1,189 research outputs found
Quantized evolution of the plasmonic response in a stretched nanorod
Quantum aspects, such as electron tunneling between closely separated
metallic nanoparticles, are crucial for understanding the plasmonic response of
nanoscale systems. We explore quantum effects on the response of the
conductively coupled metallic nanoparticle dimer. This is realized by
stretching a nanorod, which leads to the formation of a narrowing atomic
contact between the two nanorod ends. Based on first-principles time-dependent
density-functional-theory calculations, we find a discontinuous evolution of
the plasmonic response as the nanorod is stretched. This is especially
pronounced for the intensity of the main charge-transfer plasmon mode. We show
the correlation between the observed discontinuities and the discrete nature of
the conduction channels supported by the formed atomic-sized junction.Comment: Main text: 6 pages, 2 figures; Supplemental Material: 5 pages, 4
figure
Nanoplasmonics simulations at the basis set limit through completeness-optimized, local numerical basis sets
We present an approach for generating local numerical basis sets of improving
accuracy for first-principles nanoplasmonics simulations within time-dependent
density functional theory. The method is demonstrated for copper, silver, and
gold nanoparticles that are of experimental interest but computationally
demanding due to the semi-core d-electrons that affect their plasmonic
response. The basis sets are constructed by augmenting numerical atomic orbital
basis sets by truncated Gaussian-type orbitals generated by the
completeness-optimization scheme, which is applied to the photoabsorption
spectra of homoatomic metal atom dimers. We obtain basis sets of improving
accuracy up to the complete basis set limit and demonstrate that the
performance of the basis sets transfers to simulations of larger nanoparticles
and nanoalloys as well as to calculations with various exchange-correlation
functionals. This work promotes the use of the local basis set approach of
controllable accuracy in first-principles nanoplasmonics simulations and
beyond.Comment: 11 pages, 6 figure
Kohn-Sham decomposition in real-time time-dependent density-functional theory: An efficient tool for analyzing plasmonic excitations
The real-time-propagation formulation of time-dependent density-functional
theory (RT-TDDFT) is an efficient method for modeling the optical response of
molecules and nanoparticles. Compared to the widely adopted linear-response
TDDFT approaches based on, e.g., the Casida equations, RT-TDDFT appears,
however, lacking efficient analysis methods. This applies in particular to a
decomposition of the response in the basis of the underlying single-electron
states. In this work, we overcome this limitation by developing an analysis
method for obtaining the Kohn-Sham electron-hole decomposition in RT-TDDFT. We
demonstrate the equivalence between the developed method and the Casida
approach by a benchmark on small benzene derivatives. Then, we use the method
for analyzing the plasmonic response of icosahedral silver nanoparticles up to
Ag. Based on the analysis, we conclude that in small nanoparticles
individual single-electron transitions can split the plasmon into multiple
resonances due to strong single-electron-plasmon coupling whereas in larger
nanoparticles a distinct plasmon resonance is formed.Comment: 11 pages, 3 figure
Electron transport through quantum wires and point contacts
We have studied quantum wires using the Green's function technique and the
density-functional theory, calculating the electronic structure and the
conductance. All the numerics are implemented using the finite-element method
with a high-order polynomial basis. For short wires, i.e. quantum point
contacts, the zero-bias conductance shows, as a function of the gate voltage
and at a finite temperature, a plateau at around 0.7G_0. (G_0 = 2e^2/h is the
quantum conductance). The behavior, which is caused in our mean-field model by
spontaneous spin polarization in the constriction, is reminiscent of the
so-called 0.7-anomaly observed in experiments. In our model the temperature and
the wire length affect the conductance-gate voltage curves in the same way as
in the measured data.Comment: 8 page
Matter-positronium interaction: An exact diagonalization study of the He atom - positronium system
The many-body system comprising a He nucleus, three electrons, and a positron
has been studied using the exact diagonalization technique. The purpose has
been to clarify to which extent the system can be considered as a
distinguishable positronium (Ps) atom interacting with a He atom and, thereby,
to pave the way to a practical atomistic modeling of Ps states and annihilation
in matter. The maximum value of the distance between the positron and the
nucleus is constrained and the Ps atom at different distances from the nucleus
is identified from the electron and positron densities, as well as from the
electron-positron distance and center-of-mass distributions. The polarization
of the Ps atom increases as its distance from the nucleus decreases. A
depletion of the He electron density, particularly large at low density values,
has been observed. The ortho-Ps pick-off annihilation rate calculated as the
overlap of the positron and the free He electron densities has to be corrected
for the observed depletion, specially at large pores/voids.Comment: 18 pages, 8 figure
Hydrogen and deuterium decoration of In-vacancy complexes in nickel
The quantum-mechanical states of hydrogen and deuterium in pure and defected nickel have been calculated using the effective-medium theory. The defects considered include monovacancies, the substitutional In impurity, a complex of four vacancies, and a complex of an In impurity decorated with a tetrahedron of four vacancies. While the substitutional In impurity does not trap hydrogen, the vacancy and the vacancy complexes with and without In association do. The calculated binding energy to the four vacancy complex is nearly insensitive to the hydrogen isotopic mass and to the In decoration. These results, along with the dependence of the hydrogen binding energy on multiple hydrogen occupancy of the In vacancy complex, are compared with recent perturbed-angular-correlation experiments.Peer reviewe
Vacancy recovery and vacancy-hydrogen interaction in niobium and tantalum studied by positrons
Positron-lifetime measurements in electron-irradiated pure Nb and Ta show that monovacancy migration occurs at 220 and 260 K, respectively. Hydrogen impurities can be bound to vacancies, as is experimentally observed in Ta at 70 K after low-temperature α-particle irradiation. The vacancy-hydrogen complex formation shifts the vacancy migration to higher temperatures. Vacancy-hydrogen complexes retain the capability to trap positrons. Theoretical calculations performed for hydrogen and positron states at vacancies are in agreement with experimental findings.Peer reviewe
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