an additional gate lead is studied
using the developed approach. Additional resonances and minima of transmission
are indicated in the device.Comment: 11 pages, 5 figures, 2 equations are added, misprints in 5 equations
are removed, published in Journal of Physics: Condensed Matte
Exciton-plasmon states in nanoscale materials: breakdown of the Tamm-Dancoff approximation
Within the Tamm-Dancoff approximation ab initio approaches describe excitons
as packets of electron-hole pairs propagating only forward in time. However, we
show that in nanoscale materials excitons and plasmons hybridize, creating
exciton--plasmon states where the electron-hole pairs oscillate back and forth
in time. Then, as exemplified by the trans-azobenzene molecule and carbon
nanotubes, the Tamm-Dancoff approximation yields errors as large as the
accuracy claimed in ab initio calculations. Instead, we propose a general and
efficient approach that avoids the Tamm--Dancoff approximation, and correctly
describes excitons, plasmons and exciton-plasmon states
Quantifying the Underlying Causes of a Discrepancy Between Predicted and Measured Energy Use
Simulation is commonly utilized as a best practice approach to assess building performance in the building industry. However, the built environment is complex and influenced by a large number of independent and interdependent variables, making it difficult to achieve an accurate representation of real-world building energy in-use. This gives rise to significant discrepancies between simulation results and actual measured energy consumption, termed “the performance gap.” The research presented in this paper quantified the impact of underlying causes of this gap, by developing building simulation models of four existing non-domestic buildings, and then calibrating them toward their measured energy use at a high level of data granularity. It was found that discrepancies were primarily related to night-time use and seasonality in universities is not being captured correctly, in addition to equipment and server power density being underestimated (indirectly impacting heating and cooling loads). Less impactful parameters were among others; material properties, system efficiencies, and air infiltration assumptions
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