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Treatment of disorder effects in X-ray absorption spectra beyond the conventional approach
摘要： The contribution of static and thermal disorder is one of the largest challenges for the accurate determination of the atomic structure from the extended X-ray absorption fine structure (EXAFS). Although there are a number of generally accepted approaches to solve this problem, which are widely used in the EXAFS data analysis, they often provide less accurate results when applied to outer coordination shells around the absorbing atom. In this case, the advanced techniques based on the molecular dynamics and reverse Monte Carlo simulations are known to be more appropriate: their strengths and weaknesses are reviewed here.
关键词： Reverse Monte Carlo,Extended X-ray absorption fine structure (EXAFS),Molecular dynamics,Static and thermal disorder,X-ray absorption spectroscopy
更新于2025-09-23 15:23:52
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Molecular-beam resonance method with Zeeman-decelerated samples: Application to metastable helium molecules
摘要： We use a multistage Zeeman decelerator to generate slow beams (v ≈ 100 m/s) of translationally cold, spin-polarized metastable a 3Σ+u He2 molecules and perform a precision measurement of their spin-rotation fine structure. The spin polarization results from the elimination of the high-field-seeking J = N spin-rotational component of each rotational level (rotational quantum number N) by the Zeeman deceleration process. By repopulating the J = N component from the J = N ± 1 low-field-seeking components using radio-frequency radiation, we measured the spin-rotation fine structure of 13 rovibrational levels with v = 0, 1 and N = 1–21. The low beam velocity and the resulting long interaction times with the radio-frequency radiation were exploited to determine the transition frequencies with a precision of 300 Hz.
关键词： metastable helium molecules,precision spectroscopy,molecular-beam resonance,Zeeman deceleration,spin-rotation fine structure
更新于2025-09-23 15:23:52
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Vertical fine structure and time evolution of plasma irregularities in the Es layer observed by a high-resolution Ca+ lidar
摘要： The vertical fine structures and the time evolution of plasma irregularities in the sporadic E (Es) layer were observed via calcium ion (Ca+) density measurements using a resonance scattering lidar with a high time-height resolution (5 s and 15 m) at Tachikawa (35.7°N, 139.4°E) on December 24, 2014. The observation successfully provided clearer fine structures of plasma irregularities, such as quasi-sinusoidal height variation, localized clumps, “cats-eye” structures, and twist structures, in the sporadic Ca+ (Ca+s) layers at around 100 km altitude. These fine structures suggested that the Kelvin–Helmholtz instabilities occurred in the neutral atmosphere whose density changed temporarily or spatially. The maximum Ca+ density in the Ca+s layer was two orders of magnitude smaller than the maximum electron density estimated from the critical frequency (foEs) simultaneously observed by the ionosonde at Kokubunji (35.7°N, 139.5°E). A strong positive correlation with a coefficient of 0.91 suggests that Ca+ contributes forming the Es layer as well as major metallic ions Fe+ and Mg+ in the lower thermosphere. Moreover, the formation of a new Ca+s layer at 110 km and the upward motions of the Ca+s layers at 100 km and 110 km were observed just after the sunrise time at the conjugation point. Although the presence or absence of a causal relationship with the sunrise time was not clear, a possible explanation for the formation and the upward motions of the Ca+s layers was the occurrence of strong horizontal wind, rather than the enhancement of the eastward electric field.
关键词： Calcium ion (Ca+) density,Ion upward flow,Mid-latitude,Resonance scattering lidar,Kelvin–Helmholtz instability,Sporadic E (Es) layer,Vertical fine structure,Lower thermosphere
更新于2025-09-23 15:22:29
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reveals remarkable optical linewidths and fine structure for well-known damage centers
摘要： Luminescence and optical absorption due to radiation damage centers in silicon have been studied exhaustively for decades, but are receiving new interest for applications as emitters for integrated silicon photonic technologies. While a variety of other optical transitions have been found to be much sharper in enriched 28Si than in natural Si, due to the elimination of inhomogeneous isotopic broadening, this has not yet been investigated for radiation damage centers. We report results for the well-known G, W, and C damage centers in highly enriched 28Si, with optical linewidth improvements in some cases of over two orders of magnitude, revealing previously hidden ?ne structure in the G-center emission and absorption. These results have direct implications for the linewidths to be expected from single-center emission, even in natural Si, and for models for the G-center structure. The advantages of 28Si can be readily extended to the study of other radiation damage centers in Si.
关键词： 28Si,silicon,fine structure,optical linewidths,radiation damage centers
更新于2025-09-23 15:21:01
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Atomistic theory of electronic and optical properties of InAsP/InP nanowire quantum dots
摘要： We present here an atomistic theory of the electronic and optical properties of hexagonal InAsP quantum dots in InP nanowires in the wurtzite phase. These self-assembled quantum dots are unique in that their heights, shapes, and diameters are well known. Using a combined valence-force-field, tight-binding, and configuration-interaction approach we perform atomistic calculations of single-particle states and excitonic, biexcitonic, and trion complexes as well as emission spectra as a function of the quantum dot height, diameter, and As versus P concentration. The atomistic tight-binding parameters for InAs and InP in the wurtzite crystal phase were obtained by ab initio methods corrected by empirical band gaps. The low energy electron and hole states form electronic shells similar to parabolic or cylindrical quantum confinement, only weakly affected by hexagonal symmetry and As fluctuations. The relative alignment of the emission lines from excitons, trions, and biexcitons agrees with that for InAs/InP dots in the zincblende phase in that biexcitons and positive trions are only weakly bound. The random distribution of As atoms leads to dot-to-dot fluctuations of a few meV for the single-particle states and the spectral lines. Due to the high symmetry of hexagonal InAsP nanowire quantum dots the exciton fine structure splitting is found to be small, of the order a few μeV with significant random fluctuations in accordance with experiments.
关键词： fine structure splitting,electronic properties,optical properties,emission spectra,tight-binding,single-particle states,excitonic complexes,InAsP quantum dots,InP nanowires,wurtzite phase,configuration-interaction,valence-force-field,atomistic theory
更新于2025-09-23 15:19:57
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Influence of morphology on the blinking mechanisms and the excitonic fine structure of single colloidal nanoplatelets
摘要： Colloidal semiconductor nanoplatelets with a similar electronic structure as quantum wells have recently emerged as exciting materials for optoelectronic applications. Here we investigate how morphology affects important photoluminescence properties of single CdSe and core/shell CdSe/CdZnS nanoplatelets. By analyzing photoluminescence intensity-lifetime correlation and second-order photon correlation results, we demonstrate that, irrespective of the morphology, Auger recombination plays only a minor role in dictating the blinking behavior of the nanoplatelets. We find that a rough shell induces additional non-radiative channels presumably related to defects or traps of an imperfect shell. Furthermore, polarization-resolved spectroscopy analysis reveals exciton fine-structure splitting of the order of several tens of meV in rough-shell nanoplatelets at room temperature, which is attributed to exciton localization and is substantiated by theoretical calculations taking into account the nanoplatelet shape and electron–hole exchange interaction.
关键词： exciton fine structure,photoluminescence,nanoplatelets,blinking,Auger recombination
更新于2025-09-19 17:15:36
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Strain tunable quantum dot based non-classical photon sources
摘要： Semiconductor quantum dots are leading candidates for the on-demand generation of single photons and entangled photon pairs. High photon quality and indistinguishability of photons from different sources are critical for quantum information applications. The inability to grow perfectly identical quantum dots with ideal optical properties necessitates the application of post-growth tuning techniques via e.g. temperature, electric, magnetic or strain fields. In this review, we summarize the state-of-the-art and highlight the advantages of strain tunable non-classical photon sources based on epitaxial quantum dots. Using piezoelectric crystals like PMN-PT, the wavelength of single photons and entangled photon pairs emitted by InGaAs/GaAs quantum dots can be tuned reversibly. Combining with quantum light-emitting diodes simultaneously allows for electrical triggering and the tuning of wavelength or exciton fine structure. Emission from light hole exciton can be tuned, and quantum dot containing nanostructure such as nanowires have been piezo-integrated. To ensure the indistinguishability of photons from distant emitters, the wavelength drift caused by piezo creep can be compensated by frequency feedback, which is verified by two-photon interference with photons from two stabilized sources. Therefore, strain tuning proves to be a flexible and reliable tool for the development of scalable quantum dots-based non-classical photon sources.
关键词： quantum dot,on-chip,piezoelectric crystal,entangled photons,fine structure splitting,strain tuning
更新于2025-09-19 17:13:59
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Local atomic structure of the GaN-side of the Al2O3/GaN interface revealed by X-ray absorption spectroscopy
摘要： The interface between a gate insulator (Al2O3) and a semiconductor (GaN) was investigated via surface-sensitive Ga K-edge extended X-ray absorption fine structure spectroscopy, achieved by detecting the Ga LMM Auger electrons originated from the Ga K-shell absorption. This GaN-side interface study was conducted on Al2O3 thin films formed via atomic layer deposition. The determined atomic structures revealed GaN crystalline changes and the formation of Ga–O bonds due to nitrogen annealing.
关键词： Aluminum oxide (Al2O3),Interface,Post-deposition annealing (PDA),Gallium nitride (GaN),Extended X-ray absorption fine structure (EXAFS),Atomic layer deposition (ALD)
更新于2025-09-19 17:13:59
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Nonclassical Light Sources Based on Selectively Positioned Deterministic Microlens Structures and (111) In(Ga)As Quantum Dots
摘要： The results of investigations of the optical characteristics of nonclassical light sources based on selectively positioned microlens structures and single (111) In(Ga)As quantum dots grown on a (111) BGaAs substrate are presented. The single-photon nature of the radiation is confirmed by measuring and analyzing second-order correlation functions g(2)(τ); g(2)(0) = 0.07. The fine structure of the exciton states of (111) In(Ga)As quantum dots is investigated. It is shown that, in the energy range of 1.320–1.345 eV, the splitting of exciton states is comparable to the natural width of the exciton lines, which is of interest for developing photon-pair emitters based on them.
关键词： quantum dots,single-photon sources,fine structure of exciton states,photon-pair sources
更新于2025-09-16 10:30:52
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Aluminium Matrix Composites Produced by Laser Based Additive Manufacturing
摘要： 3D laser metal deposition (LMD) based on the laser cladding is an established technique for additive manufacturing tasks. Also the laser cladding is a well-known technique to produce aluminum matrix composites reinforced by Si particles. High cooling rates typical for laser treatment enable to eliminate coarse Si particles formation which is a substantial disadvantage of in-situ Al-Si composites produced by traditional manufacturing processes like casting. The coarse Si particles formation reduces Al-Si composites mechanical properties. Extremely non-equilibrium solidification conditions typical for micro LMD process allow to obtain a further structure refinement and its mechanical properties improvement. In this paper the in-situ Al-Si composites produced by micro LMD process with different parameters sets were investigated by fine structure X-ray analysis. The structure and phase composition of in-situ Al-Si composites were described. The influence of cladding parameters on Al-Si composites structure was investigated.
关键词： MMC,aluminium matrix composites,Laser micro cladding,thin-wall parts,fine structure,additive manufacturing,aluminium alloys,X-ray analysis,thin wall
更新于2025-09-12 10:27:22