153,644 research outputs found
Gauge coupling unification in E 6 F-theory GUTs with matter and bulk exotics from flux breaking
No full textWe consider gauge coupling unification in E 6 F-Theory Grand Unified Theories (GUTs) where E 6 is broken to the Standard Model (SM) gauge group using fluxes. In such models there are two types of exotics that can affect gauge coupling unification, namely matter exotics from the matter curves in the 27 dimensional representation of E 6 and the bulk exotics from the adjoint 78 dimensional representation of E 6. We explore the conditions required for either the complete or partial removal of bulk exotics from the low energy spectrum. In the latter case we shall show that (miraculously) gauge coupling unification may be possible even if there are bulk exotics at the TeV scale. Indeed in some cases it is necessary for bulk exotics to survive to the TeV scale in order to cancel the effects coming from other TeV scale matter exotics which would by themselves spoil gauge coupling unification. The combination of matter and bulk exotics in these cases can lead to precise gauge coupling unification which would not be possible with either type of exotics considered by themselves. The combination of matter and bulk exotics at the TeV scale represents a unique and striking signature of E 6 F-theory GUTs that can be tested at the LHC
Small asymmetric Brownian objects self-align in nanofluidic channels
No full textAlthough the self-alignment of asymmetric macro-sized objects of a few tens of microns in size have been studied extensively in experiments and theory, access to much smaller length scales is still hindered by technical challenges. We combine molecular dynamics and stochastic rotation dynamics techniques to investigate the self-orientation phenomenon at different length scales, ranging from the micron to the nano scale by progressively increasing the relative strength of diffusion over convection. To this end, we model an asymmetric dumbbell particle in Hele-Shaw flow and explore a wide range of Péclet numbers (Pe) and different particle shapes, as characterized by the size ratio of the two dumbbell spheres (R). By independently varying these two parameters we analyse the process of self-orientation and characterize the alignment of the dumbbell with the direction of the fluid flow. We identify three different regimes of strong, weak and no alignment and we map out a state diagram in Pe versus R plane. Based on these results, we estimate dimensional length scales and flow rates for which these findings would be applicable in experiments. Finally, we find that the characteristic reorientation time of the dumbbell is a monotonically decreasing function of the dumbbell anisotropy.Accepted Author ManuscriptComplex Fluid Processin
Constraining Dark Matter through CMB
No full textSelf-Annihilating Dark Matter (DM) candidates can modify the recombination history of the Universe, injecting additional energy into the thermal gas, and modifying its ionization state. The electron fraction history affects the CMB temperature and polarization power spectra; observations of the modifications (or of the absence) of the latter can then be used to constrain the power injected by annihilating DM, and ultimately the DM parameters themselves. DM annihilations able to modify the CMB spectra are active at redshifts 100 ×z× 1000, thus involving only a smooth density field, and permitting to ignore structure formation. Current WMAP7 data on the TT, TE and EE angular power spectra already permit to rule out interesting regions in the 〈σv〉 -mx space; the forthcoming PLANCK ones will permit to explore the "thermal WIMP" region. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution- NonCommercial-ShareAlike Licence
Semiclassical magnetotransport in strongly spin-orbit coupled Rashba two-dimensional electron systems
No full textSemiclassical magnetoelectric and magnetothermoelectric transport in strongly spin-orbit coupled Rashba two-dimensional electron systems is investigated. In the presence of a perpendicular classically weak magnetic field and short-range impurity scattering, we solve the linearized Boltzmann equation self-consistently. Using the solution, it is found that when Fermi energy EF locates below the band crossing point (BCP), the Hall coefficient is a nonmonotonic function of electron density ne and not inversely proportional to ne. While the magnetoresistance (MR) and Nernst coefficient vanish when EF locates above the BCP, non-zero MR and enhanced Nernst coefficient emerge when EF decreases below the BCP. Both of them are nonmonotonic functions of EF below the BCP. The different semiclassical magnetotransport behaviors between the two sides of the BCP can be helpful to experimental identifications of the band valley regime and topological change of Fermi surface in considered systems.National Natural Science Foundation of China [11274018]SCI(E)[email protected]
Unified Dark Matter models with fast transition
No full textWe investigate the general properties of Unified Dark Matter (UDM) fluid models where the pressure and the energy density are linked by a barotropic equation of state (EoS) p = p(ρ) and the perturbations are adiabatic. The EoS is assumed to admit a future attractor that acts as an effective cosmological constant, while asymptotically in the past the pressure is negligible. UDM models of the dark sector are appealing because they evade the so-called ``coincidence problem'' and ``predict'' what can be interpreted as wDE ≈ −1, but in general suffer the effects of a non-negligible Jeans scale that wreak havoc in the evolution of perturbations, causing a large Integrated Sachs-Wolfe effect and/or changing structure formation at small scales. Typically, observational constraints are violated, unless the parameters of the UDM model are tuned to make it indistinguishable from ΛCDM. Here we show how this problem can be avoided, studying in detail the functional form of the Jeans scale in adiabatic UDM perturbations and introducing a class of models with a fast transition between an early Einstein-de Sitter CDM-like era and a later ΛCDM-like phase. If the transition is fast enough, these models may exhibit satisfactory structure formation and CMB fluctuations. To consider a concrete case, we introduce a toy UDM model and show that it can predict CMB and matter power spectra that are in agreement with observations for a wide range of parameter values
The d-f luminescence of Eu2+, Ce3+ and Yb2+ ions in Cs2MP2O7 (M = Ca2+, Sr2+)
No full textThe efficient narrow band emission of Eu2+ in Cs2MP2O7 (M = Ca2+, Sr2+) is characterized by a large Stokes shift and a high quenching temperature which makes the material promising for application in warm white LEDs. The unusual Eu2+ luminescence properties were reported recently but an explanation for the peculiar behavior is lacking. In this paper we aim at providing new insights in the luminescence of the Eu2+ emission in Cs2MP2O7 through measurements at cryogenic temperatures (down to 4 K) and by comparison with the d-f luminescence of Ce3+ and Yb2+ in the same host. The results reveal a sharp onset of the Eu2+ emission and excitation bands at 4 K. Usually the sharp onset for narrow excitation and emission bands coincide at an energy corresponding to the zero-phonon (purely electronic) transition, but for Eu2+ in Cs2MP2O7 there is a large shift of 3500 cm-1 between the onsets, consistent with the large Stokes shift observed. The onset shift can be explained by emission from a lower energy distorted excited 4f65d1 state. For Ce3+, the f-d absorption bands are at energies expected based on the relation between the absorption energies for Eu2+ and Ce3+ reported by Dorenbos. Contrary to Eu2+, the emission for Ce3+ shows a normal Stokes shift and therefore the emission bands are at much higher energies than predicted from the energy of the Eu2+ emission and the Dorenbos relations. Based on the present results the unusually large Stokes shift for the Eu2+ emission in Cs2MP2O7 is assigned to a Jahn-Teller like deformation in the excited 4f65d1 state of Eu2+ that is not present in the 5d state of Ce3+
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