Nonlinear transport in p-type SiGe quantum well structure containing Ge quantum dots

We use magneto-transport spectroscopy to study a dramatic instability between a low and high conductivity mode in Si/SiGe-based resonant tunneling diodes with an embedded layer of self-assembled Ge hut cluster quantum dots in the Si barrier. In the low current regime a simple activation-type behavior with an astonishingly low activation energy in the order of 0.1 meV is determined. In the high current regime a region of negative differential conduction can be observed. We discuss the influence of different layer structures and magnetic fields.     

High average power large-pitch fiber amplifier with robust single-mode operation

Ytterbium-doped large-pitch fibers with very large mode areas are investigated in a high-power fiber amplifier configuration. An average output power of 294?W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62?μm. Compared to previous active large-mode area designs, the threshold of mode instabilities is increased by a factor of about 3.     

Measuring the similarity between trajectories using clustering techniques

A clustering method has been developed to group signals that display similar dynamic behavior. The procedure involves using the method of time delay embedding to construct a trajectory in state space from a time series. Certain features that characterize the geometry of the trajectory have been defined. These features were subjected to a series of statistical tests to determine their usefulness in a hierarchical clustering analysis. The latter is aimed at finding groups of similar trajectories. The trajectory-based clustering algorithm has been applied to simulated data, which included both stochastic data generated by a linear AR model, and nonlinear data generated by a Duffing oscillator. The results show that the algorithm works reliably in both cases.     

Oscillatory melting temperature of the vortex smectic phase in layered superconductors

We report on transport measurements of YBa 2Cu 3O (7-delta) single crystals with different oxygen contents in the geometry B, J ||ab (J perpendicularB). Our data show that the vortices become confined between the Cu-O planes below a well-defined temperature at which the effective size 2xi of the vortex core is approximately equal to the period of the Cu-O layers. This confinement strongly increases the vortex liquid freezing temperature. A new melting line is found separating a vortex liquid and a smectic phase, which shows an oscillatory field dependence reflecting differences between commensurate and incommensurate smectic states.     

Quenching of excited strontium atomic state measured in H2- and CO-flames

The efficiency of resonance fluorescence, Y, of the strontium resonance line (¹P₁→¹S₀ transition) at 4607.33 Å was measured in CO/N₂O, CO/O₂/Ar, and H₂/O₂/CO₂/N₂ flames at atmospheric pressure. From these data, the specific quenching cross sections, σ_qu, for CO₂ and CO were found to be (60 ± 10) Ų and  (300 ± 60) Ų, respectively. The experimental cross sections were confronted with the intermediate ionic-state curve-crossing model and chemical quenching model, respectively.     

The orientation of porphins in n-alkane Shpolskii hosts

When a solution of a porphin in n-octane is slowly frozen the guests predominantly occupy a single site (A) in which the porphin has been shown to replace a number of host molecules displaced along the b-direction; if the solution is quickly frozen the optical spectra reveal the presence of a second site (B). In the present paper it is shown that the occurrence of these two types of sites, A and B, is a common phenomenon throughout the series from n-heptane (C₇) to n-decane (C ₁₀) as hosts and phenomenological criteria are formulated to distinguish between the two kinds of sites.

The orientation of palladiumporphin (PdP) in the A and B sites of a C₇ host is determined from Zeeman experiments on the S ₁← S ₀ and T ₀ →S ₀ transitions in a single crystal of C₇ doped with PdP. The porphin nuclei in the B sites replace a number of alkanes displaced along the a-axis. In the course of the experiments signals related to a third type of site, B', grew in intensity in the spectra. This site is coplanar with the B site and it differs from the latter by an in-plane rotation of the trapped molecule over 20°. The origins of the S ₁ ←S ₀ transition for the two types of site are assigned for H₂P, ZnP and PdP as solutes in C₇, C₈, C₉ and C₁₀ as hosts.

Finally, the Zeeman shifts observed in the S ₁ ←S ₀ transition of PdP in the three sites of C₇ yield an effective orbital angular momentum for PdP in the state S ₁ of (4·4 ± 0·1)?, in excellent agreement with previous determinations.     

On the analysis of a dynamic evolutionary algorithm

Evolutionary algorithms are applied as problem-independent optimization algorithms. They are quite efficient in many situations. However, it is difficult to analyze even the behavior of simple variants of evolutionary algorithms like the (1+1) EA on rather simple functions. Nevertheless, only the analysis of the expected run time and the success probability within a given number of steps can guide the choice of the free parameters of the algorithms. Here static (1+1) EAs with a fixed mutation probability are compared with dynamic (1+1) EAs with a simple schedule for the variation of the mutation probability. The dynamic variant is first analyzed for functions typically chosen as example-functions for evolutionary algorithms. Afterwards, it is shown that it can be essential to choose the suitable variant of the (1+1) EA. More precisely, functions are presented where each static (1+1) EA has exponential expected run time while the dynamic variant has polynomial expected run time. For other functions it is shown that the dynamic (1+1) EA has exponential expected run time while a static (1+1) EA with a good choice of the mutation probability has polynomial run time with overwhelming probability.     

Reply to the ‘Comment on “Uncommon structural and bonding properties in Ag16B4O10” by A. Lobato,Miguel Á. Salvadó, and J. Manuel Recio,Chem. Sci., 2021, 12, DOI: 10.1039/D1SC02152D

Where are the excess electrons in Ag₁₆B₄O₁₀?

Ag₁₆B₄O₁₀ features an exotic scheme of chemical bonding and extends the growing family of subvalent silver oxides. These findings constitute a new general and intrinsic facet of the chemistry of silver, which has not been fully understood, yet, and definitely deserves to be analysed from different perspectives. Against this background, we distinctly appreciate the efforts made by A. Lobato, Miguel Á. Salvadó, and J. Manuel Recio (LSR) in studying these phenomena at the example of the title compound.¹ While the computational results presented in the Comment article well comply with those published in our original paper,² the interpretations follow different routes. Whereas LSR focus on the analogy of pattern of the Electron Localization Function (ELF) in position space in the title compound with those found in elemental silver, we interpreted the electronic structure of Ag₁₆B₄O₁₀, both in position and reciprocal space, also considering the interactions between cationic and anionic partial structures.     

On the equivalence of ring-coupled cluster and adiabatic connection fluctuation-dissipation theorem random phase approximation correlation energy expressions

The correlation energy in the direct random phase approximation (dRPA) can be written, among other possibilities, either in terms of the interaction strength averaged correlation density matrix, or in terms of the coupled cluster doubles amplitudes obtained in the direct ring approximation (drCCD). Although the corresponding dRPA correlation density matrix on the one hand, and the drCCD amplitude matrix on the other hand, differ significantly, they yield identical energies. Similarly, the analogous RPA and rCCD correlation energies calculated from antisymmetrized two-electron integrals are identical to each other despite very different underlying working equations. In the present communication, a direct correspondence between amplitudes and densities is established and investigated with perturbation theory arguments. Our analysis also sheds some light on the properties of recently proposed RPA/rCCD variants which use antisymmetrized integrals in part of the equations and nonantisymmetrized integrals in others.     

What is the role of acid–acid interactions in asymmetric phosphoric acid organocatalysis? A detailed mechanistic study using interlocked and non-interlocked catalysts

Organocatalysis has revolutionized asymmetric synthesis. However, the supramolecular interactions of organocatalysts in solution are often neglected, although the formation of catalyst aggregates can have a strong impact on the catalytic reaction. For phosphoric acid based organocatalysts, we have now established that catalyst–catalyst interactions can be suppressed by using macrocyclic catalysts, which react predominantly in a monomeric fashion, while they can be favored by integration into a bifunctional catenane, which reacts mainly as phosphoric acid dimers. For acyclic phosphoric acids, we found a strongly concentration dependent behavior, involving both monomeric and dimeric catalytic pathways. Based on a detailed experimental analysis, DFT-calculations and direct NMR-based observation of the catalyst aggregates, we could demonstrate that intermolecular acid–acid interactions have a drastic influence on the reaction rate and stereoselectivity of asymmetric transfer-hydrogenation catalyzed by chiral phosphoric acids.

Supramolecular acid–acid interactions lead to competing monomeric and dimeric pathways in phosphoric acid catalysis – so that stereoselectivities depend on catalyst concentration.