Preeminent role of the Van Hove singularity in the strong-coupling analysis of scanning tunneling spectroscopy for two-dimensional cuprates

In two dimensions the non-interacting density of states displays a Van Hove singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent in three dimensions. We show that due to this VHS the strong-coupling analysis of tunneling spectra in high-$T_c$ superconductors must be reconsidered. Based on a microscopic model which reproduces the experimental data with great accuracy, we elucidate the peculiar role played by the VHS in shaping the tunneling spectra, and show that more conventional analyses of strong-coupling effects can lead to severe errors.Comment: 5 pages, 4 figure

Strong-coupling analysis of scanning tunneling spectra in Bi2_2Sr2_2Ca2_2Cu3_3O10+δ_{10+\delta}

We study a series of spectra measured in the superconducting state of optimally-doped Bi-2223 by scanning tunneling spectroscopy. Each spectrum, as well as the average of spectra presenting the same gap, is fitted using a strong-coupling model taking into account the band structure, the BCS gap, and the interaction of electrons with the spin resonance. After describing our measurements and the main characteristics of the strong-coupling model, we report the whole set of parameters determined from the fits, and we discuss trends as a function of the gap magnitude. We also simulate angle-resolved photoemission spectra, and compare with recent experimental results.Comment: Published versio

Current trends in vasopressor use to the operating room : a pharmacoepidemiologic study in French teaching and military hospitals

Objectives: Phenylephrine, ephedrine and norepinephrine are the vasopressors most commonly used in the operating room to treat anaesthesia-induced hypotension. Two new diluted forms of phenylephrine were released in 2011 (500 μg/10 mL and 500 μg/5 mL). We initiated a study to evaluate trends in the use of vasopressors in the operating room in French hospitals over the period 2011–2014. Methods: We conducted a longitudinal, retrospective, observational study between 2011 and 2014 in French teaching and military hospitals. A questionnaire was sent in February 2015 to hospital pharmacists of each centre to retrospectively collect the consumption of each type of vasopressor. Yearly numbers of vasopressor ampoules were divided by the yearly numbers of anaesthetics recorded. For each vasopressor, we calculated the number of ampoules per 100 anaesthetics recorded (/100A). Results: Thirty-two hospitals (82%) completed the questionnaire. One hundred per cent of hospitals had registered the diluted form of phenylephrine (61% had chosen the dilution 500 μg/10 mL), whereas concentrated ampoules were available in 68% of hospitals. Over the period, an exponential increase in the use of diluted phenylephrine was observed (from 1.0 ampoule/100A in 2012 to 31.7 in 2014), the use of ephedrine remained stable (26 ampoules and 17 prefilled syringe/100A), and use of norepinephrine trended upwards (from 6.7 to 8.2 ampoules/100A). Conclusions: The use of diluted phenylephrine has exponentially increased without reducing consumption of other vasopressors. This trend might be secondary to practice changes in hypotension treatment following the release of French guidelines in 2013 related to fluid management, the restriction of indications of hydroxylethyl-starch solutions in 2013, and a better knowledge of the benefit of blood pressure optimisation to reduce postoperative morbidity

First direct observation of the Van Hove singularity in the tunneling spectra of cuprates

In two-dimensional lattices the electronic levels are unevenly spaced, and the density of states (DOS) displays a logarithmic divergence known as the Van Hove singularity (VHS). This is the case in particular for the layered cuprate superconductors. The scanning tunneling microscope (STM) probes the DOS, and is therefore the ideal tool to observe the VHS. No STM study of cuprate superconductors has reported such an observation so far giving rise to a debate about the possibility of observing directly the normal state DOS in the tunneling spectra. In this study, we show for the first time that the VHS is unambiguously observed in STM measurements performed on the cuprate Bi-2201. Beside closing the debate, our analysis proves the presence of the pseudogap in the overdoped side of the phase diagram of Bi-2201 and discredits the scenario of the pseudogap phase crossing the superconducting dome.Comment: 4 pages, 4 figure

Fast pyrolysis bio-oil production in an entrained flow reactor pilot

Bio-oil produced from biomass fast pyrolysis could constitute an alternative to fossil liquid fuels, especially to be combusted for local district heating. So far, only few studies have dealt with bio-oil production by biomass fast pyrolysis in an entrained flow reactor [1], yet it could constitute an alternative to the better-known fluidised bed pyrolysis process. In the context of the BOIL project with the CCIAG Company (Grenoble district heating), a new pilot based on an entrained flow reactor concept has been designed [2]. The pilot design has been carried out on the basis of woody biomass fast pyrolysis experiments and modeling performed in a drop tube reactor as a first step laboratory-scale study, and also CFD modeling [2-3]. The facility is composed of a biomass injection system with a hopper and a feeding screw, an electrically heated pyrolysis reactor, a cyclone to separate gas and char, 3 heat exchangers to cool the gas (at 30°C, 0°C and 0°C respectively) and condense bio-oil, and a post-combustion unit to burn the incondensable species. Gas temperature is maintained at 350°C from the reactor outlet to the entrance of the first heat exchanger in order to avoid bio-oil condensation. Several conditions were tested in 14 runs: 3 different biomass feedstocks, varying biomass feeding rates from 2 to 9 kg/h and two reactor temperatures 500°C and 550°C. 85 kg of bio-oil has been produced for combustion tests. Recovered bio-oil mass yield is on average 50%, its LHV is about 15 MJ/kg, its water content 26%w and its pH 2.15. We identified three main difficulties during the runs: about 15% of the bio-oil go through the heat exchanger, some char particles go through the cyclone which causes regular plugging of the first heat exchanger. Detailed analyses of the bio-oil produced have been done and the chemical and physical bio-oil characteristics have been compared to the European Standard recommendations [4]. With a regularly cleaning of the first heat exchanger, we successfully produce bio-oil with physical and chemical properties in agreement with the European Standard recommendations. Combustion tests of the bio-oil produced have been carried on by the CIRAD. They succeeded in obtaining a stable flame (without the use of a pilot flame) in a 50 kW burner and a 250 kW combustion chamber. However the physical and chemical characteristics of the bio-oil involve the use of specific pump and pulverization system adapted. In perspective for future projects, it would be interesting to perform pilot modifications in order to increase bio-oil yield and to minimize heat exchanger cleaning, and to test other resources like agricultural biomass or solid recovered fuels. Bibliography 1. J.A. Knight, C.W. Gorton, R.J. Kovac, Biomass 6, pp. 69-76, 1984. 2. Fast pyrolysis reactor for organic biomass materials with against flow injection of hot gases - US 20170166818 A1 3. Guizani, S.Valin, J.Billlaud, M.Peyrot, S.Salvador, Fuel, 2017, 207, pp.71-84. 4. C.Guizani, S.Valin, M.Peyrot, G.Ratel S.Salvador, Woody biomass fast pyrolysis in a drop tube reactor - Pyro2016 conference 5. Fast pyrolysis bio-oils for industrial boilers – Requirements and test methods – EN 1690

Magnetic glassy phase in FeSeTe single crystals

The evolution of the magnetic order in FeSeTe crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent DFT calculations both indicate that muons are implanted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase disorders progressively with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment "freezes" at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.Comment: Approved for publication in J. Phys.: Condens. Matte

Imaging the essential role of spin-fluctuations in high-Tc superconductivity

We have used scanning tunneling spectroscopy to investigate short-length electronic correlations in three-layer Bi2Sr2Ca2Cu3O(10+d) (Bi-2223). We show that the superconducting gap and the energy Omega_dip, defined as the difference between the dip minimum and the gap, are both modulated in space following the lattice superstructure, and are locally anti-correlated. Based on fits of our data to a microscopic strong-coupling model we show that Omega_dip is an accurate measure of the collective mode energy in Bi-2223. We conclude that the collective mode responsible for the dip is a local excitation with a doping dependent energy, and is most likely the (pi,pi) spin resonance.Comment: 4 pages, 4 figure

Early diagenetic vivianite [Fe-3(PO4)(2) center dot 8H(2)O] in a contaminated freshwater sediment and insights into zinc uptake: a mu-EXAFS, mu-XANES and Raman study

The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe3(PO4)2 · 8H2O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 μm) and micron-scale analysis techniques (SEM, electron microprobe, μ-EXAFS, μ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces Fe–O and Fe–P co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99–2.05 Å; 2P atoms at 3.17–3.25 Å). One analysed grain displays evidence of a significant Fe3+ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Å; 2P atoms at 3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption

Diamagnetism above Tc in underdoped Bi2.2Sr1.8Ca2Cu3O10+d

Single crystals of ${\rm Bi}_{2+x}{\rm Sr}_{2-x}{\rm Ca}_{2}{\rm Cu}_{3}{\rm O}_{10+\delta}$(Bi2223) with $x=0.2$ were grown by a traveling solvent floating zone method in order to investigate the superconducting properties of highly underdoped Bi2223.Grown crystals were characterized by X-ray diffraction, DC susceptibility and resistivity measurements, confirming Bi2223 to be the main phase.The crystals were annealed under various oxygen partial pressures to adjust their carrier densities from optimally doped to highly underdoped.The fluctuation diamagnetic component above the superconducting transition temperature $T_{\rm c}$ extracted from the anisotropic normal state susceptibilities $\chi_{ab}(T)$ ($H\perp c$) and $\chi_{c}(T)$ ($H\parallel c$) was found to increase with underdoping, suggesting a decrease in the superconducting dimensionality and/or increase in the fluctuating vortex liquid region.Comment: 6 pages, 7 figures, corrected fig.4 and references, published in J. Phys. Soc. Jpn. 79, 114711 (2010