Thermal state of superconducting films on wide substrates

The stationary thermal state and propagation of a normal zone in a long superconducting film on a wide substrate are analyzed analytically. Expressions describing voltage-current characteristics and temperature-current dependence of the film are derived for the flux creep, flux flow and normal regions. It is shown how the flux creep influences the conditions of the thermal stability. In particular, it is found that the bistability of the thermal state can appear in this regime. Under the boiling crisis, the temperature-current dependence for a film differs markedly from that for a wire and is characterized by a smooth temperature increase with the current. A "mixed" regime is analyzed where the flux flow and normal states exist simultaneously with the boundary between them parallel to the film axis. Expressions for the propagation velocity of a normal zone along a narrow film are obtained which show that this velocity in films is sufficiently higher than in wires.Comment: 17 pages, 10 figures, submitted to "Cryogenics

AC losses in superconducting composite strips in a magnetic field in the form of a standing wave

Analytical expressions for the evaluation of AC losses in a superconductor-metal composite strip in a nonuniform AC magnetic field having the form of a standing wave are derived. The considered configuration models superconducting tapes, thin films and coated superconductors. A distinctive feature of the problem is the appearance of a transverse component of the induced current that converts the problem to a two-dimensional one. Bean's critical state model for a superconductor and a low frequency approximation for a metal are used. In the framework of this approximation the influence of eddy currents in the metal on the magnetic field is neglected, and the current distribution in the superconductor is determined by an external field. Two cases were considered: (1) superconducting and metal strips electrically separated; (2) they are in an electric contact. It is shown that for most practical cases enough to take into account only the losses generated by induced longitudinal currents in a composite strip. When an electric contact between strips exists, the maximum loss density in high fields (above 2 T) can be determined by the transverse component of the current.Comment: 18 pages, 10 figure

Controlled Irradiative Formation of Penitentes

Spike-shaped structures are produced by light-driven ablation in very different contexts. Penitentes 1-4 m high are common on Andean glaciers, where their formation changes glacier dynamics and hydrology. Laser ablation can produce cones 10-100 microns high with a variety of proposed applications in materials science. We report the first laboratory generation of centimeter-scale snow and ice penitentes. Systematically varying conditions allows identification of the essential parameters controlling the formation of ablation structures. We demonstrate that penitente initiation and coarsening requires cold temperatures, so that ablation leads to sublimation rather than melting. Once penitentes have formed, further growth of height can occur by melting. The penitentes intially appear as small structures (3 mm high) and grow by coarsening to 1-5 cm high. Our results are an important step towards understanding and controlling ablation morphologies.Comment: Accepted for publication in Physical Review Letter

The NMDA receptor activation by D-serine and glycine is controlled by an astrocytic Phgdh-dependent serine shuttle

Astrocytes express the 3-phosphoglycerate dehydrogenase (Phgdh) enzyme required for the synthesis of L-serine from glucose. Astrocytic L-serine was proposed to regulate NMDAR activity by shuttling to neurons to sustain D-serine production, but this hypothesis remains untested. We now report that inhibition of astrocytic Phgdh suppressed the de novo synthesis of L-and D-serine and reduced the NMDAR synaptic potentials and long-term potentiation (LTP) at the Schaffer collaterals-CA1 synapse. Likewise, enzymatic removal of extracellular L-serine impaired LTP, supporting an L-serine shuttle mechanism between glia and neurons in generating the NMDAR coagonist D-serine. Moreover, deletion of serine racemase (SR) in glutamatergic neurons abrogated D-serine synthesis to the same extent as Phgdh inhibition, suggesting that neurons are the predominant source of the newly synthesized D-serine. We also found that the synaptic NMDAR activation in adult SR-knockout (KO) mice requires Phgdh-derived glycine, despite the sharp decline in the postnatal glycine levels as a result of the emergence of the glycine cleavage system. Unexpectedly, we also discovered that glycine regulates D-serine metabolism by a dual mechanism. The first consists of tonic inhibition of SR by intracellular glycine observed in vitro, primary cultures, and in vivo microdialysis. The second involves a transient glycine-induce D-serine release through the Asc-1 transporter, an effect abolished in Asc-1 KO mice and diminished by deleting SR in glutamatergic neurons. Our observations suggest that glycine is a multifaceted regulator of D-serine metabolism and implicate both D-serine and glycine in mediating NMDAR synaptic activation at the mature hippocampus through a Phgdh-dependent shuttle mechanism

Differential Effect of Contrast Polarity Reversals in Closed Squares and Open L-Junctions

Scene segmentation depends on interaction between geometrical and photometric factors. It has been shown that reversals in contrast polarity at points of highest orientation discontinuity along closed contours significantly impair shape discrimination performance, while changes in contrast polarity at straight(er) contour segments do not have such deleterious effects (Spehar, 2002). Here we employ (semi) high resolution fMRI (1.5 mm × 1.5 mm × 1.5 mm) to investigate the neuronal substrate underlying these perception effects. Stimuli consisted of simple elements (a) squares with contrast reversals along straight segments; (b) squares with contrast reversals in the corner (highest orientation discontinuity); (c) L-Junctions with contrast reversals along the straight ends; (d) L-Junctions with contrast reversals in the corner. Element with contrast polarity reversals are easy to distinguish though appear geometrically equivalent. For squares with contrast polarity reversals only along straight lines we find significantly lower BOLD modulation compared to any of the control conditions, which show similar responses to each other. In the light of previous psychophysical work (Elder and Zucker, 1993; Spehar, 2002) we speculate that this effect is due to closure perception. We observe this across a wide range of areas on occipital cortex

Strongly Enhanced Current Densities in Superconducting Coated Conductors of YBa2Cu3O7-x + BaZrO3

There are numerous potential applications for superconducting tapes, based on YBa2Cu3O7-x (YBCO) films coated onto metallic substrates. A long established goal of more than 15 years has been to understand the magnetic flux pinning mechanisms which allow films to maintain high current densities out to high magnetic fields. In fact, films carry 1-2 orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialisation of conductors is much closer, for both better performance and lower fabrication costs, an important goal is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route which yields a 1.5 to 5-fold improvement in the in-field current densities of already-high-quality conductors

Angular dependence of thin-film dielectric coating damage thresholds

Newman et al. reported experiments showing that the angular dependence of 351-nm laser damage thresholds in HfO{sub 2}/SiO{sub 2} multilayer dielectric reflectors was much weaker than even the 1/cos{theta} expected from simple geometric fluence dilution. Several plausible explanations were suggested, but none were convincing. We propose a simple geometric model based on a cylindrical form for the coating defect responsible for damage initiation. We have measured 248-nm damage thresholds for bare fused silica, evaporated aluminium films, and HfO{sub 2}/SiO{sub 2} and Al{sub 2}O{sub 3}/SiO{sub 2} dielectric reflectors at angles out to 85{degree}. The measured data agree well with our simple model

Evaluation of an Electro-Pneumatic Device for Artificial Capillary Pulse Generation used in a Prospective Study in Animals for Surgical Neck Wound Healing

The paper examines the development and testing of an electro-pneumatic device for wound healing therapy after surgery in the neck area. The device generates air pressure values in a miniaturized cuff using electronic circuitry to drive an electro-valve and air compressor. The device works in two distinct modes: continuous pressure mode and pulsating pressure mode. The pressure value setting can vary from 3 to 11 mmHg, and the pulsating pressure mode’s operating frequency range is approximately 0.1 to 0.3 Hz. Laboratory measurements were conducted to evaluate the device’s correct functioning in both continuous and pulsating pressure modes. A four-day prospective study with animals (n = 10) was also conducted to evaluate neck wound healing therapy using the electro-pneumatic device. Out of the twelve histological parameters analysed to reveal the differences between the experimental and control wounds, only one demonstrated a significant difference. Out of the ten animals treated with the device, three showed a significant difference in terms of benefit after therapy. We can therefore conclude that the device potentially improves the wound healing process in the neck area if the pre-set air pressure value does not exceed 8 mmHg