Light-induced removal of 180° ferroelectric domains in Rh:BaTiO₃

We show that optically-induced photorefractive space-charge fields can remove 180° ferroelectric domains in rhodium-doped barium titanate. The cross section of the domains must be small (less than 100 microns) for this process to occur

Screening of heavy quark free energies at finite temperature and non-zero baryon chemical potential

We analyze the dependence of heavy quark free energies on the baryon chemical potential (mu_b) in 2-flavour QCD using improved (p4) staggered fermions with a bare quark mass of m/T = 0.4. By performing a 6th order Taylor expansion in the chemical potential which circumvents the sign problem. The Taylor expansion coefficients of colour singlet and colour averaged free energies are calculated and from this the expansion coefficients for the corresponding screening masses are determined. We find that for small mu_b the free energies of a static quark anti-quark pair decrease in a medium with a net excess of quarks and that screening is well described by a screening mass which increases with increasing mu_b. The mu_b-dependent corrections to the screening masses are well described by perturbation theory for T > 2 T_c. In particular, we find for all temperatures above T_c that the expansion coefficients for singlet and colour averaged screening masses differ by a factor 2.Comment: 14 page

The MinK Potassium Channel Exists in Functional and Nonfunctional Forms When Expressed in the Plasma Membrane of \u3cem\u3eXenopus\u3c/em\u3e Oocytes

The minK protein induces a slowly activating voltage-dependent potassium current when expressed in Xenopus oocytes. In order to measure the levels of minK protein in the plasma membrane, we have modified the minK gene by inserting a 9 amino acid epitope into the N- terminal domain of the protein sequence. When intact live oocytes are injected with the modified minK RNA and subsequently incubated with an antibody to this epitope, specific binding is detected, indicating that the N-terminal domain is extracellular. We found that when oocytes are injected with amounts of minK mRNA up to 50 ng, the levels of protein at the surface are proportional to the amount of injected mRNA. In contrast, the amplitude of the minK current recorded in the oocytes saturates at 1 ng of injected mRNA. Although the amplitude of the currents is not altered by increasing mRNA levels above 1 ng, the kinetics of activation of the current differ in oocytes with high or low levels of minK RNA. In particular, activation is slower with higher levels of minK protein in the plasma membrane. Finally, we find that increasing intracellular cAMP levels, which increases the amplitude of minK currents, does not alter surface expression of the minK protein but produces a small increase in the rate of activation of the current. Our results support a model in which minK protein forms functional potassium channels by association with a factor endogenous to the oocyte

Modulation by cAMP of a Slowly Activating Potassium Channel Expressed in \u3cem\u3eXenopus\u3c/em\u3e Oocytes

When expressed in the Xenopus oocyte, the minK protein induces a slowly activating voltage-dependent potassium current (Isk). We studied the modulation of this current by altering intracellular cAMP levels and found that the amplitude of Isk is dramatically increased by treatments that raise cAMP levels and decreased by agents that lower cAMP levels. Preinjection of a protein inhibitor of the cAMP-dependent protein kinase blocked the effects of increased cAMP levels. There were no changes in the voltage dependence or kinetics of Isk. Mutations that eliminate a potential phosphorylation site on the minK protein did not block the effects of activating the kinase. In addition, the membrane capacitance of the oocyte increased and decreased in parallel with Isk. Our results fit a mechanism in which channel proteins are selectively inserted into and removed from the plasma membrane in response to changes in kinase activity

Electric field control of diffraction and noise in dye-doped liquid crystals

We present results on permanent gratings in highly dye-doped liquid crystal cells without polymer coatings of the cells' surfaces. The surface-mediated gratings remain in cells for months without degradation of their quality. The peak diffraction efficiency can be controlled, enhanced or decreased, by applying low voltage AC field. At low frequencies, below 1 Hz, the diffracted signal can modulated by the AC field, but the time development of the signal shows a complex response. The enhancement of diffraction can be, however, observed at all frequencies we tested (0.1 Hz – 300 kHz). The permanent gratings cannot be removed by heating above the liquid crystal phase transition temperature as on cooling the diffraction efficiency is restored

Heavy Quark Interactions and Quarkonium Binding

We consider heavy quark interactions in quenched and unquenched lattice QCD. In a region just above the deconfinement point, non-Abelian gluon polarization leads to a strong increase in the binding. Comparing quark-antiquark and quark-quark interaction, the dependence of the binding on the separation distance $r$ is found to be the same for the colorless singlet $Q\bar Q$ and the colored anti-triplet $QQ$ state. In a potential model description of in-medium $J/\Psi$ behavior, this enhancement of the binding leads to a survival up to temperatures of 1.5 $T_c$ or higher; it could also result in $J/\Psi$ flow.Comment: 8 pages, 8 Figures; invited talk at "Strangeness in Quark Matter 2008", Beijing/China, to appear in the Proceeding

Lattice constraints on the thermal photon rate

We estimate the photon production rate from an SU(3) plasma at temperatures of about 1.1Tc and 1.3Tc. Lattice results for the vector current correlator at spatial momenta k ~ (2-6)T are extrapolated to the continuum limit and analyzed with the help of a polynomial interpolation for the corresponding spectral function, which vanishes at zero frequency and matches to high-precision perturbative results at large invariant masses. For small invariant masses the interpolation is compared with the NLO weak-coupling result, hydrodynamics, and a holographic model. At vanishing invariant mass we extract the photon rate which for k \gsim 3T is found to be close to the NLO weak-coupling prediction. For k \lsim 2T uncertainties remain large but the photon rate is likely to fall below the NLO prediction, in accordance with the onset of a strongly interacting behaviour characteristic of the hydrodynamic regime.Comment: 20 pages. v2: clarifications adde

Conjugate gradient solvers on Intel Xeon Phi and NVIDIA GPUs

Lattice Quantum Chromodynamics simulations typically spend most of the runtime in inversions of the Fermion Matrix. This part is therefore frequently optimized for various HPC architectures. Here we compare the performance of the Intel Xeon Phi to current Kepler-based NVIDIA Tesla GPUs running a conjugate gradient solver. By exposing more parallelism to the accelerator through inverting multiple vectors at the same time, we obtain a performance greater than 300 GFlop/s on both architectures. This more than doubles the performance of the inversions. We also give a short overview of the Knights Corner architecture, discuss some details of the implementation and the effort required to obtain the achieved performance.Comment: 7 pages, proceedings, presented at 'GPU Computing in High Energy Physics', September 10-12, 2014, Pisa, Ital

Determination of Freeze-out Conditions from Lattice QCD Calculations

Freeze-out conditions in Heavy Ion Collisions are generally determined by comparing experimental results for ratios of particle yields with theoretical predictions based on applications of the Hadron Resonance Gas model. We discuss here how this model dependent determination of freeze-out parameters may eventually be replaced by theoretical predictions based on equilibrium QCD thermodynamics.Comment: presented at the International Conference "Critical Point and Onset of Deconfinement - CPOD 2011", Wuhan, November 7-11, 201