Microfluidic simulation of a colonial diatom chain reveals oscillatory movement

Diatoms are single-celled organisms with rigid parts in relative motion at the micro- and nanometer length scales. Some diatom species form colonies comprising many cells. In this manuscript, the results of a two-dimensional finite element computer model are presented. This model was established to discover if diatom colonies start to exhibit some kind of »pumping« behaviour when subjected to water flow. To analyze this computationally, a model diatom colony with continuously repeated units of ten cells is investigated in a fluid dynamic simulation. In this first simple model, undisturbed fluid flow is allowed for between the single cells. The cells do not move actively, and are solely moved by the water. The initial fluid velocity is assumed between 0.01 m s–1 and 1 m s–1. Acomputational result that does not change anymore with time is called a steady state solution. Such a steady state solution is reached in all calculations performed. The steady state for a chain where initially all diatoms are spaced equally (equidistant spacing) has forces that encourage the formation of cell pairs with less distance between one another. These forces result from the flow of the surrounding fluid. The steady state for a chain with initially paired cells shows the opposite effect: the pairs tend to un-pair and head for the equidistant state again. The mutual change in forces between these two states, i.e., paired and unpaired formations, suggests that these two steady states lead into each other: The computer simulations suggest that a diatom colony subjected to water flow exhibits some kind of oscillatory movement. Such movement might facilitate nutrient uptake of the diatom colony

Orbital ordering transition in Ca2_2RuO4_4 observed with resonant x-ray diffraction

Resonant x-ray diffraction performed at the $\rm L_{II}$ and $\rm L_{III}$ absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca$_2$RuO$_4$ single crystals. A large resonant enhancement due to electric dipole $2p\to 4d$ transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at $\rm T_{N}=110$ K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru $t_{2g}$ electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca$_2$RuO$_4$.Comment: to appear in PR

Continuous and Discontinuous Quantum Phase Transitions in a Model Two-Dimensional Magnet

The Shastry-Sutherland model, which consists of a set of spin 1/2 dimers on a 2-dimensional square lattice, is simple and soluble, but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry-Sutherland material, SrCu2(BO3)2, in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution x-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure.Comment: 16 pages, 4 figures. Accepted for publication in PNA

A comparison of an elliptical multipole wiggler and crystal optics for the production of circularly polarized x-rays

Recently, there has been a great deal of interest in polarization modulated x-ray diffraction and spectroscopy techniques. In particular, the importance of photon helicity in spin-dependent magnetic interactions has expanded the need for high quality circularly polarized x-ray sources with fast switching capabilities. Because circularly polarized photons couple differently with the magnetic moment of an atom than do neutrons, they are able to provide unique magnetic information not accessible by neutron techniques. The development of experiments utilizing circularly polarized x-rays, however, has been hampered by the lack of efficient sources. Two different approaches for the production of circularly polarized x-rays have attracted the most attention; (i) employing specialized insertion devices, and (ii) utilizing x-ray phase retarders based on perfect crystal optics. For soft x-rays (0.1--3.0 keV), source development has centered primarily on insertion devices because there are currently no crystal or multilayer polarizing optics available that cover that full energy range. For harder x-rays (>3.0 keV), however, phase retarding optics have been demonstrated, but whether these optics or insertion devices provide the most efficient circularly polarized x-ray source in this energy regime has remained a matter of contention. Advocates of each method have made qualitative statements about their advantages, i.e., insertion devices provide a larger flux and phase retarders provide a higher degree of circular polarization, yet a detailed quantitative comparison has been lacking. In this paper, we attempt to provide such a comparison by examining the efficiencies of an elliptical multipole wiggler (EMW) and a standard undulator followed by phase retarding crystal optics

Chromium at High Pressures: Weak Coupling and Strong Fluctuations in an Itinerant Antiferromagnet

The spin- and charge-density-wave order parameters of the itinerant antiferromagnet chromium are measured directly with non-resonant x-ray diffraction as the system is driven towards its quantum critical point with high pressure using a diamond anvil cell. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge, while the evolution of the incommensurate ordering vector provides important insight into the difference between pressure and chemical doping as means of driving quantum phase transitions. Measurement of the charge density wave over more than two orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly-coupled, BCS-like ground state. Evidence for the coexistence of this weakly-coupled ground state with high-energy excitations and pseudogap formation above the ordering temperature in chromium, the charge-ordered perovskite manganites, and the blue bronzes, among other such systems, raises fundamental questions about the distinctions between weak and strong coupling.Comment: 11 pages, 9 figures (8 in color

Charge-magnetic interference resonant scattering studies of ferromagnetic crystals and thin films

The element- and site-specificity of X-ray resonant magnetic scattering (XRMS) makes it an ideal tool for furthering our understanding of complex magnetic systems. In the hard X-rays, XRMS is readily applied to most antiferromagnets where the relatively weak resonant magnetic scattering (10−2–10−6 I c ) is separated in reciprocal space from the stronger, Bragg charge scattered intensity, I c . In ferro(ferri)magnetic materials, however, such separation does not occur and measurements of resonant magnetic scattering in the presence of strong charge scattering are quite challenging. We discuss the use of charge-magnetic interference resonant scattering for studies of ferromagnetic (FM) crystals and layered films. We review the challenges and opportunities afforded by this approach, particularly when using circularly polarized X-rays. We illustrate current capabilities at the Advanced Photon Source with studies aimed at probing site-specific magnetism in ferromagnetic crystals, and interfacial magnetism in films

A 4-unit-cell superstructure in optimally doped YBa2Cu3O6.92 superconductor

Using high-energy diffraction we show that a 4-unit-cell superstructure, q0=(1/4,0,0), along the shorter Cu-Cu bonds coexists with superconductivity in optimally doped YBCO. A complex set of anisotropic atomic displacements on neighboring CuO chain planes, BaO planes, and CuO2 planes, respectively, correlated over ~3-6 unit cells gives rise to diffuse superlattice peaks. Our observations are consistent with the presence of Ortho-IV nanodomains containing these displacements.Comment: Corrected typo in abstrac

Iterated Moire Maps and Braiding of Chiral Polymer Crystals

In the hexagonal columnar phase of chiral polymers a bias towards cholesteric twist competes with braiding along an average direction. When the chirality is strong, screw dislocations proliferate, leading to either a tilt grain boundary phase or a new "moire state" with twisted bond order. Polymer trajectories in the plane perpendicular to their average direction are described by iterated moire maps of remarkable complexity.Comment: 10 pages (plain tex) 3 figures uufiled and appende