Pinning of stripes by local structural distortions in cuprate high-Tc superconductors

We study the spin-density wave (stripe) instability in lattices with mixed low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) crystal symmetry. Within an explicit mean-field model it is shown how local LTT regions act as pinning centers for static stripe formation. We calculate the modulations in the local density of states near these local stripe regions and find that mainly the coherence peaks and the van Hove singularity (VHS) are spatially modulated. Lastly, we use the real-space approach to simulate recent tunneling data in the overdoped regime where the VHS has been detected by utilizing local normal state regions.Comment: Conference proceedings for Stripes1

Kinect Depth Sensor Evaluation for Computer Vision Applications

This technical report describes our evaluation of the Kinect depth sensor by Microsoft for Computer Vision applications. The depth sensor is able to return images like an ordinary camera, but instead of color, each pixel value represents the distance to the point. As such, the sensor can be seen as a range- or 3D-camera. We have used the sensor in several different computer vision projects and this document collects our experiences with the sensor. We are only focusing on the depth sensing capabilities of the sensor since this is the real novelty of the product in relation to computer vision. The basic technique of the depth sensor is to emit an infrared light pattern (with an IR laser diode) and calculate depth from the reflection of the light at different positions (using a traditional IR sensitive camera). In this report, we perform an extensive evaluation of the depth sensor and investigate issues such as 3D resolution and precision, structural noise, multi-cam setups and transient response of the sensor. The purpose is to give the reader a well-founded background to choose whether or not the Kinect sensor is applicable to a specific problem

Thermodynamic gauge-theory cascade

It is proposed that the cooling of a thermalized SU($N$) gauge theory can be formulated in terms of a cascade involving three effective theories with successively reduced (and spontaneously broken) gauge symmetries, SU($N$) $\to$ U(1)$^{N-1}$ $\to$ Z$_N$. The approach is based on the assumption that away from a phase transition the bulk of the quantum interaction inherent to the system is implicitly encoded in the (incomplete) classical dynamics of a collective part made of low-energy condensed degrees of freedom. The properties of (some of the) statistically fluctuating fields are determined by these condensate(s). This leads to a quasi-particle description at tree-level. It appears that radiative corrections, which are sizable at large gauge coupling, do not change the tree-level picture qualitatively. The thermodynamic self-consistency of the quasi-particle approach implies nonperturbative evolution equations for the associated masses. The temperature dependence of these masses, in turn, determine the evolution of the gauge coupling(s). The hot gauge system approaches the behavior of an ideal gas of massless gluons at asymptotically large temperature. A negative equation of state is possible at a stage where the system is about to settle into the phase of the (spontaneously broken) Z$_N$ symmetry.Comment: 25 pages, 6 figures, 1 reference added, minor corrections in text, errors in Sec. 3.2 corrected, PRD versio

Sharp contrasts between freshwater and marine microbial enzymatic capabilities, community composition, and DOM pools in a NE Greenland fjord

Increasing glacial discharge can lower salinity and alter organic matter (OM) supply in fjords, but assessing the biogeochemical effects of enhanced freshwater fluxes requires understanding of microbial interactions with OM across salinity gradients. Here, we examined microbial enzymatic capabilities—in bulk waters (nonsize-fractionated) and on particles (≥ 1.6 μm)—to hydrolyze common OM constituents (peptides, glucose, polysaccharides) along a freshwater–marine continuum within Tyrolerfjord-Young Sound. Bulk peptidase activities were up to 15-fold higher in the fjord than in glacial rivers, whereas bulk glucosidase activities in rivers were twofold greater, despite fourfold lower cell counts. Particle-associated glucosidase activities showed similar trends by salinity, but particle-associated peptidase activities were up to fivefold higher—or, for several peptidases, only detectable—in the fjord. Bulk polysaccharide hydrolase activities also exhibited freshwater–marine contrasts: xylan hydrolysis rates were fivefold higher in rivers, while chondroitin hydrolysis rates were 30-fold greater in the fjord. Contrasting enzymatic patterns paralleled variations in bacterial community structure, with most robust compositional shifts in river-to-fjord transitions, signifying a taxonomic and genetic basis for functional differences in freshwater and marine waters. However, distinct dissolved organic matter (DOM) pools across the salinity gradient, as well as a positive relationship between several enzymatic activities and DOM compounds, indicate that DOM supply exerts a more proximate control on microbial activities. Thus, differing microbial enzymatic capabilities, community structure, and DOM composition—interwoven with salinity and water mass origins—suggest that increased meltwater may alter OM retention and processing in fjords, changing the pool of OM supplied to coastal Arctic microbial communities

Sharp contrasts between freshwater and marine microbial enzymatic capabilities, community composition, and DOM pools in a NE Greenland fjord

Increasing glacial discharge can lower salinity and alter organic matter (OM) supply in fjords, but assessing the biogeochemical effects of enhanced freshwater fluxes requires understanding of microbial interactions with OM across salinity gradients. Here, we examined microbial enzymatic capabilities—in bulk waters (nonsize-fractionated) and on particles (≥ 1.6 μm)—to hydrolyze common OM constituents (peptides, glucose, polysaccharides) along a freshwater–marine continuum within Tyrolerfjord-Young Sound. Bulk peptidase activities were up to 15-fold higher in the fjord than in glacial rivers, whereas bulk glucosidase activities in rivers were twofold greater, despite fourfold lower cell counts. Particle-associated glucosidase activities showed similar trends by salinity, but particle-associated peptidase activities were up to fivefold higher—or, for several peptidases, only detectable—in the fjord. Bulk polysaccharide hydrolase activities also exhibited freshwater–marine contrasts: xylan hydrolysis rates were fivefold higher in rivers, while chondroitin hydrolysis rates were 30-fold greater in the fjord. Contrasting enzymatic patterns paralleled variations in bacterial community structure, with most robust compositional shifts in river-to-fjord transitions, signifying a taxonomic and genetic basis for functional differences in freshwater and marine waters. However, distinct dissolved organic matter (DOM) pools across the salinity gradient, as well as a positive relationship between several enzymatic activities and DOM compounds, indicate that DOM supply exerts a more proximate control on microbial activities. Thus, differing microbial enzymatic capabilities, community structure, and DOM composition—interwoven with salinity and water mass origins—suggest that increased meltwater may alter OM retention and processing in fjords, changing the pool of OM supplied to coastal Arctic microbial communities

Effect of an Electron-phonon Interaction on the One-electron Spectral Weight of a d-wave Superconductor

We analyze the effects of an electron-phonon interaction on the one-electron spectral weight A(k,omega) of a d_{x^2-y^2} superconductor. We study the case of an Einstein phonon mode with various momentum-dependent electron-phonon couplings and compare the structure produced in A(k,omega) with that obtained from coupling to the magnetic pi-resonant mode. We find that if the strength of the interactions are adjusted to give the same renormalization at the nodal point, the differences in A(k,omega) are generally small but possibly observable near k=(pi,0).Comment: 10 pages, 14 figures (color versions of Figs. 2,4,10,11,12 available upon request

Magnetic Oscillations in Dense Cold Quark Matter with Four-Fermion Interactions

The phase structures of Nambu-Jona-Lasinio models with one or two flavours have been investigated at non-zero values of $\mu$ and $H$, where $H$ is an external magnetic field and $\mu$ is the chemical potential. In the phase portraits of both models there arise infinitely many massless chirally symmetric phases, as well as massive ones with spontaneously broken chiral invariance, reflecting the existence of infinitely many Landau levels. Phase transitions of first and second orders and a lot of tricritical points have been shown to exist in phase diagrams. In the massless case, such a phase structure leads unavoidably to the standard van Alphen-de Haas magnetic oscillations of some thermodynamical quantities, including magnetization, pressure and particle density. In the massive case we have found an oscillating behaviour not only for thermodynamical quantities, but also for a dynamical quantity as the quark mass. Besides, in this case we have non-standard, i.e. non-periodic, magnetic oscillations, since the frequency of oscillations is an $H$-dependent quantity.Comment: latex, 29 pages, 8 figure

Inverse magnetic catalysis in field theory and gauge-gravity duality

We investigate the surface of the chiral phase transition in the three-dimensional parameter space of temperature, baryon chemical potential and magnetic field in two different approaches, the field-theoretical Nambu-Jona-Lasinio (NJL) model and the holographic Sakai-Sugimoto model. The latter is a top-down approach to a gravity dual of QCD with an asymptotically large number of colors and becomes, in a certain limit, dual to an NJL-like model. Our main observation is that, at nonzero chemical potential, a magnetic field can restore chiral symmetry, in apparent contrast to the phenomenon of magnetic catalysis. This "inverse magnetic catalysis" occurs in the Sakai-Sugimoto model and, for sufficiently large coupling, in the NJL model and is related to the physics of the lowest Landau level. While in most parts our discussion is a pedagogical review of previously published results, we include new analytical results for the NJL approach and a thorough comparison of inverse magnetic catalysis in the two approaches.Comment: 37 pages, 11 figures, to appear in Lect. Notes Phys. "Strongly interacting matter in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A. Schmitt, H.-U. Ye