Ising instability of a Holstein phonon mode in graphene

We study the thermal distribution of phonons in a graphene sheet. Due to the two electronic bands there are two out-of-plane phonon modes with respect to the two sublattices. One of these modes undergoes an Ising transition by spontaneously breaking the sublattice symmetry. We calculate the critical point, the renormalization of the phonon frequency and the average lattice distortion. This transition might be observable in Raman scattering and in transport properties.Comment: 5 pages, 2 figure

Inelastic scattering of atoms in a double well

We study a mixture of two light spin-1/2 fermionic atoms and two heavy atoms %in a Mott state in a double well potential. Inelastic scattering processes between both atomic species excite the heavy atoms and renormalize the tunneling rate and the interaction of the light atoms (polaron effect). The effective interaction of the light atoms changes its sign and becomes attractive for strong inelastic scattering. This is accompanied by a crossing of the energy levels from singly occupied sites at weak inelastic scattering to a doubly occupied and an empty site for stronger inelastic scattering. We are able to identify the polaron effect and the level crossing in the quantum dynamics.Comment: 12 pages, 9 figure

Internal kinematics of modelled interacting disc galaxies

We present an investigation of galaxy-galaxy interactions and their effects on the velocity fields of disc galaxies in combined N-body/hydrodynamic simulations, which include cooling, star formation with feedback, and galactic winds. Rotation curves (RCs) of the gas are extracted from these simulations in a way that follows the procedure applied to observations of distant, small, and faint galaxies as closely as possible. We show that galaxy-galaxy mergers and fly-bys disturb the velocity fields significantly and hence the RCs of the interacting galaxies, leading to asymmetries and distortions in the RCs. Typical features of disturbed kinematics are significantly rising or falling profiles in the direction of the companion galaxy and pronounced bumps in the RCs. In addition, tidal tails can leave strong imprints on the rotation curve. All these features are observable for intermediate redshift galaxies, on which we focus our investigations. We use a quantitative measure for the asymmetry of rotation curves to show that the appearance of these distortions strongly depends on the viewing angle. We also find in this way that the velocity fields settle back into relatively undisturbed equilibrium states after unequal mass mergers and fly-bys. About 1 Gyr after the first encounter, the RCs show no severe distortions anymore. These results are consistent with previous theoretical and observational studies. As an illustration of our results, we compare our simulated velocity fields and direct images with rotation curves from VLT/FORS spectroscopy and ACS images of a cluster at z=0.53 and find remarkable similarities.Comment: 13 pages, 14 figures, accepted for publication in A&A, some improvements and changes, main conclusions are unaffecte

Very high energy emission from the hard spectrum sources HESS J1641-463, HESS J1741-302 and HESS J1826-130

A recent study of the diffuse $\gamma$-ray emission in the Central Molecular Zone using very high energy (VHE, E $>$ 0.1 TeV) H.E.S.S. data suggests that the Galactic Center (GC) is the most plausible supplier of Galactic ultra-relativistic cosmic-rays (CRs) up to the knee at about 10$^{15}$ eV (PeV). However, the GC might not be the only source capable to accelerate CRs up to PeV energies in the Galaxy. Here we present H.E.S.S. data analysis results and interpretation of three H.E.S.S. sources, with spectra extending beyond 10 TeV and relatively hard spectral indices compared with the average spectral index of H.E.S.S. sources, namely HESS J1641-463, HESS J1741-302 and HESS J1826-130. Although the nature of these VHE $\gamma$-ray sources is still open, their spectra suggest that the astrophysical objects producing such emission must be capable of accelerating the parental particle population up to energies of at least several hundreds of TeV. Assuming a hadronic scenario, dense gas regions can provide rich target material for accelerated particles to produce VHE $\gamma$-ray emission via proton-proton interactions followed by a subsequent $\pi^{0}$ decay. Thus, detailed investigations of the interstellar medium along the line of sight to all of these sources have been performed by using data from available atomic and molecular hydrogen surveys. The results point out the existence of dense interstellar gas structures coincident with the best fit positions of these sources. One can find possible hadronic models with CRs being accelerated close to the PeV energies to explain the $\gamma$-ray emission from all of these sources, which opens up the possibility that a population of PeV CR accelerators might be active in the Galaxy.Comment: 8 pages, 2 figures, in Proceedings of 35th ICRC, Busan (Korea) 201

HESS J1826−-130: A Very Hard γ\gamma-Ray Spectrum Source in the Galactic Plane

HESS J1826$-$130 is an unidentified hard spectrum source discovered by H.E.S.S. along the Galactic plane, the spectral index being $\Gamma$ = 1.6 with an exponential cut-off at about 12 TeV. While the source does not have a clear counterpart at longer wavelengths, the very hard spectrum emission at TeV energies implies that electrons or protons accelerated up to several hundreds of TeV are responsible for the emission. In the hadronic case, the VHE emission can be produced by runaway cosmic-rays colliding with the dense molecular clouds spatially coincident with the H.E.S.S. source.Comment: 6 pages, 3 figures, Proceedings of the 6th International Symposium on High Energy Gamma-Ray Astronomy (Gamma2016), Heidelberg, German

Re-examining rates of lithium-ion battery technology improvement and cost decline

Lithium-ion technologies are increasingly employed to electrify transportation and provide stationary energy storage for electrical grids, and as such their development has garnered much attention. However, their deployment is still relatively limited, and their broader adoption will depend on their potential for cost reduction and performance improvement. Understanding this potential can inform critical climate change mitigation strategies, including public policies and technology development efforts. However, many existing models of past cost decline, which often serve as starting points for forecasting models, rely on limited data series and measures of technological progress. Here we systematically collect, harmonize, and combine various data series of price, market size, research and development, and performance of lithium-ion technologies. We then develop representative series for these measures and employ performance curve models to estimate improvement rates. We also develop a method to incorporate additional performance characteristics into these models, including energy density and specific energy performance metrics. When energy density is incorporated into the definition of service provided by a lithium-ion cell, estimated technological improvement rates increase considerably, suggesting that previously reported improvement rates might underestimate the rate of lithium-ion technologies' change. Moreover, our estimates suggest the degree to which lithium-ion technologies' price decline might have been limited by performance requirements other than cost per energy capacity. These rates also suggest that battery technologies developed for stationary applications, where restrictions on volume and mass are relaxed, might achieve faster cost declines, though engineering-based mechanistic cost modeling is required to further characterize this potential.Comment: 37 pages, 11 figure

Frequency splitting of intervalley phonons in graphene

We study the thermal distribution of intervalley phonons in a graphene sheet. These phonons have two components with the same frequency. The degeneracy of the two modes is preserved by weak electron-phonon coupling. A sufficiently strong electron-phonon coupling, however, can result in a splitting into an optical and an acoustic phonon branch, which creates a fluctuating gap in the electronic spectrum. We describe these effects by treating the phonon distribution within a saddle-point approximation. Fluctuations around the saddle point indicate a Berezinskii-Kosterlitz-Thouless transition of the acoustic branch. This transition might be observable in the polarization of Raman scattered light.Comment: 5 pages, 1 figur

Polytopality and Cartesian products of graphs

We study the question of polytopality of graphs: when is a given graph the graph of a polytope? We first review the known necessary conditions for a graph to be polytopal, and we provide several families of graphs which satisfy all these conditions, but which nonetheless are not graphs of polytopes. Our main contribution concerns the polytopality of Cartesian products of non-polytopal graphs. On the one hand, we show that products of simple polytopes are the only simple polytopes whose graph is a product. On the other hand, we provide a general method to construct (non-simple) polytopal products whose factors are not polytopal.Comment: 21 pages, 10 figure

Bose-Einstein Condensation in a Trap: the Case of a Dense Condensate

We consider the Bose-Einstein condensation of atoms in a trap where the density of particles is so high that the low density approach of Gross and Pitaevskii will not be applicable. For this purpose we use the slave boson representation which is valid for hard-core bosons at any density. This description leads to the same results as the Gross-Pitaevskii approach in the low density limit, but for higher densities, it predicts the depletion of the order parameter field condensate in the regions where the density of the atomic cloud is high.Comment: 6 pages RevTeX, 3 eps-figure