Melting and unzipping of DNA

Experimental studies of the thermal denaturation of DNA yield a strong indication that the transition is first order. This transition has been theoretically studied since the early sixties, mostly within an approach in which the microscopic configurations of a DNA molecule are given by an alternating sequence of non-interacting bound segments and denaturated loops. Studies of these models neglect the repulsive, self-avoiding, interaction between different loops and segments and have invariably yielded continuous denaturation transitions. In this study we exploit recent results on scaling properties of polymer networks of arbitrary topology in order to take into account the excluded-volume interaction between denaturated loops and the rest of the chain. We thus obtain a first-order phase transition in d=2 dimensions and above, in agreement with experiments. We also consider within our approach the unzipping transition, which takes place when the two DNA strands are pulled apart by an external force acting on one end. We find that the unzipping transition is also first order. Although the denaturation and unzipping transitions are thermodynamically first order, they do exhibit critical fluctuations in some of their properties. For instance, the loop size distribution decays algebraically at the transition and the length of the denaturated end segment diverges as the transition is approached. We evaluate these critical properties within our approach.Comment: 12pages,8 figures, REVTEX

Rolling friction of a hard cylinder on a viscous plane

The resistance against rolling of a rigid cylinder on a flat viscous surface is investigated. We found that the rolling-friction coefficient reveals strongly non-linear dependence on the cylinder's velocity. For low velocity the rolling-friction coefficient rises with velocity due to increasing deformation rate of the surface. For larger velocity, however, it decreases with velocity according to decreasing contact area and deformation of the surface.Comment: 7 pages, 3 figure

PPLN Waveguide for Quantum Communication

We report on energy-time and time-bin entangled photon-pair sources based on a periodically poled lithium niobate (PPLN) waveguide. Degenerate twin photons at 1314 nm wavelength are created by spontaneous parametric down-conversion and coupled into standard telecom fibers. Our PPLN waveguide features a very high conversion efficiency of about 10^(-6), roughly 4 orders of magnitude more than that obtained employing bulk crystals. Even if using low power laser diodes, this engenders a significant probability for creating two pairs at a time - an important advantage for some quantum communication protocols. We point out a simple means to characterize the pair creation probability in case of a pulsed pump. To investigate the quality of the entangled states, we perform photon-pair interference experiments, leading to visibilities of 97% for the case of energy-time entanglement and of 84% for the case of time-bin entanglement. Although the last figure must still be improved, these tests demonstrate the high potential of PPLN waveguide based sources to become a key element for future quantum communication schemesComment: 11 pages, 9 figures, submitted to the European Physical Journal D (special issue of the Quick conference

Structurally Constrained Protein Evolution: Results From a Lattice Simulation

. We simulate the evolution of a protein-like sequence subject to point mutations, imposing conservation of the ground state, thermodynamic stability and fast folding. Our model is aimed at describing neutral evolution of natural proteins. We use a cubic lattice model of the protein structure and test the neutrality conditions by extensive Monte Carlo simulations. We observe that sequence space is traversed by neutral networks, i.e. sets of sequences with the same fold connected by point mutations. Typical pairs of sequences on a neutral network are nearly as di#erent as randomly chosen sequences. The fraction of neutral neighbors has strong sequence to sequence variations, which influence the rate of neutral evolution. In this paper we study the thermodynamic stability of di#erent protein sequences. We relate the high variability of the fraction of neutral mutations to the complex energy landscape within a neutral network, arguing that valleys in this landscape are associated..

Astrophysical and Cosmological Neutrinos

Introduction The connection between neutrinos and astrophysics and cosmology is one of the traditional pillars of astroparticle physics. On the one hand side the intrinsic properties of neutrinos are dicult to measure; the \heavenly laboratories" provide invaluable complementary information [1, 2, 3]. On the other hand side neutrinos dominate the dynamics of the radiation dominated universe and of core-collapse supernovae and are important cooling agents even for ordinary stars. Knowing the intrinsic neutrino properties is crucial for our understanding of various astrophysical and cosmological phenomena. Yet the focus of neutrino astrophysics and cosmology is changing in the light of what is beginning to be the established wisdom. Pure laboratory experiments will soon overtake solar and atmospheric neutrinos at measuring the mixing parameters. While precision cosmology continues to provide the most restrictive limit on neutrino masses, the importance of astrophysics and cosmology a