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关键词： quantum dot   pump-probe   carrier capture   carrier recombination   excited-state absorption   recombination   two-photon process  

摘要： Quantum dots (QDs) allow for a significant amount of strain relaxation, which is helpful in GaN systems where a large lattice mismatch needs to be accommodated. InGaN QDs with a large indium composition are intensively investigated for light emitters requiring longer wavelengths. These are especially important for developing high-efficiency white light sources. Understanding the carrier dynamics in this large lattice-mismatched system is essential to improving the radiative efficiency while circumventing high defect density. This work investigates femtosecond carrier and photon dynamics in self-organized In0.27Ga0.73N/GaN QDs grown by molecular beam epitaxy using transient differential absorption spectroscopy, which measures the differential absorption coefficient (Delta alpha) with and without an optical pump. Due to 3D quantum confinement and the small effective mass of InGaN, the low density of states in the conduction band is easily filled with electrons. In contrast, the GaN barrier region is replete with a high density of electrons due to a large effective mass. This contrast in carrier density creates a unique phenomenon in the dynamics, showing a change in the differential absorption coefficient (Delta alpha) sign from negative to positive with time. The ultrafast microscopic processes indicate that right after the optical pump and first photon absorption, the valence (conduction) band states are depleted (replete) of electrons. This ground-state bleaching process makes Delta alpha negative, and the probe beam is not absorbed. The electrons are then gradually transferred from the GaN barrier into InGaN QDs, which absorb the second photon from the probe beam (excited-state absorption), making Delta alpha positive. The presence of excited-state carriers with a long lifetime is indicative of the enhanced availability of carriers for radiative recombination. This effect also promotes stimulated emission and amplified spontaneous emission, which can be used to de

关键词： Quartz crystal microbalances  

摘要： Despite being a fundamental tool in soft matter research and biosensing, quartz crystal microbalance (QCM) analyses of discrete macromolecules in liquids so far lack a firm theoretical basis. Quite often, acoustic signals of discrete particles are qualitatively interpreted using ad hoc frameworks based on effective electrical circuits, effective springs and trapped-solvent models with many fitting parameters. Nevertheless, due to its extreme sensitivity, the QCM technique pledges to become an accurate predictive tool. Using unsteady low Reynolds hydrodynamics we derive analytical expressions for the acoustic impedance of adsorbed discrete spheres. The present approach is successfully validated against 3D simulations and a plethora of experimental results covering more than a decade of research on proteins, viruses, liposomes, and massive nanoparticles, with sizes ranging from a few to hundreds of nanometers. The agreement without fitting parameters indicates that the acoustic response is dominated by the hydrodynamic propagation of the particle surface stress over the resonator. Understanding this leading contribution is a prerequisite for deciphering the secondary contributions arising from the relevant specific molecular and physico-chemical forces.

关键词： 自然原理   时间变慢   量子力学  

摘要： 自然是一个有自由有约束的地方,自然总是以各种现象或形式呈现出它的真容,现象是功能的体现,功能是结构的作用,结构是物质的状态,人类从诞生以来都在不停的对自然进行探索与认识,认识是认清的过程.认清自然、认清物质才是人类的最终目标.人类的进步依靠科学,科学是人类对事物与自然规律的了解与应用.总之,人类可以通过自然现象去逐步了解自然的本质,将采用通俗的语言,对时间变慢原理加以解释.

关键词： Structure of liquid perfluoro-tert-butanol   IR and Raman spectroscopies and X-ray   diffraction   Quantum DFT calculations   Molecular Dynamics simulations  

摘要： The state of aggregation of room temperature liquid perfluoro tert-butanol (TBF) has been assessed in comparison with that of its hydrogenated homologue (TBH). This study has been performed using vibrational spectroscopy (Raman scattering and infrared absorption) and X-ray diffraction, analyzed by quantum DFT calculations and Molecular Dynamics simulations. In contrast with TBH, in which large oligomers (aggregates of 4 to 6 molecules) predominate, TBF was found to mainly consist of monomers and dimers. The strongly acidic properties of this molecule, due to the electron-withdrawing effect of its CF3 groups, plays a relevant role in making it a poorly associated alcohol.

关键词： Token graph   Laplacian spectrum   Algebraic connectivity   Binomial matrix   Adjacency spectrum   Doubled odd graph   Doubled Johnson graph   Complement graph  

摘要： We study the Laplacian spectrum of token graphs, also called symmetric powers of graphs. The k-token graph F-k(G) of a graph G is the graph whose vertices are the k-subsets of vertices from G, two of which being adjacent whenever their symmetric difference is a pair of adjacent vertices in G. In this paper, we give a relationship between the Laplacian spectra of any two token graphs of a given graph. In particular, we show that, for any integers hand k such that 1 <= h <= k <= n/2, the Laplacian spectrum of F-h(G) is contained in the Laplacian spectrum of F-k(G). We also show that the doubled odd graphs and doubled Johnson graphs can be obtained as token graphs of the complete graph K-n and the star S-n = K-1,K-n-1, respectively. Besides, we obtain a relationship between the spectra of the k-token graph of G and the k-token graph of its complement (G) over bar. This generalizes to tokens graphs a well-known property stating that the Laplacian eigenvalues of G are closely related to the Laplacian eigenvalues of (G) over bar. Finally, the doubled odd graphs and doubled Johnson graphs provide two infinite families, together with some others, in which the algebraic connectivities of the original graph and its token graph coincide. Moreover, we conjecture that this is the case for any graph G and its token graph. (C) 2021 The Author(s). Published by Elsevier Inc.

关键词： Distance matrix   Distance Laplacian matrix   Spectral radius   Chromatic number  

摘要： Let G be a connected simple graph with n vertices having chromatic number chi. The distance Laplacian matrix D-L(G) is defined as D-L(G) = Diag(Tr) - D, where Diag(Tr) is the diagonal matrix of vertex transmissions and D is the distance matrix of G. The eigenvalues of D-L(G) are the distance Laplacian eigenvalues of G and are denoted by partial derivative(L)(1)(G), partial derivative(L)(2)(G), ..., partial derivative(L)(n)(G). The largest eigenvalue partial derivative(L)(1)(G) is called the distance Laplacian spectral radius. For a non-complete graph G with n vertices and chromatic number chi, Aouchiche and Hansen (2017) proved that partial derivative(L)(1)(G) >= n + inverted right perpendicular n/chi inverted left perpendicular. If G is a connected graph with n >= 4 vertices and chromatic number chi <= n - 2, we prove that partial derivative(L)(2)(G) >= n + inverted right perpendicular n/chi inverted left perpendicular and we show the existence of graphs for which the equality holds. Among all graphs with chromatic number chi satisfying n/2 <= chi <= n - 1, we show that the graph K2, 2, ..., 2}n-chi, 1, 1, ..., 1}2 chi-n has the minimum distance Laplacian spectral radius. Also, we give the distribution of the distance Laplacian eigenvalues in relation to the chromatic number chi and other graph invariants. We characterize the extremal graphs for some of these results and for others, we illustrate by examples. (C) 2021 Elsevier Inc. All rights reserved.

关键词： Extremal eigenvalues   Frustration index   Frustration number   Gain Laplacian matrix   Signed graphs  

摘要： A complex unit gain graph (T-gain graph), Phi = (G, phi) is a graph where the function phi assigns a unit complex number to each orientation of an edge of G, and its inverse is assigned to the opposite orientation. A T-gain graph Phi is balanced if the product of the edge gains of each cycle (with a fixed orientation) is 1. Signed graphs are special cases of T-gain graphs. The adjacency matrix of Phi, denoted by A(Phi) is defined canonically. The gain Laplacian for Phi is defined as L(Phi) = D(Phi) - A(Phi), where D(Phi) is the diagonal matrix with diagonal entries are the degrees of the vertices of G. The minimum number of vertices (resp., edges) to be deleted from Phi in order to get a balanced gain graph is the frustration number (resp, frustration index). We show that frustration number and frustration index are bounded below by the smallest eigenvalue of L(Phi). We provide several lower and upper bounds for extremal eigenvalues of L(Phi) in terms of different graph parameters such as the number of edges, vertex degrees, and average 2-degrees. The signed graphs are particular cases of the T-gain graphs, all the bounds established in this paper hold for signed graphs. Most of the bounds established here are new for signed graphs. Finally, we perform comparative analysis for all the obtained bounds in the paper with the state-of-the-art bounds available in the literature for randomly generated Erdos-Reyni graphs. Some of the major highlights of our paper are the gain-dependent bounds, limit convergence of the bounds to the extremal eigenvalues, and optimal extremal bounds obtained by posing optimization problems to achieve the best possible bounds. (C) 2021 Elsevier Inc. All rights reserved.

关键词： Energy   Basis sets   Chemical calculations   QM/MM   Molecules  

摘要： We propose to generalize the previously developed two-layer permuted adaptive-partitioning quantum-mechanics/molecular-mechanics (QM/MM), which reclassifies atoms as QM or MM on-the-fly in dynamics simulations, to multilayer adaptive-partitioning algorithms that enable multiple levels of theory. In this work, we formulate two new algorithms that smoothly interpolate the energy between two QM (Q1 and Q2) levels of theory. The first "permuted adaptive-partitioning" scheme is based on the weighted many-body expansion of the potential, as in the adaptive-partitioning QM/MM. Unconventional and potentially more efficient, the second "interpolated adaptive-partitioning" method employs alchemical QM calculations with Q1/Q2-mixed basis sets, Fock matrices, and overlap matrices. To our knowledge, this is the first time that such alchemical calculations are performed in QM, although they are routinely done in MM. Test calculations on water-cluster models show that both new algorithms indeed yield smooth energy curves when water molecules shift between Q1 and Q2.

关键词： Quantum statistical mechanics   Perturbation theory   Molecular dynamics   Quantum effects   Edgeworth expansion   Zero point energy   Semiclassical methods   Matsubara dynamics   Fermi resonance   Vibrational spectra  

摘要： The performance of different approximate algorithms for computing anharmonic features in vibrational spectra is analyzed and compared on model and more realistic systems that present relevant nuclear quantum effects. The methods considered combine approximate sampling of the quantum thermal distribution with classical time propagation and include Matsubara dynamics, path integral dynamics approaches, linearized initial value representation, and the recently introduced adaptive quantum thermal bath. A perturbative analysis of these different methods enables us to account for the observed numerical performance on prototypes for overtones and combination bands and to draw qualitatively correct trends for the numerical results obtained for Fermi resonances. Our results prove that the unequal performances of these approaches often derive from the method employed to sample initial conditions and not, as usually assumed, from the lack of coherence in the time propagation. Furthermore, as confirmed by the analysis reported in Benson and Althorpe, J. Chem. Phys. 130, 194510 (2021), we demonstrate, both via the perturbative approach and numerically, that path integral dynamics methods fail to reproduce the intensities of these anharmonic features and follow purely classical trends with respect to their temperature behavior. Finally, the remarkably accurate performance of the adaptive quantum thermal bath approach is documented and motivated. Published under an exclusive license by AIP Publishing.

关键词： Electron beams & optics   Lasers   Light-matter interaction   Photon statistics   Quantum optics   X-ray beams & optics  

摘要： We solve the long-time dynamics of a high-gain free-electron laser in the quantum regime. In this regime each electron emits at most one photon on average, independently of the initial field. In contrast, the variance of the photon statistics shows a qualitatively different behavior for different initial states of the field. We find that the realization of a seeded quantum free-electron laser is more feasible than self-amplified spontaneous emission.