Buneman instability in a magnetized current-carrying plasma with velocity shear

Buneman instability is often driven in magnetic reconnection. Understanding how velocity shear in the beams driving the Buneman instability affects the growth and saturation of waves is relevant to turbulence, heating, and diffusion in magnetic reconnection. Using a Mathieu-equation analysis for weak cosine velocity shear together with Vlasov simulations, the effects of shear on the kinetic Buneman instability are studied in a plasma consisting of strongly magnetized electrons and cold unmagnetized ions. In the linearly unstable phase, shear enhances the coupling between oblique waves and the sheared electron beam, resulting in a wider range of unstable eigenmodes with common lower growth rates. The wave couplings generate new features of the electric fields in space, which can persist into the nonlinear phase when electron holes form. Lower hybrid instabilities simultaneously occur at $k_{\shortparallel}/k_{\perp} \sim \sqrt{m_e/m_i}$ with a much lower growth rate, and are not affected by the velocity shear.Comment: Accepted by Physics of Plasm

Open charm tomography of cold nuclear matter

We study the relative contribution of partonic sub-processes to D meson production and D meson-triggered inclusive di-hadrons to lowest order in perturbative QCD. While gluon fusion dominates the creation of large angle DD-bar pairs, charm on light parton scattering determines the yield of single inclusive D mesons. The distinctly different non-perturbative fragmentation of c quarks into D mesons versus the fragmentation of quarks and gluons into light hadrons results in a strong transverse momentum dependence of anticharm content of the away-side charm-triggered jet. In p+A reactions, we calculate and resum the coherent nuclear-enhanced power corrections from the final state partonic scattering in the medium. We find that single and double inclusive open charm production can be suppressed as much as the yield of neutral pions from dynamical high-twist shadowing. Effects of energy loss in p+A collisions are also investigated phenomenologically and may lead to significantly weaker transverse momentum dependence of the nuclear attenuation.Comment: 24 pages, 21 figure

Characterizing the Hofstadter butterfly's outline with Chern numbers

In this work, we report original properties inherent to independent particles subjected to a magnetic field by emphasizing the existence of regular structures in the energy spectrum's outline. We show that this fractal curve, the well-known Hofstadter butterfly's outline, is associated to a specific sequence of Chern numbers that correspond to the quantized transverse conductivity. Indeed the topological invariant that characterizes the fundamental energy band depicts successive stairways as the magnetic flux varies. Moreover each stairway is shown to be labeled by another Chern number which measures the charge transported under displacement of the periodic potential. We put forward the universal character of these properties by comparing the results obtained for the square and the honeycomb geometries.Comment: Accepted for publication in J. Phys. B (Jan 2009

Observation of discrete energy levels in a quantum confined system

Low temperature scanning tunneling microscope images and spectroscopic data have been obtained on subnanometer size Pb clusters fabricated using the technique of buffer layer assisted growth. Discrete energy levels were resolved in current-voltage characteristics as current peaks rather than current steps. Distributions of peak voltage spacings and peak current heights were consistent with Wigner-Dyson and Porter-Thomas distributions respectively, suggesting the relevance of random matrix theory to the description of the electronic eigenstates of the clusters. The observation of peaks rather than steps in the current-voltage characteristics is attributed to a resonant tunneling process involving the discrete energy levels of the cluster, the tip, and the states at the interface between the cluster and the substrate surface.Comment: 4 pages, 4 figure

The two components of the SO(3)-character space of the fundamental group of a closed surface of genus 2

We use geometric techniques to explicitly find the topological structure of the space of SO(3)-representations of the fundamental group of a closed surface of genus 2 quotient by the conjugation action by SO(3). There are two components of the space. We will describe the topology of both components and describe the corresponding SU(2)-character spaces by parametrizing them by spherical triangles. There is the sixteen to one branch-covering for each component, and the branch locus is a union of 2-spheres or 2-tori. Along the way, we also describe the topology of both spaces. We will later relate this result to future work into higher-genus cases and the SL(3,R)-representations

Partial survival and inelastic collapse for a randomly accelerated particle

We present an exact derivation of the survival probability of a randomly accelerated particle subject to partial absorption at the origin. We determine the persistence exponent and the amplitude associated to the decay of the survival probability at large times. For the problem of inelastic reflection at the origin, with coefficient of restitution $r$, we give a new derivation of the condition for inelastic collapse, $r<r_c=e^{-\pi/\sqrt{3}}$, and determine the persistence exponent exactly.Comment: 6 page