On k-stacked polytopes

AbstractIt is proved that equality in the Generalized Simplicial Lower Bound Conjecture can always be obtained by k-stacked polytopes

Generalised BPS conditions

We write down two E11 invariant conditions which at low levels reproduce the known half BPS conditions for type II theories. These new conditions contain, in addition to the familiar central charges, an infinite number of further charges which are required in an underlying theory of strings and branes. We comment on the application of this work to higher derivative string corrections

Generalised space-time and duality

In this paper we consider the previously proposed generalised space-time and investigate the structure of the field theory upon which it is based. In particular, we derive a SO(D,D) formulation of the bosonic string as a non-linear realisation at lowest levels of the semi-direct product of E11 with the first fundamental representation l1. We give a Hamiltonian formulation of this theory and carry out its quantisation. We argue that the choice of representation of the quantum theory breaks the manifest SO(D,D) symmetry but that the symmetry is manifest in a non-commutative field theory. We discuss the implications for the conjectured E11 symmetry and the role of the l1 representation.Comment: 17 pages, a few typographical errors correcte

Optical in situ monitoring of hydrogen desorption from Ge(100) surfaces

Molecular hydrogen strongly interacts with vicinal Ge(100) surfaces during preparation in a metal organic vapor phase epitaxy reactor. According to X-ray photoemission spectroscopy and Fourier-transform infrared spectroscopy results, we identify two characteristic reflection anisotropy (RA) spectra for H-free and monohydride-terminated vicinal Ge(100) surfaces. RAS allows in situ monitoring of the surface termination and enables spectroscopic hydrogen kinetic desorption studies on the Ge(100) surface. Comparison of evaluated values for the activation energy and the pre-exponential factor of H desorption evaluated at different photon energies reflects that H unevenly affects the shape of the RA spectrum

MMP13 mediates cell cycle progression in melanocytes and melanoma cells: in vitro studies of migration and proliferation

Contains fulltext : 87376.pdf (publisher's version ) (Open Access)BACKGROUND: Melanoma cells are usually characterized by a strong proliferative potential and efficient invasive migration. Among the multiple molecular changes that are recorded during progression of this disease, aberrant activation of receptor tyrosine kinases (RTK) is often observed. Activation of matrix metalloproteases goes along with RTK activation and usually enhances RTK-driven migration. The purpose of this study was to examine RTK-driven three-dimensional migration of melanocytes and the pro-tumorigenic role of matrix metalloproteases for melanocytes and melanoma cells. RESULTS: Using experimental melanocyte dedifferentiation as a model for early melanomagenesis we show that an activated EGF receptor variant potentiates migration through three-dimensional fibrillar collagen. EGFR stimulation also resulted in a strong induction of matrix metalloproteases in a MAPK-dependent manner. However, neither MAPK nor MMP activity were required for migration, as the cells migrated in an entirely amoeboid mode. Instead, MMPs fulfilled a function in cell cycle regulation, as their inhibition resulted in strong growth inhibition of melanocytes. The same effect was observed in the human melanoma cell line A375 after stimulation with FCS. Using sh- and siRNA techniques, we could show that MMP13 is the protease responsible for this effect. Along with decreased proliferation, knockdown of MMP13 strongly enhanced pigmentation of melanocytes. CONCLUSIONS: Our data show for the first time that growth stimuli are mediated via MMP13 in melanocytes and melanoma, suggesting an autocrine MMP13-driven loop. Given that MMP13-specific inhibitors are already developed, these results support the evaluation of these inhibitors in the treatment of melanoma

MOVPE growth of GaP/GaPN core-shell nanowires: N incorporation, morphology and crystal structure

Dilute nitride III-V nanowires (NWs) possess great potential as building blocks in future optoelectronical and electrochemical devices. Here, we provide evidence for the growth of GaP/GaPN core-shell NWs via metalorganic vapor phase epitaxy, both on GaP(111)B and on GaP/Si (111) hetero-substrates. The NW morphology meets the common needs for use in applications, i.e. they are straight and vertically oriented to the substrate as well as homogeneous in length. Moreover, no parasitical island growth is observed. Nitrogen was found to be incorporated on group V sites as determined from transmission electron microscopy (TEM) and Raman spectroscopy. Together with the incorporation of N, the NWs exhibit strong photoluminescence in the visible range, which we attribute to radiative recombination at N-related deep states. Independently of the N incorporation, a peculiar facet formation was found, with {110} facets at the top and {112} at the bottom of the NWs. TEM reveals that this phenomenon is related to different stacking fault densities within the zinc blende structure, which lead to different effective surface energies for the bottom and the top of the NWs.This work was supported by the Deutsche Forschungsgemeinschaft (DFG, proj. no. HA 3096/4-2 & DA 396/6-2). We thank D Roßberg and D Flock for preparation of the TEM lamellae via FIB, as well as A Müller for technical support of the MOVPE system and W Dziony for AES measurements. We appreciate fruitful discussions with A Paszuk and A Nägelein

In situ control of As dimer orientation on Ge(100) surfaces

We investigated the preparation of single domain Ge(100):As surfaces in a metal-organic vapor phase epitaxy reactor. In situ reflection anisotropy spectra (RAS) of vicinal substrates change when arsenic is supplied either by tertiarybutylarsine or by background As4 during annealing. Low energy electron diffraction shows mutually perpendicular orientations of dimers, scanning tunneling microscopy reveals distinct differences in the step structure, and x-ray photoelectron spectroscopy confirms differences in the As coverage of the Ge(100): As samples. Their RAS signals consist of contributions related to As dimer orientation and to step structure, enabling precise in situ control over preparation of single domain Ge(100): As surfaces

Impact of Rotational Twin Boundaries and Lattice Mismatch on III-V Nanowire Growth

Pseudomorphic planar III-V transition layers greatly facilitate the epitaxial integration of vapor liquid solid grown III-V nanowires (NW) on Si(111) substrates. Heteroepitaxial (111) layer growth, however, is commonly accompanied by the formation of rotational twins. We find that rotational twin boundaries (RTBs), which intersect the surface of GaP/Si(111) heterosubstrates, generally cause horizontal NW growth and may even suppress NW growth entirely. Away from RTBs, the NW growth direction switches from horizontal to vertical in the case of homoepitaxial GaP NWs, whereas heteroepitaxial GaAs NWs continue growing horizontally. To understand this rich phenomenology, we develop a model based on classical nucleation theory. Independent of the occurrence of RTBs and specific transition layers, our model can generally explain the prevalent observation of horizontal III V NW growth-in lattice mismatched systems and the high crystal quality of horizontal nanowires.This work was financially supported by the BMBF (Project No. 03SF0404A) and partly by the Spanish Ministry of Economy (Project TEC2014-54260-C3-2-P). C.K. and L.W. acknowledge the Thuringia Graduate School for Photovoltaics “Photograd” for financial support. The authors would like to thank A. Paszuk and A. Nagelein for valuable discussions as well as A. Muller and M. Biester for technical support, T. Nieszner for supporting the determination of the spatial direction of NWs, and D. Roßberg for preparing the TEM lamella