Raman solitons in transient SRS

We report the observation of Raman solitons on numerical simulations of transient stimulated Raman scattering (TSRS) with small group velocity dispersion. The theory proceeds with the inverse scattering transform (IST) for initial-boundary value problems and it is shown that the explicit theoretical solution obtained by IST for a semi-infinite medium fits strikingly well the numerical solution for a finite medium. We understand this from the rapid decrease of the medium dynamical variable (the potential of the scattering theory). The spectral transform reflection coefficient can be computed directly from the values of the input and output fields and this allows to see the generation of the Raman solitons from the numerical solution. We confirm the presence of these nonlinear modes in the medium dynamical variable by the use of a discrete spectral analysis.Comment: LaTex file, to appear in Inverse Problem

Solubility of precursors and carbonation of waterglass-free geopolymers

Geopolymers have the potential to function as an environmentally friendly substitute for ordinary Portland cement, with up to 80% less CO$_{2}$ emission during production. The effect is best utilized for geopolymers prepared with amorphous silica instead of waterglass (Na$_{2x}$Si$_{y}$O$_{2y+x}$) to adjust the Si:Al ratio. The reactivity of the precursors with the alkaline activator affects the final mineralogical properties of the binder. The purpose of the present study was to investigate the amount of different phases formed during geopolymerization and to understand the quantitative evolution of carbonation during geopolymer synthesis by determining the solubility of metakaolinite and amorphous SiO$_{2}$ in NaOH at various concentrations. The solubility was studied by ICP-OES measurements. X-ray diffraction was used for qualitative and quantitative phase analysis of the geopolymers. The solubility of the precursors increased with calcination temperature of metakaolinite, reaction time for amorphous SiO$_{2}$, and at higher NaOH concentrations. Partial dissolution resulted in free Na$^{+}$, which is a source for the formation of carbonates in the geopolymers. Thermonatrite occurred prior to trona formation in all samples

Intermediate phase, network demixing, boson and floppy modes, and compositional trends in glass transition temperatures of binary AsxS1-x system

The structure of binary As_xS_{1-x} glasses is elucidated using modulated-DSC, Raman scattering, IR reflectance and molar volume experiments over a wide range (8%<x<41%) of compositions. We observe a reversibility window in the calorimetric experiments, which permits fixing the three elastic phases; flexible at x<22.5%, intermediate phase (IP) in the 22.5%<x<29.5% range, and stressed-rigid at x>29.5%. Raman scattering supported by first principles cluster calculations reveal existence of both pyramidal (PYR, As(S1/2)3) and quasi-tetrahedral(QT, S=As(S1/2)3) local structures. The QT unit concentrations show a global maximum in the IP, while the concentration of PYR units becomes comparable to those of QT units in the phase, suggesting that both these local structures contribute to the width of the IP. The IP centroid in the sulfides is significantly shifted to lower As content x than in corresponding selenides, a feature identified with excess chalcogen partially segregating from the backbone in the sulfides, but forming part of the backbone in selenides. These ideas are corroborated by the proportionately larger free volumes of sulfides than selenides, and the absence of chemical bond strength scaling of Tgs between As-sulfides and As-selenides. Low-frequency Raman modes increase in scattering strength linearly as As content x of glasses decreases from x = 20% to 8%, with a slope that is close to the floppy mode fraction in flexible glasses predicted by rigidity theory. These results show that floppy modes contribute to the excess vibrations observed at low frequency. In the intermediate and stressed rigid elastic phases low-frequency Raman modes persist and are identified as boson modes. Some consequences of the present findings on the optoelectronic properties of these glasses is commented upon.Comment: Accepted for PR

Magnetoelastic nature of solid oxygen epsilon-phase structure

For a long time a crystal structure of high-pressure epsilon-phase of solid oxygen was a mistery. Basing on the results of recent experiments that have solved this riddle we show that the magnetic and crystal structure of epsilon-phase can be explained by strong exchange interactions of antiferromagnetic nature. The singlet state implemented on quaters of O2 molecules has the minimal exchange energy if compared to other possible singlet states (dimers, trimers). Magnetoelastic forces that arise from the spatial dependence of the exchange integral give rise to transformation of 4(O2) rhombuses into the almost regular quadrates. Antiferromagnetic character of the exchange interactions stabilizes distortion of crystal lattice in epsilon-phase and impedes such a distortion in long-range alpha- and delta-phases.Comment: 11 pages, 4 figures, Changes: corrected typos, reference to the recent paper is adde

Theory of Pump Depletion and Spike Formation in Stimulated Raman Scattering

By using the inverse spectral transform, the SRS equations are solved and the explicit output data is given for arbitrary laser pump and Stokes seed profiles injected on a vacuum of optical phonons. For long duration laser pulses, this solution is modified such as to take into account the damping rate of the optical phonon wave. This model is used to interprete the experiments of Druhl, Wenzel and Carlsten (Phys. Rev. Lett., (1983) vol. 51, p. 1171), in particular the creation of a spike of (anomalous) pump radiation. The related nonlinear Fourier spectrum does not contain discrete eigenvalue, hence this Raman spike is not a soliton.Comment: LaTex file, includes two figures in LaTex format, 9 page

Existence of superposition solutions for pulse propagation in nonlinear resonant media

Existence of self-similar, superposed pulse-train solutions of the nonlinear, coupled Maxwell-Schr\"odinger equations, with the frequencies controlled by the oscillator strengths of the transitions, is established. Some of these excitations are specific to the resonant media, with energy levels in the configurations of $\Lambda$ and $N$ and arise because of the interference effects of cnoidal waves, as evidenced from some recently discovered identities involving the Jacobian elliptic functions. Interestingly, these excitations also admit a dual interpretation as single pulse-trains, with widely different amplitudes, which can lead to substantially different field intensities and population densities in different atomic levels.Comment: 11 Pages, 6 Figures, presentation changed and 3 figures adde

P35. Intratumoral patterns of clonal evolution in meningiomas

Núm. a Art Públic: 1815Digitalitzat per Tecnodo

A Quantum Broadcasting Problem in Classical Low Power Signal Processing

We pose a problem called ``broadcasting Holevo-information'': given an unknown state taken from an ensemble, the task is to generate a bipartite state transfering as much Holevo-information to each copy as possible. We argue that upper bounds on the average information over both copies imply lower bounds on the quantum capacity required to send the ensemble without information loss. This is because a channel with zero quantum capacity has a unitary extension transfering at least as much information to its environment as it transfers to the output. For an ensemble being the time orbit of a pure state under a Hamiltonian evolution, we derive such a bound on the required quantum capacity in terms of properties of the input and output energy distribution. Moreover, we discuss relations between the broadcasting problem and entropy power inequalities. The broadcasting problem arises when a signal should be transmitted by a time-invariant device such that the outgoing signal has the same timing information as the incoming signal had. Based on previous results we argue that this establishes a link between quantum information theory and the theory of low power computing because the loss of timing information implies loss of free energy.Comment: 28 pages, late

The lesson of causal discovery algorithms for quantum correlations: Causal explanations of Bell-inequality violations require fine-tuning

An active area of research in the fields of machine learning and statistics is the development of causal discovery algorithms, the purpose of which is to infer the causal relations that hold among a set of variables from the correlations that these exhibit. We apply some of these algorithms to the correlations that arise for entangled quantum systems. We show that they cannot distinguish correlations that satisfy Bell inequalities from correlations that violate Bell inequalities, and consequently that they cannot do justice to the challenges of explaining certain quantum correlations causally. Nonetheless, by adapting the conceptual tools of causal inference, we can show that any attempt to provide a causal explanation of nonsignalling correlations that violate a Bell inequality must contradict a core principle of these algorithms, namely, that an observed statistical independence between variables should not be explained by fine-tuning of the causal parameters. In particular, we demonstrate the need for such fine-tuning for most of the causal mechanisms that have been proposed to underlie Bell correlations, including superluminal causal influences, superdeterminism (that is, a denial of freedom of choice of settings), and retrocausal influences which do not introduce causal cycles.Comment: 29 pages, 28 figs. New in v2: a section presenting in detail our characterization of Bell's theorem as a contradiction arising from (i) the framework of causal models, (ii) the principle of no fine-tuning, and (iii) certain operational features of quantum theory; a section explaining why a denial of hidden variables affords even fewer opportunities for causal explanations of quantum correlation