370 research outputs found
Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dots
When an electron is confined to a triangular atomic thick layer of graphene
[1-5] with zig-zag edges, its energy spectrum collapses to a shell of
degenerate states at the Fermi level (Dirac point) [6-9]. The degeneracy is
proportional to the edge size and can be made macroscopic. This opens up the
possibility to design a strongly correlated electronic system as a function of
fractional filling of the zero-energy shell, in analogy to the fractional
quantum Hall effect in a quasi-two-dimensional electron gas[10], but without
the need for a high magnetic field. In this work we show that electronic
correlations, beyond the Hubbard model[6,7] and mean-field density functional
theory (DFT) [7,8] play a crucial role in determining the nature of the ground
state and the excitation spectrum of triangular graphene quantum dots as a
function of dot size and filling fraction of the shell of zero-energy states.
The interactions are treated by a combination of DFT, tight-binding,
Hartree-Fock and configuration interaction methods (TB-HF-CI) and include all
scattering and exchange terms within second nearest neighbors as well as
interaction with metallic gate. We show that a half filled charge neutral shell
leads to full spin polarization of the island but this magnetic moment is
completely destroyed by the addition of a single electron, in analogy to the
effect of skyrmions on the quantum Hall ferromagnet [11-14] and spin
depolarization in electrostatically defined semiconductor quantum dots[15-18].
The depolarization of the ground state is predicted to result in blocking of
current through a graphene quantum dot due to spin blockade (SB) [18].Comment: v2: minor corrections, new forma
Effect of disorder on transport properties in a tight-binding model for lead halide perovskites
The hybrid organic-inorganic lead halide perovskite materials have emerged as
remarkable materials for photovoltaic applications. Their strengths include
good electric transport properties in spite of the disorder inherent in them.
Motivated by this observation, we analyze the effects of disorder on the energy
eigenstates of a tight-binding model of these materials. In particular, we
analyze the spatial extension of the energy eigenstates, which is quantified by
the inverse participation ratio. This parameter exhibits a tendency, and
possibly a phase transition, to localization as the on-site energy disorder
strength is increased. However, we argue that the disorder in the lead halide
perovskites corresponds to a point in the regime of highly delocalized states.
Our results also suggest that the electronic states of mixed-halide materials
tend to be more localized than those of pure materials, which suggests a weaker
tendency to form extended bonding states in the mixed-halide materials and is
therefore not favourable for halide mixing.Comment: 24 pages (preprint), 11 figure
Theory of highly excited semiconductor nanostructures including Auger coupling: exciton-bi-exciton mixing in CdSe nanocrystals
We present a theory of highly excited interacting carriers confined in a
semiconductor nanostructure, incorporating Auger coupling between excited
states with different number of excitations. The Coulomb matrix elements
connecting exciton, bi-exciton and tri-exciton complexes are derived and an
intuitive picture of breaking neutral multi-exction complexes into positively
and negatively charged multi-exciton complexes is given. The general approach
is illustrated by analyzing the coupling of biexciton and exciton in CdSe
spherical nanocrystals. The electron and hole states are computed using
atomistic tight binding Hamiltonian including an effective crystal
field splitting and surface passivation. For each number of electron-hole pairs
the many-body spectrum is computed in the configuration-interaction approach.
The low-energy correlated biexciton levels are broken into charged complexes: a
hole and a negatively charged trion and an electron and a positively charged
trion. Out of a highly excited exciton spectrum a subspace coupled to
bi-exciton levels via Auger processes is identified. The interaction between
correlated bi-exciton and exciton states is treated using exact diagonalization
techniques. This allows to extract the spectral function of the biexciton and
relate its characteristic width and amplitude to the characteristic amplitude
and timescale of the coherent time evolution of the coupled system. It is shown
that this process can be described by the Fermi's Golden Rule only if a fast
relaxation of the excitonic subsystem is accounted for.Comment: 9 figure
A geometric approach to elliptic curves with torsion groups Z/10Z, Z/12Z, Z/14Z, and Z/16Z
We give new parametrisations of elliptic curves in Weierstrass normal form y2 = x3 + ax2 + bx with torsion groups Z/10Z and Z/12Z over Q, and with Z/14Z and Z/16Z over quadratic fields. Even though the parametrisations are equivalent to those given by Kubert and Rabarison, respectively, with the new parametrisations we found three infinite families of elliptic curves with torsion group Z/12Z and positive rank. Furthermore, we found elliptic curves with torsion group Z/14Z and rank 3 - which is a new record for such curves - as well as some new elliptic curves with torsion group Z/16Z and rank 3
Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate
Electrochemical reduction of carbon dioxide (CO2RR) to formate provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks powered using renewable electricity. Here, we hypothesized that the presence of sulfur atoms in the catalyst surface could promote undercoordinated sites, and thereby improve the electrochemical reduction of CO2 to formate. We explored, using density functional theory, how the incorporation of sulfur into tin may favor formate generation. We used atomic layer deposition of SnSx followed by a reduction process to synthesize sulfur-modulated tin (Sn(S)) catalysts. X-ray absorption near-edge structure (XANES) studies reveal higher oxidation states in Sn(S) compared with that of tin in Sn nanoparticles. Sn(S)/Au accelerates CO2RR at geometric current densities of 55 mA cm−2 at −0.75 V versus reversible hydrogen electrode with a Faradaic efficiency of 93%. Furthermore, Sn(S) catalysts show excellent stability without deactivation (<2% productivity change) following more than 40 hours of operation. With rapid advances in the efficient and cost-effective conversion of sunlight to electrical power, the development of storage technologies for renewable energy is even more urgent. Using renewable electricity to convert CO2 into formate simultaneously addresses the need for storage of intermittent renewable energy sources and the need to reduce greenhouse gas emissions. We report an increase of greater than 4-fold in the current density (hence the rate of reaction) in formate electrosynthesis compared with relevant controls. Our catalysts also show excellent stability without deactivation (<2% productivity change) following more than 40 hours of operation. The electrochemical reduction of carbon dioxide (CO2RR) offers a compelling route to energy storage and high-value chemical manufacture. The presence of sulfur atoms in catalyst surfaces promotes undercoordinated sites, thereby improving the electrochemical reduction of CO2 to formate. The resulting sulfur-modulated tin catalysts accelerate CO2RR at geometric current densities of 55 mA cm−2 at −0.75 V versus RHE with a Faradaic efficiency of 93%
The local adsorption structure of methylthiolate and butylthiolate on Au(1 1 1): a photoemission core-level shift investigation
Measurements of the core-level shifts in Au 4f photoemission spectra from Au(1 1 1) at different coverages of methylthiolate and butylthiolate are reported. Adsorption leads to two components in addition to that from the bulk, one at lower photoelectron binding energy attributed to surface atoms not bonded to thiolate species, while the second component has a higher binding energy and is attributed to Au atoms bonded to the surface thiolate. The relative intensities of these surface components for the saturation coverage (mainly (√3 × √3)R30°) phases are discussed in terms of different local adsorption sites in a well-ordered surface, and favour adsorption of the thiolate species atop Au adatoms. Alternative interpretations that might be consistent with an Au-adatom-dithiolate model are discussed, particularly in the context of the possible influence of reduced coverage associated with a disordered surface. Marked differences from previously-reported results for longer-chain alkylthiolate layers are highlighted
Possibilities of improving paraxial brightness in rf ion sources
The paraxial brightness in rf ion sources can be improved by redistributing the beam phase density, increasing
the beam current density and using beam extraction systems with low aberrations. Experimental data are presented
for the central region of a hydrogen or helium ion beam extracted from a helicon rf ion source with permanent
magnets. A high-voltage (~6 kV) extraction structure of the rf ion source was investigated with imposed external
magnetic field. The evolution of phase sets in the extraction structure with emission hole size approaching Debye
radius was calculated for the helium plasma density of ~10¹² cm⁻³ with imposed nonuniform external magnetic field.
The calculations were performed involving data on ion energy spread (~25 eV) of the helicon rf ion source with
permanent magnets.Розглянуто можливості підвищення осьової яскравості ВЧ-джерела іонів за рахунок перерозподілу
фазової щільності пучка, підвищення щільності струму пучка, застосування систем екстракції з низькими
абераціями. Приведено експериментальні дані по дослідженню характеристик привісьової області пучка
іонів водню, гелію, аргону, що витягається з геліконового ВЧ джерела іонів із системою постійних магнітів.
Приведено результати дослідження високовольтної (~6 кВ) системи екстракції ВЧ-джерела іонів при
накладенні зовнішнього магнітного поля. Проведено розрахунок еволюції фазових множин у системі
екстракції з емісійним отвором порядку радіуса Дебая для щільності гелієвої плазми (~10¹² см⁻³) при
накладенні зовнішнього неоднорідного магнітного поля. У розрахунках використані дані про енергетичний
розкид іонів (~25 еВ) геліконового ВЧ-джерела іонів із системою постійних магнітів.Рассмотрены возможности повышения осевой яркости ВЧ-источника ионов за счет перераспределения
фазовой плотности пучка, повышения плотности тока пучка, применения систем экстракции с низкими
аберрациями. Приведены экспериментальные данные по исследованию характеристик приосевой области
пучка ионов водорода, гелия, аргона, извлекаемого из геликонового ВЧ-источника ионов с системой
постоянных магнитов. Приведены результаты исследования высоковольтной (~6 кВ) системы экстракции
ВЧ-источника ионов при наложении внешнего магнитного поля. Проведен расчет эволюции фазовых
множеств в системе экстракции с эмиссионным отверстием порядка радиуса Дебая для плотности гелиевой
плазмы (~10¹² см⁻³) при наложении внешнего неоднородного магнитного поля. В расчетах использованы
данные об энергетическом разбросе ионов (~25 эВ) геликонового ВЧ-источника ионов с системой
постоянных магнитов
Plasma density measurement of RF ion source
For a radiofrequency (27.12 MHz) inductively coupled ion source (3 cm diameter, 7 cm long, without external magnetic field, working gas-hydrogen, helium, argon), measurements of the average plasma density were made using an 8 mm microwave interferometer. The range of neutral gas pressure is 2-30 mTorr and RF-power is in the range 20-400 W. It is found that the plasma density increases with increased gas pressure and RF-power. A global discharge model is applied to relate the electron densities and the electron temperature in an argon plasma to the pressure and input power ranges of interest. The model calculations are compared to measured plasma density, showing fair agreement.Для високочастотного (27.12 МГц) індуктивного джерела іонів (3 см у діаметрі і довжиною 7 см, без зовнішнього магнітного поля, робочий газ - водень, гелій, аргон) були виконані виміри середньої щільності плазми за допомогою 8-ми міліметрового інтерферометра. Тиск робочого газу змінювався в діапазоні 2-30 мТорр, ВЧ-потужність - в діапазоні 20-400 Вт. Встановлено, що щільність плазми росте зі збільшенням робочого тиску і вхідної ВЧ-потужності. Щоб розрахувати електронну щільність і електронну температуру аргонової плазми в цікавлячому нас діапазоні тисків і ВЧ-потужності, була застосована глобальна модель плазменного розряду. Обчислена по глобальній моделі щільність плазми знаходиться у гарний згоді з експериментально обмірюваною величиною.Для высокочастотного (27.12 МГц) индуктивного источника ионов (3 см в диаметре и длиной 7 см, без внешнего магнитного поля, рабочий газ - водород, гелий, аргон) были выполнены измерения средней плотности плазмы с помощью 8-ми миллиметрового интерферометра. Давление рабочего газа изменялось в диапазоне 2-30 мТорр, ВЧ-мощность - в диапазоне 20-400 Ватт. Установлено, что плотность плазмы растет с увеличением рабочего давления и входной ВЧ-мощности. Чтобы рассчитать электронную плотность и электронную температуру аргоновой плазмы в интересующем нас диапазоне давлений и ВЧ-мощности, была применена глобальная модель плазменного разряда. Вычисленная по глобальной модели плотность плазмы находится в хорошем согласии с экспериментально измеренной величиной
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