Isotopic composition and concentration measurements of atmospheric CO2 with a diode laser making use of correlations between non-equivalent absorption cells

Correlations between a sample and a sealed reference cell of a tunable diode laser spectrometer for the measurement of the isotopic composition (δ ¹³C) and the concentration of atmospheric carbon dioxide in air have been investigated. Likely due to fluctuations of the laser emission profile, these correlations have been used to improve the performance of the instrument. In a comparison with isotope ratio mass spectrometer and gas chromatographic measurements, an accuracy of 0.15‰ for δ ¹³C and 0.05 ppmv for the CO₂ concentration is demonstrated for 40 s integration time. Long-term stability and field deployment of the instrument have been investigated during a few days measurement campaign in Paris.     

Collective shape oscillations of SiGe islands on pit-patterned Si(001) substrates: a coherent-growth strategy enabled by self-regulated intermixing

The shape of coherent SiGe islands epitaxially grown on pit-patterned Si(001) substrates displays very uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower "dome" and steeper "barn" morphologies. Correspondingly, the average Ge content in the alloyed islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the islands to keep growing in size while avoiding plastic relaxation.     

Ultrafast magnon generation in an Fe film on Cu(100)

We report on a combined experimental and theoretical study of the spin-dependent relaxation processes in the electron system of an iron film on Cu(100). Spin-, time-, energy- and angle-resolved two-photon photoemission shows a strong characteristic dependence of the lifetime of photoexcited electrons on their spin and energy. Ab?initio calculations as well as a many-body treatment corroborate that the observed properties are determined by relaxation processes involving magnon emission. Thereby we demonstrate that magnon emission by hot electrons occurs on the femtosecond time scale and thus provides a significant source of ultrafast spin-flip processes. Furthermore, engineering of the magnon spectrum paves the way for tuning the dynamic properties of magnetic materials.     

Ions colliding with cold polycyclic aromatic hydrocarbon clusters

We report the first experimental study of ions interacting with clusters of polycyclic aromatic hydrocarbon (PAH) molecules. Collisions between 11.25 keV 3He+ or 360 keV 129Xe20+ and weakly bound clusters of one of the smallest PAH molecules, anthracene, show that C14H10 clusters have much higher tendencies to fragment in ion collisions than other weakly bound clusters. The ionization is dominated by peripheral collisions in which the clusters, very surprisingly, are more strongly heated by Xe20+ collisions than by He+ collisions. The appearance size is k=15 for [C ??H??](k)2+.     

GaN/LiNbO3 (0 0 0 1) interface formation calculated from first-principles

The stable adsorption sites for both Ga and N ions on the ideal and on the reconstructed LiNbO₃ (0 0 0 1) surface are determined by means of first-principle total energy calculations. A single N layer is found to be more strongly bound to the substrate than a single Ga layer. The adsorption of a GaN monolayer on the polar substrate within different orientations is then modeled. On the basis of our results, we propose a microscopic model for the GaN/LiNbO₃ interface. The GaN and LiNbO₃ (0 0 0 1) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [1 0 0]_GaN‖ [1 1  0]_LiNbO3. In this way the (0 0 0 1) plane lattice mismatch between GaN and LiNbO₃ is minimal and equal to 6.9% of the GaN lattice constant. The adsorbed GaN and the underlying LiNbO₃ substrate have parallel c-axes.     

A note on the surjectivity of operators on vector bundles over discrete spaces

In this note we give a short and self-contained proof for a criterion of Eidelheit on the solvability of linear equations in infinitely many variables. We use this criterion to study the surjectivity of magnetic Schrödinger operators on bundles over graphs.     

On the number of inequivalent Gabidulin codes

Maximum rank-distance (MRD) codes are extremal codes in the space of \(m\times n\) matrices over a finite field, equipped with the rank metric. Up to generalizations, the classical examples of such codes were constructed in the 1970s and are today known as Gabidulin codes. Motivated by several recent approaches to construct MRD codes that are inequivalent to Gabidulin codes, we study the equivalence issue for Gabidulin codes themselves. This shows in particular that the family of Gabidulin codes already contains a huge subset of MRD codes that are pairwise inequivalent, provided that \(2\leqslant m\leqslant n-2\).     

On Lander’s conjecture for difference sets whose order is a power of 2 or 3

Let p be a prime and let b be a positive integer. If a (v, k, λ, n) difference set D of order n = p ^b exists in an abelian group with cyclic Sylow p-subgroup S, then \({p\in\{2,3\}}\) and |S| = p. Furthermore, either p = 2 and v ≡ λ ≡ 2 (mod 4) or the parameters of D belong to one of four families explicitly determined in our main theorem.     

Codimension-two critical behavior in vacuum gravitational collapse

We consider the critical behavior at the threshold of black-hole formation for the five-dimensional vacuum Einstein equations satisfying the cohomogeneity-two triaxial Bianchi type-IX ansatz. Exploiting a discrete symmetry present in this model we predict the existence of a codimension-two attractor. This prediction is confirmed numerically and the codimension-two attractor is identified as a discretely self-similar solution with two unstable modes.