Ab initio calculation of the Hoyle state

The Hoyle state plays a crucial role in the hydrogen burning of stars heavier than our sun and in the production of carbon and other elements necessary for life. This excited state of the carbon-12 nucleus was postulated by Hoyle [1] as a necessary ingredient for the fusion of three alpha particles to produce carbon at stellar temperatures. Although the Hoyle state was seen experimentally more than a half century ago [2,3], nuclear theorists have not yet uncovered the nature of this state from first principles. In this letter we report the first ab initio calculation of the low-lying states of carbon-12 using supercomputer lattice simulations and a theoretical framework known as effective field theory. In addition to the ground state and excited spin-2 state, we find a resonance at -85(3) MeV with all of the properties of the Hoyle state and in agreement with the experimentally observed energy. These lattice simulations provide insight into the structure of this unique state and new clues as to the amount of fine-tuning needed in nature for the production of carbon in stars.Comment: 4 pp, 3 eps figs, version accepted for publication in Physical Review Letter

Multiphoton Bloch-Siegert shifts and level-splittings in spin-one systems

We consider a spin-boson model in which a spin 1 system is coupled to an oscillator. A unitary transformation is applied which allows a separation of terms responsible for the Bloch-Siegert shift, and terms responsible for the level splittings at anticrossings associated with Bloch-Siegert resonances. When the oscillator is highly excited, the system can maintain resonance for sequential multiphoton transitions. At lower levels of excitation, resonance cannot be maintained because energy exchange with the oscillator changes the level shift. An estimate for the critical excitation level of the oscillator is developed.Comment: 14 pages, 3 figure

Generation and evaluation of business continuity processes using algebraic graph transformation and the mCRL2 process algebra

Critical business processes can fail. Therefore, continuity processes are needed as back-up solutions. Today, those continuity processes are set up and maintained manually. They are mostly based on best practices that focus on specific continuity scenarios, Nevertheless, failures can occur in new and unforeseen combinations. As a consequence, a given business continuity plan needs to handle such situations as well. For this purpose, we present a technique for the generation and validation of the universe of continuity processes given a critical business process at Credit Suisse. The presented approach uses a combination of formal methods in the area of algebraic graph transformation and process algebra encompassing modal logic. The overall approach prepares for a sound evaluation of the effectiveness and efficiency of such plans. It uses formal tools, not standard software engineering solutions, to benefit from formal guarantees that facilitate the implementation of local and global security requirements. Keywords: business continuity, business process, algebraic graph transformation, process algebra, generation, evaluation, enterprise modelin

Possible mechanism for achieving glass-like thermal conductivities in crystals with off-center atoms

In the filled Ga/Ge clathrate, Eu and Sr are off-center in site 2 but Ba is on-center. All three filler atoms (Ba,Eu,Sr) have low temperature Einstein modes; yet only for the Eu and Sr systems is there a large dip in the thermal conductivity, attributed to the Einstein modes. No dip is observed for Ba. Here we argue that it is the off-center displacement that is crucial for understanding this unexplained difference in behavior. It enhances the coupling between the "rattler" motion and the lattice phonons for the Eu and Sr systems, and turns on/off another scattering mechanism (for 1K < T < 20K) produced by the presence/absence of off-center sites. The random occupation of different off-center sites produces a high density of symmetry-breaking defects which scatters phonons. It may also be important for improving our understanding of other glassy systems.Comment: 4 pages, 1 figure (2 parts) -- v2: intro broadened; strengthened arguments regarding need for additional phonon scattering mechanis

Jahn-Teller Distortions and the Supershell Effect in Metal Nanowires

A stability analysis of metal nanowires shows that a Jahn-Teller deformation breaking cylindrical symmetry can be energetically favorable, leading to stable nanowires with elliptic cross sections. The sequence of stable cylindrical and elliptical nanowires allows for a consistent interpretation of experimental conductance histograms for alkali metals, including both the shell and supershell structures. It is predicted that for gold, elliptical nanowires are even more likely to form since their eccentricity is smaller than for alkali metals. The existence of certain metastable ``superdeformed'' nanowires is also predicted

Optical signature of the pressure-induced dimerization in the honeycomb iridate α\alpha-Li2_2IrO3_3

We studied the effect of external pressure on the electrodynamic properties of $\alpha$-Li$_2$IrO$_3$ single crystals in the frequency range of the phonon modes and the Ir $d$-$d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure $P_c$=3.8 GPa. With increasing pressure an overall decrease in spectral weight of the Ir $d$-$d$ transitions is found up to $P_c$. Above $P_c$, the local (on-site) $d$-$d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The non-local (intersite) Ir $d$-$d$ transitions show a monotonic blue-shift and decrease in spectral weight. The changes observed for the non-local excitations are most prominent well above $P_c$, namely for pressures $\geq$12 GPa, and only small changes occur for pressures close to $P_c$. The profile of the optical conductivity at high pressures ($\sim$20 GPa) appears to be indicative for the dimerized state in iridates.Comment: 10 pages, 6 figures; accepted for publication in Phys. Rev.

Prostaglandin E Positively Modulates Endothelial Progenitor Cell Homeostasis: An Advanced Treatment Modality for Autologous Cell Therapy

Aims: The mobilization of endothelial progenitor cells (EPC) and their functioning in postnatal neovascularization are tightly regulated. To identify new modulators of EPC homeostasis, we screened biologically active prostaglandin E compounds for their effects on EPC production, trafficking and function. Methods and Results: We found that EPC are a rich source for prostaglandin E 2 (PGE 2), stimulating their number and function in an auto- and paracrine manner. In vivo blockade of PGE 2 production by selective cyclooxygenase-2 inhibition virtually abrogated ischemia-induced EPC mobilization demonstrating its crucial role in EPC homeostasis following tissue ischemia. Conversely, ex vivo treatment of isolated EPC with the clinically approved PGE 1 analogue alprostadil enhanced EPC number and function. These effects were mediated by increased expression of the chemokine receptor CXCR4 and were dependent on nitric oxide synthase activity. Most importantly, ex vivo PGE 1 pretreatment of isolated EPC significantly enhanced their neovascularization capacity in a murine model of hind limb ischemia as assessed by laser Doppler analysis, exercise stress test and immunohistochemistry. Conclusions: The conserved role for PGE in the regulation of EPC homeostasis suggests that ex vivo modulation of the prostaglandin pathway in isolated progenitor cells may represent a novel and safe strategy to facilitate cell-based therapies. Copyright (C) 2009 S. Karger AG, Base