Quantum phases in a resonantly interacting boson-fermion mixture

We consider a resonantly interacting boson-fermion mixture of 40K and 87Rb atoms in an optical lattice. We show that by using a red-detuned optical lattice the mixture can be accurately described by a generalized Hubbard model for 40K and 87Rb atoms, and 40K-87Rb molecules. The microscopic parameters of this model are fully determined by the details of the optical lattice and the interspecies Feshbach resonance in the absence of the lattice. We predict a quantum phase transition to occur in this system already at low atomic filling fraction, and present the phase diagram as a function of the temperature and the applied magnetic field.     

New source of dense, cryogenic positron plasmas

We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10(-9) mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10(-13) mbar. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than 1.2 x 10(9) positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 x 10(6) positrons were compressed to a density of 2.6 x 10(10) cm(-3). This is a factor of 6 improvement over earlier measurements.     

Omega- and Omega+ production in central Pb + Pb collisions at 40 and 158A GeV

Results are presented on Omega production in central Pb+Pb collisions at 40 and 158A GeV beam energy. For the first time in heavy ion reactions, rapidity distributions and total yields were measured for the sum Omega(-) + Omega(+) at 40A GeV and for Omega(-) and Omega(+) separately at 158A GeV. The yields are strongly underpredicted by the string-hadronic UrQMD model but agree better with predictions from hadron gas models.     

Dynamical arrest in attractive colloids: the effect of long-range repulsion

We study gelation in suspensions of model colloidal particles with short-ranged attractive and long-ranged repulsive interactions by means of three-dimensional fluorescence confocal microscopy. At low packing fractions, particles form stable equilibrium clusters. Upon increasing the packing fraction the clusters grow in size and become increasingly anisotropic until finally associating into a fully connected network at gelation. We find a surprising order in the gel structure. Analysis of spatial and orientational correlations reveals that the gel is composed of dense chains of particles constructed from face-sharing tetrahedral clusters. Our findings imply that dynamical arrest occurs via cluster growth and association.     

Picosecond thermometer in the amide I band of myoglobin

The amide I and II bands in myoglobin show a heterogeneous temperature dependence, with bands at 6.17 and 6.43 microm which are more intense at low temperatures. The amide I band temperature dependence is on the long wavelength edge of the band, while the short wavelength side has almost no temperature dependence. We compare concepts of anharmonic solid-state crystal physics and chemical physics for the origins of these bands. We suggest that the long wavelength side is composed of those amino acids which hydrogen bond to the hydration shell of the protein, and that temperature dependent bands can be used to determine the time it takes vibrational energy to flow into the hydration shell. We determine that vibrational energy flow to the hydration shell from the amide I takes approximately 20 ps to occur.     

Single-molecule pump-probe detection resolves ultrafast pathways in individual and coupled quantum systems

We report the first experimental study of individual molecules with femtosecond time resolution using a novel ultrafast single-molecule pump-probe method. A wide range of relaxation times from below 100 up to 400 fs is found, revealing energy redistribution over different vibrational modes and phonon coupling to the nanoenvironment. Addressing quantum-coupled molecules we find longer decay times, pointing towards inhibited intramolecular decay due to delocalized excitation. Interestingly, each individual system shows discrete jumps in femtosecond response, reflecting sudden breakup of the coupled superradiant state.     

Characteristics of distortion product otoacoustic emissions in the frog from L1,L2 maps

For a given set of stimulus frequencies (f1 ,f2), the level of distortion product otoacoustic emissions (DPOAEs) varies with the levels of the stimulus tones. By variation of the stimulus levels, L1,L2-maps for DPOAEs can be constructed. Here, we report on L1 ,L2-maps for DPOAEs from the frog ear. In general, these maps were similar to those obtained from the mammalian cochlea. We found a conspicuous difference between the equal-level contour lines for low-level and high-level DPOAEs, which could be modeled by a saturating and an expansive nonlinearity, respectively. The transition from the high-level to the low-level response was accompanied by a DPOAE phase-change, which increased from 0 to pi rad with increasing frequency. These results suggest that in the frog low-level and high-level DPOAEs are generated by separate nonlinear mechanisms. Also, there was a conspicuous difference in the growth of the low-level emissions from the two anuran auditory papillae. In the basilar papilla, this growth was expansive for the lowest stimulus levels and saturated for intermediate levels. This is consistent with the behavior of a Boltzman nonlinearity. In the amphibian papilla this growth was compressive, suggesting the additional effect of a compressive amplification mechanism on the generation of DPOAEs.     

Provable First-Order Transitions for Nonlinear Vector and Gauge Models with Continuous Symmetries

We consider various sufficiently nonlinear vector models of ferromagnets, of nematic liquid crystals and of nonlinear lattice gauge theories with continuous symmetries. We show, employing the method of Reflection Positivity and Chessboard Estimates, that they all exhibit first-order transitions in the temperature, when the nonlinearity parameter is large enough. The results hold in dimension 2 or more for the ferromagnetic models and the RP^N–1 liquid crystal models and in dimension 3 or more for the lattice gauge models. In the two-dimensional case our results clarify and solve a recent controversy about the possibility of such transitions. For lattice gauge models our methods provide the first proof of a first-order transition in a model with a continuous gauge symmetry.Acknowledgement We thank in particular E. Domany and A. Schwimmer who suggested to us to consider lattice gauge models, and also L. Chayes, D. v.d. Marel, A. Messager, K. Netocný, S. Romano and A. Sokal for stimulating discussions and/or correspondence. S.S. acknowledges the financial support of the RFFI grant 03-01-00444.     

Drag reduction in bubbly Taylor-Couette turbulence

In Taylor-Couette flow the total energy dissipation rate and therefore the drag can be determined by measuring the torque on the system. We do so for Reynolds numbers between Re=7 x 10(4) and Re=10(6) after having injected (i) small bubbles (R=1 mm) up to a volume concentration of alpha=5% and (ii) buoyant particles (rhop/rhol=0.14) of comparable volume concentration. In case (i) we observe a crossover from little drag reduction at smaller Re to strong drag reduction up to 20% at Re=10(6). In case (ii) we observe at most little drag reduction throughout. Several theoretical models for bubbly drag reduction are discussed in view of our findings.     

Neutral-pion photoproduction off protons in the energy range 0.3 GeV<E(gamma)<3 GeV

Single pi(0) photoproduction has been studied with the CB-ELSA experiment at Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental setup covers a very large solid angle of approximately 98% of 4pi. Differential cross sections dsigma/dOmega have been measured. Complicated structures in the angular distributions indicate a variety of different resonances being produced in the s channel intermediate state gammap-->N(*)(Delta(*))-->ppi(0). A combined analysis including the data presented in this letter along with other data sets reveals contributions from known resonances and evidence for a new resonance N(2070)D15.